1997 Mercury Mystique surges while highway driving

DANTERRI21

MEMBER
1997 MERCURY MYSTIQUE

I've got a 1997 Mystique 2.0 liter with 164,000 miles and have had very few problems over the years. I have a couple of recent problems I'm trying to solve but the biggest one happened late last week while I was driving home from work and had my car on cruise control (typical for me since I drive a lot of freeway miles). While driving at highway speeds of between 65 - 70 mph, my car suddenly and without warning reved higher than what it was set at. The tach was reading approx. 2700 rpm for this speed under normal conditions and it was surging upwards to something close to 3200 - 3500 then comes back down to where it was. To my knowledge, I have no trouble codes setting the ECM check engine light so I'm at a loss for what the problem could be.

I have checked for the obvious - vacuume leaks, disconnected sensor wiring etc and could find nothing wrong. I thought possibly the transmission might be slipping but this does not happen at lower speeds (under 55 for example). There is no slipping in any of the gears coming out of park or reverse so I don't think that the transmission would be the problem. As I stated, this issue Just Started and without warning !

Over the weekend, I changed plugs and plug wires, because I have had an idle surge and an engine miss (sputter) as well and thought the problem was ignition wiring related. I even pulled the MAF sensor and cleaned that thinking the sensor was gummed up or something. Nothing. I did have an oil leak and changed the timing belt over the weekend and resolved the oil leak part of the problem because the valve cover gasket was bad. Afterwards, I took the car out for a test drive and the problem has not disappeared so I could use some help on this one. Especially the surge that is occuring at highway speed. I've had the car with the cruise control enabled and disabled - it does it either way. Any help soon would be appreciated !

Thanks

Dan

2 Replies

MERLIN2021

MECHANIC
17,250 POSTS

Have a pressure test on tranny. Altho a good tranny guy and a test drive will probably tell you the OD is going.

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Saturday, April 28th, 2007 AT 10:32 PM

DANTERRI21

MEMBER
2 POSTS

Hello all,
As an update for anyone with a similar problem in the future, I made a trip to the local dealership and this proved to be a bit of a pain t otrace and figure out.

First of all, the problem was intermitant - the service dept found no error codes and thought that the timing was off and proceded to check for a few other things. After being told that the problem was the timing, I picked up my car and drove home. The following day, I took it out on the highway and sure enough, the problem reappeared. This time, I went back to the dealership and they got the tech to ride with me - in a matter of 5 minutes he saw what I was talking about, the engine would suddenly surge higher with or without the cruise control being activated. Needless to say, the car went back into the service dept and extensive troubleshooting led the tech to the root of the problem. The transmission has a sensor mounted to it that talks to the engine ecm. In my case, the sensor would act normal until things warmed up to operating temp and then would short itself out internally.

Bottom line - the sensor is about a $65.00 part through the dealer - It will not trigger a code in the engine ecm to set the service engine light even though the ecm sees the info the sensor broadcasts back and the ecm tells the sensor tells the ecm the transmission is going to make a shift in the gears or if the transmission is in a range that the engine would think of a nutral.

For those that run into this problem, check this first !