Jeep 4.0 HOT temp. Need help

Van-go

Van-go

Not an old man
Joined
Dec 11, 2014
Location
Greensboro
Previous owner to my yj swapped a 1996 4.0 into it instead of the 2.5.
It has ran flawlessly at 200F for about 8 months now. Until last weekend.
I noticed the temp gauge was nearing 240 on the highway.
I replaced the thermostat, front temp sensor, water pump, radiator and cap and flushed the engine.
Previous owner installed an electric fan that is wired to the ignition so it runs all the time.
I burped the cooling system by parking on incline and removing rear temp sensor until only coolant leaks out instead of air.

Gauge stays pegged at max temp now...
I have replaced the whole cooling system... What else could I do to fix this? I've read about every page I can find on this.
Only thing left I can think of would be a head gasket but there is no water in my oil.
Trying to make it to the flats Saturday if at all possible.
 
I'd be checking for combustion gas in the coolant. Sounds like a blown head gasket. Is the cooling system pressurized? Is the coolant a muddy color with odd odor?

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Also possible to have a bad tstat, or the cap isn't holding pressure, or maybe the water pump is wrong, etc. But I'd question the gauge first. It should be close to boiling over at 240F.
 
Yeah, this is likely a sending unit problem or gauge problem. If it's pegged out, that can also mean a local steam pocket, but that would not happen when the engine is cold, etc., so a gauge or sender problem is the likely culprit if it's pegged out regardless of operating temperature. Sending units are usually cheap, but they're also easy to test if you can find what their resistance profile is. You can test it most of the way in the kitchen with a pot of water on the stove and a multimeter. Same thing with the thermostat, pull it at the same time as the temp sender (when you've already got the coolant drained) and test the thermostat for proper function in a pot of water..

I never install a thermostat without testing it first, because I've had new OEM units fail right out of the package. I won't touch non-OEM ones with a 10-foot pole, unless I know the company is a direct OEM supplier (and then I still test them too). 5 minutes in the kitchen and you know it works.
 
Fabrik8 said:
Yeah, this is likely a sending unit problem or gauge problem. If it's pegged out, that can also mean a local steam pocket, but that would not happen when the engine is cold, etc., so a gauge or sender problem is the likely culprit if it's pegged out regardless of operating temperature. Sending units are usually cheap, but they're also easy to test if you can find what their resistance profile is. You can test it most of the way in the kitchen with a pot of water on the stove and a multimeter. Same thing with the thermostat, pull it at the same time as the temp sender (when you've already got the coolant drained) and test the thermostat for proper function in a pot of water..

I never install a thermostat without testing it first, because I've had new OEM units fail right out of the package. I won't touch non-OEM ones with a 10-foot pole, unless I know the company is a direct OEM supplier (and then I still test them too). 5 minutes in the kitchen and you know it works.
Click to expand...
Agreed, aftermarket thermostats are junk. You could always just take a test light and check the gauge/ect circuit. Much faster than checking the ect sensor

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mysterync said:
You could always just take a test light and check the gauge/ect circuit. Much faster than checking the ect sensor
Click to expand...

But you're not really testing the proper function of the sensor with a test light, you're just doing a binary test of whether it passes voltage through or not, which it likely will even if it's failed. A test light is only good for the most basic of troubleshooting, because it only tells you the most basic of things. That's like using a 12V light bulb to test whether a battery is charged or not. Lighting a bulb doesn't tell you anything about what the battery voltage actually is. I'm not trying to be a troubleshooting snob, it's just that a test light doesn't tell you enough for analog sensors.


Do Jeeps have a separate ECT and gauge sending unit? If they do have both, the ECT is not the issue. Many cars have both, with the ECT only going to the ECU and not the gauge.
 
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mysterync said:
I'd be checking for combustion gas in the coolant. Sounds like a blown head gasket. Is the cooling system pressurized? Is the coolant a muddy color with odd odor? Sent from my SM-N910V using Tapatalk
Click to expand...
The coolant is all normal in color and odor. I doubt it's the head gasket, but I guess its possible.
shawn said:
Also possible to have a bad tstat, or the cap isn't holding pressure, or maybe the water pump is wrong, etc. But I'd question the gauge first. It should be close to boiling over at 240F.
Click to expand...
I know the t-stat works, I tested it on the stove in water with a meat thermometer before I installed it. It opens at correct temp. I purchased a new cap, It seems to function properly. How do I test to be sure it is holding pressure?

Fabrik8 said:
Yeah, this is likely a sending unit problem or gauge problem. If it's pegged out, that can also mean a local steam pocket, but that would not happen when the engine is cold, etc., so a gauge or sender problem is the likely culprit if it's pegged out regardless of operating temperature. Sending units are usually cheap, but they're also easy to test if you can find what their resistance profile is. You can test it most of the way in the kitchen with a pot of water on the stove and a multimeter. Same thing with the thermostat, pull it at the same time as the temp sender (when you've already got the coolant drained) and test the thermostat for proper function in a pot of water..

