My '98 Bonne had its first failure at ~70,000 miles. The plenum was badly fried: it looked like burnt sugar near the EGR pipe. Upper gasket failed. The mechanic replaced the plenum, upper and lower gaskets with aftermarket parts of unknown brand.

Second failure was a month ago ~90,000 miles. This time it was a lower gasket. The plenum was again melted and visibly deformed near the EGR pipe.

I have a new mechanic now... he drives and repairs race cars, and owns 7 GM cars himself. (One's a Bonne with more miles than mine, which has never had UIM/LIM issues.) He swears by OEM parts.

I found out about GM's redesigned LIM gasket (with the aluminum frame) on one of your forums... THANK YOU! We installed two of these new gaskets, also upper gasket and a new plenum... all OEM parts.

My mechanic acknowledges there is a heat problem around the EGR pipe, but does not want to talk aftermarket parts, hence no reduced diameter stovepipe.

Here are a few questions:

(1.) Why do some of these 3800's fail early and/or repeatedly, while others seem to run "forever"? Has anyone figured this out?

(2.) Is there any way to go back to a metal plenum on this engine? Why or why not? Would that be a good thing?

(3.) What's the reason for the EGR system? What would happen if the EGR plumbing were plugged/blocked, so no hot exhaust gas ever went up through that EGR pipe (thus protecting the plenum from heat)? What would be the effect on efficiency, performance, etc.? Might this cause other problems with the engine at a later time? Would the engine computer compensate by adjusting the mix, timing, or whatever?

THANKS IN ADVANCE for any further information.

This thread is extremely good reading. Mkaake works for GM directly. He is the guy with the inside track on most things and was able to share this one with us.

The end of UIM deaths?

For GM to retool or rework after production was halted is a big thing. Couple the lack of coolant in the upper with the aluminum gaskets and I think it may be a great motor.

Personally I've plugged three L36 upper manifolds, the first was approximately 6-7 months back with all running great.

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97 SSEi 13.185

blocking the egr will net you a few HP probably but you'll also probably throw a code. if you want to learn more about EGRs etc try searching on google. EGR deletes are very popular on many cars.