Yet another "my Jeep is running hot" thread!

[acerocknroll]

As I mentioned in my build thread, my 91 4.0 HO Eliminator has been apparently running a little hot according to the gauge. It had been pointing straight up at 210 until recently, now it's typically right around here:

 

 

It does fluctuate a little bit, but not much. Sometimes goes down a tiny bit when accelerating but not back to 210 even. I checked the fan clutch, seems to be working. Replaced the radiator cap, no change. Fluid level appears ok, bright slimer green.

 

I tried to confirm the gauge was reading accurately to begin with but wasn't 100% sure what to measure off of. I used my IR thermometer on the thermostat housing, is this a reasonable place to take a reading? Temp was reading 210 plus minus 5 degrees or so depending on what part I aimed at.

 

 

Was gonna replace the thermostat next, I'd read here that you should drill a hole in it first - what size? Hole should be oriented to top, correct? Anything else I should check?

[gogmorgo]

My '91 4.0 has done something similar for a while. It doesn't ever get hot enough to call it overheating, just goes back and forth from straight up to the next line over. I can't say when exactly it started doing this (I don't remember it doing so when I first got the truck) but it might have coincided with swapping out the thermostat... but it was a couple months after the swap (also completely new hoses) before I noticed it doing it. At the time I did notice, every part of my cooling system except the engine block, tstat housing, and heater core were less than eight months/5000 miles old. Last summer I swapped out the engine, leaving on the water pump and tstat from the used donor motor. I'd have to check back, but I think I also used the "new" motor's sender. It still does it.

It never smells or otherwise seems like it's overheating. The gauge moving doesn't even seem to coincide with the aux fan switching on/off. It's not relevant to the outside temperature (happens at 80F or -30F) or to a/c use. I'm beginning to suspect a sender or wiring issue, but until it actually starts overheating or the gauge becomes definitely erroneous, I've decided there are more important things to worry about.

[HOrnbrod]

The thermostat is one of those "should use" factory parts. It has the hole and jiggle valve built in. Get a 195* factory stat from the dealer.

[acerocknroll]

Ok thanks, I'll snag a Mopar t-stat and see what happens.

[Megadan]

I have a non-factory T-stat that I installed with no hole drilling or other modifications and it holds rock solid and even runs a bit cool on all but the days where it gets around 90 or hotter., and it will hit 210 and then my fan comes on and it slowly falls back down.  I should note that I put in a new radiator and water pump at the same time as that T-stat.  Honestly, my biggest complaint is that it struggles to get up to temp when it's below 40 degrees, and below 20 it almost never does.

[gogmorgo]

What temp is the tstat rated at? If it's much less than the factory 195°, it'll pop open way early and struggle to warm up. The parts-store one I put in is 195 and had no problem getting up to temp, even approaching -40° with no added air control or even if I forgot to plug in the block heater. Five minutes idle and 1 minute driving and it was up to temp this weekend when I started it at 0°F... but then the tstat in it now is of unknown origin or condition.

[Megadan]

Maybe I should have said, "it takes along time" rather than "Never Does."  It will get up to temp, but if I am going 60 on the highway, it loves to stick right around the tick mark about 1/4 way up the scale (maybe 160) and might slowly climb to 180-185.  The T-stat is Rated at 195. Stant 45359.  It's not stuck open, I checked that just to be sure.  Ran the truck at idle and waited for it to open, hose went cold to hot quickly when it opened.  Honestly, My truck has always ran on the cool side ever since I did that new radiator. even on a 100 degree day it has to sit for a long time at idle before it gets hot enough to kick on the electric fan.  

FWIW, before I did the new radiator I also did an acid flush of the system to clear out any rust or other deposists from the block, and then did a complete block drain (what came out was NASTY), ran the base solution, block drained, and ran straight distilled water to rinse it out.

[cruiser54]

I have a non-factory T-stat that I installed with no hole drilling or other modifications and it holds rock solid and even runs a bit cool on all but the days where it gets around 90 or hotter., and it will hit 210 and then my fan comes on and it slowly falls back down.  I should note that I put in a new radiator and water pump at the same time as that T-stat.  Honestly, my biggest complaint is that it struggles to get up to temp when it's below 40 degrees, and below 20 it almost never does.

With a factory stat, at 20*F outside, I have cabin heat in less than 3 miles of driving. 

[Megadan]

 

I have a non-factory T-stat that I installed with no hole drilling or other modifications and it holds rock solid and even runs a bit cool on all but the days where it gets around 90 or hotter., and it will hit 210 and then my fan comes on and it slowly falls back down.  I should note that I put in a new radiator and water pump at the same time as that T-stat.  Honestly, my biggest complaint is that it struggles to get up to temp when it's below 40 degrees, and below 20 it almost never does.

With a factory stat, at 20*F outside, I have cabin heat in less than 3 miles of driving. 

 

The truck heats up from cold in about that same amount of time, maybe 5 min.  It is possible it isn't closing all the way, but otherwise it does operate.

[acerocknroll]

Update:

I flushed the coolant, installed new Mopar t-stat, replaced upper and lower rad hoses and... still running hot, maybe a touch hotter.

