89 LeSabre Intermittently Running Rough, No Codes

[timrice]

I think my '89 (169K) may now be a part-time five-cylinder. It intermittently runs rough at idle (in P or N) or at highway speed. But it will run smooth sometimes when warmed up, too. It always runs rough when started cold. It rarely if ever will actually die, but it will sure slug down when it's unhappy. No matter what happens though, there are no codes to point me in any particular diagnostic direction.

I took it to a local shop that has done pretty reliable work for me in the past. They hooked it up to the Sun machine, showed me a printout of the coil firings and told me I had a misfire on Cyl. 3. But then they told me that to fix it, it would require a new belt, a transmission mount, plug wires, a fuel filter, a motor "vac" (supposed to clean your injectors / rail chemically, I suppose), plus cleaning the throttle body and MAF sensor by hand, for a total estimate of $662. Not being familiar with the motor vac's effectiveness one way or the other (this car had one 30K ago), I declined it, and just had them put new plug wires on; they said the plugs were good. I was very skeptical that the belt or transmission mount were going to fix a misfire, and even more so when they suggested putting transmission fluid "additive" in and using some toothpaste-like goo for the torque converter. Maybe they weren't as laser-focused on the "misfire" (if that's what it is) as I was...anyway:

Fuel filter / pump have 11K on them.
PCV valve has 28K on it.
02 sensor has 29K on it.
Crank sensor has 11K on it. (replaced needlessly in fuel pump fiasco)
Belt has 27K on it.
Spark plugs are new - I put them in.
Battery and [+] post are new.

(By "fuel pump fiasco" I mean that three aftermarket-brand fuel pumps shelled out in this car over a two-year period, and on the fourth, I insisted on an OEM part. The symptom with that problem was different...the engine wanted to run rough and would actually die when it was hot outside and after running the car for a while. It would start right up after you let it cool a couple hours. After the OEM pump went in, I haven't noticed any obvious fuel pump problems.)

I swapped the HEI module (coil pack too) with my '90, and neither car's behavior changed, so I feel that's ruled out...

I put a bottle of injector cleaner additive in my last tank, but I've only run about 20% of the tank through the engine, so it's probably way too early to tell if that will even help.

I've run a compression check, and found nothing that seems out of whack: Cyl #1 155 Cyl #4 150
Cyl #2 140, 145 Cyl #5 155
Cyl #3 150 Cyl #6 155

I "stethoscoped" the injectors with a metal rod that had a wooden handle on it, and all of them sounded the same, at least to my untrained ear. I would have to say that when I did this test, the engine was running smoothly, probably, because I didn't hear the engine stumbling or laboring at the time.

I'm getting ready to run a fuel pressure test with a gauge, and check to see if I can find any loss of fire in the plug wires with a timing light / inductive pickup.

Depending on how these tests turn out, I am considering buying a new injector and putting it in #3. I suppose it could be the ECM or wiring / HEI terminals, too...

Any ideas on how to narrow this one down?

[edg]

Check your EGR valve for proper operation. If it's not closing completely due to carbon buildup or other problem, it will cause a rough idle.

[timrice]

Thanks for the reply, edg. Hadn't thought about the EGR. I couldn't get the engine to act up today, so my timing light experiment didn't yield any new information. And with it running OK, there seemed to be no point in hooking up the fuel pressure gauge. The intermittency is the worst part of this problem...

[gwoehrel]

Originally Posted by timrice

Thanks for the reply, edg. Hadn't thought about the EGR. I couldn't get the engine to act up today, so my timing light experiment didn't yield any new information. And with it running OK, there seemed to be no point in hooking up the fuel pressure gauge. The intermittency is the worst part of this problem...

Did the ERG work. My Lasabre is doing the same thing. I have never worked on one of the 3800 model v6. any information would be helpful.

[RABarrett]

This "diagnosis" and repair sounds like throwing out the baby with the bath water. Some insight: when listening to the injectors. listen for two things. The first is to listen for a solid metallic tap, which indicates a clean seat for the pintle. If the seat is dirty, it will sound like tapping your fingertip on a table, a little "soft". Listening carefully will show the difference if there is one. The other thing to listen for is a steady tapping, rather than the occasional "miss". Do all of the injectors sound steady? I would focus on #3, unless the test was done improperly, and the determined miss is in another cylinder. Sometimes, a plug examination will reveal something. The complementary plug to #3 is #6. The coils fire cyls1+4, 2+5, 3+6. Sun machines, if properly equipped, can also reveal 4 gas readings per cylinder. If I have those as well, I might be able to steer you in the right direction. Since the EGR has already been mentioned, I won't mention it again other than to say that it is a possibility. Is the timing chain noisy? It is possible for slack in the chain to affect the cam sensor's readings in relation to the crank sensor, causing timing errors as well. Food for thought. Ray

[timrice]

Swapped the ECM with the one from my '90. No change in either car. Fuel pressure testing revealed no problem. Never did put a new injector in cylinder 3. As much as I hate to admit it, I finally gave up on this car and sold it. I hope the next owner figures it out.