99 Grand Prix Shakes

kendrid

I have a 99 GT that has run flawlessly for 85,000 miles.

In the last three months, about once a month when I start the car it will shake. If I give it a little gas the car will stop shaking and then run fine. This happens when the engine is cold and also after it has been driven.

Last week when I started it up it shook so bad I shut it off after about 3 seconds. After turning it back on it was fine.

Any ideas as to what this might be? I am obviously out of warranty and unfortunatley I do not have a dealer I trust.

As a side note: I used to drive a 99 Intrigue w/ 3.8 (the 99GP was my wifes car...she now has a 03 Envoy XL SLT). My Intrigue did the 'shake' three or four times in the last year I owned it. Seems like it might be something others have run into with the 3.8 Series II.

ruski

plug misfire?

Mr_Shiftright

Sounds like either an engine miss (bad wires?) or a bad motor mount in combination with an engine miss.

ruski

you should post your question at another place. It is a forum. It starts with Club. It ends with GP.

kevitra

Thanks for the replies. I was thinking (hoping) it might be plug related. I am going to have them replace and see what happens.

This is cross-posted in the GP forum...

swschrad

if no other way, have somebody else start the car and see how much the engine moves. because of possible screwups on the production line, minor fender-benders, etc., an engine should never get close to a fender or firewall, there should always be room to get the width of a tom clancy paperback between them. if you are closer than that when punching it up, a motor mount deteriorating is likely enough to be looked at by a mechanic.

stubborn1

I would be curious to see what you find out. Ours only does the shake after its been sitting a few days without running. It only shakes for two seconds at the most and then goes into a regular idle. I have noticed that the rpms are only about 400-500 when I get the shake and it sounds like plugs firing out of sequence. When the engine is warm there are no problems.

swschrad

that could do it, leaky valve stem seals. what do the plugs look like? I bet it's only one or two cylinders... now.

Mr_Shiftright

That's a good guess actually. Valve stem seals which are worn or hard will allow a small amount of oil to drip down into the cylinder. So when you start up, the oil momentarily fouls the plugs.

kendrid

I won't be able to get the car into the local shop for a couple of weeks. I will let everyone know what they find (or don't find).

I don't know much about car engines, but oil getting on the plug makes sense. I've had my lawn mower have that happen and cleaning the plug fixed the issue. The car usually only shakes for 2-3 seconds, which is probably enough time to burn off any oil.

q45man

Depending upon the exact composition of the valve stem seals [Viton is the best and most expensive] [doubt it is on your engine]. BG Quick Clean for Engines can soften the seals or at least dissolve the heat induced varnish [on the upper stems]that has deformed them.

As the rings/seals wear more and more start up/cruise oil is sucked into the cylinders under vacuum. The plug change interval has to be adjusted downward to compensate.

But 85,000 miles is young if you have been changing the oil every 90 days. [small filters can't hold much dirt]. Check and clean PCV/EGR/plenum system to minimize additional dirt/oil.

kendrid

I drive about 16,000 miles a year, so I change my oil every 2.5 months.

Thanks for the advice.

kendrid

I had my 99 GP in the shop today for the following:
Engine coolant flush
Trans fluid flush (which actually seems to have made it shift smooter...I wasn't expecting a change)
new belt
fuel filter
air filter
spark plugs
front brakes
brake fluid

I mentioned the occasional shaking to them. They said the plugs were pretty 'worn', but the wires looked fine. Their first guess was that the wires could have corroded and moisture was building up on one of the plugs.

They said it could be a number of other things, but since I can't reproduce the problem and it happens infrequently they suggested waiting to see if it still occurs. I agree with that, even though I am going to guess that the shakes will come back.