84 Fiero 2.5L Losing power and stalling out (Page 2/2)
ironpuke84 OCT 30, 12:35 PM

quote
Originally posted by 1985 Fiero GT:


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.
ironpuke84 OCT 30, 12:48 PM

quote
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.
1985 Fiero GT OCT 30, 01:09 PM

quote
Originally posted by ironpuke84:


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).]

82-T/A [At Work] OCT 30, 02:44 PM

quote
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.
olejoedad OCT 30, 03:10 PM

quote
Originally posted by ironpuke84:


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.



You are incorrect.
ironpuke84 OCT 30, 03:38 PM
retracted

[This message has been edited by ironpuke84 (edited 10-30-2025).]

ironpuke84 OCT 30, 03:46 PM

quote
Originally posted by 82-T/A [At Work]:


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.



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.
ironpuke84 OCT 30, 03:55 PM

quote
Originally posted by 82-T/A [At Work]:


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.



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
82-T/A [At Work] OCT 30, 04:30 PM

quote
Originally posted by ironpuke84:

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.

ironpuke84 OCT 30, 05:25 PM

quote
Originally posted by 82-T/A [At Work]:


#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.