So I have been daily driving my 84 fiero for a few weeks now that I have my license (it's the 4 speed manual and that has been one heck of a challenge for me but I'm doing pretty good with it). However, unfortunately I have come across a terrible problem, which has happened to me a few times now and left me in some interesting situations. The best way I can describe the issue is that when it happens (when is only when the engine is warmed up) it loses power completely with no response from throttle, no matter if it's 1/4 or wide open, and then the power will come back, for about a second or 2, and this fluctuation with continue to occur until the engine stalls (if it's idling) or is turned off. The previous owner removed the EGR, and the catalytic converter has also been removed. This most commonly happens to me when I come to a stop after driving, though if I let it idle for long enough (~30 minutes) it will also happen. I'm honestly a little bummed out on this one, so if anyone has any good ideas on what might cause this please let me know. Not sure if this has anything to do with the issue but just in case, this morning when I was throttling down on the engine, and then suddenly hit the brakes, which caused the low oil light to come on (despite the oil being full), and after restarting the engine the low oil light was gone. any help or advice is greatly appreciated. Thanks!!
You don't say whether or not the engine will immediately restart after it's stalled.
quote
Originally posted by ironpuke84:
Not sure if this has anything to do with the issue but just in case, this morning when I was throttling down on the engine, and then suddenly hit the brakes, which caused the low oil light to come on (despite the oil being full), and after restarting the engine the low oil light was gone.
If the engine stalled, then it's normal for the oil light to come on.
Friend, its always nice to talk about issues with a 41 year old antique vehicle and discuss repair tips but there needs to be a starting point on your end. Losing power and stalling out could be any number of problems. Problems such as you have reported are usually based on lack of fuel or poor spark. If you have good fuel pressure Then you focus on the ignition system. if the problem is Spark replacing the ignition module often does it but just throwing on parts to solve a problem without proper diagnosis doesn't usually cut it. Time to invest in a few tools, and a good repair manual. that will point you to the diagnostic steps. IMO, If you are looking for daily driver/rock solid reliability sadly an antique 41 year old car won't provide it
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So I have been daily driving my 84 fiero for a few weeks now that I have my license (it's the 4 speed manual and that has been one heck of a challenge for me but I'm doing pretty good with it). However, unfortunately I have come across a terrible problem, which has happened to me a few times now and left me in some interesting situations. The best way I can describe the issue is that when it happens (when is only when the engine is warmed up) it loses power completely with no response from throttle, no matter if it's 1/4 or wide open, and then the power will come back, for about a second or 2, and this fluctuation with continue to occur until the engine stalls (if it's idling) or is turned off. The previous owner removed the EGR, and the catalytic converter has also been removed. This most commonly happens to me when I come to a stop after driving, though if I let it idle for long enough (~30 minutes) it will also happen. I'm honestly a little bummed out on this one, so if anyone has any good ideas on what might cause this please let me know. Not sure if this has anything to do with the issue but just in case, this morning when I was throttling down on the engine, and then suddenly hit the brakes, which caused the low oil light to come on (despite the oil being full), and after restarting the engine the low oil light was gone. any help or advice is greatly appreciated. Thanks!!
There's a few things it could be... the first thing I would ask is, are you getting any error codes?
Report back and let us know if you have any error codes.
Next, my first thought is that it's like a fuel pressure issue... but let me know first if you have any errors. I will say that, when running well, the Iron Duke is lots of fun to drive with when it's a 4 cyl. My daughter is almost done completely restoring one, and it drives absolutely fantastic. Doesn't have the same power as the V6 does, but it's still a lot of fun to drive. You just have to work out all the bugs because of how old the car is.
You don't say whether or not the engine will immediately restart after it's stalled.
If the engine stalled, then it's normal for the oil light to come on.
