I have searched the forum as there are several threads related to engine miss but either they have no conclusion or they did not solve my issue. Here is what I have and what I have done.
Vehicle:1988 Fiero coup with the 2.8 V6 and 5 speed Getrag. Rebuilt stock engine with less than 2000 miles.
Symptoms: Idle fluctuates between 900 and 1150 RPM. Trying to hold any steady RPM causes the engine to miss, the higher the RPM the worse the miss. Hard acceleration while driving and it runs as expected but try to hold it steady at any RPM and it kicks and jerks severely.
Distributor has been replaced with a rebuilt distributor from The Fiero Store. Timing is set at 10 degrees BTDC and fires up instantly.
Disconnected tach filter, no change.
Cap, rotor, wires, plugs (NGK steel core properly gapped at .045”) have all been replaced with new. Plug wires have dielectric at both ends.
All ignition wires misted with salt water after dark and there are no visible sparks.
ICM swapped with another new ICM, no change.
ECM swapped, no change.
I have an old Snap On MT2500 scanner that feeds back info on the sensors. They read as follows:
MAP = .6 at idle, 4.9 at wide open throttle. Replaced with new and no change.
MAF I don’t remember the numbers but replacing it didn’t change anything.
IAC = 50 and fluctuates a little as the RPM changes at idle.
TPS = .5 and moves smoothly through the range. Replaced, no change.
O2 = 450 mv and never changes. This keeps the engine from going into closed loop so it will be replaced next but there is no O2 code.
I had 2 extra coils from the previous owner so I swapped those in. Problem got worse so I replaced the coil with a new coil. It performed the same as the coil that was installed when the problem started.
Fuel pressure does not build with key on but does when cranking, 38 psi. I know it should turn the fuel pump on for 2 seconds at key on so that’s an issue but I don’t believe it’s my idle issue. Depending on demand, fuel pressure is between 38 and 50 psi.
Checked for vacuum leaks by using carb cleaner over entire vacuum circuit with no change to performance.
EGR has been replaced. No vacuum at EGR because engine is staying in open loop mode. No apparent leaks in the EGR tube.
Cleaned all ground wire connections including at the ECM. Added an additional ground from engine to frame.
If I hook the timing light to each individual plug wire, hold RPM around 1500 and watch the flash pattern, there is a very noticeable miss on cylinders 1, 3, and 5 and an occasional miss on 2, 4, and 6.
I have a new EGR tube and stainless-steel vacuum lines ordered from Rodney just to make sure those aren’t the issue.
So many times I was sure I was headed in the right direction to find out I wasn’t. I’m down to just throwing money at it and I’m not really in a position to do that so any input or thoughts would be greatly appreciated.
How quickly does the fuel pressure drop when the engine is turned off.
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Originally posted by Another One:
Timing is set at 10 degrees BTDC...
With ALDL jumper installed? And have you verified that the outer ring of your harmonic balancer has not slipped? If it has (and this isn't uncommon), your ignition timing will be all off.
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Originally posted by Another One:
O2 = 450 mv and never changes. This keeps the engine from going into closed loop so it will be replaced next but there is no O2 code.
Fuel pressure is 50 psi when the engine is shut off and holds for at least 30 seconds. I didn't time it because I figured 30 seconds of the pump not running meant any leak was minimal. If that's wrong please let me know. According to your numbers, it is a little low at idle. Any increase in RPM causes the pressure to increase over 40 psi.
I forgot to mention that the fuel injectors were cleaned and spray pattern looks great. I have a buddy that has a contraption that hooks up to the injectors (1 at a time out of the engine), applies about 60 psi of cleaner and turns them on/off till all the cleaner has run through them.
Ignition timing was set with the ALDL jumper installed. How can I check the harmonic balancer to see if it's slipped as I have not checked that?
I agree on the O2 sensor but I only found about it last night as it is not setting a code. Headed to parts store shortly.
Fuel pressure is 50 psi when the engine is shut off and holds for at least 30 seconds. I didn't time it because I figured 30 seconds of the pump not running meant any leak was minimal. If that's wrong please let me know. According to your numbers, it is a little low at idle. Any increase in RPM causes the pressure to increase over 40 psi.
Fuel pressure should slowly dissipate over several minutes. 30 seconds is nothing. You're losing pressure somewhere.
