Car loses power after running for awhile


Asked by Oct 23, 2008 at 06:21 PM about the 1991 Pontiac Grand Prix

Question type: Maintenance & Repair

I have a 91 grand prix with the 3.1.  If the car is cold it runs like a dream.  if I let it idle for say 10 minutes and I get on the highway it really lacks for power especially when I hit a hill.  If I get on the highway without letting it warm-up it will start acting up after about 10 miles.  It jerks in and out of gear on the hills.  Also when I'm on a flat stretch it will start shaking at about 70 miles per hour, which at whatever gear that is doesn't take a lot of gas pedal, to get rid of the shake I just have to give it gas.  When it idles it sounds rough, like it was missing because a bad plug or wire.  They are brand new wires and plugs, and I checked them anyway just in case, they seem fine.  Also, this probably doesn't matter, but I have a constant low coolant light on.  Hope I wasn't too long winded.  Please help.

9 Answers


Well If you are driving up steep hills, I strongly advise that you take the car out of overdrive and put it in third. if it's steeper, you put it in second. if you are in a mountainous area, you put it in first if necessary. Whenever you're in a mountainous area, GET OUT OF OVERDRIVE!! If you think I am talking about manual Transmission, here is a Diagram of which gears to use. P R N [D] > 3 > 2 > 1 If you still experience problems, I think you should go to a mechanic nearest to you. Just try my advice on this when you go out on the road. also I think you should get your warning lights fixed because if everything is fine but the lights stay on continuously, there is a malfunction in the Lighting panel or a problem is actually happening but it doesn't seem like it, and you weren't too Long winded because this is important and it could put your safety at risk if something goes wrong with the car. I'm the one that is Long winded. If I were a car, I'd be a Rambler:)

1 of 1 people found this helpful.

1st of all does the car down shift when you step on the gas going up hill? it should the coolant issue is probably a separate issue, this could be a transmission issue, or engine issue. have you checked the coil, you could have a weak coil?


Since the vehicle is an automatic if operating properly it will downshift out of overdrive when necessary, such as going up hill.


it does downshift but very aggressively, i think possibly because it doesn't have the power to hold to the right gear.


I'll check the coil tonight.


I definitely live in a mountainous area. So you're saying that the automatic transmission won't work correctly in this area.


2 questions and 1 part answer does the car idle ok when it is cold or miss then also when it is warmed up does it run ok at slower speeds with less load(no hills) answer= all of your problemsecept coolant light could be cuased by 4 things I can think of 1 a bad sensor especially a MAF or MAP 2 cracked head or gasket 3 a partialy restriced catalitic convertor 4 a weak fuel pump or clogged filter I could list serveral other posibilities but d rather chk fuel psi at the injector rail and then hook up a scan tool to determine what the computer is seeing

2 of 2 people found this helpful.

the car idles ok when I first start it. In fact I don't really notice it idling bad for about 10 to 12 miles on the highway, then when I stop its really pronounced, really sounds like it is missing. When it is warmed up it runs decent at slow speeds and no load. No load is key. Just running through town the only thing I notice is lack of power, and it will stutter and hesitates.

1 of 1 people found this helpful.

Sounds to me like you have 1: either head gasket leak causing coolant to burn in your cylinders and overheat and cause vapour lock or you have 2: a cracked head either will be exspensive to fix.

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