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Running too cold

OkieXJ

OkieXJ

NAXJA Forum User
NAXJA Member
Location
Muskogee, OK
'91, 4.0, aw4, np231.
My Jeep runs too cold, usually on the second strike of the gauge which is around 150-160°. This has been confirmed with a IR thermometer so I'm pretty sure my gauge is functioning properly. I've been through 3 thermostats and am currently running a mopar 195° purchased from the dealership. The other types I've tried were a stant from rock auto and a cheapo murphey from O'rielleys. They all worked the same. I just replaced the water pump last night with no change in behavior whatsoever. The radiator is a 3 core of some sort, not sure what brand but it is not stock. I'm running a 50/50 mix of coolant right now for the winter.

Is it possible to install a thermostat upside down? That is what one of my coworkers was suggesting.

I am not sure what else to try besides a piece of card board in front of the radiator. I believe this is killing my gas mileage cause it is around 12mpg and it used to be much better before this problem. BTW, the Jeep runs colder in the winter. Any suggestions are appreciated.
 
You can install the thermostat upside down but that will just prolong the time that it takes to get the air out of the system. You cannot put the thermostat in backwards though.

I would take the IR thermometer to the thermostat housing/upper hose/head to check if the thermostat is opening at ~195°.
 
You're definitely running cold and it is almost certainly causing your gas mileage issue... it could be that and something else as well though.

I would try a piece of cardboard in front of the radiator to see if things get better. If the mileage goes back to normal (or at least close) look into it further.
 
So basically the water flowing through the hose should be 195° since the thermostat is 195 huh. I'll give it a try tonight after I get home.

Where should I put the cardboard? I guess I might have to take my grill off or something.
 
OkieXJ said:
So basically the water flowing through the hose should be 195° since the thermostat is 195 huh. I'll give it a try tonight after I get home.

Where should I put the cardboard? I guess I might have to take my grill off or something.
Click to expand...

Just unscrew the phillips screws holding the grill slats in, insert cardboard between header and radiator, replace grill. Takes like 10 minutes. I did it on my 87 because it took like 5 thermostats to get one that worked.

even after I did my closed/open cooling conversion/water pump, thermostat housing I kept that t-stat.

I blocked off 50% of the radiator on the passive side of mine (no elec fan for me) so that the mech fan would cool it if I idled for a long time.
 
One thing I'll add to this: if you can, use corrugated plastic instead of cardboard. Placed properly (zip-tying it to the grille helps) you won't have issues with it melting from engine / radiator heat, and it won't turn to pulp if the weather's nasty. If you end up needing more cooling after it goes in, just cut out slits with an X-Acto knife or similar.

Pretty much anywhere that makes signs can supply this; I got mine at Staples.
 
87manche said:
I did it on my 87 because it took like 5 thermostats to get one that worked.
Click to expand...

Interesting that it took you 5 thermostats to find one that works. I'll keep that in mind.

One thing I'll add to this: if you can, use corrugated plastic instead of cardboard. Placed properly (zip-tying it to the grille helps) you won't have issues with it melting from engine / radiator heat, and it won't turn to pulp if the weather's nasty. If you end up needing more cooling after it goes in, just cut out slits with an X-Acto knife or similar.

Pretty much anywhere that makes signs can supply this; I got mine at Staples.
Click to expand...

This sounds like a good idea, I'll give it a try. We have a bunch of corrugated plastic here at work that they use to ship fat sheets of PVC to us.
 
I must have gotten lucky, my first thermostat I ever installed, a crappy duralast from autozone, worked perfectly. I've got 2 or 3 stants sitting around I haven't installed yet, I keep buying one and meaning to install it, losing it, buying another, etc.
 
OkieXJ said:
So basically the water flowing through the hose should be 195° since the thermostat is 195 huh. I'll give it a try tonight after I get home.

Where should I put the cardboard? I guess I might have to take my grill off or something.
Click to expand...

195 degrees is the temperature at which the thermostat will open. Then engine should achieve an operating temperature of between 200-210 degrees.

If you have an auxiliary fan make sure it isn't running all of the time.

Check your mechanical fan clutch to make sure it hasn't seized.
 
joe_peters said:
195 degrees is the temperature at which the thermostat will open. Then engine should achieve an operating temperature of between 200-210 degrees.

If you have an auxiliary fan make sure it isn't running all of the time.

Check your mechanical fan clutch to make sure it hasn't seized.
Click to expand...

Thanks for the tips.
The electric fan isn't running all the time, I've had it running with the hood open so I'm pretty sure that isn't the cause. My fan clutch spins but it does feel a bit stiff even when it is cold outside (that's when its supposed to be loose right?). I suppose that might affect it now that I think about it. I have a ZJ fan clutch I bought a while back that I might compare to my current fan clutch.
 
OkieXJ said:
Interesting that it took you 5 thermostats to find one that works. I'll keep that in mind.
Click to expand...

Yeah, I had no heat for half a winter and kept taking them back to vatozone. When I finally got one that worked I've kept a hold of it, even through multiple cooling system replacement/maintenances.

kastein said:
I must have gotten lucky, my first thermostat I ever installed, a crappy duralast from autozone, worked perfectly. I've got 2 or 3 stants sitting around I haven't installed yet, I keep buying one and meaning to install it, losing it, buying another, etc.
Click to expand...

