Misfire mystery

I

I_bRAD

New member
1997 SR5 4X4 manual. 350k kms.

Issue:

No problems until recently, and then got a surprise check engine light. Codes are for random (P0300) and specific cylinder (P0301,2,3,4,5) but no patterns. Assuming no #6 purely by chance. Light comes back if I clear it.

Diganostic:

-all three coils were new last summer
-new plug wires last summer
-spark plugs within 50k kms. Pulled them last night and it looks like they've been in for a week (i.e. they look fine)
-fuel filter within 50k kms
-air filter within 20k kms. Looks fine. MAF and TB are clean.
-no obvious vacuum leaks
-no fuel leaks
-no indication of bad head gasket. Coolant is full.

No drive ability issues at all. In fact if I hadn't got the light I'd never suspect anything was up. No stumbles. No hard starts.

Additional info:

check engine light does not reset if disconnecting the battery for a while? Is that normal?

ONCE, about 3 weeks ago I came to a stop sign and the truck died. Wouldn't crank. Eventually started up a few seconds later just fine and haven't had any issues since. I brushed it off as being lazy on the clutch pedal, but now I'm wondering if that was a symptom.

So. What next? My best guess is fuel pump since it affects all cylinders rather than a single injector. Same goes for a bad valve. Is there some way I can test a fuel pump that's just starting to go but otherwise working fine?

Anyone else have something similar happen? Any help appreciated!

Thanks

EDIT- PROBLEM WAS CAUSED BY A WEAK ALTERNATOR.
 
Last edited:
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No idea if this would apply or not, but I had similar issues with mine and it ended up that my cam gears were off by a couple of teeth on each side. I am assuming that previous owner had the t-belt done and they did not get it set properly. When I replaced the belt and set the cams properly, all was good.
 
Are all the parts replaced OEM?

I had misfire and lean codes when my pump failed but i had major drivablilty issues.
 
JayRolla said:
Are all the parts replaced OEM?

I had misfire and lean codes when my pump failed but i had major drivablilty issues.
Click to expand...

All OEM parts. Even the air filter!

When I pulled the plugs I found a couple of loose hoses on the air intake that might have had potential for vacuum leak. Tightened those up and got a friend with a reader to clear the code. Nothing has come back yet, but I've only driven it maybe 40kms. Fingers crossed that was it!

Haven't had that air intake off since Christmas though so not sure why it wouldn't have started then...

Never had any lean codes. Not sure what the parameters are to trigger a lean code but it sounds reasonable that if the misfires were due to fuel starvation I'd get codes for both the misfire and the lean as you did.
 
low fuel pressures/dirty injectors will cause random miss fires. wiring / ignition coils also .

350K miles I would say that is at the high end of life on injectors. may need to be removed and cleaned / tested or replaced.

if you had a vacuum leak the engine it will run rich on fuel .. O2 sensors will tell PCM to increase injector ON time.

fuel trims will be too high ...
 
catrains said:
low fuel pressures/dirty injectors will cause random miss fires. wiring / ignition coils also .

350K miles I would say that is at the high end of life on injectors. may need to be removed and cleaned / tested or replaced.

if you had a vacuum leak the engine it will run rich on fuel .. O2 sensors will tell PCM to increase injector ON time.

fuel trims will be too high ...
Click to expand...

350 kms! That's only about 220 miles...

I think I'm going to have to bite the bullet and get my own diagnostic tool!
 
catrains said:
low fuel pressures/dirty injectors will cause random miss fires. wiring / ignition coils also .

350K miles I would say that is at the high end of life on injectors. may need to be removed and cleaned / tested or replaced.

if you had a vacuum leak the engine it will run rich on fuel .. O2 sensors will tell PCM to increase injector ON time.

fuel trims will be too high ...
Click to expand...

If the vacuum leak is after the MAF (most are) than the car will run lean. The air is being sucked in without being metered. So the ECU does not know to add fuel for this unmetered air causing a lean condition. Not rich. The o2 will try and compensate but usually not enough. ECU trusts the air coming in the motor more than the metered air coming out.
 
JayRolla said:
If the vacuum leak is after the MAF (most are) than the car will run lean. The air is being sucked in without being metered. So the ECU does not know to add fuel for this unmetered air causing a lean condition. Not rich. The o2 will try and compensate but usually not enough. ECU trusts the air coming in the motor more than the metered air coming out.
Click to expand...

the ecm/pcm uses the MAF air volume and the O2 sensor reading to determine a incorrect fuel ratio. so if its off and out of range that it cannot correct it then the codes are created.

a vacuum leak causes the engine to run rich with fuel injectors.
I believe his engine does not have a carb..if it does have one then it would run lean.

the ecm/pcm knows everything unless the sensors are damaged.

O2 sensors do not do anything but report the oxygen in the exhaust the ecm/pcm does the fuel trims not an O2 sensor.
 
catrains said:
the ecm/pcm uses the MAF air volume and the O2 sensor reading to determine a incorrect fuel ratio. so if its off and out of range that it cannot correct it then the codes are created.

a vacuum leak causes the engine to run rich with fuel injectors.
I believe his engine does not have a carb..if it does have one then it would run lean.

the ecm/pcm knows everything unless the sensors are damaged.

O2 sensors do not do anything but report the oxygen in the exhaust the ecm/pcm does the fuel trims not an O2 sensor.
Click to expand...

Ive been tuning cars for 15 years. I know how it works. The ecu bases fuel off the air metered by the maf. If air gets sucked in at a vacuum leak this air is unmetered and ecu can't compensate causing a lean condition.
 