The engine didn't stall when the oil light came on, I just turned it off immediately after it came on, fearing the worst. Also, the engine usually won't start first crank after it stalls out, it just cranks, but on the second try, it starts up and sometimes still has the issue, sometimes not. When it stalled on me in and intersection (not a great time), it started up and was lurching forward the same as when it stalled, but thankfully I was able to limp it home. Another time when my dad drove it, it started right back up like nothing (albeit the first time it was low on gas). On that not I should have mentioned I don't have a working fuel gauge and I think I ran it very low on gas yesterday (when it happened) but then after filling it up it still does it.
Friend, its always nice to talk about issues with a 41 year old antique vehicle and discuss repair tips but there needs to be a starting point on your end. Losing power and stalling out could be any number of problems. Problems such as you have reported are usually based on lack of fuel or poor spark. If you have good fuel pressure Then you focus on the ignition system. if the problem is Spark replacing the ignition module often does it but just throwing on parts to solve a problem without proper diagnosis doesn't usually cut it. Time to invest in a few tools, and a good repair manual. that will point you to the diagnostic steps. IMO, If you are looking for daily driver/rock solid reliability sadly an antique 41 year old car won't provide it
Not my daily driver, I have a second (more practical car), however after getting my license I've been driving daily driving the car for fun considering I've worked to even get it somewhat road worthy for the past 3 years, just kind of having fun with it at the moment. I plugged in an old GM OBD 2 tool to it and it doesn't really appear to be a problem with the O2 sensor or Map sensor I don't think. Pretty sure it's a fuel issue (though I could be wrong). My best guess at the moment is that the oil pressure switch is bad, so today at some point (preferably when it stops raining) I'm going to jump the wires and bypass the oil pressure switch and see if it still does it. In conclusion, I don't think it's an issue with spark or air, pretty sure it's a fuel issue (should have mentioned in the original post, my bad).
Report back and let us know if you have any error codes.
Next, my first thought is that it's like a fuel pressure issue... but let me know first if you have any errors. I will say that, when running well, the Iron Duke is lots of fun to drive with when it's a 4 cyl. My daughter is almost done completely restoring one, and it drives absolutely fantastic. Doesn't have the same power as the V6 does, but it's still a lot of fun to drive. You just have to work out all the bugs because of how old the car is.
I do find the 2.5 duke to be quite a fun little car do drive around, especially paired with the Muncie 4 speed, which has a bit more pep than the automatic version. Many a V6 Fiero owners like to say nasty things about the duke, which is unfortunate, yet besides the point. I have no codes at the moment, just a recurring one that comes on on long decelerations because of the EGR delete. I am pretty sure it's a fuel pressure issue, and my best guess at the moment is that the oil pressure switch is getting finicky at standard operating temperature and the oil viscosity change is throwing it off, making it cut the power to the fuel pump since the fuel pump is run through the oil pressure switch on Fieros. I should mention that not too long ago I put a thermostat in the car, and prior to that I was driving it without a thermostat, which made it run cold and it usually never got past the 2nd line on the temperature gauge. Thanks for the assistance once again.
making it cut the power to the fuel pump since the fuel pump is run through the oil pressure switch on Fieros.
The fuel pump is run by both the ECM and the oil pressure sender, in parallel, as a "backup"system. The ECM should be what controls the fuel pump 100% of the time through its relay, the oil pressure switch only provides an additional path for more current to flow, or in the event of the ECM fuel relay failing, as a backup power source. A bad oil sender won't do anything unless the ECM relay is also damaged.
(I'm not 100% certain what's what on the 84s, I could be wrong, but I'm fairly certain)
The oil pressure switch will not turn off the fuel pump on a Fiero. It acts as a backup circuit in case the fuel pump relay fails.
The oil pressure light comes on when oil pressure falls below a certain value, that circuitry in in the gage assembly, and has nothing to do with the switch.
Neither the oil pressure sending unit nor the switch interface with the ECU.
