1990 GrandPrix?

I have a 1990 Grand Prix with a 3.1 v6 engine. I have been taking care of it I changed the oil, antifreeze, I changed its spark plugs 1 1/2 years ago with autolite platium plugs. Sometimes When I drive it for about 12 miles when the engine is at operating temp it starts to make a knocking noise. I noticed this when the plug were 3 months old.The noise will go away when you rev it up alittle. it also comes and goes. The engine does not use oil and its full. Im just wondering if the autolite platium plugs are causing this and also the transmission has a hard Reverse when warm it makes a clunk noise only in reverse. Drive and other gears don't do this. This is a 4 speed hydramatic. Was there a spring upgrade for this? I know its way overdue for a transmission fluid change and will do it when its warm outside.


I read online about platinum can cause knocking on a older car since they run hotter. I have used regular champions without a problem with this car it seems to have showed up when the Autolites platinum were put in/

3 Answers

  • 1 decade ago
    Favorite Answer

    The plugs you need are AC Delco plugs. Platinum if I recall correctly. The Autolites will work but stay with the OE equipment in your car and you can't go wrong.

    The rattling you hear could be a timing issue, nothing you can adjust, bad gas, or some really dirty and corboned combustion chambers. Try using premium on your next fill and seeing if the rattling goes away.

    I woould check the engine/transmission mounts about the hard clunking. Sound like you have something loose somewhere.

  • 1 decade ago

    The plugs aren't your problem. I would start checking for loose pulleys. Chances are you have a alternator or water pump that is going bad. Based on the fact that it starts making noise when it gets warm I would check the water pump first. Examine the belt for wear and then take it off. Grab a hold of the water pump pulley with your hands and attempt to wiggle it up and down and side to side. Any knocking noise or pulley movement will indicate that this part needs to be replaced.

    As far as your reverse Issues, check your motor and transmission mounts. I would also think about a transmission fluid and filter change.

  • 1 decade ago

    im not too sure about the tranny. but a fluid filter change im sure won't hurt. however, pending the milage on your 3.1L V6, i don't think the plugs would cause such a noise. it sounds more like you have piston slapping, or a loose connection rod. any sort of knocking sound from any engine is no good. u should have that checked before it might leave you stranded. good luck

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