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4.0 running too cool...

ColoCherokees

NAXJA Forum User
Location
Littleton, CO
Specs - '96 XJ, 4.0, AX15, 181K. Original radiator, water pump and heater core - heat works fine, no leaks/coolant loss, totally unmodified engine/transmission/exhaust.

I just replaced the original thermostat, because when driving, the temp would climb to approx 210, then drop dramatically - in the winter could be as much as a 50* drop, and then would slowly oscillate between ~160* and 210* (as indicated on temp gauge). I assumed a faulty thermostat was to blame.

When I replaced the thermostat (Mopar unit from dealer, installed with bleed hole at 12 o'clock), I did a "flush & fill" - seems to be bled properly, and no leaks or coolant loss (one week later). Now, the engine will only warm to approx. 180*...but in the recent 90*+ weather, if idling for a few minutes (AC on), the temp will climb to near 210* (a hair or two below the 210 mark on the gauge), but as soon as I begin to move, the temp drops back to the ~180* area.

To sum up, the engine appears to be running cooler after new thermostat, but still with the same dramatic drop in temp once it warms up. I don't have reason to suspect a faulty gauge, but I'm going to borrow an IR temp sensor from a friend to verify gauge is telling the truth.

What's wrong with this thing?
 
Does the temp rise the same speed as with the old thermostat ? I can almost tell where I am on my drive to work by the rise in temp of the engine.From a cold motor-70* morning to 150* on gauge will take 5 min,to 210* about 8-10 min,I can watch gauge to know when t-stat opens> drop to 195* > rise back to 210* which is my normal opp temp.

I did put a thermostat in incorrectly(slipped out of groove) and while the temp would come up it took way too long-even threw code it was too long to opp temp.
You may have a leak around the thermostat ?

Wayne
 
the engine will only warm to approx. 180*...but in the recent 90*+ weather, if idling for a few minutes (AC on), the temp will climb to near 210* (a hair or two below the 210 mark on the gauge), but as soon as I begin to move, the temp drops back to the ~180* area.

that sounds about right to me, maybe 10 degrees low which could be explained away by faulty gauge sender

FWIW the water pump and viscous fan pulley are both driven by the belt--idling generates the least water and air flow, while revving increases both significantly
 
I agree that verifying actual heat with a point and shoot infrared thermometer is a good idea. Don't want to be chasing your tail.

Let us know what you find. You really need accurate data before you replace any more parts. 205-215 is the normal range of a hot 4.0
 
My stroker runs from 190 to 220
 
There is a 180* replacement available from Mopar.Could be you got one of those.

Wayne
 
Wayne: The temp does seem rise at about the same rate as with the old thermostat, it now just operates within a lower temp range than before. If nothing else seems a likely fix, I'll pull the housing to make sure the thermostat is seated properly. Is there a way to visually determine if its a 180* or a 195* unit? I don't think I still have the packaging to get a part number (whoops).

Will report back when I get the IR thermometer readings.
 
unless you're pulling it to get the number off the thermostat, I wouldn't bother to open it to see if it was seated correctly-- the water outlet has a ridge cast into it and if the therm. wasn't seated it would be leaking like a sieve. If it was installed backwards, it'd never open and you'd just overheat and boil over.
 
unless you're pulling it to get the number off the thermostat, I wouldn't bother to open it to see if it was seated correctly-- the water outlet has a ridge cast into it and if the therm. wasn't seated it would be leaking like a sieve. If it was installed backwards, it'd never open and you'd just overheat and boil over.

I put one in that slid out of the recess ,and it did not leak.Just took too long for the engine to heat up.
Should have a code # on the "pellet" xx195x or the like.
Wayne
 
I'd try the point and shoot infrared first to see what the REAL temps are. If the temps are good, I'd check the GAUGE sending unit at the rear of the head and then the CPU temp unit in the T-Stat housing. I have had problems with both of these sensors, on an otherwise correct operating stroker.
 
Moves to 210 at idle,then cools when you rev it? It's working.

It does sound like the old one needed replacing. What you describe now sounds about factory perfect. Mine runs too cool - I can't get to 210 with a 195 stat and three core radiator, even idling in a fast food line for half an hour in 98 degree weather. I can't even get the thermo switch to kick in the fan. I'm considering putting in a restrictor in the upper hose.

From my perspective, I'd consider it done and drive it.
 
This post is mostly for the searchers, but thanks to all that replied with suggestions.

Used the IR gun to measure temps on various parts of the thermostat housing. I let the XJ idle to warm up, ambient air temp approx 92-93*F, relative humidity below 10% (yes, I love living at 5280 feet ASL), but sitting in a small, one car garage (nose to the back). First (most important point), the engine IS getting up to, and staying at proper temp. Dash gauge is accurate as to WHEN t-stat opens, and then coolant cools a little. Dash gauge is NOT accurate as to what that temp range is (in MY case).

Hash mark on dash gauge in between 100* and 210* measured at ~180-183* at t-stat housing. Halfway between hash mark and 210* mark, t-stat housing measured at 208*-211*, after t-stat opened and ~10 minutes at idle.

At about the 10 min mark, I did these checks:

With heat on, and fan on highest setting, dash gauge showed about a third of the way between first hash mark and 210* mark - IR gun showed t-stat housing at 204*-208* - nice hot air blowing from vents.

Turned heat off, and the AC on - dash gauge rose to approx 2-3 mm below 210* mark - IR gun showed 215*-219* (again, this is after 10+ mins at idle in at least 85*+ heat in an enclosed space).

Obviously, my t-stat is working properly, and my dash gauge is reading low...low enough that from now on, if I see it at or much over the so called "210*" mark, I know I'm running pretty hot and ought to take necessary precautions.

Again, for the searchers: The t-stat was purchased at my local dealer - stock 195* Mopar unit, part number 83501426AC (for my 1996 4.0L), installed with the hole at the 12 o'clock position. T-stat came with 3 "paper" gaskets (very obvious which one was the correct fit for my application). I used a little "Indian Head" to keep the gasket stuck to the housing during install - did not use any kind of RTV or other sealant. Going on 2 weeks with normal mix of city/highway driving, and no leaks or coolant loss

This is IN NO WAY some kind of a dash gauge calibration - seems that everyone's XJ reads slightly different on the dash gauge - and I would highly encourage anyone with an engine temp issue to use an "independent" method of measuring coolant temp before trusting the dash gauge.
 
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