mis-fire nightmare/im stumped

[pfmeeks]

I purchased a 04 SE with the 3.8 with 138k miles yesterday. It has a mis-fire that sometimes is bad enough to cause the engine to stall. The only code that comes up is the o2 sensor. I drove it about 3 hrs. home and on the highway it does pretty good unless you get on a hill it wants to mis-fire. It stalls worse usually in gear and sitting still.

I know I should replace the o2 since its throwing a CEL but I don't think that's the main problem. Usually a bad o2 will cause crappy gas mileage or run a lil rough at worse but not this bad.

Ive worked on cars for almost 30yrs and cant figure this one out. I read about the crank sensor and replaced it and didn't fix it. I put new A/C delco plug wires on it today, the old ones were original.

My neighbor has a monte carlo with the same engine so I decided to swap out the MAF, TPS, and Idle air control valve, coil packs, and ignition module, off his car which I know runs perfect.. and that still didn't fix it !

I checked the fuel pressure and while running its a steady 45 psi. and holds pressure after shutting the engine off.

I sprayed starting fluid all around the intake and doesn't show any vacuum leaks. The PO says he had the intake gaskets replaced. I can shut it off, start it back up and itll run "ok" for about 30 sec and then mis fire badly.

When driving down the street itll mis-fire as if its not getting fuel and I can mash about 3/4 down on the pedal and all the sudden.. it will take off like a rocket.

So.. it has a new crank sensor, swapped the TPS, MAF, IAC, coil packs, ignition module, checked for vac leaks, New plug wires, plugs looked good. A/C delco's, checked fuel pressure. IM STUMPED !!!!!!

Any ideas anyone ? thanks for any help !!

[FiendScurro]

Map sensor/pcv valve? Air filter?
Reset the pcm by disconnecting battery for 10 mins.
Clogged/restricted cat?
Hope you can figure it out. Looks like you already tried most of the things we would tell you.

[MKMike]

If the oxygen sensor is constantly telling the engine to lean out the mixture, then it could be the entire problem--although the symptoms are rather atypical.
Maybe it is simply a combination of things.
A clogged fuel filter that allows normal flow only at idle.
Clogged pickup sock for the fuel pump.
Faulty fuel injector(s).
A faulty fuel pressure regulator that doesn't close, so the engine is starved for fuel while under load.
PCV or EGR stuck open creating a vacuum leak.
Defective plug that isn't always firing--perhaps due to a hairline crack or carbon tracking that causes no spark when under load as the spark travels to the block instead.
Clogged air filter choking the engine even at idle.
Timing chain jumped a tooth.

Hope you figure this one out ASAP before something gets damaged.

[pfmeeks]

Well.. I broke down and took to a shop and they diagnosed it as a bad MAF connector. I went to NAPA and purchased a new connector and installed it and STILL DIDNT FIX IT. I probed the wires and im not getting 12 volts to the MAF.
I decided to use a jumper wire and supply 12 volts to the MAF and then probed the wire that should show the voltage change as you move the throttle and its staying stuck at 2.75v.
So.. I guess I have two issues.. not getting 12 volts to the MAF with the key on and the MAF output signal is not changing while the engine is running so bad MAF ?

Anyone have any idea what supplies the MAF with 12 volts ??

[pfmeeks]

well.. figured out the fuse for the MAF was blown, replaced the fuse and got 12 volts to it now. Im a little concerned about the output readings im getting. Im gonna check on that some more.

The o2 sensor I can disconnect it and the car will idle without stalling and when i plug it back up it will start running worse so I guess it is the root cause of the problem. Ive never seen a o2 cause that much trouble.

[MKMike]

Glad you likely have gotten it all figured out.

Your experience with paying a shop to diagnose it is the type of experience that motivates many of us from paying for diagnoses.
If the mechanic was worth his salt, it would have been money well spent but, unfortunately, you usually don't know until after you pay the bill and get or don't get the result you expected.
At least when you do it yourself it is most often cheaper even if you change a part that was good.

[max harris]

Glad you likely have gotten it all figured out.

Your experience with paying a shop to diagnose it is the type of experience that motivates many of us from paying for diagnoses.
If the mechanic was worth his salt, it would have been money well spent but, unfortunately, you usually don't know until after you pay the bill and get or don't get the result you expected.
At least when you do it yourself it is most often cheaper even if you change a part that was good.

That's pretty much about right, I had been fighting a PO301 and PO201 code with an occasional PO300 code thrown in. One shop in Louisiana where I was working at the time said it was a dirty MAF. The problem re-occurred the next week, changed out the injector, coil pack, spark plugs, wires, and finally the ICM after a parts store said it was going bad. After losing my job out there I had a friend tow it back here to Florida, the misfire problem was only intermittent. The shop I normally use here wouldn't even look at the car until it was actively misfiring. That took almost a year to get the misfire to occur when they where open. They told me it was a blown head gasket and wanted $1500 for the repair, being unemployed I told them I did have more time than money. Got that job done took my time and cleaned everything up so it looked nice, the fourth day back on the road the misfire started again. Since the misfire was always on the number one cylnder and checking around some other sources I figured out I was the crankshaft position sensor since it's on primary responsibility is firing number 1 cylinder. I have been driving the car around for a week so far so good.

[pfmeeks]

It still has a very slight misfire at idle for some reason..

[redzmonte]

any way to scan the car and see if it has a misfire count on what cyl? that should help narrow it down to what plug, wire, coil to look at.

S