I've got a Buick Park Avenue with the 3.8 that stumbles under load. Usually happens after the engine is warmed up. Seems to get worse on a hot day above 80º. Generally happens at low RPM'* in 4th gear going up a hill but can happen in 2nd or 3rd gear. Sometimes it will backfire a few times while stumbling. When you step into the throttle enough it usually cleans up and runs better. Have not seen any black or blue smoke when it happens.

So far I have replaced the spark plugs, the plug wires, the oxygen sensor, the crankshaft position sensor, the mass airflow sensor, the fuel filter and the throttle position sensor. Have also checked the air filter. The engine does not seem to have a manifold absolute pressure sensor. The car is equipped with OBD 1.5 which means the engine is OBD 1 and the under dash connector is the OBD 2 type. The check engine light does not come on. I disconnected the battery a few weeks ago which cleared the computer. Not sure if a code would turn up anyway. The coil packs look good upon inspection but have not tried replacing any of them yet.

Looking for suggestions of what to check next.

I would start by checking the coils. Free test, easy to remove them.

https://www.gmforum.com/trouble-shooting-test-procedures-141/testing-coils-spark-plug-wires-278818/

Being that the issue only occurs when hot, you may wish to compare the coil readings when cold, and when hot.

Given the nature of the issue, I would suspect an ignition issue over a fueling issue. Also, what kind of shape are your vacuum lines in? A vacuum leak could cause the symptoms as well. With these cars, it is usually not a bad idea to have a spare ICM (Ignition Control Module) laying around, this can be had at your local wrecker, relatively inexpensively. While you are in there, it wouldn't hurt to grab some coils off of a Series 2 supercharged. I believe they fire just a little bit hotter, and again, spares are not a bad idea in this regard as well.

Also, have you checked the fuel pressure just to confirm that is behaving properly? Does A/C vs no A/C have any bearing on the issue?

Oh, and welcome to GM Forum!

I had a 87 park ave with a 3.8, mine would do the same thing when it got warmed up. SES Light would not come ! I was out of town when this happened to me, i could go maybe 10 miles at a time and eng would start acting up. The park ave had a front tilting hood so I rased it up, this resembled a large air scoop and cooled eng and it ran fine then, I made it home!

After I got it home I warmed the eng in the driveway until it started to run bad and would die, I took a glass of water and poured it on the sensor above water pump, when I did this the car would run just fine. When the sensor warmed it would lose it contact. You can test for this with a Ohm meter.

i replaced sensor with a new one And I was good to go

The issue you are describing, as well as the diagnosis, is the typical Crankshaft Position Sensor failure. The original poster has already replaced this part as part of troubleshooting as mentioned at the beginning, without any change in the issue. Also, when they fail, they usually don't work at all intermittently, and usually don't cause a stumbling issue in this manner. Car dies, and won't restart until it feels like it.

Thanks for the input however, and any further suggestions are welcome.

Hi Randall; Thanks for the welcome. Thanks also for your post and the link for checking the coils. I just checked the coils per the instructions on the link with the engine cold. I'll check the readings again tomorrow with the engine hot. The resistance reading on the 2/5 coil seem a little off on both the primary and secondary but I didn't get a null or infinity reading.

As for the vacuum, I inspected the lines the other day and replaced a T fitting that was cracked even though the crack was at the end beyond where it seated. I can leave the car sitting for an hour, then disconnect a vacuum line and hear the vacuum being released so the system will hold a vacuum without the engine running.

I haven't checked the fuel pressure but I don't really suspect that because I can give it more gas and the stumble goes away.

As for d ryan'* post, not sure what sensor he was referring to. He might have been referring to a Manifold absolute pressure sensor which on some cars is located above the water pump area on the front of the intake manifold. My Buick does not have one (being a '95). The cam sensor is located below the water pump so I don't think that was what he was talking about. My crankshaft sensor was intermittent when it was going bad. Occasionally the engine would die while driving and sometimes wouldn't start on the first few tries.

I'll also see if turning off the A/C makes any difference tomorrow.

At any rate, thanks for the tips!

You might also want to test your charging system. I have stumbles, and I suspect its my alternator, but I haven't done anything because of school but when I tested my battery the voltage was jumping everywhere.

I am pretty sure he was referring to the Crankshaft Position Sensor, I failed to mention which poster I was addressing with my reply regarding it.

Also, when testing the coils, you also are looking for consistancy in the readings, as well as a reading in a specific range (mentioned in the link). Ideally, all 3 coils should have the same readings. If it already seems a little off, I would expect the problem to be apparent when hot. As was mentioned, you can easily source these from junkyards, and if you bring your multimeter with you, test them as you pull them off of the donor car.

Do you get a flashing SES light? Chances are there is a bad coil. The one you mention with a off ready may go bad when hot. Coils do not like heat and may test good cold, then fail hot. You also don't need a open for a coil to be bad. One coil with numbers way off from the others would indicate a problem.

This car is a '95, wielding the OBD 1.5 system. I don't believe that it is capable of indicating misfires in the same manner that the OBD 2 system is, as used in '96 and beyond.

Please correct me if I am wrong.