95 with 3800 lacks power on highway with A/C on

I would appreciate any tips for the following problem:

My '95 transport has a 3800 V6 with 80,000 miles.

I recently bought it, so I don't have any history on it.

The symptom is quite straightforward: I'm on the highway in the relative flatlands of michigan. With the high temps outside, I've got the the air on, accelerating up to 70MPH, I put the cruise in when I reach 70. The transmission has shifted into 4th, and I can see the RPM drop as the TCC has locked. The speed is maintained for a short time, but within a few minutes, it is apparent that that the car has slowed down to 65, even though the cruise is still in and engaged ( I am not pressing the gas myself).

Two observations:

1. If I put the car into 3rd manually, it quickly gets back up to 70.

-or-

2. If I turn the A/C off, leaving the tranny in 4th, it regains speed.

The symptoms are also worse when going up a slight grade (such as an approach to a large bridge).


After purchasing the vehicle, I replaced the plugs, wires, fuel filter, air filter, O2 sensor. All fluid levels are good. Scanning provided no stored codes in the PCM.

The van does sometimes "shudder" or "chuggle" in 4th gear: In a steady cruise, when trying to slightly accelerate. It just doesn't have power under load. Fuel economy is not as good as our '94 Transport 3800.

Please, anyone with ideas - let me know. I don't know where to turn.

Followup: The fuel pump is new.

LMP, I saw a previous note from you on a dirty MAP sensor. Could this be it? I don't have any codes.

Also, while under the vehicle, I noticed a rattle when shaking the catalytic converter (from its inside). Could a plugged CC be part of the issue? How would one check this out?

I guess I could install a test-pipe to gauge how restrictive it is. This vehicle (I THINK) doesn't check the downstream O2, so dropping the CAT won't screw up the PCM or light a code - RIGHT?

The MAP sensor is a place to start. However, I never recommend removing cats just because in most places it is illegal to do so. The cat is probably internally damaged and needs to be replaced. If you heard a rattle don't worry about testing it further because the rattle tells that the medium has broken up. It is possible that it is blocking the exhaust and that is why you have a loss of power. Removing the cat and putting a high flow unit on will help remove the exhaust from the car better and should improve the performance issue. You can buy a universal cat that will work for less than $70 from a variety of places. Get it welded into place and try it, I believe your problems will go away.

The MAP sensor is a place to start. However, I never recommend removing cats just because in most places it is illegal to do so. The cat is probably internally damaged and needs to be replaced. If you heard a rattle don't worry about testing it further because the rattle tells that the medium has broken up. It is possible that it is blocking the exhaust and that is why you have a loss of power. Removing the cat and putting a high flow unit on will help remove the exhaust from the car better and should improve the performance issue. You can buy a universal cat that will work for less than $70 from a variety of places. Get it welded into place and try it, I believe your problems will go away.

Thanks for the recommendations.

I saw on the LeSabre page that a broken motor mount may yield a "shudder" symptom. Any possibility on this one? How does one check a motor mount?

I had the same problem on my 96 w/ 3.4. The studder was caused by a bad egr valve. replaced it and everything is fine. This problem will not throw a code, so try unplugging the valve first and see if it runs better. It took me about 25-30 miles to have the cel come on but car ran great so I replaced the valve. Go online to find replacement as the stealer will rape you!
Steve

I had the same problem on my 96 w/ 3.4. The studder was caused by a bad egr valve. replaced it and everything is fine. This problem will not throw a code, so try unplugging the valve first and see if it runs better. It took me about 25-30 miles to have the cel come on but car ran great so I replaced the valve. Go online to find replacement as the stealer will rape you!
Steve

Interesting. Unplugging the EGR doesn't cause a code? I'll have to try that.

Interesting. Unplugging the EGR doesn't cause a code? I'll have to try that.


No when you unplug it , it will throw a code. Gm built in so many variences that under conditions like you were describing it won't throw a code.

3800 uses a MAF (Mass Air FLow), not a MAP (Manifold Absolute Pressure)..but I guess that is what you meant. Just one thing not clear : will the drop in speed occur only with the cruise control ..that is, if you throttle yourself (slowly so as not to shift gears), will it maintain speed? Occasionally mine will do that (loose speed) on cruise, going slightly uphill, while I can maintain speed if I throttle myself. I cannot repeat that at will, just random, so I have not really investigated the cause.

No when you unplug it , it will throw a code. Gm built in so many variences that under conditions like you were describing it won't throw a code.

Okay - 2 hr. drive with the EGR disconnected. You were absolutely correct! The power problem went away.. Brilliant!

The MAP was cleaned as well

The car no longer lags when the A/C is on - keeps speed, much improved power. After a while the Trouble light came on (as expected).

I still get the shudder though.. When the TCC is locked, it doesn't seem to want to unlock when slightly accelerating.

Somebody told me it might be due to a vacuum leak. Any way to check this?

Any other possibilities on the shudder?

Shudder problem was resolved - bad spark plug cable. Changed all plugs and wires - A-OKAY!