And you're saying that your fuel pressure goes up to 50 PSI, yet is below the minimum reading of 30.5 PSI with the engine idling? Something isn't right.
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Originally posted by Another One:
How can I check the harmonic balancer to see if it's slipped as I have not checked that?
With the spark plug removed from cylinder #1, use something like a plastic straw inserted into the hole to find TDC while moving the crank by hand. When this has occurred, the wide timing mark should be lined up with 0° on the timing scale.
Patrick, I apologize, I misunderstood the fuel pressure numbers you posted. Attention to detail is everything and I failed today.
There is no fuel pressure at key on so that is the next problem to solve. If you have any input, I will gladly listen. Bump the ignition with out starting the engine and fuel pressure is between 38 and 42 psi. At idle the fuel pressure is between 40 and 42 psi.
Key off and the fuel pressure was 48 psi. Two minutes later it was 47 psi. 5 minutes later it was 44 psi. My fuel pressure gauge is in 2 psi increments so these numbers are a close approximation. Basically about 1/2 psi per minute. If you believe this could still be a problem then I can and will test it longer.
O2 sensor has been replaced and there is no change in condition. The Snap On MT2500 scanner shows the O2 sensor reading between 423 mv and 458 mv depending on throttle position. I'm not sure what numbers should be seen on the O2 sensor but it is changing now as the old one was not. Still not making it to closed loop mode. I'm not sure what it takes to go to closed loop mode.
I did not check the harmonic balancer but it fires up real quick so if it is off, it's not by much. I will check it tomorrow to confirm. I have a long reach micrometer (.001 mm or .00024 inch) that will hopefully reach the piston. If not I will use the stick by eye method.
OleJoedad, The rebuilt distributor, ICM, rotor, and cap were bought from the Fiero store so I presume they are correct although I have swapped the ICM (voiding the TFS warranty) to eliminate that as a possible cause. Car was running good for awhile as another Fiero fan saw it in town and posted a picture of it a few years ago. I read somewhere on the internet (possibly here?) that copper core vs steel core plugs increased resistance and stressed the already weak GM ignition system. I understand that it may be inaccurate so I will get copper core tomorrow and try that as well.
I'll swap to the original distributor tomorrow and see what happens although it has run good on this distributor. It was swapped because I believed the pick up coil was going bad. I know the logic doesn't make sense but I ran out of logic just before I ran out of money. I've given up on being able to explain why, just want it running good again.
Key off and the fuel pressure was 48 psi. Two minutes later it was 47 psi. 5 minutes later it was 44 psi. My fuel pressure gauge is in 2 psi increments so these numbers are a close approximation. Basically about 1/2 psi per minute. If you believe this could still be a problem then I can and will test it longer.
The fuel pressure drop sounds fine.
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Originally posted by Another One:
There is no fuel pressure at key on so that is the next problem to solve. If you have any input, I will gladly listen.
Electronics aren't my strong suit, but have you checked the fuel pump relay mounted on the firewall? If you have A/C, you can swap the relays for testing purposes
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Originally posted by Another One:
Still not making it to closed loop mode. I'm not sure what it takes to go to closed loop mode.
Coolant temperature for one. What temperature thermostat do you have installed? What temperature does your scanner tell you that the engine is reaching?
The A/F mixture will be influenced by both the MAT and coolant temperatures (a colder engine requires a richer mixture), so it's important to know what readings are being sent to the ECU when the engine is fully warmed up.
I also like to occasionally check the MAT and coolant temperatures first thing in the morning after the engine has sat all night, to make sure these two readings are very similar. The engine isn't running for this test. If the readings of the cold engine aren't close, at least one of the temperature sensors is probably faulty.
[This message has been edited by Patrick (edited 05-01-2022).]
"One Wire" O2 sensor is a Myth... Is a Tan "Ground" bolted to block that is the Return Line for this. Sim problem w/ "ground" between ECM and ICM, See my Cave, Ground "Myth" notes
Clean/Fix All "Grounds" bolted/screwed to engine and frame. Coat ends w/ brake or dielectric grease. Do that before replacing more parts. Because New O2 will have problems w/ iffy Tan "Ground."