I never install a thermostat without testing it first, because I've had new OEM units fail right out of the package. I won't touch non-OEM ones with a 10-foot pole, unless I know the company is a direct OEM supplier (and then I still test them too). 5 minutes in the kitchen and you know it works.
Click to expand...
Agreed, I always test them.
As for What I said with it being pegged, When I start the jeep, it seems to heat up fast, but does not jump to 260 in a couple seconds, it seems to take about 30 sec to reach it, but it stays there.

I am thinking that it has to be a gauge or sending unit like y'all are saying cause I have tested everything before install and after and installed all new parts... everything is functioning as it should and nothing is boiling over.
I don't see how it could be a head gasket, cause the coolant is the yellow green as it should be and there is not water in my oil. I will check compression to ensure this though.

This might be a dumb question, but the temp sensor that is on the back of the motor is the one for the gauge, correct? Also, mine only has one wire running to it instead of two and the plug only has one prong in it, where the replacements i've found have two...
 
The sensor for the gauge is the one towards the back of the motor. The sensor for the computer is on thermostat housing. At least that is how my 95 4.0 is. I cant remember for sure how many wires, but I believe it should only be one.
 
Loganwayne said:
Believe it's on the front on the thermostat housing if it is like mine.

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Click to expand...
I have two sensors, one on t-stat housing, one on the back same as this google image.
, Also, see this video. It is somewhat informative...
ai25.photobucket.com_albums_c61_Holder350_Tech_TempSenderOutlineIMG_9297.jpg
 
Thanks everyone. I appreciate it:beer:
 
Fabrik8 said:
But you're not really testing the proper function of the sensor with a test light, you're just doing a binary test of whether it passes voltage through or not, which it likely will even if it's failed. A test light is only good for the most basic of troubleshooting, because it only tells you the most basic of things. That's like using a 12V light bulb to test whether a battery is charged or not. Lighting a bulb doesn't tell you anything about what the battery voltage actually is. I'm not trying to be a troubleshooting snob, it's just that a test light doesn't tell you enough for analog sensors.


Do Jeeps have a separate ECT and gauge sending unit? If they do have both, the ECT is not the issue. Many cars have both, with the ECT only going to the ECU and not the gauge.
Click to expand...
Actually I can verify the circuit is in proper working order, and can verify the gauge or ecm sees a change. I don't think you understand the testing method I'm speaking of. So let's say we're working on a 97 4.0 grand Cherokee - the sensor is a pull down design - meaning that one pin is a sensor ground and the other pin is the ect signal. The ect signal is 5v high impedance feed from the ecm, as the resistance pulls the 5v to the respective value in regards to resistance. When this occurs the ecm basically has a dvom in the circuit and watches for this change and uses the map to determine the associated temp. By taking a test light (not a led, requires a incandescent bulb to carry the required current - mines roughly 35mA) to ground to the signal wire, I can max the value in one direction. When lacking the presence of bias voltage the same can be done with test light to positive. But in pull down designs the computer provides a bias voltage to generate sensor readings. The same testing methods can be used for gauges as well.
I'd suggest you do some research on scanner danner, he shows what I'm speaking of in much greater detail.

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mysterync said:
Actually I can verify the circuit is in proper working order, and can verify the gauge or ecm sees a change. I don't think you understand the testing method I'm speaking of. So let's say we're working on a 97 4.0 grand Cherokee - the sensor is a pull down design - meaning that one pin is a sensor ground and the other pin is the ect signal. The ect signal is 5v high impedance feed from the ecm, as the resistance pulls the 5v to the respective value in regards to resistance. When this occurs the ecm basically has a dvom in the circuit and watches for this change and uses the map to determine the associated temp. By taking a test light (not a led, requires a incandescent bulb to carry the required current - mines roughly 35mA) to ground to the signal wire, I can max the value in one direction. When lacking the presence of bias voltage the same can be done with test light to positive. But in pull down designs the computer provides a bias voltage to generate sensor readings. The same testing methods can be used for gauges as well.
I'd suggest you do some research on scanner danner, he shows what I'm speaking of in much greater detail.

