Coolant temp. going up and down at highway speed.

As stated, The temp guage will go from 200* down to 150* and back up and so on. When the temp drops to 150* the engine misses and stumbles. I've had a cold miss and stumble and a low "closed loop" idle since I put in the newly rebuilt engine.

The entire cooling system is new and I'm not loosing any coolant. I do not know why the coolant system is doing this other than the new thermostat may be bad. I don't think a bad T-stat would cause the temp to go from "normal" to "low". But I do know when this happens the engine even sounds different, has alot of detonation and I loose alot of power.

My mileage is great, go figure!

All the RENIX sensors are new, except the MAT which can no longer be bought.
These are new;
TPS
CTS
CPS
O2
IAC
Knock sensor
Sync Sensor and Distributor.
None of the old sensors tested bad, but I replaced them anyway.

The intake is clean and I've tried three different brands of plugs and wires!

All new battery cables have been installed on my new Red Top Optima, I've added a extra ground to the chassis from the - side post.

All connectors have been cleaned and the remaining grounds have been cleaned.

I'm about to give up, this is my 3rd post on similar problems with this 90'.

I hate to say it, but I'm a 20 year seasoned mechanic with auto and diesel certs, this XJ is kicking my @#$. I know there are people on here that know more about the RENIX XJ's than me so please help!!

sent pm

ok i wonder if its a faulty temp sender, i know its new but u never know, if u havent thrown away ur old one put it in and try it.

I went through the RENIX manuel today and re-performed all tests and came up "normal".

I still do not know why the coolant temp is fluctuating. It has started doing this at city driving speeds now also.

Anybody have any ideas!

have u traced the wires to see if there is a short.

I started poking around today, no luck there yet, ran out of light but will try again tomorrow evening. I was playing around with some vacuum and found that the line going to the thermactor was blocked. opened that up and the door opened up to pull air in from the heat riser. It may make some difference, we will see.

Also, I have 3 IAC's, 2 original equipement and one Holley. When I put the Holley IAC in, the engine starts and then dies, I have to "goose" the throttle to pull up the idle and then it runs fine kicking the idle as it should. I checked the Power Latch relay and it is good so this may be some parts incompatability issue's, not sure.

I tested the MAT and got excellent readings both cold and warm so I'm rulling that one out.

Still on the hunt and I'm not going to give up!

Bubble floating around in the system? Gauge reads proper when covered in anti freeze, low when the bubble comes around????

Thats what my experiance and common sense is telling me also but I've jacked the rear up, pulled the temp sensor at the rear of the head. It seems to be "burped" pretty good. The coolant level is staying the same with little or no air at the top of the resovoir and it hasn't spewed any excess out of the cap yet either. This coolant problem has started since I put in the new CTS 3 weeks ago, somehow I knew then that I might have problems getting it burped as good as I did when I dropped the motor back in.

I'm going to "burp" it again tomorrow for the #&!! of it!

The cold miss/stumble and "low" warm idle(300-400 rpm) has been a problem since day one on this motor rebuild.

The cold miss/stumble happens only when the temp guage reads below 175*. When the engine is cold or when the temp guage drops below 175* the problems start.

Like I stated before when its running good it runs great, and I'm getting good mileage(18mpg city). But when it acts up its miserable to drive.