At a loss... Cooling system

I have been fighting a cooling system problem for about 2 years now that has, in my opinion gotten progressively worse. It will quickly heat up to 225-235 on the gauge. Hot weather and running the A/C makes the problem worse and will send it into the red.

The entire cooling system has been replaced.
-New 3-row Radiator barn radiator
-New water pump
-New hoses
-New thermostat
-New radiator cap
-New fan clutch
-New temp sending units
-New O2 sensor

Running out of options I thought the head gasket might be cracked so I replaced it. I had the head decked (12 thousandths ) and checked out. Head was otherwise good, no cracks and the valves are good.

When I pulled the head I noticed that the pushrods for the #5 and #6 cylinders where very sludged up compared to the rest of the pushrods. It looked like the oil had been 'baked' on.

Current symptoms

From a cold soak the engine warms up very quickly, at least according to the gauge. I pulled my IR temp gun out and started shooting temps from the cold soak and this is what I found.
The dash gauge will show 230 before the thermostat (195) opens. At this point the head right by the thermostat housing is reading 190F. On the sparkplug side of the head the temps range from 180 by the #6 to 170 by the #1.
I've checked the sending unit with a multimeter and the values it is showing seem to confirm the dash gauge.
After warming up for a while the temps seem to be what I would expect. 200f at the thermostat housing. 195 at the radiator inlet. 165-170 at the outlet. Yet I'm seeing 230+ on the gauge.

It's clear to me that there is something going on at the back of the motor that is just creating a god-awful amount of heat. That much is clear from when I pulled the head.

I'm gonna go get a auto meter mechanical temp gauge and install it. My plan is to drill and tap the thermostat housing on the front face (it kinda seems the spot was designed for another sensor) right next to the upper bolt. That will let me keep an eye on the water temp exiting the head.

notes:
I've tried swapping the injectors to see if one was causing a cylinder to run lean. No joy.
The electric fan is hardwired and stays on all of the time.


I am completely out of ideas.

Sound to me like the gauge is your problem. The only place you are seeing excessive temps is on the gauge. Maybe you don't have a problem, in the engine compartment but in the cabin.

Well, it looks like you've covered everything that I would look for.

How are the plugs? Any signs of a super lean or rich mixture?

Could it be further down the line, like a clogged cat causing back pressure? Power OK?

Either way if you're getting IR readings that look normal, and gauge readings that are way different, I'd suspect the gauge sender OR the gauge itself. Especially if the thing says it's reaching 230 before the t-stat opens at 192-195.

As far as drilling and tapping the t-stat housing, it's super easy(er) to go to the junkyard or even NAPA and get a post 91 housing. They're cheap enough.

Edit: P.S. I assume since you don't have a bung in your t-stat housing that your rig is a closed system (RENIX). If you haven't done so already, check for bottle cracks/loss of pressure at that end...and the infamous air in the system.

While you're at the JY pull a gauge cluster as well if you suspect the gauge itself.

I've tried swapping the gauge along with the sender. I've confirmed via multimeter that the gauge is correct based off of the values the temp sender is seeing.

Plugs are good. Power is good.

Its a 91 with the temp sensor in the thermostat housing. I'm talking about drilling and tapping the other flat spot on the housing for an after market gauge.

The normal temps with the IR gun after warming up only last until I start driving it around. Then it quickly starts climbing up into the red and I see elevated temps at the thermostat housing.

menos said:

Its a 91 with the temp sensor in the thermostat housing. I'm talking about drilling and tapping the other flat spot on the housing for an after market gauge.

The normal temps with the IR gun after warming up only last until I start driving it around. Then it quickly starts climbing up into the red and I see elevated temps at the thermostat housing.

Click to expand...

So, correct me if I'm wrong, please. On a 91 to ? I thought the temp sender for the gauge is still on the back of the head by the firewall. The gauge for the computer is mounted on the t-stat housing.

On a 90 there are 3 ~ temp gauge sender/sensor/switch ~ in 91 they did away with the sensor on the lower block and replaced it with the one on the t-stat housing. That's the one that tells the computer what's going on.

The sender your looking for is mounted on the rear of the head. Yet having never looked at any 4.0 besides a RENIX, I'm not an authority.

I'm not looking for the sender for the stock gauge, I know where that is. I'm talking about adding an additional sender for an aftermarket gauge so I can see the water temps exiting the head and compare them to the temps I am seeing from the stock temp gauge at the back of the head.

My concern is that there is something wrong with the motor that is creating a s***-ton of heat around the #5-#6 cylinder.

Well just because you do not mention it. Did you bleed the system of all air when you did all this? If there is an air pocket trapped at the gauge sensor it will be much hotter than the rest of the areas.

What fan clutch did/are you using. I replace 3 regular Hayden fan clutches in a span of about 7 months. They all died. I upgraded to the one suggested here on .

On the lower hose.... the replacement. did it have a spring like deal inside it. If not under load the hose could be collapsing and not allowing it to slow.

Its a 91 with the temp sensor in the thermostat housing.

