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'88 Oldmobile Toronado Trofeo: Bucks and chugs at 2K+ RPM only after car warms up

So, my car has had this issue for quite a while now. I decided to finally fix it, but have no idea what is causing the issue. I am hoping someone can point me in the right direction.

When I first start my car in the morning to head to work, all is great. It has full power, gets up and goes like a typical Buick 3800 engine does. Once the engine gets hot, though, I notice a significant loss of power. It chugs, bucks, and hesitates going up hills, accelerating, etc. Anything over 2000 RPM will make the car do this AND in any gear (so I know it isn’t the tranny). If I stop the engine and then restart it, it acts as though it is going to stall (low RPMs, then recovers, then low RPMs again). After doing a little bit of research, I discovered that the O2 sensor is the likely culprit. So, I replaced it. Doing so has had no effect whatsoever on the issue. I really don’t want to dive straight into replacing the ECM, but that is really the only thing I can think of as the next step.

I am hoping some of you may have had a similar experience and could help point out the culprit.

Oh, I should also mention another thing that just came to mind:

I can actually FEEL when the issue starts if I am sitting at a light or stop sign. The RPMs of the engine drop. Idle RPMs prior to occurrence: ~1000-1200. On occurrence: ~800. That’s about the time I say to my girlfriend “Here it goes again…”

Not sure if that helps, but there it is.

Is the check engine light on?

Ignition module

Your vehicle has the OBDI engine management system. So unless you have at least the GM TECHII diagnostic program for a scanner it can be pretty hard to tell what’s happening.

But from what you’re describing the problem might be with the coolant temp sensor for the computer.

When the engine is started/operated cold, the computer adjusts the fuel mixture/timing from the signals from a few sensors. One being the coolant temp sensor. This is called the open loop mode. Once the engine warms up the computer goes into the closed loop mode. This when the computer makes adjustments from the signal from the O2 sensor.

If the coolant temp sensor for the computer is sending an erroneous signal to the computer when the engine actually reaches operating temperature, the computer thinks the engine is still cold and never goes into the closed loop mode. This results in the engine running too rich. This can cause a lack of power, bucking, and chugging.


+1 to Tester’s post.
I was unable to find the temp sensor output curve, but I did find them online for this engine (see link) for under $20. It might be worth it to just change it and see if that solves the problem. Sometimes changing a part (shotgunning) is cheaper than testing it.

Agree with Tester. You do realize that you can go into diagnostic mode on the CRT and get all of the sensor readings etc., standing still or driving. O2, fuel trim, air temp, engine temp sensor etc.?

EDIT: I don’t have one anymore and couldn’t remember which buttons on the CRT you push to go into diagnostics but pilfered a comment from a Reatta owner playing with his CRT on youtube: “If you go to the climate control menu and hold down OFF and WARM at the same time, it will put you in service diagnostic mode and you can pull Trouble codes and monitor live engine data better than any scan tool. This is NOT in the owners manual, but WAS in the service manuals”

Ah. Thanks all. My model does not have the CRT screen. I am fairly certain I can test Tester’s theory, though. I’ll see if I can grab a temp sensor at my local Oreilly’s while keeping the ignition module in the back of my mind.

I also have a sneaking suspicion that I will need to replace the cat fairly soon as well - Some under-car inspections make it appear as though it may be the original…I’ll have to dig through the extensive paperwork I have on the car to disprove my belief.

I’ll let you all know how it goes. Thanks for the help!

While it would be unusual for these symptoms to start when the engine is warm only, it might also be worth checking the fuel pressure when the symptoms begin, and also for exhaust back pressure, like a clogged catalytic converter. If the fuel filter hasn’t been changed recently, I’d add that to your maintenance list.

Fuel pressure check would be a good idea. The regulator is external to the tank. Restricted exhaust would be another check. Fuel pressure check just requires a gauge placed on the fuel rail and the exhaust back pressure can be read by screwing a gauge in the exhaust.

The thing is if it does it only when warmed up, it is after the engine goes into closed loop and is thus using the input from the various sensors. If it did it all the time, then it wouldn’t necessarily be sensor related-like fuel pressure. Doing it only after closed loop seems to narrow it down to bad sensor input. Question is which one but they are all pretty cheap. Too bad you don’t have the CRT, its a hoot.

Another thing to check out, if the other things already suggested don’t seem to be a problem, is a fault in the ignition system when things get hot.