 

So what's next? Fan clutch? I did the hand-spin test when hot and it seemed ok but if there's a better test I'll try it. Also, should a 91 Eliminator have come with an aux electric fan? Because I don't have one

 

Should I install one? How difficult is that?

[Minuit]

Very easy as long as the connector is there (it should be). 2 pin Weatherpak connector, green and black wires I believe. Use a 94-96 fan with 8 curved blades. There are 2 speed nuts across the top of the radiator support that hold it in on top and 2 plastic tabs on the bottom. Since you have a '91 it's also very easy to make it manually controllable as well.

[acerocknroll]

Hmmm this is the only unused connector I see in that area...

 

 

A local junkyard actually just got in a 95 XJ (very rare to see XJ's in jy's around here) so I'll check it out tomorrow and see if I can snag a fan!

[HOrnbrod]

^^   That's not it, it's the diagnostic connector.

[Minuit]

Might be tucked up under the airbox. In any case it's closer to the headlight.

[acerocknroll]

Quick update, I replaced the fan clutch today - way louder, seems like it's moving more air but gauge still showing hot.

 

On the plus side, I do in fact seem to have an electrical harness for an aux fan! I plugged it in, put a fuse for the aux fan in the fusebox but I'm confused about the relay. Inside the fuse box there is a space listed for a "cooling fan" that was empty, so I put the relay there.

 

However, stuff I'd read online said the relay should be near the driver side fender. Anyone confirm where to put it on a 91 4.0? Are there any other fuses/etc I need to plug in before this thing should start kicking on above 210? Or should I just rig it up manually?

[HOrnbrod]

You have an HO; the aux fan relay is in the PDC as in your pic. The relay in the driver side fender is for Renix stuff.

 

Provided the aux fan harness is intact, you can test it by either driving it until the temp reaches ~230 or so and see if the fan comes on, or by applying a ground signal to ECU pin 31 (or aux fan relay pin 5) as shown on the wiring diagram here:  http://comancheclub.com/topic/48804-electric-cooling-fan-issue/?hl=aux+fan&do=findComment&comment=496745

[acerocknroll]

Thanks hornbrod, I'll see if I can get the e-fan up and running.

 

Another thought: the first thing I did was replace the rad cap, the parts store gave me a 13lb one and from what I'm seeing now people seem to think a 16lb is actually correct? Is it possible that could cause this type of issue? I would have replaced it and tried already but all three parts stores in my area are out of stock... Will report back tomorrow hopefully!

[mikekaz1]

you can also "trick" the fan on by unplugging the temp sensor on the t-stat housing. that should kick the fan on immediately.

 

on my 1991, I had an alternative issue.  my electric fan wouldn't turn on till ~250*f  instead of ~216*f  as it should.(and it would frequently cycle to that point)  tried new sensors and swapped nearly everything, contributed it to just old wiring and connectors and a raised resistance preventing /slowing the activation of the fan. I ended up parallel wiring in a stand alone temp sensor to kick the e fan on.  now I stay 160-210*f no problem.

[shelbyluvv]

you can also "trick" the fan on by unplugging the temp sensor on the t-stat housing. that should kick the fan on immediately.

 

Don't do this and drive it. It will put the ECU in limp mode. It is really bad for your truck. You can damage the engine or your cat.

[mikekaz1]

 

you can also "trick" the fan on by unplugging the temp sensor on the t-stat housing. that should kick the fan on immediately.

 

Don't do this and drive it. It will put the ECU in limp mode. It is really bad for your truck. You can damage the engine or your cat.

 

 

yes, sorry should have wrote for test/diagnostic purposes only.

[acerocknroll]

Cool, thanks guys. I'm gonna replace the radiator and the water pump today. I'd like to get it to run @ 210 without the efan if possible since that's how it is originally configured, and should be able to do so. If those don't fix it... there's not much left to replace haha. Will try testing the out the efan once I'm done. Thanks again!

[acerocknroll]

After replacing radiator, water pump and serpentine belt - temp still high... but at least drops down a bit when I get up to speed, then fluctuates a little. 16lb cap is ordered (still can't believe no one locally carries them), will try that next, then replace the gauge sending unit again with a MOPAR one.

 

Also, hooked the e-fan up, would not run at normal operating temp but DID kick on when I disconnected the ECU. So at least it works!

[mikekaz1]

The fan relay is fed by two fuses. The fan fuse supplies the line voltage for it. But the coil voltage is sent from another in the line up. If I'm not mistaken. But I forget which one exactly. Double check all slots have a fuse and/or that they are not blown. because that was part of my issue too, it knocked out my reverse lamps and a couple other functions

[Minuit]

Mine doesn't turn itself on at any sane temperature either. I know I've heard it come on by itself in the past but I have no idea what temperature it turns on at as it doesn't get that high anymore.

[cruiser54]

Mine doesn't turn itself on at any sane temperature either. I know I've heard it come on by itself in the past but I have no idea what temperature it turns on at as it doesn't get that high anymore.

I think it's 218*.