Note: O2 sensor will not switch when < ~ 600°F. Depending on location, can take a while to reach that starting when cold or even engine hot but off so exhaust cools. In some setups, can drop back to open loop even while running at idle for too long. Is why some state inspection have a "Warning" to bump the gas for ~ 1 minute before E-test to keep O2 and Cat Hot.
get another ECM scanner or make sure you use right program for this... Fiero Engines does Not have a MAF and Other Field(s) can be wrong having no data or wrong data.
Test MAP needs hand vac pump w/ gauge. See my Cave, Sensors Quick Ref
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
I did cap the EGR vacuum with no change. I have ordered another EGR tube and gaskets incase there is something there I can't see. Also ordered the SS vacuum lines to help eliminate that as a problem. Some of the vacuum fittings are in pretty poor shape so I'll replace those with generics from O'Reilly, unless someone has a better idea.
I cleaned and added a ground connection from engine to frame but I did not dielectric grease them. Hope to get to that today. While I'm at it I will confirm "Tan" wire bolted to block is clean and dielectric greased.
I tried the MAP test using the vacuum from air powered brake bleeder but it is to much vacuum so I need to repeat the test with a hand vacuum pump.
I said MAF sensor when I meant ACT sensor (on the side of the air filter can). I checked all temp sensors this morning with the scanner and they are within 1 C of each other.
theogre Thanks for your input. There is a "Tan" wire bolted to the head under the exhaust cross over. It is crimped in a ring terminal (looks factory) with a black wire in the same harness. I removed the terminal from the head, inspected it for damage (none) pulled on it to make sure the crimp was good, sanded it shiny and around the hole in the head. Dielectric in the hole, on the bolt and all over the terminal ring. Did the same thing for the battery ground to the head, battery ground to the frame, head to the frame, frame to trunk lid, black wire from the same wire bundle as the tan wire to transmission. I couldn't find any others.
Idle fluctuation improved from around 300 RPM to 150 RPM (900 - 1050 now) and at higher RPMs the miss is not as bad. O2 runs between 437 mv and 452 mv depending on throttle work.
1 good backfire coming down from 2000 RPM opened the original muffler so that's now on the list.
Still need to do the MAP vacuum test and swap to the copper core plugs.
Again thanks to everyone for the input. For the first time in months it feels like I'm making progress.
I agree on the O2 sensor but I only found about it last night as it is not setting a code.
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Originally posted by Another One:
O2 sensor has been replaced and there is no change in condition.
O2 won't throw a code unless its beyond dead, maintenance item that should be replaced due to age, the cross counts on a good 02 should fluctuate like crazy.
And nowadays the AC Delco and Bosch O2s are both garbage, the Denzo is the only one you want (not universal, make sure it has OEM connector).
1 good backfire coming down from 2000 RPM opened the original muffler so that's now on the list.
Ouch! Maybe you can find a good used original stainless muffler to swap it out (cheaper than aftermarket route).
Your condition sounds like a classic ignition coil even though you replaced it with several used and a new one, what brand did you buy?
For a stock replacement ignition coil the Standard Blue Streak made in Poland is better build quality than most these days (they are black now and no longer blue).
New coil is a blue streak. Not sure where RockAuto got it from. Firing order has been verified. Pulling any one wire makes a noticeable difference. Not sure on the O2 sensor. Original was stuck at 450 mv. New O2 sensor changes but not like suggested above. I'm hoping the new vacuum lines and EGR tube fix the remaining miss issue.
I did cap the EGR vacuum with no change. I have ordered another EGR tube and gaskets incase there is something there I can't see. Also ordered the SS vacuum lines to help eliminate that as a problem. Some of the vacuum fittings are in pretty poor shape so I'll replace those with generics from O'Reilly, unless someone has a better idea.
I cleaned and added a ground connection from engine to frame but I did not dielectric grease them. Hope to get to that today. While I'm at it I will confirm "Tan" wire bolted to block is clean and dielectric greased.
I tried the MAP test using the vacuum from air powered brake bleeder but it is to much vacuum so I need to repeat the test with a hand vacuum pump.
I said MAF sensor when I meant ACT sensor (on the side of the air filter can). I checked all temp sensors this morning with the scanner and they are within 1 C of each other.
Thanks again for the ideas.
