Idle Missfire

[vampyr]

OK I have been searching, and I am not really finding what I need. Here is what I have going on,

Earlier this year I picked up a 1997 Camaro 3.8, auto with a little over 180,000 miles and a blown head gasket. Got a KILLER deal on the car. Head gaskets have been replaced, heads resurfaced in the process.
Pain in the butt, but not a real problem.

The problem is this, at idle it misses, the check gauges light NEVER comes on but if I plug my AutoTap in it says I have "random multiple misfires". I that that code. I have replaced just about EVERY thing I can think of (I luckily have a donor car). I have replaced all of the coils, new plugs and wires, the ICM, ALL of the sensors accept the O2 sensors, the computer, all of the vacuum lines, the FPR, I have checked to verify that I don't have a leaking injector, fuel pressure is fine at idle.

The car runs down the road just fine (does not miss according to Auto Tap), but SOME of the time at a stop light if is VERY rough it bucks and generally sounds like crap. Other times I can feel the miss but not very bad. I am out of ideas on this one. Please help... Any constructive ideas would be appreciated.

[97rs4life]

Re: Idle Missfire

did you have the heads checked when you replaced the head gasket for cracks or warpage? Maybe some coolant leaking into cylinders yet

[vampyr]

Re: Idle Missfire

Heads were tanked and resurface when I did the gaskets. I am not showing ANY signs of a blown gasket. (not loosing water, no dirty mayo in the water or oil).

[97rs4life]

Re: Idle Missfire

maybe a faulty/defective new part? Does your scanner monitor each cylinder and tell ya how many misfires per cylinder? If it does, track that, then if it is two cylinders, see if those two cylinders share the same coil. Or switch the plugs to another cylinder and see if the misfire moves

[vampyr]

Re: Idle Missfire

Unfortunantly it does not track by cylinder. Fortunantly I know the parts are good because they are directly off of another running car, (all accept the plugs and wires). I have been reduced to the part swap game. Every part I swap from one car to the other doe not change EITHER car. As in the "donor car" is still running fine the "new" car is still running like crap. Hope that made since. LOL

I suppose it could be one of the plugs or wires, but the reason I changed them was to solve this and it did not fix (or change) the problem. in addition I would think that if it was a bad plug or wire it would happen ALL the time not just at idle correct?

Could a BAD O2 sensor, bad cat act like this?

[97rs4life]

Re: Idle Missfire

a bad cat usually will act up while under a load as well like going up a hill. A bad o2 sensor may but then you would have an o2 sensor could or a lean/rich code.

I have seen brand new plugs cause issues like you are having though, just takes one to be defective and give you grief. That is why if you had a way to monitor which cylinder(s) was missing you could eliminate some other items.

[vampyr]

Re: Idle Missfire

I hear ya. I guess I could do the single plug swap. Or take it to a dealer (really don't have the 100 bucks) and have it scanned. What is confusing me is the "sometimes" it is bad sometimes not thing. Also the fact that nothing changed when I changed the plugs and wires. *sigh* LOL


I love the challenge of a car like ours, but sometimes you just want it to WORK.... AHHHHHHH I am just getting frustrated.

[Blizzard242]

Re: Idle Missfire

This if for my general knowledge too, but could the IAC have anything to do with it?

[cam98aro]

Re: Idle Missfire

This if for my general knowledge too, but could the IAC have anything to do with it?

yes, def. and its the first thing i would replace if my car was doing that.

[Bandit127]

Re: Idle Missfire

I ran 'round and 'round with a misfire in my 3.4 until I took a step back from it and treated it like a customers car.

FIRST: What is needed for combustion?
1. Air.
2. Fuel.
3. Spark.
4. Compression.

We know air is in the clear, so move to fuel.

Fuel Tests:
To test if you have a sticking injector (what I had, and I had the same exact problem as you), you first need to get a ignition on- engine off fuel pressure reading. Then, one cylinder at a time, measure fuel pressure drop with a injector pulser, and reset fuel pressure after each test. Every injector needs to be within 10% of each other. I had an injector spraying 4lbs extra and it was causing a horrid misfire at idle and low rpms under load.

Next test on fuel injectors is to do an ohms test, you need to measure resistance across the terminals on the injector. I'm not 100% how this effects running, but it has to do with injector temp and will help tell if you've got a soon-to fail injector. (look ohms specs up)
That should cover injectors and fuel delivery.

