I just purchased a 1998 Mystique and began to have problems almost immediately. (I wish I had taken a longer test drive!!) I don't know much about cars and hope someone can help.

The car runs great...for about 15 miles. At that point, it does some strange things:

- Tachometer spikes as the engine revs but there's no power (as if transmission isn't engaging
- The car begins to jerk very strongly
- These problems seem to only/mostly occur at or above 2000 RPM (ie. when I'm accelarating or going up a hill)
- Once the problem occurs, it continues on a frequent periodic basis. For example, it may rev/jerk for a few seconds, then stop for a 15-30 second, then do it again.

Other notes:
- The engine temperature gauge shows that the car isn't heating up (even though it clearly is)
- The speedometer was dropping to zero while the car was moving; Speed sensor was cracked and replacing it made the speedometer work, but didn't solve the revving/jerking problem
- At one point, my mechanic/transmission specialist drove the car. It started the revving/jerking after some time. He plugged in the diagnostic computer and it ran fine. When he unplugged the computer, the problems happened again. Based on this, he put in a new (i.e. used) computer, but the problem is continuing.
- My mechanic/transmission specialist doesn't know what the problem is, but based on 30 years of experience, he doesn't believe there is a problem with the transmission itself. He is going to look at the solenoids and wiring schematic next, and also check to see if the prior owner may have done something with (or removed) the thermostat.

Can anyone offer some insights?

Thanks!

dirty(internally)/faulty throttle position sensor can cause this. Possibly a trans pressure sensor (if there is one... not too sure but have heard of such a beast). Anything else that tells the computer to compensate by downshifting for more power.

Slymer, thanks for the feedback.

Here are the results from two test drives on separate days. Any more thoughts?

Test Drive #1

*Country road w/ decent hills and some straightaways early morning
*Drive 11 miles no problems, then pulling out of an intersection on a flat stretch the car made a single jerk/shudder
*Drove 39 more miles with no problems or symptoms, then out at another flat intersection it jerked/shudder (single strong jerk, then several small ones)
*Drove 6 more miles; jerked/shuddered again pulling out of a level intersection
*End drive


Test Drive #2

Late afternoon
Flat stretch followed by hilly country roads 9 miles w/ no problems
Interstate for 6 miles, no problems
Suburban shopping for about 5 miles with several stops at stores, At one store, a fast acceleration onto the road drove the tach to 6000 RPM, no wheels spinning; Driver took foot off gas, tach dropped and then pulled out normally, no jerking/shuddering
At the last store, it made a very hard single jerk pulling out of the parking lot
About 100 yards later it jerked/shuddered pulling out of an intersection onto slight uphill
About 100 yards later it jerked/shuttered while accelerating after making a turn
About 5 miles of no problems, then jerked/shuddered twice while driving 40 mph on a flat straightaway
About 2 miles with no problems
End drive
Jerk/shudder on straight away was preced by the tach quickly rising from about 2000 RPM to 2500-3000 RPM
Other possible issue - temperature gauge seems to never to rise much above the very lowest reading; Not sure if this is normal, but I'm used to temperature gauges settling at the 1/3 or 1/2 mark.

it's almost like it's got a low voltage issue and the computer is losing it at points. Have the battery and voltage regulator (alternator) been tested? The battery may be exhibiting a heat induced dropped cell (I had a car do that on me, but it would just not start... older car though). I do know that all kinda of weird stuff will happen if the electricity doesn't flow right. The only thing else I can think of is a slipping torque converter/bands in the transmission.

The strange transmission problem is solved.

The mechanic was able to capture the failure mode on the scanner while driving. The problem was a faulty transmission range sensor. It was essentially trying to shift into neutral and even reverse while driving. A new transmission range sensor (no new connector needed) and labor was about $180.

bah... silly sensors. sometimes the newer transmissions just bug me with all the gizmos they shove in there for checking this that and the other.