ULTM8Z

I've been picking the brains of folks on Thirdgen.org and Nastyz28.com in an attempt to solve this almost-year-old problem I've been having. It's a misfire at idle speed that appeas to be localized to the #2 cylinder. I'm hoping someone here might have an idea. I narrowed it down to #2 by pulling injector plugs and spark plug wires off one at a time and monitoring the O2 sensor response on my scanner. It's definitely at #2.


I'm asking a few of my buddies if they know anyone with a good shop that has a scope. But what would it tell me if someone was to do it?

I've verified all engine internals are good (cam, heads, lifters, pushrods, etc.- by inspection and with leak down and compression tests), sensors, plugs, wires, cap, coil, rotor, distributor, injectors, wiring harness, ECM, PROM chip, vaccum lines, fuel pump and filter, pressure regulator. How could this freakin' thing still be misfiring?? There are no trouble codes. The only indication that something is wrong is a really noticeable misfire at idle speeds and a bad stumble anywhere below 2500 rpm. I've also spent an inordinate amount of time with my new CATS Real Time Tuner trying to "tune" the problem out, but to no avail.

Before this problem started up, the car was running very well with this engine combination. As of now, I'm at a total loss.

My setup consists of a ZZ4 (with approximately 30,000 miles), AFR 180cc heads, a new Comp Cams 268 roller, MiniRam intake with a 1990 Camaro fuel injection setup. I've tried the factory calibration along with the ones I've been burning and the misfire is always there. I know the ECM has no individual cylinder control on spark or fuel, so what gives?

I'm hoping someone here has had a similar problem and found a solution?

lordmetalz28

im having the similar problam below 1000rpm it feels like its gonna stall. and in reverse to badly it happened a few months ago then went away. i degreased my motor this weekend and it reoccured but this time wont go away. i have not looked into it yet but should.

90rocz

I suggest you start over with the basics. Swap parts from a known GOOD cylinder, one at a time, such as: spark plug, plug wire, injector etc.
Verify the distributor cap is properly seated in the center of the groove of the module. A tilted cap can be a frustrating miss.

One of the things you've mention MUST be bad.
How are you checking each part???

aklim

How did you do this O2 sensor reading? AFAIK, 3rd Gens have one sensor and that is in the driver side before the collector. Whatever the passenger side #2 is doing, the sensor cannot read.

ULTM8Z

To verify parts, I did one of three things: swapped it with known good part from another car, tested it with a multi-meter, or simply bought a new one. I've also tried to move the misfire to other cylinders by moving around injectors, plugs, wires, etc. But it only seems to be acting at the #2. It makes absolutely no sense.

As far as O2 sensors, I put another one in. The engine typically runs of the driver's side sensor, but I put one in on the passenger side to help diagnose this problem. I spliced in a connector near the ECM and built a special harness to go from the passenger side O2 sensor to the ECM. Both driver and passenger side O2's are heated sensors.

aklim

Sounds like you have covered all the bases. What does the #2 plug look like in comparison to the others? According to you, the compression and leakdown is good. I'm assuming that the compression is within spec of the others.

Try grounding the spark plug and see what happens. I would ground the metal base to the neg pole. If that doesn't change things, we know that it is getting ground so it should be able to fire. You say the injector is good, that leads to the next question. Is it getting a fire signal or is the injector plug bad. Check the voltage you are getting and see if it has come loose (wires).

You saying that the ECM is getting 2 readings while it thinks it should have 1? I mean, won't that double the voltage it sees? I have not heard of that one being done but that is interesting.

ULTM8Z

It's wierd, but there's no noticeable difference between the #2 plug and the others. Maybe I should let it idle for a long time and then look.

The spark plug ground thing sounds interesting... I'm going to try that one this weekend.

Quote: You saying that the ECM is getting 2 readings while it thinks it should have 1? I mean, won't that double the voltage it sees? I have not heard of that one being done but that is interesting

I'm assuming you're refering to the O2 sensor. Actually, the ECM never sees more than one at a time. I simply rigged it such that I can switch from one side to the other in order to see what each side is doing.