You can completely rule out the oil pressure sensing system from your diagnosis of the problem you are having.
quote
Originally posted by ironpuke84:
I do find the 2.5 duke to be quite a fun little car do drive around, especially paired with the Muncie 4 speed, which has a bit more pep than the automatic version. Many a V6 Fiero owners like to say nasty things about the duke, which is unfortunate, yet besides the point. I have no codes at the moment, just a recurring one that comes on on long decelerations because of the EGR delete. I am pretty sure it's a fuel pressure issue, and my best guess at the moment is that the oil pressure switch is getting finicky at standard operating temperature and the oil viscosity change is throwing it off, making it cut the power to the fuel pump since the fuel pump is run through the oil pressure switch on Fieros. I should mention that not too long ago I put a thermostat in the car, and prior to that I was driving it without a thermostat, which made it run cold and it usually never got past the 2nd line on the temperature gauge. Thanks for the assistance once again.
I do find the 2.5 duke to be quite a fun little car do drive around, especially paired with the Muncie 4 speed, which has a bit more pep than the automatic version. Many a V6 Fiero owners like to say nasty things about the duke, which is unfortunate, yet besides the point. I have no codes at the moment, just a recurring one that comes on on long decelerations because of the EGR delete. I am pretty sure it's a fuel pressure issue, and my best guess at the moment is that the oil pressure switch is getting finicky at standard operating temperature and the oil viscosity change is throwing it off, making it cut the power to the fuel pump since the fuel pump is run through the oil pressure switch on Fieros. I should mention that not too long ago I put a thermostat in the car, and prior to that I was driving it without a thermostat, which made it run cold and it usually never got past the 2nd line on the temperature gauge. Thanks for the assistance once again.
Yeah, the EGR delete definitely wouldn't cause that. Can you tell me what error code it does show (regarding the EGR)?
The EGR on the 4 cyl Fiero isn't computer controlled, so I'm wondering what error code it shoots. As you know, it's just a direct vacuum port to the throttle body. But yeah, I'm definitely thinking the fuel pump. It's hard to check fuel pressure on these things since there's no Schrader valve, but my guess is that the pump is either failing, or the hose that connects the pump to the pickup tube (in the tank) is deteriorating and thus ... not able to maintain pressure since it's just squirting all out.
Another thing you could try is rebuilding your throttle body. It's super easy to do, and the kit to do it is $50 bucks on Rock Auto. My daughter has a nice video she put on YouTube on how to rebuild the Rochester 300 TBI:
She also has other videos for removing the fuel tank, etc... so if you need to see that, check out other videos (she has one for removing the tank, and also replacing the pickup tube).
The fuel pump is run by both the ECM and the oil pressure sender, in parallel, as a "backup"system. The ECM should be what controls the fuel pump 100% of the time through its relay, the oil pressure switch only provides an additional path for more current to flow, or in the event of the ECM fuel relay failing, as a backup power source. A bad oil sender won't do anything unless the ECM relay is also damaged.
(I'm not 100% certain what's what on the 84s, I could be wrong, but I'm fairly certain)
True but that backup system is a one way thing, if the fuel relay goes bad is will run off the oil pressure switch, but not the other way around. The design was that if you lose oil pressure (which could indicate big engine problems) the engine would automatically shut off to prevent damage. however if the switch was bad then it would shut off on you. that just made me think though, i haven't seen the oil light come on... is it run to a different sensor/switch?? this is all really confusing to me honestly because i am illiterate at diagnosing stuff (or in other words, i'm stupid). thanks again.
Originally posted by 82-T/A [At Work]: Yeah, the EGR delete definitely wouldn't cause that. Can you tell me what error code it does show (regarding the EGR)?
The EGR on the 4 cyl Fiero isn't computer controlled, so I'm wondering what error code it shoots. As you know, it's just a direct vacuum port to the throttle body. But yeah, I'm definitely thinking the fuel pump. It's hard to check fuel pressure on these things since there's no Schrader valve, but my guess is that the pump is either failing, or the hose that connects the pump to the pickup tube (in the tank) is deteriorating and thus ... not able to maintain pressure since it's just squirting all out.