JayRolla said:
Ive been tuning cars for 15 years. I know how it works. The ecu bases fuel off the air metered by the maf. If air gets sucked in at a vacuum leak this air is unmetered and ecu can't compensate causing a lean condition.
Click to expand...


The MAF is just one sensor that reports to the ECM what's going on. The upstream O2 sensors are the final feedback , that does create a signal voltage to ecm so the programmed fuel ratio is correct to prevent pollution and converter damage....

The forum member with his detonation problem and his comment on the use of a scan tool will get his engine to be repaired with no guessing , other forum members past issues with their engines operation is going to create more expense and confusion .
 
catrains said:
I said 350K miles not KM.. we down in the US use the long miles LOL...

K = 1,000 like in electronic tech stuff..
Click to expand...

And I said my truck has 350k kms! So if injectors typically go bad at 350 miles as you suggest, then I'm a long way from that. I have my doubts about all of the injectors going at the same time, but the low fuel pressure thing is where I was coming from with the original fuel pump question.
 
I_bRAD said:
And I said my truck has 350k kms! So if injectors typically go bad at 350 miles as you suggest, then I'm a long way from that. I have my doubts about all of the injectors going at the same time, but the low fuel pressure thing is where I was coming from with the original fuel pump question.
Click to expand...

high mileage and age does effect the injectors as well as the other sensors . scanning the engines operation is how to find out what's wrong.

so far no testing so not gonna get much help .. if you post the testing data then you will get some help.
 
catrains said:
high mileage and age does effect the injectors as well as the other sensors . scanning the engines operation is how to find out what's wrong.

so far no testing so not gonna get much help .. if you post the testing data then you will get some help.
Click to expand...

No tester = no data! If I had data I probably wouldn't have started the thread!

If the potential leaks I found and fixed solved the problem then that was the answer. If not looks like it's fuel delivery related. I reset everything after fixing the leaks and the light didn't come back on, but I haven't driven the truck for a couple of days now so basically still waiting to see if it's fixed.
 
I_bRAD said:
No tester = no data! If I had data I probably wouldn't have started the thread!

If the potential leaks I found and fixed solved the problem then that was the answer. If not looks like it's fuel delivery related. I reset everything after fixing the leaks and the light didn't come back on, but I haven't driven the truck for a couple of days now so basically still waiting to see if it's fixed.
Click to expand...

scan tool will not fix it but it will lead you to where the problem is OBDII scan tools that read engine data does save on time and $$$.. this info will show the engines issues before CEL comes on.. high mile old vehicles like you have , can be multiple components going out of normal range so the tool does work on viewing what's up.

the tool I use all the time is able to be up dated to be used on newer vehicles and multiple vehicles. cost me $200 USD probably $300 CA dollar. 350K KM miles is high mile vehicle US miles its over 200,000 miles.
 
catrains said:
The MAF is just one sensor that reports to the ECM what's going on. The upstream O2 sensors are the final feedback , that does create a signal voltage to ecm so the programmed fuel ratio is correct to prevent pollution and converter damage....

The forum member with his detonation problem and his comment on the use of a scan tool will get his engine to be repaired with no guessing , other forum members past issues with their engines operation is going to create more expense and confusion .
Click to expand...

I understand that but most compensation is done by the metered air coming into the motor from how I always understood it. Its been many years since tuning maf car. We usually convert to speed density or gm maf and then disable o2, simulating input to ecu to eliminate it completely so you may be right.
 
The early stages of a fuel pump failing may not throw a lean code while the ECM tries to compensate, but with all the 30X codes it should have thrown a random lean if that were the issue. You can check fuel pressure if you think your pump may be going bad. You can also check the resistance of your AF sensors (pre and post cat) pretty easily.

You said you replaced the coils, were they OEM/Denso coils or Amazon specials, 3 for $50?

Buckaroo said:
JayRolla & Catrains -

I was enjoying your conversation regarding fuel/air control but, unless I missed it, didn't see fuel trims mentioned. For folks new into this, following video very good regarding ecm control of fuel/air mixture although some of the concepts go beyond the capabilities of the T4R Third Gen control scheme....

https://workshop.searchautoparts.co...cClsAJkLld79gFGVkyNDGnI0oQm9Qg&_hsmi=51456038

Buckaroo
Click to expand...

Love a good forum cockfight, but love a good link more. Thanks Buck :thumb3:

BTW, catrains, what is the K in "electronics tech stuff" that equals 1,000? :snacker:
catrains said:
K = 1,000 like in electronic tech stuff..
Click to expand...

Good luck OP :D
 
Buckaroo said:
JayRolla & Catrains -

I was enjoying your conversation regarding fuel/air control but, unless I missed it, didn't see fuel trims mentioned. For folks new into this, following video very good regarding ecm control of fuel/air mixture although some of the concepts go beyond the capabilities of the T4R Third Gen control scheme....

https://workshop.searchautoparts.co...cClsAJkLld79gFGVkyNDGnI0oQm9Qg&_hsmi=51456038

Buckaroo
Click to expand...

trims mentioned post #5 low fuel pressures/dirty injectors will cause random miss fires. wiring / ignition coils also .

350K miles I would say that is at the high end of life on injectors. may need to be removed and cleaned / tested or replaced.

if you had a vacuum leak the engine it will run rich on fuel .. O2 sensors will tell PCM to increase injector ON time.

fuel trims will be too high ...
 
No Trims NO Fixee.

Did you check Freeze Frame Data to see when CEL was triggered????
350k (350,000.0 miles) being high end of life for injectors???? Plausible at best... doubt they all went bad at once! CHECK grounds and Fuel Pressure.
 
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