Engine miss

JIMRITT

MEMBER
1996 BUICK ROADMASTER
V8
2WD
AUTOMATIC
250,000 MILES

96 buick roadmaster
after it warms up, and goes into closed loop operation, the engine has an irregular miss. I read the codes, and found one O2 sensor faulty. Replacing it cleared the code, but did not fix the problem. I also tried a different PCM, with no improvement.
The engine runs fine until it warms up.

6 Replies

HMAC300

MECHANIC
48,601 POSTS

Check for a vacuum leak and bad plug wires/spark plugs. This can also be low fuel pressure as well as a bad injector which won't set codes until it sees a miss. But with the mileage you have check compression on all cylinders, you may have a burnt valve. You shold have 41-47 psi. Fuel pressure. All compression readings should be within20% of each other.

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Thursday, August 11th, 2011 AT 4:46 PM

JIMRITT

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4 POSTS

Here is a little more info:
This started suddenly about a month ago. I thought it was the fuel pump because it acted this way about 2 years ago and the fuel pump was the cause. I also changed the fuel filter at that time. When the car is cold, it runs fine. After I changed these, I thought it was fixed because I drove it about 5 miles and it worked OK. My wife drove to town (20 miles) and it started doing it again. I scanned the PCM (OBD2)and I found the bank 1 sensor 2 O2 sensor had stopped reporting. Replacing the sensor fixed that problem. The engine still missed, but still ONLY AFTER IT WARMS UP. The things you suggest would also affect it when it is cold. I also tried another PCM, to no avail. If I stop, turn it off, then restart, it is OK until it goes into closed loop operation again, then starts chugging and missing randomly. Also, reverse doesn't work well (after it goes into closed loop). Any other ideas?

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Thursday, August 11th, 2011 AT 8:57 PM

HMAC300

MECHANIC
48,601 POSTS

Actually a partially clogged injector wouldn't because in open loop the enigne runs richer and hides stuff like vacuum leaks and air ratio isn't set in open loop whereas it is in closed loop (warmed up) you also have that distributor under the water pump and those are notorious for failing all of a sudden. It still can be a vacuum leak as well. If you are restarting it after itis warm thenit is in closed loop right away so it would suggest to me a distributor problem as it only goes for a while. Before returning.

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Friday, August 12th, 2011 AT 12:05 AM

JIMRITT

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OK. I will check the distributor, make sure the vac line is still connected at both ends. (It pulls fresh air from the intake to keep out moisture build-up) I will run some injector cleaner through the fuel. I will get back to you. Thanks.

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Friday, August 12th, 2011 AT 12:47 PM

JIMRITT

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4 POSTS

I'm back.
My wife had surgery, so I was busy. I looked under the distributor, and the vac lines are intact. I don't know what else I can do to check out the distributor except replace it. (Expensive) I will put a bottle of injector cleaner in the tank, and drive it. Hope it fixes the problem.
Any other suggestions?
Jim