EGR: I blocked mine off at the tube going into the upper intake manifold with a cutout from a coke can. That fixed my high idle issue till the new tube came in, but I dunno that it will cause a misfire. No fuel pressure at key on: There are two relays in the engine bay on the driver side. One is the AC relay and the other is the fuel pump relay. Your fuel pump relay is out. To test you can swap them.
(I am chasing a miss myself at the moment)
On my sons 88 it was missing bad! This was fixed by pulling the fuel rail and using a shrinkwrap contraption/w carb cleaner and a 9V battery to clean the injectors. Both ways.... Replaced those and the car ran a lot better afterward...still missed on one cylinder. Screwdriver on the injector and my ear to it confirmed the injector was fried. (Dunno how this was missed while cleaning...I blame the kid.)
Have you checked that all the injectors are ticking? 1 3 and 5 are easy to get to....3 and 5 for sure. Tickles my brain a bit that they are not firing and they all ride the same fuse. Have you pulled the fuel rail and cleaned it/the injectors? Injectors might be toast or clogged. How old is your fuel pressure regulator? Might wanna swap that out while you're in there.
[This message has been edited by Forrest (edited 05-03-2022).]
Forrest, I appreciate the input. All injectors were cleaned at about 75 psi with solvent. I have a friend that has a device that holds solvent and feeds it to the injector at 75 psi and fires the injector every few seconds (out of the engine). All spray patterns were great. Nice even ring of mist when they fired. In the engine, all 6 injectors are ticking away. I believe even when there is no spark to burn the fuel.
EGR tube and gaskets have been ordered as well as the SS vacuum lines from Rodney. The EGR is new and has no vacuum on it but that doesn't fix a tube leak..
Cleaning and greasing all grounds (Thanks Ogre) made a big improvement but didn't correct it all. I'm hoping the new vacuum lines, fittings, and EGR tube remedy the issue.
I'm used to load on the engine making the problem worse but in this case it gets better. I got a second gear squeal during hard acceleration the other day (I was smiling for a moment). In my opinion, the ECU is getting bad data from somewhere. Just don't know how to determine where. Possible vacuum leak but hosing down the engine with carb cleaner didn't change anything.
I still need to do the MAP test with a hand vacuum pump. Where did I put that damn thing. It's a small garage after all. Where did all this *stuff* come from?
Fix ground other places like G201 G202 in the cabin too. See my Cave, Wire Service
F-pump and more goes thru them. Need long screw/nut driver w/ magnet tip to get at 1 behind radio.
Test volts at battery and alt out. You still have Hunting Idle and Flaky Power is a good way to have this problem. Battery & Alt out should be ~ 14.7v w/ engine running. And should have same or Very close to same. ECM should see about same when scanned. If not still have power ground or both w/ problems. (Scan read is often bit different vs a meter on wires.) Still reports volts wrong could be ECM dying slowly. Just 1 Crack solder joints or part in it can do odd things or completely kill ECM.
Note: Fixing/cleaning grounds etc can cause ECM to reset idle and not run right until drive above 35mph. (Some reset idle even run while parked but can't be sure engine sees right conditions but drive > 35mph will make sure.)
[This message has been edited by theogre (edited 05-04-2022).]
New EGR tube and SS vacuum lines installed with no change. Except the coolant leak I created on one of the throttle body rubber coolant hoses.
Voltage at alternator and battery is 14.5V. Scanner says the ECU is reporting 14.2V so I'll go after the cabin grounds next.
Passenger headlight only went about 1/2 way up and lots of chugging when it moved. Acted like stripped gear teeth so that's been added to the list as well.
Headlight grounds (1 on each side), ECM ground, and that mother behind the stereo all cleaned and dielectric grease applied in liberal amounts. No change.
I did read a tip for setting the timing while I was searching for answers. Put your timing light on the coil to cap wire, then set your timing to 10* BTDC. It makes averaging the 1 and 4 cylinder a lot faster and easier.
I have a Fluke 100 mHZ oscilloscope to check for missing pulses from the ECM but I'm not sure where to check. If anyone has any input I would be extremely grateful. I don't think I can check the pick up coil while it's running but should be able to check ICM to ECM and ECM to coil
Oh yeah, the no fuel at key on was the relay. Several people mentioned that. Thanks to all of them.
[This message has been edited by Another One (edited 05-16-2022).]
Originally posted by Another One: Passenger headlight only went about 1/2 way up and lots of chugging when it moved. Acted like stripped gear teeth so that's been added to the list as well.
