Overheating

Got a new motor which I did not get to hear run. As an FYI this is in a 93 jeep.

Put a new A/C delco water pump, thermostat (with the bleeder holes) radiator and hoses on. Ran the motor and it ran warm 230 ish and the fan did not turn on. I thought the sending unit on the new motor might be bad so I replaced it. Ran the motor for 5 mins tops and the temp kept climbing until I turned off the motor at approx 230-240 (the mark before the red). There was no dip in the temp when I figured the thermostat would open. Replaced thermostat with a new stant and water pump with a NAPA unit, verifying the correct impeller and verifying I had flow through the block by using a garden hose to check flow from the empty thermostat housing out the water pump location. Buttoned everything up, filled the system with 50/50, removed the sending unit on the back of the head to bleed the system, started the motor and the temp kept climbing until I turned it off around 240. I am confident the belt is routed correctly as I compared it to my other jeep and the diagram (water pump spins counter clockwise).

Took the thermostat out, kept the upper hose off, started the motor and verified I had flow out of the upper hose. It was not a constant steady flow at idle, more of a surge of flow, a lull, surge, ect. Started it and ran it for about 5 minutes without the thermostat and it warmed up to around 220-230 with 50/50 coolant. Verified fan clutch was good by a rag being pulled into the fins on the radiator and staying against the side opposite the fan.

Throughout all this the electric fan has not turned on, and I have verified the fan is working by removing the connector from the CTS and the fan turns on. I checked the resistance at the CTS and it indicated the temp the senor read was approx 180-190 and I verified this by using the thermocouple on my multimeter and it read the same temp through conduction by touching the TC to the thermostat housing while the gauge has read 220-230.

Tonight frustrated, I put a thermostat back in the motor, pulled the upper hose, put the garden hose in the radiator and started the motor. It ran up to about 220 or so after 2-3 minutes, without a drop in temp when the thermostat should have opened, and I shut it down. While it was running, less than a dribble of water was coming out of the upper hose. I am not sure if that was because the water was boiling in the block and no flow was exiting the thermostat or not. Drained all the water from the lower hose and the water was extremely hot coming out of the block indicating the temp sensor is reading a hot temp.

I also checked the compression tonight. All six cylinders were 135-140. The plugs looked good, but they are all of an hour or so old. I want to try a CO2 test, but I am not sure if I will see much flow out the thermostat to measure for that.

I will admit I will probably be slow to respond so bear with me. I have done a good deal of searching and I am stumped as to what is causing this issue and I apologize for the novel, but I wanted to mention all I have done.

Did you figure this out? Sounds like problem with the dash gauge or dash sensor or both to me. The sensor needs to ground well to the block, no thread sealant!!!

Also, get dealer T-stat, the aftermarket ones are getting bad reports !!!!

Just a thought, The heater exit at the thermostat housing back to the pump is for the heater but is also a bypass for the thermostat. You get a glob of cold coolant behind the thermostat, without the bypass and it can heat very slowly from the rear. The coolant behind the thermostat will eventually get hot enough to open the thermostat, until you get another surge of cool coolant through the block and it happens all over again. The end effect is you get real hot (at the sender), then cold, then real hot and the temp gauge takes wild swings.

It can happen if the heater valve is plugged or if you use the wrong gasket for the thermostat. The universal gasket supplied with many thermostats can cover the thermostat bypass. A Jeep OEM type gasket is the only real solution other than making your own.

The temperature at the rear of the block has little to do with the temperature at the front of the block or at the thermostat housing.

A little trick I use is to run the motor up to temperature (until the thermostat is open) and press the top radiator hose together, let it go, push again and use it kind of like a suction pump, it seems to get most of the air to move to the top and/or the filler neck.

You may have a combination of things going on, no flow near the thermostat and trapped air that isn't moving.

Ok.

OBD I--coolant temp sensor, feeds the PCM. Then there is a temperature sending unit, and there are two types--one for a light, and one for a gauge. Which sensor/sender are you checking? Which sender do you have--light or gauge?

BTW, in 4 years now, I have yet to find an aftermarket rear head temp sensors that display properly on any of My Renix dash gauges. I use the hand held infra red for my data, and then just use the dash gauge +/_ a calculated bias number. My gauges or gauge grounds or ??? may be the issue. many have reported that the Jeep dash coolant temp gauges from 87-95 are just not reliable.

Thank-you for the responses!

