temp guage acting quirky

97 block in an 87 4.0 (renix electronics with sensor in t-stat housing) open system. New(ish) radiator, water pump, t-stat, and temp sensor. My guage keeps moving up and down from 150-60 to 195-200 degrees about every minute or so, but never goes above the 210 mark. Ive gone through 3 thermostats, 1 spare and 2 new temp gauges trying to figure it out (the spare one didnt read above 120 no matter what). No leaks, builds pressure fine, nothing in the oil. Anyone ever had this issue before that can help or is there something I might have overlooked?

Thanks,

Jmeti000

Is the cap opening and closing to the overflow bottle, weak cap?

Check the grounds, and the other problem may be a lack of a good ground on the temp sensor. I have read that using thread tape or sealant on the sensor threads kills the sensor body ground (Renix and OBD-I one wire sensor gauge combo,.... the HO/ OBD-II uses a two wire sensor ), and dorks with the signal. Might be even worse on the T-stat housing than on the head.

I had a 86 which did that, turned out to be steam built up before the T-Stat would open and release it. I suspect I had a leaking head gasket.

So, assuming that isn't your trouble...

The issue seems to coincide with the T-Stat opening and closing because the pickup is in the T-Stat housing as it leaves the engine, right?

You may have air trapped in the cooling system.

Does your T-Stat have a relief hole in it or a notch in the side? I believe it should.

Also, since it is a 87, did you bleed all the air out of the system when you changed engines? The Renault style cooling systems had an issue with trapping air at the highest point in the cooling system. Renaults had air bleeds, XJs didn't because AMC was too cheap to put them in I have heard of folks put the front end up on ramps, popping the cap off the recovery bottle, opening the heater all the way and massageing the hoses until the air is worked out.

-Ron

He said he has an open system, HO era plumbing, not Renix closed system. Somewhat of a hybrid, part Renix, part HO.

I actually have two sensors both located in the t-stat housing. One runs the temp guage (single wire), and the other runs the e-fan (two wire). I did put some sealant on the sensor when I put it in...will clean it up and see if that helps. I also went to release the pressure the other day and think the cap isnt sealing correctly. The minute I pulled up on the release valve a bunch of coolant shot out from under the cap. Back to the auto parts store...again. Thanks all, will keep you posted.

I'm having the same problem

Have you considered this:

http://www.go.jeep-xj.info/HowtoRadiatorRestrictor.htm

hmmm, interesting. Well, I havent dont anything to it yet...just been to lazy. Im thinkin the sensor isnt grounded well because I slathered it with sealant. Im gonna take it out, clean it up, and see if that helps at all...probably in the next couple days. Will let you know if that cures it.