Sent from my SM-N910V using Tapatalk
Click to expand...
Edit: here's a video that shows this method pretty well, it should be noted that if you don't have a wiring diagram (or don't understand DC electronics), you should always use a test light to prevent damage!
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mysterync said:
Actually I can verify the circuit is in proper working order, and can verify the gauge or ecm sees a change. I don't think you understand the testing method I'm speaking of. So let's say we're working on a 97 4.0 grand Cherokee - the sensor is a pull down design - meaning that one pin is a sensor ground and the other pin is the ect signal. The ect signal is 5v high impedance feed from the ecm, as the resistance pulls the 5v to the respective value in regards to resistance. When this occurs the ecm basically has a dvom in the circuit and watches for this change and uses the map to determine the associated temp. By taking a test light (not a led, requires a incandescent bulb to carry the required current - mines roughly 35mA) to ground to the signal wire, I can max the value in one direction. When lacking the presence of bias voltage the same can be done with test light to positive. But in pull down designs the computer provides a bias voltage to generate sensor readings. The same testing methods can be used for gauges as well.
I'd suggest you do some research on scanner danner, he shows what I'm speaking of in much greater detail.
Click to expand...

I know what you're talking about (I worked for the ecu and controls systems division of two companies in the past) but using a multimeter will tell you so much more. In the case of an analog pulldown sensor, you can directly ground the output to signal ground, which is really what you're doing with a test light. A light bulb has fairly low resistance and the 5v signal output is protected with a resistor to the 5v ecu supply rail to protect the ecu and the analog to digital converter input.

Basically almost everything you're talking about can be done with a piece of wire because everything is protected, and a multimeter to see what's actually happening.

I do understand your point, but I don't know why you'd want to forego a multimeter for a test light.
 
Temp gun FTW.

They're like $10 on Amazon. Everybody should have one. Shoot the top tank on the radiator, confirm that it's 210F, go wheeling.
 
Van-go said:
The coolant is all normal in color and odor. I doubt it's the head gasket, but I guess its possible.

I know the t-stat works, I tested it on the stove in water with a meat thermometer before I installed it. It opens at correct temp. I purchased a new cap, It seems to function properly. How do I test to be sure it is holding pressure?

Agreed, I always test them.
As for What I said with it being pegged, When I start the jeep, it seems to heat up fast, but does not jump to 260 in a couple seconds, it seems to take about 30 sec to reach it, but it stays there.

I am thinking that it has to be a gauge or sending unit like y'all are saying cause I have tested everything before install and after and installed all new parts... everything is functioning as it should and nothing is boiling over.
I don't see how it could be a head gasket, cause the coolant is the yellow green as it should be and there is not water in my oil. I will check compression to ensure this though.

This might be a dumb question, but the temp sensor that is on the back of the motor is the one for the gauge, correct? Also, mine only has one wire running to it instead of two and the plug only has one prong in it, where the replacements i've found have two...
Click to expand...
I've experienced many failures where combustion gasses enter the cooling system with no notable loss of compression. The only way to detect this failure is with a 5 gas or a leak down test. There are "block testers" available which detect combustion byproducts in the cooling system but they are rarely effective. I assumed you were experiencing additional symptoms of overheating. If not most definitely suggest manually manipulating the gauge circuit to determine if the sensor has failed. It's is odd, because alldata shows the vehicle in question should have a single sensor system, the signal is processed by the pcm and then sent via CCD to the cluster.

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Fabrik8 said:
I know what you're talking about (I worked for the ecu and controls systems division of two companies in the past) but using a multimeter will tell you so much more. In the case of an analog pulldown sensor, you can directly ground the output to signal ground, which is really what you're doing with a test light. A light bulb has fairly low resistance and the 5v signal output is protected with a resistor to the 5v ecu supply rail to protect the ecu and the analog to digital converter input.

Basically almost everything you're talking about can be done with a piece of wire because everything is protected, and a multimeter to see what's actually happening.

I do understand your point, but I don't know why you'd want to forego a multimeter for a test light.
Click to expand...
Because when dealing with someone who may not have a wiring diagram or could possibly unplug a output it's a whole lot less likely they fry a transistor. And a multimeter is pointless in testing like this, you know it, I know it. A DSO, yes absolutely we can detect minor glitches or abnormalities due to the faster sampling rate - a multimeter we can only verify the the module is sending voltage in this case (figured I'd correct myself before you had a chance - we obviously can check the resistance of the sensor and determine if it's close to its map/chart or if it's open). Otherwise that's pretty pointless wouldn't you say? As a shop owner who's required to diagnose issues such as this quickly, why wouldn't I use a test light or jumper wire to complete a circuit test in 30 seconds? I've verified the vehicle side functions as designed and we have a sensor issue or vice versa, the voltage reading provides no additional data for this procedure in its infancy. Fast is the name of the game! No offense but you'd argue with a fence post lol.

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Key on, engine off- what does the gauge do?
With the key on, disconnect the sender connector (driver's side rear of head) and run the wire to ground. This should peg the gauge full hot.
Report back so we can tell if you have a gauge issue or an engine issue
 
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