88 4.0 Converted to Open System, STILL HOT!!

My 88 MJ 4.0 is running hot.
Its been a constant battle since day one.
It's been converted to an OPEN SYSTEM
and the Heater Core is bypassed.

The Details:
New OPEN STYLE OEM Autozone replcement radiator. No leaks. Not plugged with mud or anything else.
Good hoses, no leaks or crimps.
New 180 Thermostat. Not Clogged.
New OEM replacement Water Pump. Working in the Proper Direction.
New Belt, routed correctly.
Twin electric fans - 16" and 10".. no clutch fan.
Both pull TONS of air, and are both wired correctly.

When driving around town, moving faster than a crawl.. it runs at
a constant 210... even with only 1 fan running.

Immediately, when stopped... temp will start to rise.
Put the vehicle in Low Range... and start through the trail
at slow speeds the TEMP IMMEDIATELY starts to creep up.

Move up to 230, 240 in a matter of minutes.
Both fans cranking full blast.
After about 10-15 mintues of slow speed it will be up to 260-70
and ready to boil over.
The engine gets so hot the two electric fans stop working.

Im at my wits end with this problem.
I will be getting a 3 Row radiator from Advance Auto
today, and trying this. Its my last resort.

Ive seriously tried everything.
Could the block be warped?
Could the head gasket be blown?
Could the head gasket be interfering with the water ports?

Have you done a complete flush of your cooling system, with say, that Prestone stuff that's supposed to clean all the gunk out? Could be plugged up gunk in the block/engine...

oh and i know someone else will post it, but everyone complains about thermostats other than the stock 195 one. the debate will begin about whether or not your t-stat is stuck open or closed.....

I tried it with just the thermastat housing (No guts), and with a 180. Same thing both ways is happening still.

JNickel101 said:

Have you done a complete flush of your cooling system, with say, that Prestone stuff that's supposed to clean all the gunk out? Could be plugged up gunk in the block/engine...

Click to expand...

No i have not done a complete flush. How do I do that? With a garden hose?

there are two different ones...there's one they sell with a T-connector that you connect a hose to....then there's another one where you just put the bottle of cleaner in your cooling system...run it for a while...then drain, refill with water...run it again.....drain, refill with water, run it again......then drain, refill with antifreeze/water mix.

Not sure which one works better, maybe someone else knows....?

when new, my 89 with the closed cooling did the same thing with rising temps until i upgraded to a 3 row radiator. is your new radiator a stock 1 row replacement. for mine, the stock radiator didnt have enough flow

I was having this common problem. I did the following:

-Install 3-row all metal radiator
-New waterpump
-New belt
-New 195* thermostat
-New Upper & Lower Hoses
-System flush (mine was actually pretty clean for 225k on the motor)

I now run right where it should be all day long. Even when slow moving on the trails the temp doesn't budge! It is SO nice!!!

So, to flush it since you have an open system now...buy the t-fitting and Prestone system flush, run flush for a day, install t-fitting, you will then hook up a garden hose to it and let the car run and it will flush all that crap out. Just follow the directions, it's pretty easy to do.

By the way, alot of people, including myself, don't care for the t-fittings since they are plastic and you are splicing it into your hose. Just keep an extra hose with you just in case the worst happens.

Or make one yourself out of brass fittings - which is what I did.

N.B. - if you rinse the flush out with a garden hose, be sure to drain all the water you can out of the system! There should be a block drain plug and a coolant temperature sensor on the driver's side of the engine block, behind the exhaust header (I know, pain in the arse...) - pull one or the other of these to drain.

Why for? Because tap water has dissolved metals and solids in it, and will crap up your system - rendering your flush work useless. Oops.

Mix your coolant with either distilled or reverse osmosis-filtered water - no solids, no metals.

Given the behaviour of his system, I'm inclined to say the thermostat is working properly - however, you'll want to make sure there's not an air pocket behind the thermostat once you fill up - that's why I just drill the flange on the thermostat out of habit whenever I put one in. Two 1/16" holes, 180* apart, and one up top and the other down bottom (as installed) makes the system purge itself of air.

There is nothing "wrong" with the RENIX "closed" cooling system as designed - it just requires maintenance. There is a little quirk with the OEM radiators - replacing with aftermarket will usually take care of that (OEM radiator service life is about 150-180Kmiles. Even the Jeep techs around here use aftermarket...)

Still, it looks like you've hit all the likely suspects. A couple of things...

1) When you get your coolant and new radiator, get a bottle of Water Wetter. The stuff works.

2) Normally, I'd say to pull the radiator cap while running (get to op temp and be bloody careful!) I'm not sure you'll be able to see flow with a converted system anyhow. If you've got access to a "non-contact" IR thermometer, it may be instructive to take readings across the radiator as it warms up to see if there might be blockage from a manufacturing defect.

3) "Since day one." How long has it been, and did the PO say anything to you?

try running it w/o a thermostat.

acousticrawk said:

try running it w/o a thermostat.

Click to expand...

I didn't want to suggest that at first - since I've only run across three engines in about 30 years that do not like having thermostats (and the one that didn't like having a thermostat, but needed a restrictor plate, instead.)

I'm going to vote on a blown head gasket. You don't have to be loosing very much water for it to start over heating.

Bottom line....you have covered all of the obvious things....

Have you pulled the plugs to see if any of them have any unusual deposits on them?

Pull the fuel pump relay and block the TB...then crank the engine over for about 30 seconds....then pull the plugs and look for any water.

I am going out on a limb... Both of your electric fans? Pushers or Pullers? Do you have them wired properly.. I have seen a couple of guys wire the fans so they were actually blowing threw the radiator and out the grille which is wrong.. Make sure your fans are pulling air threw the grille and radiator.. 2. Make sure you have bleed all the air out of the system.. Its still a renix motor and air does get traped in the back of the head.. DO not run with out a t-stat because that will cause it to overheat as well..

It's been converted to an OPEN SYSTEM and the Heater Core is bypassed.

A couple of observations. By-passing the heater core would actually make the vehicle run hotter. Less coolant capacity and the heater core being a heat exchanger removes latent heat from the coolant.

I am scratching my head over your why youi ask the last three questions. Did the PO mention that the engine was rebuilt? Did you rebuild it?

Could the block be warped?
It could be, but why would you think that?

Could the head gasket be blown?
If there is no visible sign of external leak, do a compression test to check the head gasket, remove the rad cap and look for bubbles in the coolant.


Could the head gasket be interfering with the water ports?
Yes the incorrect or incorrectly installed head gasket could close off passages.
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The fan shroud is often overlooked as an important component of a cooling system. Do the electric fans have shrouding to not just "pull air" but to pull it through the radiator? Missing fan shroud leads to inefficient air flow.

Exhaust, you didn't mention performance, but do you feel this vehicle has good take off or does it feel sluggish? A restricted exhaust, such as a bad convertor could cause a vehicle to over heat.

Good Luck,
Tom