ScaryTruth

'97 Grand Cherokee Ltd 5.2, 190,000 miles, Canadian build, in fine shape and I love it.
Runs great, garaged since new, well-maintained, no dents, rust or faded paint. Deep Amythest Pearl, (navy blue/deep purple). Handles and drives like new, sweet interior, all the options work great, the sunroof and key fobs even work perfectly.
But.
When moving/accelerating, at low speed mainly, (even in reverse), the speedometer jumps to 40, 60 even up to 120 mph -- and the engine surges at the same time.
When this happens, (a lot and more and more frequently), the system voltage drops too -- as in the blower speed drops and the lights dim slightly.
Damnest thing I've ever seen -- just started doing it out of the blue.
New speed sensor on the tranny, and it can't be a defective cluster/speedo unit as the engine surges, (like the injectors briefly open up wide).
Lasts for a second or two at a time and more pronounced when cold -- although it's doing it all the time now.
Even will do it with no pedal applied as long as the vehicle is in motion, (which makes a defective TPS unlikely, right?).
This is a real concern, especially in congested areas, and it's a good thing I brake with my left foot.
Short of having a degree in electrical engineering, the wiring schematic may as well be in Greek.
Also, as a footnote, it's hard to start after 20 - 30 minutes, especially overnight. If I turn the key on and off 4 - 5 times, it fires right up -- otherwise, it would take 10 - 15 seconds of spinning the engine to start. it.
No codes come up, although the on-board display continuously reads "coolant sensor bad", but it's been replaced.
Any advice would be wonderful, (and much apprciated), as I'm pretty sure there's a Jeep tech out there that has encountered this before.

dave1123

When you say the coolant sensor has been replaced, do you mean the one on the thermostat housing, or the one under the intake manifold? the one up front is for the guage and the other one is for the engine control unit. My 97 ZJ reads the same, but I haven't changed it yet. The only problem I have is once in a while, if it's idling in park, the engine will shut off momentarily, dropping below idle speed and than coming back before it stalls. Spooky, huh? Try going to JUSTANSWER.COM and talk to a jeep mechanic. Worked for me. I had a lighting problem and they helped me out. Costs a few bucks but was well worth it. Plus I could ask unlimited questions for the rest of the month!

ScaryTruth

I appreciate the advice.
I had no idea that there's another coolant sensor under the intake. I replaced the one next to the goose neck, so I'll look for the other.
Any idea where to look?
Yeah, and mine stalls sometimes too, out of the clear blue. No pattern.
I tried JUSTANSWER.COM with mixed results, (another vehicle some time back).
I may try it again though.
Thanks for the suggestions. My quest for a solution will continue -- and I''ll research that sensor thing for sure.
Thanks so much,
Michael