Help - Can't find problem

[bc6t]

Been trying to solve this for weeks now....(1998 STS with 103K miles)

One morning the car starting idling rough in "D" and "P" out of the blue.

Sometimes it idles o.k. (when hot) and then I get what appears to feel/sound like a miss. The tach is steady at about 650 and then bumps down quickly to 600 and then it is ok again for 10 seconds or so and then repeats. When cold it does this at a higher frequency - every 2-4 seconds. It never surges above 650 - it only dips abruptly giving the car the shakes when it does.

When in motion - I don't seem to feel any abnormal rpm dips, etc.

I have changed out the plugs and wires (which are now only 7mm - way to go GM - I am not impressed) with OEM AC/Delcos and cleaned the EGR valve. Changed the air filter too...Before cahnaing the plugs, the car would also miss and heisate during acceleration and driving. This is why I thought that for sure it had to be the plugs/wires.

Funny thing though, when I changed the plugs it ran really well for 1 day and then back to old. The new wires actually seem to have made it run a little poorer. (changed wires 2 weeks after plugs)

Of course, there are no related codes to indicate the failure of anything and I have WOT many times thinking it's carbon buildup. What other diagnosis/fixes might be in order?

I thought of replacing the EGR, maybe a failing ISC motor, possibly needs a throttle body cleaning....but I could not see how these would only be issues at idle and not in motion, especially when the car actually will run fine once in a blue moon.

Any input appreciated....

[BodybyFisher]

You said, "wires made it run poorer"

Make sure that they are routed properly and not side by side next to any injector wires or injectors. They MUST follow the correct routing.

Make sure that you didnt accidently crack a plug and that all boots are seated fully on both ends.

IF you used dilectic grease on the boots, THAT can cause a misfire as it will complete the circuit to ground. The manual clearly states NOT to use it.

DID you pull the fuel rail and attempt to clean the injector tips? DON'T be tempted to do so you will clog them, if you did clean them, that would be your problem. If you pulled the fuel rail make sure its seated. Make sure that all injectors are connected properly and that all vacuum hoses are connected and that one did not crack.

Pull the vanity cover off and in pitch dark idle engine and spritz the wires with water and look for arching, stand to side and carefully raise the idle looking for arching.

Did you use injector cleaner? Its not advised. Use only TECHRON and ONLY on a full tank and ONLY on a trip to run it out PRONTO.

Pull each wire and look into each end to see if you see the core burned out..you could have a bad wire. Measure each wires resistence with an ohm meter.

You commented on GM "way to go GM", if they validated that 7mm is OK for them, its OK for me... In this case THICKER is not necessarily better.

[bc6t]

You said, "wires made it run poorer"

Make sure that they are routed properly and not side by side next to any injector wires or injectors. They MUST follow the correct routing.

Make sure that you didnt accidently crack a plug and that all boots are seated fully on both ends.

IF you used dilectic grease on the boots, THAT can cause a misfire as it will complete the circuit to ground. The manual clearly states NOT to use it.

DID you pull the fuel rail and attempt to clean the injector tips? DON'T be tempted to do so you will clog them, if you did clean them, that would be your problem. If you pulled the fuel rail make sure its seated. Make sure that all injectors are connected properly and that all vacuum hoses are connected and that one did not crack.

Pull the vanity cover off and in pitch dark idle engine and spritz the wires with water and look for arching, stand to side and carefully raise the idle looking for arching.

Did you use injector cleaner? Its not advised. Use only TECHRON and ONLY on a full tank and ONLY on a trip to run it out PRONTO.

Pull each wire and look into each end to see if you see the core burned out..you could have a bad wire. Measure each wires resistence with an ohm meter.

You commented on GM "way to go GM", if they validated that 7mm is OK for them, its OK for me... In this case THICKER is not necessarily better.

I replaced each wire 1 at a time and followed the path of the original ones. The AC Delco replacemnts were not exactly the same as the original ones - the lengths were close but not the same and not numbered by cylinder. The wires already had dilectic grease on them - green stuff. I left it alone.

I made sure that every wire was seated correctly, but I didn't torque down the plugs - I just used equal pressure all around - I will remove them on the weekend to check for any cracks just in case I tightened/broke any....

Didn't touch the injectors or fuel rail or anything else - i will test for arching as well on the weekend....It does sound fishy though that after repalcing plugs and wires that the engine did not improve in its idling but in fact got a liittle worse. It did improve under acceleration and general cruising.....

One question: Is it possible that the coil pack has started to fail? Are these symptoms consistant with a failing coil pack?

[zonie77]

If you find a shop that has a scope and knows how to use it you could pin down the problem.

[BodybyFisher]

Check the routing again, make certain that all of the injector wires are not near the ignition wires they should cross at 90 degrees. Reminder: keep the ignition wires away from the HVAC Fan high voltage leakage/transients can damage the fan controller.

Check your plug gaps carefully. I am surprised that there was GREEN dielectric grease on your wires, typically its impregnated and you normally don't see it, what you describe seems very odd... Never pull the wire to remove it grip the boot. How your coil would spontaneously go bad is beyond me. I would dry as much of that green stuff off of my plugs as possible. Look at the plugs carefully for cracked porcelain or compression leaking (have seen defective plugs).

Make sure you torque the plugs to spec, for you car that is 11 ft. lb (15 N.m).

[bc6t]

Well, I couldn't wait for the weekend - the car finally threw me a P0300 code - misfire...so I took everything apart tonite.

What I found was that one of the back plugs had a very small hairline crack running down 2/3 of the porcelain. I probably did it reaching in to tighten it and it was now quite visible with the green dielectric grease highlighting it. Too bad I threw the old plugs out....I simply moved this plug to the front where I will get a single replacement tomorrow.

As for the wires, I re-checked their fit and routing carefully and it seems fine.

Of course, having removed the coil pack now 3-4 times I was bound to hit some mishap - I dropped one of the mounting bolts - heard it hit a few parts and of course didn't come out the bottom... jacked the car up and searched for 2 hours... no luck.... I will have to get another from a hardware store or car shop.....

As for the grease, I ordered the part through a local shop that deals with ACDelco - they are indeed OEM....

Another odd thing the shop book says to gap the plugs (1998 STS) at .050 in. but it also says 150mm. These numbers do not add up - the metric conversion is around .060 in.

Which is the correct gap?

[BodybyFisher]

According to the info I have on your car the gap is .050, but check the tag on the hood of your car and see if it states it there. The tag on my car is straight up dead center at the front of the hood.

Glad you found the culprit.... Mike