Next is Spark.
First to do is to test the ignition coil.
Get a foot of vacuum line, and cut it into 6, 1inch pieces and stick the pieces on to posts of the coil packs. Then stick your wires ON TOP of the vacuum line and start your engine.

Now, you should notice that its not running any different, because it can easily make the arc at idle. Now, you take a 12v basic light on power tester, and after grounding it, move the point to each vacuum line, and you will notice the spark will now arc directly to the probe and NOT the spark plug. At this point, you need to listen to the engine and by listening and moving the tester to each vac line, you should be able to tell which cylinder isnt firing (engine sound doesnt change) and that each coil is working properly.

You've put new plugs on, and wires, so we'll skip that, but basically the same idea.

Now you need to measure compression on EVERY cylinder and they should all be withing 10% of each other.

Now the best thing I can tell you is that a scan tool is never true 100%.
A scan tool is, and always will be, a post processed and processed again data.
PCM's are never 100% true either, because its processed DATA, if the data is wrong i.e. bad sensor, then your scanner will never be right either.

Computers have come a long way, but they havent come so far as to really know whats going on.

Doing these gauge and meter tests will give you a much better idea than pulling codes and throwing parts (MONEY AND TIME) at the car when a simple 12v probe and vac lines might tell you 10x as much as using a scanner. As one of my Snap-On teachers taught, the worst word in the world is FREE because, if you put a part on and it doesnt fix it, you gotta eat it and they get it for FREE.

Extensive hands on testing > scan tools.

[timthescarred]

Re: Idle Missfire

When I was having a misfire that showed up at idle like yours does, I took it to the dealer and they plugged their Tech-II scanner in and told me how many misfires per cylinder for free.

Incidentally, mine turned out to be a bad FPR. I also had a hard start issue

[vampyr]

Re: Idle Missfire

Blizzard and Cam98: that was my initial thought as well. The IAC has been changed for a known good unit with NO result.

Bandit: Thanks for the info, and I agree with you on the trouble shooting steps. As I said I am lucky to have a running "donor car" to get parts off of, or else this car would have been history a while ago. I am leaning toward a possible vacuum leak or a possible injector problem. I am ruling out the vacuum this weekend. I got a vacuum gauge and I am getting 15.5 to 16 (book says 17 to like 22 is norm) inches of vacuum, but when I shut the car off it is NOT holding ANY vacuum (is this normal?). Some of the rubber couplings are trash so I am going to replace them and see what I get outa that.
The coils are a known good entity as well, and I ran a compression test after the head gaskets and all are within spec.
As for the computer thing, again I agree but I have swapped this as well and had NO change. I am a little curious as to why my "check gauges" light NEVER comes on (bulb has been checked and it does come on at start up) Even when my Auto Tap says the light is on...I just find that a little odd.

tim: how much did they stick you for the scan?

[timthescarred]

Re: Idle Missfire

.

tim: how much did they stick you for the scan?

Nothing, I walked in, asked them if they would pull up the DTC's they plugged it in in the parking lot, told me what it was and then offered to bring the car in for a diagnostic.

If I would have left the car with them they were gonna charge $90 to do a Diagnostic to tell me what was causing the misfire.

Just scanning and telling me misfires the guy did for free.

[Bandit127]

Re: Idle Missfire

It might be, I've always tested vacuum with a smoke machine by pumping smoke into the brake booster to intake mani hose. Then using a flashlight, scour the entire upper engine area looking for wisps.

Now, after I changed my two bad injectors. (one dumped fuel, other failed ohms test) I had TWO more die on me, and these two dumped 15lbs too much, and my engine backfired insanely and I believe I blew an upper intake manifold on my 3.4, causing a "im not going to idle unless my rpms are above 1000 ****) and then my motor went due to fuel in the oil. So, I stress making sure its NOT an injector issue, because fuely oil sucks badly.

[vampyr]

Re: Idle Missfire

Is there a way to get aroun the "special tool" that is needed to remove our fuel rail from the fuel lines? I would rather just swap the entire rail then one injector at a time. I don't have a way to pressure test the injectors other than the test I already did on them. I know it is not a difinitive test but I changed the oil a couple of weeks ago and it did not have a fuel smell to it. It also was not overly thin.