That ability was missing in 98, 99, and early 00 models but it's in the other years. It has to do with the PCM (engine computer) that performs the diagnostics for other years.
Coolant Temperature Below 210
- Thread starter Bowhunter
- Start date
If your engine ran hotter with the old thermostat it was solely because it wasn't operating correctly... in other words it must not have been opening all the way. The thermostat's two jobs are 1) to help the engine warm up faster and 2) to set its minimum operating temperature in cold conditions. It definitely has nothing to do with how hot an engine can or will run in warm or hot conditions.
In warm conditions installing a lower temperature thermostat will NOT lower the engine's operating temperature. If that was the case we could get along with smaller radiators by just installing a lower temperature thermostat... which would not help. In hot conditions a 165, 180, and 195 degree thermostat are all wide open and switching between them will not make any difference in the engine temperature at all. None whatsoever. When can a lower temperature thermostat cause an engine to run cooler? In cold conditions, but not in warm or hot conditions.
The factory specified thermostat temperature for the TJ is 195 degrees and its computer is programmed around that temperature. In cold conditions the engine would not warm up as fast or run as warm as you'd like to get the heater working if you installed a lower than specified temperature thermostat.
I think that only works for 01-06 models when they switched to PCI bus. My 02 did it but my 00 does not.
*EDIT* I missed where you said it excluded early models, my bad
I have been using a 180 degree thermostat for over a year. The studies I have seen there is little if any difference in wear between 180 and 210. Below 180 wear goes up pretty fast. I would like to see a test with higher intake temps. Things I have read point towards poor vaporization being the cause of wear by washing oil off of the cylinders. Our Jeep's design with the intake manifold very close to the exhaust heat the air more than other designs. By heating the air, fuel vaporizes faster and more completely.
Why? Do you believe it affects how warm/hot your Jeep runs at in warm/hot weather? It does not.
Looks identical to mine (first post pic). I bet the PO put in a 180 stat in mine. My scanner gets here tomorrow so I will test it out and see if it’s a lower thermo or an inaccurate dash gauge
If I saw that needle position on mine where I didn't before, I would consider it abnormal and not wait for the PCM to throw a code.
To follow up, I received my scan tool and took the Jeep out for a 15 miles trip.
At first it was city driving for a few miles down hill at 35mph and the Jeep stayed around 192 degrees
Then back the same way but up hill and it was reading 195
Then hit the highway at 55/60mph and the temp increases to 208
Then a mile of dirt road at 10mph and temp went down to 200
Then a few miles back on the highway at 55/60 and the temp got up to 215
Then parked inside my garage and let it idle for a few minutes and the temp pulled down 201 degrees (pic is where the gauge sat at 201 degrees)
*edit* also just notice in the picture that my oil pressure is fairly low...
At first it was city driving for a few miles down hill at 35mph and the Jeep stayed around 192 degrees
Then back the same way but up hill and it was reading 195
Then hit the highway at 55/60mph and the temp increases to 208
Then a mile of dirt road at 10mph and temp went down to 200
Then a few miles back on the highway at 55/60 and the temp got up to 215
Then parked inside my garage and let it idle for a few minutes and the temp pulled down 201 degrees (pic is where the gauge sat at 201 degrees)
*edit* also just notice in the picture that my oil pressure is fairly low...
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On a side note.... now the Jeep won’t start.
It’s been have some issues starting when it is cold or has been sitting more than 10 minute but I got by the last week or so by turning the key on and off a few times. Never had an issue starting when it was warm.
I’ve tried several times turning the key on and off to let the fuel pressure build but it will just crank and crank without start
97’ 4.0 with the 3 speed auto. No check engine lights and nothing seems to be abnormal
It’s been have some issues starting when it is cold or has been sitting more than 10 minute but I got by the last week or so by turning the key on and off a few times. Never had an issue starting when it was warm.
I’ve tried several times turning the key on and off to let the fuel pressure build but it will just crank and crank without start
97’ 4.0 with the 3 speed auto. No check engine lights and nothing seems to be abnormal
Sometimes a broken ignition switch will do that. When you are cranking the engine over is the instrument cluster working? It'll often just catch as you release the key when that is the problem as well.
So I just went back out after letting it cool in the garage for 20 or so more minutes (temp was around 150 on the gauge) and turned the key on and off 8 times to prime the fuel pump. Then turned it over and after about 1-Mississippi second it started. Gauges all worked while the Jeep was cranking. Let it idle for a few min and it ran normal.
Then turned it off and let it sit for 30 second . Key on and off two times and then turned it over and it started right up. Did this twice and it started both times with little to no issues
I texted the PO and he said he has replaced the plugs, wires, distributor, starter and fuel pump all within the last 500 miles. Says the only thing he didn’t replace on the ignition system was the ignition control module
@Jerry Bransford recently described the fuel bleed down and which tests you should do, I hope he comments if I screw it up. If it bleeds down in a few minutes like when you go into a store that could be the valve in the fuel regulator on top of the tank and if it bleeds down overnight that could be the one in the pump that's in the tank. Either way a fuel pressure gauge would help you check those things.
Until today I used to only have the over night start issue. Start up wasn’t too difficult if I primed the pump by turning the key off and on a few times.
Now it’s seems that I’m starting to have issues if I try to start it after turning it off and running into store. But if I turn it off and then instantly turn it back on, it will fire up no problem.
I’m reading through some of Jerry’s other threads on staring issue topics and I’m thinking I may need to try to replace the fuel pump with a Bosh (PO likely replaced it with a Cheap autozone pump 500 miles ago)
A simple test you can do to confirm, when the jeep is cold take a fuel line disconnect tool and remove the fuel line at the fuel rail connection. You should have liquid from the fuel line end if you hold it pointed down when disconnected, if not then the check valve is not doing it's job of retaining the liquid and it running back to the tank. Having the fuel line full is what really matters.
Pagrey had it right above. No starts shortly after shutting it of, like after getting gas, usually means the fuel pressure regulator that sits outside the tank on top of the fuel pump assembly has gone bad. Slow starts after it has been sitting like over night normally means the check valve inside the fuel pump itself has gone bad.
Sounds like both of yours have gone TU.
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