'99 Camaro 3800 Seriesll automatic.

Started 3mo. ago with #1 misfire code. Checked wire - it was bad -replaced it -all OK

week later misfire NO CODES - checked all wires - hard and brittle- replaced all wires AOK

Then 3 wks ago started getting stumbling in overdrive when RPMs are around 1800-2000, going up a slight hill and you want to give gas. NO CODES. Then paid attention and stumble is pretty much under low rpm load. More paying attention- stumble even in Park/Neutral when you goose the gas. More paying attention. It happens only when warm/hot.

OK current symptoms:

-Alternating P0303 and P0300. Only throws code after warm/hot.
-Idles fine, high speed/steady speed fine
-stumbles when under load whether actual mechanical load or just goosing throttle in Park. then picks up and runs fine.
- Accelerates like a rocket when cold but that only lasts 3-5 minutes.

What I've done:

Ignition:
- New AC platinum plugs @ .60 gap. Old plugs all seem OK - slight tan. #3 plug looked good except for a slight deposit on side electrode and threads a little darker than the rest.
- New wires. double and triple checked wire routing. and boot connections. Old ones were brittle / stiff and I could snap them so I knew it was the problem ....NO!
- Had left over ICM and 3 coil packs from an engine change. Both ICM's checked out at auto store. rotated all 6 coil packs and 2 ICM - nope! . Bought one new coil pack and rotated to different positions. Did all this b/c thought coils going bad under heat. It is possible two used ICMs and 6 used + (1 new) coil packs bad but not likely.
- double checked ICM ground and bolts for corrosion. Cleaned ICM body to heat sink.


Fuel:
- Fuel pump replaced last year for unrelated reason.
- Fuel filter replaced
- Fuel pressure - KOEO (key on engine off) - about 48psi. Pressure holds pretty steady at 40 after key off for about 15-20 min. Engine on - pressure 45-50.
- no fuel in fuel press reg vac line.
- all injectors show 13.1 ohm at warm
- noid light shows flashing on all harness connectors
- replaced all injector o-rings
- running injector cleaner thru for a couple tank fulls.

Vacuum-:

-Found pretty big leak in line to fuel pressure regulator - Fixed it, sure this was the problem - NO!
- Found another leak - NO!
- DId propane check around manifold, throttle body and injectors but idle doesn't change ( b/c it idles fine) and have had a hard time finding helper to goose throttle. ( This is drive by wire throttle)

EGR: Pulled off to make sure no stuck pintle but haven't checked electrically.

MAF: pulled off known working vehicle - no change

O2: All three O2 sensors replaced last year for unrelated reasons.

What I haven't done:

Compression- Not done yet but this doesn't seem likely as this was a new used guaranteed motor and was running fine.
Check TPS for flat spot
Check crank sensor for rubbing/loose
Driven this car off the tallest cliff I can find.

I think it runs good cold 'cause its in start mode (open loop?) and getting extra gas like its supposed to but then goes into computer mode (closed loop) and then maybe leans out due to bad fuel injector or a vacuum leak I haven't found? So far I have gotten off easy dollar-wise as all this has cost about $125 and it needed this stuff anyway. I have access to '99 LeSabre that shares many parts so I can start swapping parts.

Any help appreciated .

 

if it has factory exhaust the cats could be plugging up. unbolt the y pipe from the manifolds and see if this fixes it. if so, get new exhaust.

 

hmm, if that was the cause it would do it right off the start. He said it runs fine when first started for the first few minutes. A clogged cat is "always" clogged.

Check your fuel pressure after it warms up. If you can hook it up and fix it where you can watch it as you drive would be perfect(not sure if you have that much of a gap in the hood or not).

Edit: btw those or misfire codes if I'm not mistaken. did it run fine be for you changed spark plugs or is that after the fact?

 

Could be cat. It is an aftermarket, but kinda thought a plugged cat would be bad all the time. Like I said, you can stomp on it cold and it hauls *ss for about 5-7 minutes then it starts acting up.

Watched fuel guage taped to wiper. 45 lb running and jumps to 50lb when stomp on the gas. I think it does the same on the Buick which is running fine.

Ah, was getting codes before plug change but they migrated from specific cylinder to random misfires. Now after fixing the vac leaks ( I don't know if I caught all of them) seems like Random P0300 has gone away and code is exclusively P0303 cyl #3.

Going to pull the new #3 plug and check it out. Also will swap out injector from the LeSabre. I've had THREE coils on the 3/6 position. Two used and one new. Pretty sure it's not the coilpacks.

It's possible got a bad new plug or new wire. I'm still hung up on the fact that stomping on the gas when cold works fine. The other thing is the last code yesterday happened in closed loop when hot, then fixed vac leaks and the P0303 happen again when warm but in open loop. I thought open loop was only when cold. This was the first time it's happened. Code always throws when warm and FF data always shows closed loop.

Another strange thing. Tonight cruising on highway CEL went OUT! That never happens. I always have to clear with scanner. Code is still there and stumble is still there but light is out. What's up with that?

Thanks for the tips.

 

"only happens when warm"

if it's not ignition then you're running lean. Check the O2's regardless of when they were replaced

 

O2 sensors quick check seem OK. P0303 keep showing. Lean condition sounds right.

Freeze frame fuel trim data short and long term show computer trying to lean mixture. ( mostly negative values but still under 10% swing - usually around 0.0 % to -0.3 to -0.5%)

(pulled and reconnected crank and cam sensors - don't seem broken or loose)

Since the noid test shows OK I'm guessing #3 injector sticking ( or PCM pulse too long which I can't check) which is dumping extra fuel in so computer trying to correct by running lean. Sound right?

Also, there is a good site called " easy autodiagnostics" to check lots of things including TPS but I have the drive by wire and the TPS is in cabin above the gas pedal. Any links on how to check the TPS either at the tps or in the 5-6 wires coming out of throttle body? This TB unit is riveted in. I am not afraid to drill and re-tap, but would rather not.

I know the TPS is usually a resistor and can be checked via VOM meter but I have no schematics and can't seem to find info on TPS mounted in cabin.

Thanks

 

my first thought is injector or cam/crank sensor

 

Well I guess it's on to the crank and cam sensors. New #3 injector did not fix. As mentioned, most codes have been P0303 but right after the new injector I got P0300 and later in the day two more P0303.

FF data very strange too with this happening at very low temp (95 to 110) which can't be because engine is hot. All codes before injector change happened around 185-190.

Will try cam sensor first as I have a spare and it's easy to get to I think. Then we'll try the crank sensor. Also am wondering if temp sensor flaky which might be sending PCM into and out of closed/open loop as the last code was thrown in open loop but the motor was hot.

 

what brand of 02 sensors are in it? you cleaned the heat sink, did you recover with heat sink grease?

 

Don't know brand . O2 sensors put in last year. Guess I could ask the mech. Yes thin coating of heatsink grease was put between ICM and the sink.

Just read thread about someone who went through this for quite a while and spent tons of ($$$) Turned out to be crank sensor.