Another thing you could try is rebuilding your throttle body. It's super easy to do, and the kit to do it is $50 bucks on Rock Auto. My daughter has a nice video she put on YouTube on how to rebuild the Rochester 300 TBI:
She also has other videos for removing the fuel tank, etc... so if you need to see that, check out other videos (she has one for removing the tank, and also replacing the pickup tube).
Okay, thanks for the info! I can try and work on finding that code in future, but at the moment I'm not too keen on driving it for fear of stalling out in the middle of a busy intersection (again) and since it only happens on long decelerations and goes away a few seconds after i don't think i can get it at the moment, but I will try in future. The previous owner installed a fuel filter with a Schrader valve on it, but we took it off, thankfully we still have it, so maybe i might attach it back on and test what's going on. Also I've seen it stall when idling in my driveway and I didn't notice any gas or fluids on the ground, so I don't think its a fuel line. Thanks again. I may try rebuilding the throttle body, since my gas pedal is a little bit stiff at times, so I will keep the video in mind. In all honestly though, now that I think about it, I don't think I've seen that check engine light come on in a hot minute though, so I honestly don't know. I think I'll just start testing some stuff and see what I can figure out.
True but that backup system is a one way thing, if the fuel relay goes bad is will run off the oil pressure switch, but not the other way around. The design was that if you lose oil pressure (which could indicate big engine problems) the engine would automatically shut off to prevent damage. however if the switch was bad then it would shut off on you. that just made me think though, i haven't seen the oil light come on... is it run to a different sensor/switch?? this is all really confusing to me honestly because i am illiterate at diagnosing stuff (or in other words, i'm stupid). thanks again.
It would be smart if it was designed that way, but it isn't, this is well documented on this forum and in the diagrams, at least for the v6, like I said maybe 84 was weird and different, but I don't think so in this case, the 2 switches are parallel, run from the same fuse, to the fuel pump, neither one is "connected" to the other except at the very beginning and very end, they're 2 parallel systems. The ECM should run the fuel pump for a few seconds when you first turn the key on but don't start it, that's with no oil pressure, but the pump is running, same when you first start cranking, it should fire right up, even though it would take a half second or more for oil pressure to build, then another half second or more for the fuel pump to build pressure, which would result in a long delay when starting (although that can also be caused by other things to)
[This message has been edited by 1985 Fiero GT (edited 10-30-2025).]
Originally posted by ironpuke84: Okay, thanks for the info! I can try and work on finding that code in future, but at the moment I'm not too keen on driving it for fear of stalling out in the middle of a busy intersection (again) and since it only happens on long decelerations and goes away a few seconds after i don't think i can get it at the moment, but I will try in future. The previous owner installed a fuel filter with a Schrader valve on it, but we took it off, thankfully we still have it, so maybe i might attach it back on and test what's going on. Also I've seen it stall when idling in my driveway and I didn't notice any gas or fluids on the ground, so I don't think its a fuel line. Thanks again. I may try rebuilding the throttle body, since my gas pedal is a little bit stiff at times, so I will keep the video in mind. In all honestly though, now that I think about it, I don't think I've seen that check engine light come on in a hot minute though, so I honestly don't know. I think I'll just start testing some stuff and see what I can figure out.
So just a couple of things:
- Computer Codes: I'm not sure your OBD2 scanner is doing what it should. Even if you have the right connector, the system your Fiero has is OBD1. I recommend you use the paper clip. If you have an error code, it's stored in the system, even if only happens once. So, if the check engine light came on, your code would still be there. Please go to the link I posted above, and use the paperclip method. You check the codes with the key in the ON position, but the car not running. You actually don't want the car to be running when you read the codes.
- Fuel Line: The fuel line I'm talking about is inside the fuel tank. It's the connector between the fuel pump and the fuel pickup. It's a rubber hose that's about 1.5" to 2"s long. The problem is that if your fuel pump is original, this line deteriorates inside the tank because it was never designed to work with fuel that has any ethanol in it... and the ethanol destroys the rubber both inside and outside (since it's submerged), causing it to leak internally. The fuel pump pushes fuel up into the pickup, and a significant amount of the fuel squirts out of the line through little pores... almost like a sponge, so, it effectively reduces your fuel pressure.