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the one in the thermostat housing is not for the gauges. It is for the fans. It sends a signal to the computer then the computer tells the fans when to come on.
The one for the gauge is on the rear drivers side of the head. It's a 1 wire hook up and with the connector removed it looks like a threaded part of a bolt. At least all the way up to 96 I'm sure as my buddy has the one on the thermostat disconnected(runs custom electric fans) and the one on the rear of the head is connected and his gauge works, well till we disconnect the wire on the sensor on the rear of the head.

Ok. So I took it for a drive yesterday when the outside air temp was about 94F. It quickly warmed up with the gauge pointing into the red. I shot the following temps with my IR gun.

Passenger side of the head by the #6 - 240F
Passenger side of the head by the #1 - 225F
Thermostat housing - 220F
Return hose from the radiator - 197F


The cooling system has been bled.
It currently has a standard fan clutch on it right now. I've got the ZJ one sitting in a box in the garage.
The lower hose does not have the spring, but its a newer style one that has some internal support or something. It's pretty firm.

I'm clear on the gauges and which sender sends to what. My plan is two add a 3rd sender for an AutoMeter gauge I have in the garage. But this may be a moot point seeing as how I am definitely seeing increased temps, verified with the IR gun.

The head's good but how about the block is it possible that it's cracked?
Does your elec fan kick on when it get's to temp and also when a/c is on?
How well does your a/c system work, if it only blows somewhat cold it might need to be recharged which could be allowing to much heat on the condensor that's in front of the rad. Before I removed my a/c system mine would start running hot in traffic and as soon as the condensor was removed it never overheats now.

ParadiseXJ said:

So, correct me if I'm wrong, please. On a 91 to ? I thought the temp sender for the gauge is still on the back of the head by the firewall. The gauge for the computer is mounted on the t-stat housing.

On a 90 there are 3 ~ temp gauge sender/sensor/switch ~ in 91 they did away with the sensor on the lower block and replaced it with the one on the t-stat housing. That's the one that tells the computer what's going on.

The sender your looking for is mounted on the rear of the head. Yet having never looked at any 4.0 besides a RENIX, I'm not an authority.

Click to expand...

this sounds right to me but Im in an 88.
have you tried a round piece of electrical tape?.

menos said:

Ok. So I took it for a drive yesterday when the outside air temp was about 94F. It quickly warmed up with the gauge pointing into the red. I shot the following temps with my IR gun.

Passenger side of the head by the #6 - 240F
Passenger side of the head by the #1 - 225F
Thermostat housing - 220F
Return hose from the radiator - 197F


The cooling system has been bled.
It currently has a standard fan clutch on it right now. I've got the ZJ one sitting in a box in the garage.
The lower hose does not have the spring, but its a newer style one that has some internal support or something. It's pretty firm.

I'm clear on the gauges and which sender sends to what. My plan is two add a 3rd sender for an AutoMeter gauge I have in the garage. But this may be a moot point seeing as how I am definitely seeing increased temps, verified with the IR gun.

Click to expand...

I just read this whole thread. So far all I see is a potential 30 F error reading in the Dash gauge, possibly a 15 F error in the sensor plus a 15 F error in the dash gauge. Mine combo reads about 30 F lower that it really is on my 87, and about right on my 89 currently. The rear sensor should be reading about 10 to 20 F lower than the T-stat reading. The only numbers I have seen in the this entire post that concern me is the single 240 F on #6 on the pasanger side. Seems a little high, maybe 20 F higher than usual, but even that may be OK. Many of the temps you posted are great, not too high. I don't trust the dash gauge and rear head sensor combos on jeeps as far as I can spit, they are notrious liers.

Have you had the exhaust system checked?

Tried a vacuum gauge test at 3000 rpm to check for exhaust restrictions?

Is the exhaust header cracked, or leaking and blowing on the rear of the block?

Have you checked the actual O2 sensor readings with an analog meter to see if the air fuel ratio is too lean, or OK?

ALSO! Were these tests with AC on, or off?

The ZJ clutch should help some more.

Bringing an old thread back to life... I've bee fighting similar symptoms on my ride, its been driving me nuts. I have a 90, converted to the open cooling system. I've put in a new CSF 3row radiator, new water pump, 195 thermostat, all new hoses with a universal one with the wire at the outlet side so I dont have to worry about the hose collapsing.
I'm staying cool around town but am running much warmer in the valley where its a lot hotter than the Bay Area. Last week, was pulling the grade up 50 headed east from Placerville and my temp started to rise. I have an AutoMeter mechanical gauge in the water neck and it was showing upwards of 230 on the gauge. My dash gauge was in the red, but I can't say that I trust it, even after I put in a new sender. It reads a tick over 210 when the mechanical gauge is reading 195. According to this, the temp at the rear of the head where the factory sender is should be approximately 210 degrees when I'm pushing 230 at the water neck? I haven't verified the temp with an IR gauge yet, thats's my next step. I just picked up a new sender from NAPA and will see if this one reads accurately.

I just had the air conditioning compressor replaced and the shop I took it to checked for a cracked head or bad head gasket. All seems well.

What are the other specs on your rig? When I was way undergeared, it gave the cooling system hell.

You are aware that the stock gauge is "calibrated" with a glued on magnet, correct?

