Misfire, please help (Page 1/1)
Caroline.58 JUN 07, 05:44 PM
I own an 87 2.5 Fiero with 47,000 original miles and I am having a chronic misfire problem on all four cylinders that I cannot diagnose. The misfire happens at idle and at driving speeds. When I hook up my timing light to each individual spark plug wire, it shoes that they fire a dozen times or more correctly, then one spark disappears. I have stalled at stop lights a couple of times. You can hear the engine running rough when the misfire occurs. I have replaced the plugs (gapped at 56), wires, both ignition coils, the ignition module, the crank shaft position sensor, ECM, MAP and MAT sensors, thermostat and coolant temperature sensor. Checked the grounds. I am waiting for a new gas tank and sending unit to be available for me to replace my originals, which have rust on the inside of the tank and the inside of the sending unit. I cleaned them up as much as I could, and put a new screen and fuel filter on. That seemed to help the performance somewhat, especially when the gas is low in the tank, but my light still shows the misfiring on every cylinder. A code 42 came up one time when I stalled at a light, but since then it has not reappeared (even though the spark is still missing.) If anyone has any suggestions about what to check next, please let me know. Thanks.

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Caroline K

Spoon JUN 07, 09:47 PM
I don't believe you mentioned ignition wires and type of spark plugs. Heat range of plugs.

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

theogre JUN 07, 10:19 PM
Timing light is near useless for DIS engines.

see https://web.archive.org/web...cast.net/~fierocave/ 4cyl engine

ECM maybe dying...
ECM gets hot and some part moves, solder crack opens, etc.
One fail is run one day, park over night and won't start because driver for EFI is "dead" but rest of ECM seems to work other ways.
If "dead" then save the PROM in it for new ECM.

See ECM Heat and a lot of rest of electric section.
Note: Many ECM scanners won't work for this. Maybe reddevilriver will... use search

42 maybe good or bogus.
Check/fix/clean all grounds in engine bay at minimum.

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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)

The Ogre's Fiero Cave

Caroline.58 JUN 07, 11:57 PM
My new ignition wires were expensive and don't arc in the dark, the plugs were 666 # I think AC Delco (from my memory). I put in a new ECM. I use the timing light connected to positive and negative battery posts with the lock on probe connected to each ignition wire separately. That's how I can see the consecutive spark flashes and the random misses of spark that keeps occurring. throughout the entire range of rpms. I found 2 grounds on the engine block that look connected to the wire harness involved in the ignition module. They are clean. Thanks for the ideas.
jelly2m8 JUN 08, 12:39 AM
A faulty EGR valve stuck partially open on the 2.5 can cause an idle misfire and low speed miss.

[This message has been edited by jelly2m8 (edited 06-08-2023).]

theogre JUN 08, 11:01 AM

quote
Originally posted by jelly2m8:
A faulty EGR valve stuck partially open on the 2.5 can cause an idle misfire and low speed miss.

Or iffy EGR just mess w/ intake fuel mix and seems to miss under any load.
I had this happen 2x just on mine.

EGR is bad or just gasket bad allowing easy "sucking" exhaust.

Or Vac Line to the Valve has problems can to same. Or Something stopping EGR to work can do same.

While ECM has no clue what EGR is doing,,, It assumes the EGR is working and timing is programed for that detail. Blocking EGR flow for any reason when ECM assume flow is bad as leaking flow when ECM assume 0 flow.

Again, Timing Lights are often Useless or Wrong because DIS and Waste Spark making more sparks for 1 wire and plug.
1-4 & 2-3 pairs of plugs fire 2x per rev and many Timing Light don't have enough time to recharge the cap to flash the light every time even at idle.

Often it doesn't matter what type pickup the light uses. Inductive or High Volt types still need time to charge the cap to flash the Xeon bulb. Many Inductive things have problems even on "normal" ignitions and often miss sparks while plugs fire.

More proof? Many at DMV et al can have problems just trying 2 speed Emission Test for same reason. Inductive Probes can't see Engine RPM right and large headache for everyone. Last time finally dug thru some book at inspection barn to find data and instructions to test my car after another lane w/ operator refuse to test after he had problem reading and didn't want to try. Every time I go have problems doing this test for Many Years and I'm Not In CA w/ CARB testing rules.

You have to fix etc All grounds in engine bay at minimum. Likely others in the cabin too. And protect w/ silicone or Permatex Green Break Grease. See wire service in cave.
Caroline.58 JUN 08, 11:43 AM
Thanks you! I will work on it later today. My Dad says he has found stuck EGRs in the past, but those were totally stuck open or closed not just leaking or stuck a little. I'll look for more grounds too. Luckily the car is in pretty good shape (rust-on-bolts wise.)
Caroline.58 JUN 08, 05:08 PM
The EGR appears to be in perfect condition. I took it off and the diaphragm holds the air in when pressed with the little hole blocked with my finger. I don't want to spend $65 on a new one if mine is okay. I'm going to drive the car more tomorrow and see how it is.
Dukesterpro JUN 08, 05:36 PM
Coming from another duke user that just chased a duke misfire, if your egr is holding its probably okay.

Here is what caused my misfire. Remove your injector, clean it. reinsert. That was the golden ticket for me.
Caroline.58 JUN 11, 03:40 PM
I think my rusty gas tank and rusty gas is the main culprit. Hopefully Rodney Dickman will get those new tanks in soon! Thanks guys