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87 Grand National

I have an 87 Buick Grand National and have a problem I can not figure out. Up to 65 MPH it runs great but anything above that it will start to hiccup or even completely stall for up to 3 seconds and then come back to life. I did find a bad coil pack and replaced that, I have also replaced the ignition module, crankshaft position sensor, the camshaft position sensor, the Mass Air Flow sensor, and the Throttle Position Sensor. Beginning to think computer may be bad or a bad chip? Anyone have any ideas or solutions??

Thanks in advance.

Frustrated

Run a fuel pressure check for starters and replace the filter if that has not been done in recent memory.
Turbo boost pressure ok? Just wondering if the turbocharger impeller could be dragging or if there’s an air leak somewhere.

What a predicament to be in; a GN owner! I envy you. :wink:

I would try a set of iridium plugs.

Iridium plugs don’t give you any better performance then regular plugs. They just last longer…nothing else.

A malfunctioning RPM limiter perhaps?? Today, you need an EE degree and intimate knowledge of the electronics involved to troubleshoot these 25 year old cars…

This is pure guess work. Is it possible you have poor grounding to the ECU or somewhere between?

I had a similar issue in a VW that turned out to be restripping a ground wire and a $0.10 wire clamp. My dealer repair for $25.

Have you checked the fuel pressure regulator? Long shot, but it could be the culprit. Awesome car BTW.

Could be a partially plugged cat. A backpressure test can be done to determine this.

Thanks for all the responses. One thing I forgot to mention (and is the main reason for my frustration) is that it doesn’t do this all the time. Somedays I can drive it all day without a problem but have to admit it is happening more often than not. The first thing I replaced was fuel pump and filter. I just checked again and fuel pressure is an even 30 psi and max’s at 39 psi under acceleration. I have checked the ground but will check again and my luck it is “somewhere in between” lol. I have thought about the cat but have yet to get it checked. It is for sale if you know anyone interested. Very nice and clean factory car with 60K miles and only this one problem.

Correction on the fuel pressure…39 psi static without the vacuum line attached.