Things you have not mentioned are, the E-Fan, is it running, is the fan clutch new, have you tried upgrading to the ZJ fan clutch, check for exhaust leaks blowing on the block at the manifold gasket, cracks in the header itself, or leaking flange donute. Have you checked the O2 sensor to see if the jeep is running lean, checked for low fuel pressure, dirty air filter, checked the radiator and condensor for dirt blocking air flow, made sure the stock fan cowl is in place, stuffed some polyethylene foam in areas where air might bypass the radiator to do lift on the jeep, like between the condenser and radiator, is the water pump rotation correct, is the Transmission slipping, and overheating on the grade, and is the AC overheating things. I had to up grade my AC condenser on my 87 to a 97 cross flow condenser to get my AC head pressures and temps down, but my real problem turned out to be an exhaust leak on the block. I got mine to stay cool even with the exhaust leak before I found the exhaust leak, LOL!

I changed my rad 2 days ago because of a leaking starboard side tank.

I was checking things over (using a flashlight) while I had everything apart and I noticed
that I could barely see through the AC condenser. Mind you I regularly blast the rad
from behind with a strong stream of water but I could never access the back of the AC
condenser while the rad was in place. While everything was apart I blasted out the AC
condenser from the rear side using a strong jet of water from my garden hose. I was
amazed to see how much stuff came out of there. Then I meticulously picked out most
of the most stubborn tiny rocks from between the fins carefully using a dental pick.

Deleted.

What are the other specs on your rig? When I was way undergeared, it gave the cooling system hell.

You are aware that the stock gauge is "calibrated" with a glued on magnet, correct?
__________________

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I regeared, running 4:56s with 33" tires. My rig is a 5 speed, new Centerforce clutch...no slippage. And yes, I know the stock gauge is not a pre4cision instrument, just more of a rudimentary indicator that something is not quite right. I just want to get to the bottom of things and be able to drive this thing when it gets to be over 80 degrees out.

Things you have not mentioned are, the E-Fan, is it running, is the fan clutch new, have you tried upgrading to the ZJ fan clutch, check for exhaust leaks blowing on the block at the manifold gasket, cracks in the header itself, or leaking flange donute. Have you checked the O2 sensor to see if the jeep is running lean, checked for low fuel pressure, dirty air filter, checked the radiator and condensor for dirt blocking air flow, made sure the stock fan cowl is in place, stuffed some polyethylene foam in areas where air might bypass the radiator to do lift on the jeep, like between the condenser and radiator, is the water pump rotation correct, is the Transmission slipping, and overheating on the grade, and is the AC overheating things. I had to up grade my AC condenser on my 87 to a 97 cross flow condenser to get my AC head pressures and temps down, but my real problem turned out to be an exhaust leak on the block. I got mine to stay cool even with the exhaust leak before I found the exhaust leak, LOL!

Click to expand...

The e-fan is running and fires up witht eh A/C on. I have a brand new fan clutch, replaced it when I did the water pump. I flushed the heck out of the system, even used the stuff Ford used for the Taurus when they put out a TSB for faulty cooling systems on that car. The previous owner had gummed everything up with stop leak and it took me forever to get the stuff out. It even clogged up the radiator I had in there before I put in the CSF. My next step is to upgrade to the ZJ clutch, worth a try. The O2 sensor is brand new, had to change it out to pass smog here on the left coast. I was running lean before under load, casuing it to fail the dyno portion of the test. Its good now, but i have not checked fuel pressure at the rails. Oh, and I had to replace the exhaust manifold due to cracks that made me fail smog the first time I tried.

I can pull a grade around here with no symptoms of running lean. Air filter is brand new as well. Water pump rotation is correct, and its a 5 speed so no tranny fluid in the radiator to heat things up.

What I did not consider is the possibility of an exhaust leak at the collector. The exhaust has been on and off a few times and I might have a leak going on there. I does take a while for the temp to creep up and hot exhaust on the block may be the culprit. It cooled off fairly quickly after I pulled off the highway.

Also on this trip I lost the hydraulics on the clutch. I swapped in an external slave AX15 and the later style hydraulics with the plastic hose.Took a look at it, looks like the plastic line failed somewhere and the heat from the exhaust might be the culprit as well. I limped it home and haven't had time to look at it since. That's tomorrow's project.

Just wanted to update this thread. Drove up to Sierra fest last weekend in over 100 degree heat and the Jeep stayed cool. Looks like I finally licked the overheating gremlins I've been fighting for a while. I changed out the stock fan clutch for a ZJ clutch...that thing REALLY moves a lot of air! I verified my fuel pressure to be within spec (29 PSI, 37 with vacuum disconnected at regulator) to confirm I wasn't running lean. I had the down pipe redone from the exhaust collector to the catalytic converter in order to eliminate any exhaust leaks. once it was off, I took a look at the flange...it was pretty well worn out and more than likely not sealing, letting hot exhaust escape. After all this, I managed to pull a long, steady grade with a fully loaded Jeep and my AutoMeter mechanincal guage topped out at 218 degrees. Thanks EcoMike for the tip on the exhaust leak!:thumbup:

:cheers: