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Why is my car doing this?

I travel to get to work, and on the way I use the interstate. Speed is posted at 55, naturally, but in the fast lane - the left-most lane of 4 - you have to keep up, you understand. Fast lane is usually 65 to 70.
So I’m traveling on the Interstate and in order to use the faster lanes than the right lane I depress the accelerator, and the car - 1998 Pontiac Grand Prix - speeds up, as you would expect. However, when it reaches about 45 to 50 mph the car begins to … cough. Just a little. If I step on the gas harder so that overdrive kicks in the car stops ‘coughing’. As I decelerate there is no coughing, only when I am speeding up and hit that 45 to 50 mph barrier.
What is causing this? Should I worry?

From the little bit you’ve told me, I think you might have a misfire. Possible causes include worn plugs, clogged fuel filter, tired fuel pump, and others.


  1. How many miles on the '98 GP?
  2. Is the Check Engine Light on?
  3. When were the spark plugs last changed? Fuel filter?

…As a WAG, I like spark misfire. Full throttle (and high cylinder pressures) ask a lot of the ignition system, and it’s where a worn plug would first manifest itself. We need a lot more information–including what exactly you mean by “cough”–to do anything more than guess, mind you.

I “second” meanjoe’s post. You’d be amazed at how many times these type of problems end up being simply plugs that are too old, a misfiring coil-on-plug (or coil pack, if that’s what you have), a dirty injector, or something basic like that.

Start by having the ECU codes “read” and post them here. Or, if you won’t be doing the work anyway, take the car to a trusted independent shop and have him/her take a look.

Post back with the results. We do care.

Yep, its called a miss or hesitation under load. When I had that it turned out to be a bad crank sensor. Not real expensive but if you haven’t had a tune up and the wires and coils checked, that would be a start.

Cloud (yes, I named her) has 296,467 original miles on her. Check engine light is not lit, but it will come on from time to time, and I always pay attention to those things and get her checked. The spark plugs … well, they haven’t been changed since … summer 2011, I think. Or was it summer 2010? And the fuel filter could probably use replacing. I will check the ECU codes as well, and post them here. I was sorta hoping it was something simple-ish so I could do the work myself, but I do have a good mechanic I could take her to, an independent fellow who will either do the work when I ask him to or tell me what to do when I can do it myself.
I just discovered this forum from listening to Car Talk. Thanks guys! I’ll get back to you when I know more.

You probably don’t need plugs. If you used the OEM plugs to replace the old ones, they need to be changed every 100,000 miles or so. I had a Buick Regal with the 3.8L engine and that’s what I did.

She named her car!!

Well I guess it’s ok, the girls have been calling me names for years.

“Hey you with the hairy back”!!!