Cylinder #1 Low Firing at Idle

[Ilan]

Took my 87 Carrera w/ 97K miles to my wrench because of rough, though steady idle. Previously, he discovered that the PO had disconnected the O2 sensor, and changed the CO settings to pass IM inspection, thereby having a somewhat unstable idle, and the engine running too rich and therefore too hot. My wrench readjusted all the settings to spec, and the idle was steady, oil temp even in hot weather was good, but there was a bit of "shake". Don't want to bore you with all the details, but suffice it to say that I was there most of the day with the Wrench, and he went about diagnosing the problem methodically, and in line with a lot of the threads I reviewed. He quickly narrowed it down to Cylinder #1 not firing during idle, and suspected the fuel injector, but no go. Found a bunch of cracks in the distributor cap and had a no-name rotor, so replaced both with Bosch according to spec. Also, for good measure, replaced all of the plugs. Checked for vacum leaks, but everything was fine. Fuel spray and spark voltage were excellent. He even did a leakdown, and it was a fabulous 6%. Checked and cleaned engine ground connections too. Finally decided to check valve adjustment and springs on that bank of cylinders, and everything was fine. (Had a complete valve adjustment about 6K miles ago). Now, he admitted he was beginning to grasp at straws, and I suggested we replace the fuel relay on the DME, since I had a new one as a spare. We looked at the old relay, and he told me it was a good thing we replaced it, because the original one had a loose solder connection, and it was only a matter of time before I'd be stuck.

By now, the symptoms still were there, but not as apparent. Matter of fact, when I took the car for a run, the "shake" was pretty well gone, but my wrench wasn't satisfied. CO was still too high in front of the catalytic convertor (about 2.2 instead of .04-.06) so he wanted to check if Cylinder #1 was running hotter than the rest, in order to check for any internal problems. Using a pyrometer, we noticed that actually, Cylinder #1 was running a bit cooler than the rest of the cylinders. At this point, we decided that I'll just drive the car. Frankly, now the idling is great, I definitely feel more power and smoothness as I accelerate, but my wrench and I are frustrated that we didn't absolutely pin-point the problem.

Any thoughts?????

[Early_S_Man]

Ilan,

Did he get good readings from the O2 sensor after hooking it back up? That kind of mileage sounds like a new O2 sensor would be called for.

Also, did he check for an updated two-wire head temp sensor?

[john70t]

This is just hypothetical but.. perhaps a plug wire/plug is non-functioning.
The the non-converted oxygen from the #1would fool the O2 into constently richening the mixture.
The raw gas from #1burning in the cat would also help cause the high CO. It would also cool the cylinder down as rich mix=cooler.
The shaking from one clyinder misfire wouldn't be noticed off idle because of the engine balance.

[Ilan]

Warren:

O2 sensor was replaced a few thousand miles ago when we found the old one disconnected. Not sure whether he actually checked the readings off the new one. Don't know about the 2-wire head temp sensor. I'll call him next week. Thanks for the input.

[Ilan]

John:

Forgot to mention that he did check all the plug wires, and also replaced the ignition coil wire. The original plug in cylinder #1 was ok, but we did replace all the plugs and he verified that they all sparked. The way he diagnosed #1 not firing was that when he disconnected the plug wire to #1, the shaking would go away. As I understand it, by disconnecting #1, the DME would stop hunting and readjusting. Forgive me, but I may be phrasing things incorrectly because this is definitely way above my level of expertise. And to clarify, the CO reading in front of the cat was high, but the cat was definitely doing its job by reducing the CO coming off the back end. Of course, this could cause the cat to overheat and damage over time.