Easy fix. The gears do not strip normally. There are plastic bushings in there that are prolly shredded. You need new these.... https://www.ebay.com/itm/16...2:g:7rYAAOSwytRh-VPb Some folks also just use fuel line.
I put the scope on the white wire between the IM and the coil and it is definitely missing pulses. I see the same thing with the scope on the disconnected tach filter wire. Not sure which wire is from the ECU to IM so if anyone knows, please let me know.
Signal looks like it has switch bounce when going to ground and then spikes to around 60 volts positive. Not sure if that 60 volt spike is normal/acceptable or if it means the ECU is/has failed. I have tried a separate ECU but they are both 1988 with 150,000 miles plus.
I checked for AC voltage at the alternator and the battery and the Fluke meter shows .002 so the alternator is good.
Unfortunately it will be a few more weeks before I can try anything else as the 1986 MR2 needs attention now.
I don't think this is the cause of your misfire, but it seems your O2 sensor is acting like it's not getting hot enough to cycle like it should. When you replaced the O2, did you put grease on the electrical connector? I've read that can cause it to malfunction, that it needs air to pass through there. Do you have aftermarket headers? Do you have one of those infrared pyrometer gizmos? If so, try checking if the exhaust (and by extension the O2 sensor) is getting to a normal temperature.
Separate issue: you should be getting fuel pressure as soon as the key is turned on. For it to wait until you bump the starter might be a bad fuel pump relay. Have you tried swapping relays as suggested by Patrick further up in the thread? But again that's probably not your misfire, assuming it holds fuel pressure when running and it sounds like it does.
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Originally posted by Another One:
I put the scope on the white wire between the IM and the coil and it is definitely missing pulses. I see the same thing with the scope on the disconnected tach filter wire. Not sure which wire is from the ECU to IM so if anyone knows, please let me know.
Signal looks like it has switch bounce when going to ground and then spikes to around 60 volts positive. Not sure if that 60 volt spike is normal/acceptable or if it means the ECU is/has failed. I have tried a separate ECU but they are both 1988 with 150,000 miles plus.
I checked for AC voltage at the alternator and the battery and the Fluke meter shows .002 so the alternator is good.
Unfortunately it will be a few more weeks before I can try anything else as the 1986 MR2 needs attention now.
That mini-harness from the coil to the ICM is notorious for getting toasted over the years and often needs replacement.
[This message has been edited by armos (edited 06-15-2022).]
Armos, Thanks for the input. Fuel pressure was the relay, that has been corrected. Fuel pressure varies with manifold pressure but stays consistent within normal/expected pressure readings.
I've checked and replaced the mini harness from the coil to the ICM with no change. I've also checked and replaced the wires from the pick up coil to the ICM with no improvement. If I'm understanding the print you included correctly, putting the scope on the PPL/WHT wire should let me know if the pick up coil is working consistently.
I did learn that the scope signal I was seeing on the tach wire is what I should be seeing. The high voltage is the spark starting, reduced voltage that I thought might be switch bounce is actually the flame front. YouTube has lots of generic info on using an oscilloscope on an ignition system. Just not why I'm missing pulses.
Our issues are exactly the same lol , my spark plug arching was caused because of insufficient fuel ,,, my problem is my fuel is system is perfect preasure and my fuel injectors work as they should ,,, my motor is in time so why on earth is the car still running lean ,,, I’ve tried multiple sensors ,,, just not making sense lol , my scan tool shows sensor is working as they should , all I can say is keep me up to date if you find solutions, as I will respond here if I figure it out
Pictures are always a plus. Parts for the MR2 project are still weeks out so I may be pulling the Fiero back up to the front of the line and check the signal to the ECU from the ICM. I know it's missing pulses from the ECU to the coil, just don't know why. I may post a youtube video as you did and see if that helps generate ideas.
Sorry I’ve been super busy , I’ll post pics when I get a chance , I’m about to test the car after changing some parts so wish me luck hope I figured it out lol
Mystery solved ,, turns out my new distributor was faulty there was no way to scope a pickup coil so I can’t prove it but after warranty and installing another distributor all is good
Thanks for the update. Unfortunately I voided the warranty on mine by swapping out the ICM. That and it was bought some time ago so I'm sure time has elapsed.