I recently bought a dealer thermostat, gasket and rad cap. Before putting in the new thermostat, I ran the motor with no thermostat and the upper hose removed from the radiator. Put the garden hose in the radiator with a low flow and started the motor. At first I noted the same surging of flow out of the upper radiator hose, it would flow a small amount, then stop, flow, stop. I uppped the flow on the garden hose and kept the motor running. Shortly I noticed the flow out of the upper hose was steady, even when I turned the flow of the garden hose down. I am assuming I had a clog/blockage or something which finally let loose. I ended up using a flush and ran the motor for 10 mins or so to flush the block of any other possible crud.

Put the dealer thermostat and gasket as well as an electrical gauge I bought at the local auto parts store installed at the back of the head. Ran the motor for a while verifying I had flow out of the upper hose (the top of tank on the radiator read 180 with the TC on multimeter which was close enough, I am sure the TC is not too accurate) and the gauge reading the head temp was around 170 which could have been a result of the standoff I needed to get the sender to fit and the gauge not being the best.

Long story short I think there was some sort of blockage that was causing me a problem and that I messed up, Mike you called it, I put thread sealant on the sending unit when I replaced it. I started thinking about how the sensor was working and where the ground was coming from and realized tonight I probably caused some of my problem. I am going to clean the sending unit and try to clean the threads in the block and report back.

Thanks again for the replies, I have been cranking away trying to check off all of the possible causes of the problem, this one really threw me for a loop.

joe_peters said:

Ok.

OBD I--coolant temp sensor, feeds the PCM. Then there is a temperature sending unit, and there are two types--one for a light, and one for a gauge. Which sensor/sender are you checking? Which sender do you have--light or gauge?

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Joe,

I have a coolant temp sensor on the t-stat housing which I was measuring the resistance between the two legs after shutting the motor off to correlate the temperature per the FSM. This is how I was able to determine I had a big delta because the sending unit on the back of the head for the gague, was reporting a high temp. But as mentioned I probably caused some of the headache by putting thread sealant paste on the sending unit causing it not to ground properly.

Ecomike said:

BTW, in 4 years now, I have yet to find an aftermarket rear head temp sensors that display properly on any of My Renix dash gauges. I use the hand held infra red for my data, and then just use the dash gauge +/_ a calculated bias number. My gauges or gauge grounds or ??? may be the issue. many have reported that the Jeep dash coolant temp gauges from 87-95 are just not reliable.

Click to expand...

Mike,

Like I said above I think you hit the nail on the head for my gauge giving me fits. I think I compromised the ground for the sending unit using thread sealant. I psyched myself out because prior to changing the motor the gauge was surprisingly accurate, but with the "new" motor, the sending unit was probably not grounding well. I removed it and put the sealant on when re-installing it which probably compounded my problem.

As a sanity check, the grounds on the 93 should be:

battery to block
battery to body
block to body by the firewall

Am I missing any others?

Well I have narrowed it down to the wiring into the instrument cluster. I verified the sender is operating correctly by measuring the resistance at the sender itself. I verified the gage was not the problem by swapping the cluster from my other jeep. I believe I have two grounds for the gauges, and comparing the resistance readings from these wires to the battery versus the corresponding two wires on my other jeep I get different readings. However the resistance reading from the sender to the gauge is correct, so it leaves me with my grounds. Unfortunately my manual (not the FSM and that's my problem) is little help.

Could I get a little help on where these two wires ultimately terminate to clean/check them on a 93 xj.

I have cleaned checked all the grounds that I know of under the hood (the block to firewall, battery to block by the coil, battery to body on both sides of the engine compartment) with no change.

Thank-you

Ask Kastein.

Ecomike said:

Ask Kastein.

Click to expand...

Will do thank-you.

As an FYI I cleaned the ground under the dash for the cluster and did the trick running a separate wire from that ground to the chassis with no luck. I am confused a bit because when I ring out the signal from ground to the ground wire at the cluster harness I get a reading of 1 or less ohms.

Ecomike said:

BTW, in 4 years now, I have yet to find an aftermarket rear head temp sensors that display properly on any of My Renix dash gauges. I use the hand held infra red for my data, and then just use the dash gauge +/_ a calculated bias number. My gauges or gauge grounds or ??? may be the issue. many have reported that the Jeep dash coolant temp gauges from 87-95 are just not reliable.

Click to expand...

This. All the parts house sensors were 30+ degrees off. I finally got one from the dealer and it is around 10 degrees off.