Hot start misses bad

[Young1]

Hot start bad miss first occurred after a high rpm autocross when packing up to go home. It clears up after about 3-4 miles.

Fuel pressure is perfect. 53 at red connector. 43 at idle.

Pulled plenum. Two coils had corroded high volt terminals and corresponding plug wires. Two coils had carbon tracks. Replaced 4 coils and wires and plugs. Checked injectors with harness removed. All 12.0. Ran smoother but yet hot starts misses bad with DMF clanking.

No check engine light. Pulled crank sensor. Ohms are fine but it is original with 3 cracks in end and oil in connector. Replaced it. Runs even better and will balance a nickel! BTW Cam sensor is Phils from 3 years ago. But hot start bad miss continues. I just checked for codes. None. Only 1-2.

Cold starts are normal. It seems like a heat soak thing.
What page of FSM do I need to run through? Stumped. ECM bad solder joints? I am afraid to leave home with it until resolved.

[tccrab]

Had similar issue, drove me nuts.
I wound up re-pinning the ECM plug with the injector connections.
No more hot restart missfire at all.
Turns out I had stretched out the female connections when trying to diagnose faulty fuel injectors with my DVM probe.

Crabs

[WARP TEN]

Had similar issue, drove me nuts.
I wound up re-pinning the ECM plug with the injector connections.
No more hot restart missfire at all.
Turns out I had stretched out the female connections when trying to diagnose faulty fuel injectors with my DVM probe.

Crabs

Tom, I wouldn't touch that one with a 10 foot probe, er I mean, pole. --Bob

[viper107]

injector ohms can change from hot to cold/ are injectors originals? if they are they are no good

[XfireZ51]

injector ohms can change from hot to cold/ are injectors originals? if they are they are no good

Need to bring motor to operating twmps. Then allow to heat soak for >30min before doing ohm check.

[Young1]

Hot injector check is good. 13.6 plus or minus .1 across the group. All good per MH article.
However after hot start the miss reapoears . My timing light at idle tells a tale. But the miss moves around to different cylinders. I will check and find two with a miss then when come back around those two are fine and it is others. Appears to be ignition. What about the ground on the ECM? Possibility? Where is it?

[Paul Workman]

Sounds like a leak-down (fuel pressure test) is in order!

IF an injector (one or more) is leaking fuel when at rest (after shutting the engine off), it WILL lead to the symptoms you describe (I say from personal experience!).

Connect a pressure gauge to the fuel rail and turn the ignition switch to ON long enough to pressurize the rail(s), and then turn the switch to OFF. The pressure should hold to within one PSI for several minutes - e.g., 10-15 min or so (in my case) before dropping only a pound or so.
However, if the fuel pressure drops to zero (worst case) or even several pounds, then an injector (one or more) is leaking raw gas into the cylinder(s) which may (as you say) take a minute or so to clear - depending on how bad the leak is!

NOTE: BOTH pumps have fuel check valves - which can also allow gas to leak back into the tank. This would not normally affect idle only after start-up (It would be constant - but you already monitored pressure at idle and under power and there was no issue to suggest your symptoms were resulting from lack of fuel pressure [in this case].).

[Young1]

I did the pressure test a week ago. Holds pressure for a good long time. I did it while plenum was off and looked into ports with flashlight. No prob there. What do you think Paul about my timing light showing misfires at idle when heat soaked?

[richarde]

Hi - Your comments regarding the fact that the misfire moves around cylinders sounds as though it could point to an ECM / Engine Ground issue, maybe a loose Ground which only presents under certain conditions. The Grounds are bolted to the rear right and left hand sides of the engine block directly above the bell housing. Really difficult to get to unless the plenum is removed. Hope this may be of help.


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[Young1]

Yes I shall tackle the ground under the battery and the two on the rear of the block next. They have not been touched in 30 years. Time they get love and caresses. Stay tuned for update.

[XfireZ51]

Hot injector check is good. 13.6 plus or minus .1 across the group. All good per MH article.
However after hot start the miss reapoears . My timing light at idle tells a tale. But the miss moves around to different cylinders. I will check and find two with a miss then when come back around those two are fine and it is others. Appears to be ignition. What about the ground on the ECM? Possibility? Where is it?

Ignition principally has to do w ICM. I would check 14 pin connector to ICM, making sure no bent pins. Also ignition coil housing and connectors there.
I?m a bit confused by ur description of what?s happening. Are you saying that at old start motor does not miss while at hot start it does? At cold start, does it continue to run fine even after it reaches operating temps, or does it miss once it reaches operating temps?

[Young1]

Cold start. Fine no miss.
Operating temp. Fine no miss.
After a 30 minute heat soak. Misses bad. Miss will clear 5 to 7 operating minutes later.

[Subfixer]

Almost sounds like a leaking injector. Acts like an old carb motor that's flooded.

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[XfireZ51]

Cold start. Fine no miss.
Operating temp. Fine no miss.
After a 30 minute heat soak. Misses bad. Miss will clear 5 to 7 operating minutes later.

So does that mean that it misses w coolant temps at higher than operating temps?

[richarde]

Another possible cause may be an intermittent fault in the crank sensor caused by being situated right next to a hot header with no circulating air to cool it. Once on the move again, air circulates and it resumes its normal operation. Being an intermittent problem, this poses it's own problem on how to test it other than trying a known good sensor.


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[Young1]

Subfixer- injectors don?t leak. Holds pressure and was viewed with plenum off. Excess fuel would burn off in 20 seconds. This bad miss goes on for minutes.

XFire - runs fine at highest operating temp when fans are running. Only on start up after a heat soak does it miss.

Richard - no headers, stock exhaust, crank sensor replaced two weeks ago when it gave me hard start symptoms. Cured that but the miss after heat soak continued.

Keep thinking gentlemen. I am going to clean 30 year old ground connections on Friday to get that eliminated.

[viper107]

What year is this car?

[viper107]

If it's 92 down with original ecm, replace it with a 95 ecm it needs it anyway

[Young1]

From XFire
So does that mean that it misses w coolant temps at higher than operating temps?

After thinking about your question a moment the answer is yes. After a heat soak the temperature would be higher than operating then after running with the fans on for a few minutes it would come to a normal temp. That makes sense.

Car is 91. Original ECM. No bent ICM pins. Coil base has all been ohm checked. I am second owner and all things were original when I bought it.

[Young1]

I called Marc H. He instructed me to get a spark tester to isolate which cylinder was missing. I bought at autozone a tester with the adjustable gap where you see the spark. I set it at a 3/4 inch opening for 30k voltage and number 2 had the miss. It?s twin number three was fine. Pulled plenum for the third time and replaced it with a known good used one. All is well. Five years ago I replaced original coils with AC Delco. Two went bad. Replaced all four 3 weeks ago with Standard Blue Streak. One went bad. I am a believer in Paul Workman?s mantra. Just because it is new does not mean it is good. Back to basics, got a miss? Get a cheap simple tester.

[garyhugo]

Great to hear you got it figured out and repaired.