- Oil Pressure / Fuel Pump Relay: As 1985 Fiero GT said... it's unfortunately not designed like you said, it's actually the exact opposite... a little. If the fuel relay fails... the oil pressure sending unit (which also has a switch in it) serves to engage the fuel pump IF the fuel pump relay has failed. That's because the assumption is... GM wanted you to still be able to drive the car if the fuel relay died while driving.
True but that backup system is a one way thing, if the fuel relay goes bad is will run off the oil pressure switch, but not the other way around. The design was that if you lose oil pressure (which could indicate big engine problems) the engine would automatically shut off to prevent damage. however if the switch was bad then it would shut off on you.
- Computer Codes: I'm not sure your OBD2 scanner is doing what it should. Even if you have the right connector, the system your Fiero has is OBD1. I recommend you use the paper clip. If you have an error code, it's stored in the system, even if only happens once. So, if the check engine light came on, your code would still be there. Please go to the link I posted above, and use the paperclip method. You check the codes with the key in the ON position, but the car not running. You actually don't want the car to be running when you read the codes.
- Fuel Line: The fuel line I'm talking about is inside the fuel tank. It's the connector between the fuel pump and the fuel pickup. It's a rubber hose that's about 1.5" to 2"s long. The problem is that if your fuel pump is original, this line deteriorates inside the tank because it was never designed to work with fuel that has any ethanol in it... and the ethanol destroys the rubber both inside and outside (since it's submerged), causing it to leak internally. The fuel pump pushes fuel up into the pickup, and a significant amount of the fuel squirts out of the line through little pores... almost like a sponge, so, it effectively reduces your fuel pressure.
- Oil Pressure / Fuel Pump Relay: As 1985 Fiero GT said... it's unfortunately not designed like you said, it's actually the exact opposite... a little. If the fuel relay fails... the oil pressure sending unit (which also has a switch in it) serves to engage the fuel pump IF the fuel pump relay has failed. That's because the assumption is... GM wanted you to still be able to drive the car if the fuel relay died while driving.
Okay that makes a bit more sense. Also, I mentioned I scanned it with an old GM OBD 1 scanner that my dad had in his shop, not an OBD 2. After reading what you said about the fuel line, that definitely is a possibility as well. Definitely gonna have to do some more testing and diagnostics. Thanks once again for your time.
- Computer Codes: I'm not sure your OBD2 scanner is doing what it should. Even if you have the right connector, the system your Fiero has is OBD1. I recommend you use the paper clip. If you have an error code, it's stored in the system, even if only happens once. So, if the check engine light came on, your code would still be there. Please go to the link I posted above, and use the paperclip method. You check the codes with the key in the ON position, but the car not running. You actually don't want the car to be running when you read the codes.
- Fuel Line: The fuel line I'm talking about is inside the fuel tank. It's the connector between the fuel pump and the fuel pickup. It's a rubber hose that's about 1.5" to 2"s long. The problem is that if your fuel pump is original, this line deteriorates inside the tank because it was never designed to work with fuel that has any ethanol in it... and the ethanol destroys the rubber both inside and outside (since it's submerged), causing it to leak internally. The fuel pump pushes fuel up into the pickup, and a significant amount of the fuel squirts out of the line through little pores... almost like a sponge, so, it effectively reduces your fuel pressure.
- Oil Pressure / Fuel Pump Relay: As 1985 Fiero GT said... it's unfortunately not designed like you said, it's actually the exact opposite... a little. If the fuel relay fails... the oil pressure sending unit (which also has a switch in it) serves to engage the fuel pump IF the fuel pump relay has failed. That's because the assumption is... GM wanted you to still be able to drive the car if the fuel relay died while driving.
