Car will not go pass seventy mph

JOSE MONTES

MEMBER
2002 MERCURY MARQUIS
4.8L
V8
RWD
AUTOMATIC
128,000 MILES

I have a issue with my car, will not go pass seventy mph under light acceleration only in hard acceleration. I think it might be cylinder not holding compression or transmission issue.
The reason I say it might be a loss of compression issue is the number four cylinder spark plug was cross threaded and ash gets all over the spark plug and ignition coil, and now the reason I believe it the transmission is the only way I can go passed seventy mph is hard acceleration or by pressing the overdrive button to the off position which I believe shifts the car to fourth to third gear with all cylinders active. What your opinion?

If it is the transmission would this be called transmission slip?

I am doing a transmission change this week to see if there any difference.

4 Replies

JOSE MONTES

MEMBER
5 POSTS

Also, forgot to mention that there are no engine codes and the idle rpm's are stable at 800. Car sometimes shakes at idle with rpm's still at 800.

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Wednesday, July 19th, 2017 AT 1:19 AM

HARRY P

MECHANIC
2,292 POSTS

Well if it were being caused by that one cylinder then you would probably have a misfire code for that cylinder. Also, one weak cylinder is not going to keep that car from going over seventy anyway. If the plug is staying the engine, then the cross-threading is a not a problem. The push from compression would have blown it out of there by now.

The transmission could cause some things like this, but you would probably know it was happening. It might feel like it is in neutral for a second or two when it was happening. So I kind of doubt that that is the issue. But go ahead and check the fluid to see what color it is. If it is red, then it is good. If it is brown, then it is time to change it. If it is black, then it is way past time to change it and probably too late. Let us know what you see.

I would check the air filter, change the PCV valve, and look for vacuum leaks to start with. These are all things that can cause a car to be sluggish (which is what you are describing). They also might not trigger any codes.

Let us know what you see.