95 Lesabre rough idle, high speed miss

[ChemMan]

Update. Again.

The last few weeks the problem has been intermittant, coming and going every few days, and is not affected at all by rain. I checked every inch of wire between the #4 injector and the computer and the fuse. No corrosion, no bad insulation, no resistance in any of the wires. I replaced the computer in the car and have not had the problem reoccur yet. It is nice to have the car functioning properly again. I did 70 in overdrive on the freeway yesterday and she had plenty of power and ran nice and smooth. I guess I'll keep my fingers crossed for a while. If the problem comes back I'll update again.



Update.

I purchased a noid light and a spark tester. Spark to #4 is OK, but noid light indicates that injector #4 is not being fired by the computer. I checked resistance of fuse to injector connector and computer to injector connector. Both have near zero resistance. Seems as though the computer is bad at this point. Anyone ever had a computer fail like this? Any other checks I could do to confirm that the computer is the faulty part? Thanks.

I have a 1995 Buick LeSabre V6-231 3.8L VIN L SFI with the following symptoms: rough idle, seems sluggish/underpowered, a severe miss at 45-60 mph(dashboard, steering wheel visibly shaking), above 60 mph the car runs smoothly with no sign of the misfire. If I push my foot to the floor and cause the car to downshift, the miss disappears and accelerates with full power.
Using an autoxray ez scan 6000 the following trouble code is obtained from the computer: random misfire detected and misfire #4 cylinder (Please note this is from my memory and may not be word for word)
Initially i replaced #4 injector. Inside of fuel rail seemed clean, no visible dirt. Also replaced fuel filter. This solved the misfire at that time. A few months later same problem returned with same trouble code(s). This time replacing the #4 injector had no effect and problem remains to this day. New fuel filter, new plugs, plug wires installed. All three coils secondary winding resistances check out. #4 plug has some funny deposits on it that I have not seen before.

Any advice or help is greatly appreciated. If I have left anything out please ask.

[HotZ28]

Change the #4 plug and see if it runs better or if the "funny deposits" come back.

[ChemMan]

I took out all the plugs today. The #4 plug had a small amount of black tar-like deposits on it. I could literally smear the deposit off of the plug with the gapping tool. The gap was correct and the electrodes where pretty clean, so I reinstalled the plug. All the other plugs were very clean, including #1 which is in series with #4. The plugs are only 3 months old and have about 1000 miles on them.
I removed the 1,4 coil and examined it for any problems. There was a good amount of sand and dirt under the coil but the primary contacts seem clean. I swapped the 1,4 coil with the 2,5 coil(the one directly above it)and I'll see if maybe the fouling moves from one plug to another.
I also dragged my old oscilliscope out of the closet and backprobed all six injector wires with the engine idling and saw something interesting. On injectors 1,2,3,5,6 I observed a square wave looking pulse of about negative .5 volt peak for about 10ms with a 10ms gap between each pulse. In addition I observed a faint voltage spike of about 25 volts positive peak and 10 volt negative peak. The spike lasted about 10 ms and occured in place of one of the smaller square waves. So I saw 3 of the smaller square waves and one of the large spikes displayed on the oscilliscope screen.
Now on injector #4 I did not notice the large spike, but instead saw 4 of the small square waves.
I'm guessing that I was seeing the injector pulses, but what does the lack of that large spike signify on the #4 injector? Could it be the that the computer is not firing #4 injector correctly, or is the the coil that is starting to go bad and causing #4 plug not to fire properly, but not effect #1 plug at all?

[auto trainy]

Are the plugs and wires A C Delco?

[ChemMan]

Yes. I called my local autozone about getting the ignition module and coils tested, and they said that their tester is down until tuesday. Guess I'll get them tested next weekend.

[HotZ28]

Just cleaning the deposits from the plug will not cure your problem. Replace it! It sounds like the cylinder is pumping oil either from the valve guide or the rings. (Bad problem). If it continues to corrode the plug, you have an internal engine problem. Injector problems will not cause a tar like substance to accumulate on your plug!!

[ChemMan]

I highly doubt it is an oil comsumption problem. The engine has only 45000 miles on it and does not consume oil. The level on the dipstick does not drop at all during the 3 month interval between oil changes. My 78 Ford uses about 1/2 quart every three months and that has no effect on that engine at all. I've heard about these engines getting manifold leaks and using coolant, but the coolant level hasn't dropped below the full cold mark in the 2 years I've had this car. The deposits on the plug were definately not oil deposits, I've seen my fair share of oil fouled plugs. Further more, I can't imagine how minimal oil comsuption can cause a rough idle that turns into a full miss at speed.
At this point I'm really beginning to suspect a bad coil. I've swapped the 4-1 coil with the 5-2 coil, to see if the fouling moves with the coil. When I drove the car after swapping the two coils it did seem as though the vibration at speed had changed(I might just be nuts). I'll see what autozone has to say about the ignition module and coils this upcoming weekend. Hopefully my hunch is right, but it usually isn't

[HotZ28]

Have you checked the resistance on the #4 wire? I had a car in the shop the other day that had new plugs & wires, however the ceramic on one of the plugs was cracked and caused the plug to fire to ground instead of the combustion chamber. Without seeing the plug or knowing the age and condition of the engine, I assumed by your description of the "black tar-like deposits" that either it was oil, or the plug has not fired in quite some time. One dead cylinder can cause a “rough idle that turns into a full miss at speed”. Never assume that a new part is flawless!!

Move the #4 plug to another cylinder and see if it burns clean after it is relocated. (That is, if you don't have a new one to try on #4).

[LeSabre97mint]

Originally Posted by HotZ28

Have you checked the resistance on the #4 wire? I had a car in the shop the other day that had new plugs & wires, however the ceramic on one of the plugs was cracked and caused the plug to fire to ground instead of the combustion chamber. Without seeing the plug or knowing the age and condition of the engine, I assumed by your description of the "black tar-like deposits" that either it was oil, or the plug has not fired in quite some time. One dead cylinder can cause a “rough idle that turns into a full miss at speed”. Never assume that a new part is flawless!!

Move the #4 plug to another cylinder and see if it burns clean after it is relocated. (That is, if you don't have a new one to try on #4).

I had a plug with cracked ceramic and it caused a miss during acceration. I should of known not to ignore that little snap when I was installing them.

Dan

[jmiller17777]

I had the exact issue you described. I took the car in and they spent a day and a half checking the wires, the plugs, fuel pressure, etc. but nothing worked.

Then they noticed that the "main vacuum distribution" was loose. They tightened that up and the problem was solved.

I hope your problem is as easily solved as this.

[ChemMan]

Updated, see first post

[cartoooning]

so chemman, what are you saying i need to have checked on my lesabre 95 then, i am confused