You lucked out.

joe_peters said:
195 degrees is the temperature at which the thermostat will open. Then engine should achieve an operating temperature of between 200-210 degrees.

If you have an auxiliary fan make sure it isn't running all of the time.

Check your mechanical fan clutch to make sure it hasn't seized.
Click to expand...

This is also true, it should start to open around 180ish and not be fully opened until boiling.
 
OkieXJ said:
I have a ZJ fan clutch I bought a while back that I might compare to my current fan clutch.
Click to expand...

If it's the ZJ HD fan clutch, expect it to be a bit stiffer than stock - mine was, FWIW. It's hard to definitively say how much 'a bit' is in this context (especially if comparing to one that's shot), but it should be somewhat noticeable.
 
casm said:
If it's the ZJ HD fan clutch, expect it to be a bit stiffer than stock - mine was, FWIW. It's hard to definitively say how much 'a bit' is in this context (especially if comparing to one that's shot), but it should be somewhat noticeable.
Click to expand...

I don't think he has the ZJ clutch installed.

Ok, fan clutch:

When cold the silicone fluid will drain out of the reservoir. That is why when you start a cold engine the fan clutch will engage until the silicone fluid is spun back into the reservoir where it is held in by the bi-metal spring controlled valve.

So, the fan clutch will be stiff when cold.

After the engine has run for a few minutes and spun the silicone fluid back into the reservoir, but before the air that is passing through the radiator and hitting the bi-metal spring on the front of the fan clutch reaches 170 degrees to start opening the reservoir valve, the fan clutch should spin freer than when stone cold.
 
I had the same problems with stant thermostats, the standard ant the superstat. I finally installed a oem mopar t- stat and bingo, hey it works! I couldnt even get it to warm with plastic covering the grille
 
casm, you're right about the ZJ clutch being stiffer, reminded me thats why it cools better. I have access to a torque screw driver that I could use to get some torque values off the fan clutch. I might have to do that sometime.

Some additional background. It was running too hot, so I installed the ZJ fan clutch and it didn't fix it. So I replaces the thermostat and flushed the system. After I did that is when it started running too cold so I uninstalled the ZJ clutch which didn't help anything. Been fighting this even since.

87manche, I checked the thermostat last night in a pot of boiling water. I guess those things don't open very far do they? It was clear that it was open at boiling, but at ~180° it was pretty tough to tell if it opened at all. I guess that means it pretty close to working properly? If thats the case... I'm not sure what that means about my cooling problem.

bcmaxx, I'm glad the mopar one fixed your issue. I'm running one of those right now, to no avail. But as described above, it might be working properly and something else could be wrong.

joe_peters, thanks for the lesson on the fan clutch. I didn't know that was how those worked.
 
Last edited:
joe_peters said:
I don't think he has the ZJ clutch installed.
Click to expand...

Yep, read it the same way. Was mentioning it as a 'by the way' sort of thing.

OkieXJ said:
I have access to a torque screw driver that I could use to get some torque values off the fan clutch. I might have to do that sometime.
Click to expand...

Out of idle curiosity, I'd be interested to see how the ZJ HD one compares to the stock XJ unit - though it would be best to compare them installed and under belt tension, and you'd have to make sure that the XJ fan clutch was still functional in order to get good numbers.
 
OkieXJ said:
87manche, I checked the thermostat last night in a pot of boiling water. I guess those things don't open very far do they? It was clear that it was open at boiling, but at ~180° it was pretty tough to tell if it opened at all. I guess that means it pretty close to working properly? If thats the case... I'm not sure what that means about my cooling problem.
Click to expand...

maybe 3/8" total travel? basically whatever the length of the wax pellet is.

How did you verify the temp with the IR gun? at the therm housing? I suppose it's possible you've got a big air bubble, but usually that results in overheating.

Does the heat work decent?
 
And according to Joe_peters, it would have to have been running so the fluid is is the reservoir and at the same temp on both fan clutches etc. Sounds like a lot
of work.

Heat works pretty well, I haven't noticed anything wrong with it. I definitely didn't see the thermostat move 3/8" Thats pretty far, it maybe moved 1/8". I shot the thermostat housing to confirm that it was roughly in line with my gauge to make sure the gauge was working properly. I had the same thought about an air bubble. I guess it could be there. I can try and bleed it out like people say to do through the rear temp sender thing.
 
Last edited:
Do the HO ones still have a sender in the block that the ECU reads?

regarding the fan clutch testing, an electric motor setup on the bench with a belt would be a lot easier and give more consistent numbers/repeatability.

I feel your frustration, I spent half a winter with BS heat and a cold motor. I checked everything I could think of and then just started replacing thermostats until one worked.
 
87manche said:
Do the HO ones still have a sender in the block that the ECU reads?
Click to expand...
91-96 do. Temp senders on XJs:
* on the driver side of the block under the manifolds (RENIX, think this one is for the ECU?)
* on the back driver side of the head (RENIX and 91-96) - for the gauge cluster
* somewhere on one of the hoses (RENIX, for the e-fan?)
* on the tstat housing (91-01. 91-96 it is only for the ECU, 97-up they eliminated the separate gauge sender and the ECU feeds its temp sender signal to the IP via the CCD bus.)
 
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