My '97 Grand V8 is driving me crazy. Sometimes while driving it starts missing under 3500 rpm and fuel mileage drops drastically. I can actually watch the fuel gauge drop. This seemed to happen mostly after the vehicle sat in the rain for a few days without being driven, but not always. After talking to a mechanic that put a new ox sensor in it last November, he suggested that the plug wires, rotor, and cap probably need changed since they hadn't been since the vehicle was new, 130k miles. I changed the wires etc., and everything seemed to work ok for a big 5 days. On a completely dry day I took it for a ride and could feel a little drag on acceleration, then ignition missing, high fuel usage, and finally the check engine light. I tried to do the 3 key clicks to get the code with no response. I took the vehicle to the repair place to have it checked. I gave them the valet key to run the vehicle. Astoundingly, when they unlocked the door with that key the horn started beeping and would not stop. Unfortunately, the mechanic disconnected the battery to stop the horn and guess what....yep, erased the check engine code. They have driven the vehicle to get it to screw up but it's running fine. Does the missing/high fuel usage/check engine light problem sound familiar to anybody? Any suggestions for repair? Thanks in advance.
Matt

If your check engine light is missing or no code response..then it could be an intermitten bad computer itself.
I seen this somewhere else in the this forum before and the guy that owns the GC had computer replaced and fixed the problem.
cant say this is it but might be worth checking.
Hope this helps.

No clue man sorry, try what this guy above me suggested to you.

here is the post by imaboo2 on 04-08-2004

Engine won't start gauges show nothing - trouble found

Yesterday I came here to see if my problem was listed. I found many similar items but nothing perfect to my situation. The mechanic found my trouble today and I wanted to post for others trying to troubleshoot as an FYI.

97 GC 6-cylinder - 157,000 miles

No previous troubles...ever.

Drove the car yesterday then tried to use it later that night and it wouldn't start. The engine cranked but it wouldn't turn over.

Lights, radio, and computer display worked.
None of the gauges registered anything!
When key was on, all dummy lights EXCEPT "check engine" came on.

Mechanic found that my ECM (engine control module) is dead/bad. He said this was the reason I did not see the "check engine" light as this was controlled by ECM.

ECM part number = 56044407AB

Thais was the thread i was talking about...it could be related to your problem.

Thanks for the suggestion. So far, I've been driving it in heavy rain and dry days and no problems. There was a thing on the news yesterday regarding bad gas in a few counties around where I live, with customers having car problems which sounded like mine. Maybe that is it, but I doubt it.

It's possible, but I doubt it too.