Forgive me, there are a few codes, 24 VSS no signal (I'm missing the plastic gear) 35 Idle Speed Error 44 Lean Exhaust. Sorry about the confusion
Okay that makes a bit more sense. Also, I mentioned I scanned it with an old GM OBD 1 scanner that my dad had in his shop, not an OBD 2. After reading what you said about the fuel line, that definitely is a possibility as well. Definitely gonna have to do some more testing and diagnostics. Thanks once again for your time.
Haha... you did actually say OBD2 (look above), which is why I thought it was weird. But there are some really cool OBD2 compatible scanners that can plug into OBD1 ports and pull the codes. I have a Tech-2, which is what GM line techs used to use. You can buy them brand new on eBay now too... which is pretty cool.
But yeah, I definitely think that could be it... but on this...
quote
Originally posted by ironpuke84:
Forgive me, there are a few codes, 24 VSS no signal (I'm missing the plastic gear) 35 Idle Speed Error 44 Lean Exhaust. Sorry about the confusion
#24 - VSS is definitely something you want to take care of. That's needed of course, to power your speedometer (doesn't work without it), but that's not all it's for. After it goes to the gauge cluster, it then goes to the ECM, and the ECM also uses that to determine fuel and timing adjustments at different speeds... so you really, really want to get that fixed. Even... even if you get an incorrect plastic gear and it shows the wrong speed. Right now, it's showing nothing... it's better for your ECM to get 80% there, than 0% there.
#35 - Idle Speed Error ... so, this tells me that you probably have a vacuum leak, or a failing Idle Air Control valve. This code is set when the idle speed of the car cannot be maintained and goes more than 150-200 rpms over say, I think 900 rpms, for longer than 10 minutes or so. A failing idle air control valve can ABSOLUTELY cause your car to stall out in an intersection. That's because you've got your foot off the gas, and you're decellerating, and now it can't maintain proper idle.
#44 - Lean Exhaust, this also could mean you have a vacuum leak. When your car is idling... can it idle perfectly at 900 rpms? Or... is the idle higher?
Looking at all of these in totality... lean exhuast and idle speed could be because you're not getting enough fuel, and these codes are being set, not because your O2 sensor is bad (lean exhaust) or a vacuum leak, or a failing idle air control valve... but because you just cannot get enough fuel to keep the car running at certain times. You absolutely must fix the VSS though.
#24 - VSS is definitely something you want to take care of. That's needed of course, to power your speedometer (doesn't work without it), but that's not all it's for. After it goes to the gauge cluster, it then goes to the ECM, and the ECM also uses that to determine fuel and timing adjustments at different speeds... so you really, really want to get that fixed. Even... even if you get an incorrect plastic gear and it shows the wrong speed. Right now, it's showing nothing... it's better for your ECM to get 80% there, than 0% there.
#35 - Idle Speed Error ... so, this tells me that you probably have a vacuum leak, or a failing Idle Air Control valve. This code is set when the idle speed of the car cannot be maintained and goes more than 150-200 rpms over say, I think 900 rpms, for longer than 10 minutes or so. A failing idle air control valve can ABSOLUTELY cause your car to stall out in an intersection. That's because you've got your foot off the gas, and you're decellerating, and now it can't maintain proper idle.
#44 - Lean Exhaust, this also could mean you have a vacuum leak. When your car is idling... can it idle perfectly at 900 rpms? Or... is the idle higher?
Looking at all of these in totality... lean exhuast and idle speed could be because you're not getting enough fuel, and these codes are being set, not because your O2 sensor is bad (lean exhaust) or a vacuum leak, or a failing idle air control valve... but because you just cannot get enough fuel to keep the car running at certain times. You absolutely must fix the VSS though.
at idle the rpms usually fluctuate a little bit, but usually run pretty close to 900rpm when it gets warm. starting cold, the engine usually starts out at ~2200 rpm and comes down gradually. I am trying to get a VSS gear, and I have a guy who has it, but unfortunately he hasn't gotten it to me as of yet, and I've had to rely on the tachometer for my speed. So that's good to narrow it down, Vacuum leak or faulty IAC valve. Thanks a bunch.
