Coolant Temp Sensor

So awhile back I was noticing my temps lightly increasing up to 225-230 degrees in normal city driving......maybe normal and mixed "heavy on the pedal" driving and such. I noticed my eFan wasn't coming on. At 225-230 degrees, the fan should have come on much earlier.....

So I was a lso getting a soft code telling me the engine was "cool too long - replace coolant temp sensor."

My guess is that the coolant temp sensor is fubar......thinking the engine is too cool and making it warm up when it doesn't need to be. That would also explain why the eFan wasn't coming on because it wasn't aware it was that warm.

This is a 95 XJ so the temp gauge sender is seperate from the coo lant temp sender.

Would the heating up of the engine sound like a symptom of a FUBAR coolant temp sensor?

If I stopped the temperature would just stay where it was - around the 225-230 mark, as well.

which temp sender did you replace? I'm not sure but since the o2 sensor controls the amount of fuel (adding more in the warm-up) it might be a bad o2. If you have a bad injector stuck open in #6 it could cause the temp sended area in the rear of the head to be hotter.

hot during city driving and in traffic may mean its time for a new fan clutch or your rad is stopped up. Also if you don't have a fan shroud get one (only about $30 from the stealership).

sorry.....double post

I didn't replace any temp sensors......

Fan clutch is new as of last October, radiator is new as of last July, fan shroud is new as of this last October as well. o2 is new too. Good point about the #6 injector......but how about the engine code I was getting for "cold too long - replace coo lant temp sensor?"

Got that pic with my phone today. Driving around, normally..temp will rise to this. eFan is not coming on (it works with A/C, though) so that is what also fuels my suspicion to believe that the temp sensor is slightly DOA.

I have 2 used sensors in my "rubbermaid container O' Jeep parts" which I will try before buying anything brand new

Ok, another update. Swapped out the coolant temp sensor with one of the used ones I had lying around. Everything was great as I drove around. So I said, what the hell - and floored it for a few seconds.

Well after that the temp rose to 225....and then it even went into the red-zone, so I shut it down, let it cool off some, then drove back home. No coolant was boiling, FYI......also the electric fan was on, so I guess my old sensor was partially DOA to say the least. I drove home the temp stayed around 225-230 the rest of the way home. When I was restarting it it seemed to be sort of funny - seemed like the starter was slower than usual and I had to put the pedal down all of the way (I know that turns off the injectors - I don't know how it could be flooded though since I was just driving around?)....any ideas on that?

Any other ideas on what's causing this? I'll try some injector cleaner in the next tank of gas......like I said water pump, thermostat, thermostat housing, oxygen sensor, radiator, fan shroud, fan clutch, upper + lower radiator hoses are all new.....also coolant is new and I entirely flushed the block when I did the water pump. I indirectly flushed the heater core as well, not directly though - I didn't want to take a chance with it leaking.

Any ideas?

It sounds like you have a flow problem. You should check the tightness of your belt and replace the water pump and you might as well replace the thermostat while your at it. My trail rig's e-fan did not want to come on and I ran a manual switch to it...you might want to try that as well.

Did you see I already replaced the water pump and thermostat last fall?

The fan is coming on now that I replaced sensors.

I recently bought a $20 IR temperature gauge (mini gun) at Harbour tools, on sale for $10. It allowed me the check the coolant temperatures all along the block, the thermo housing, rad hoses and the radiator inlet and outlet. I found out that the electric fan comes on at a radiator exit temperature of 185 F (the engine thermostat housing exterior was like 202 F) and the electric fan turned off when the raidator exit temperature reached 170 F! So the gauge temperature and the thermo housing sensor that feeds temperature data to the computer is independent of the electric fan temperature switch. You might want to buy one of these gauges and check the electric fan temperature switch on and off actuation temperatures, and verify that it is repeatable and correct. Mine is failry new, about 12 months old, and it cycles on at 185 F and off at 170 F which seems to be right, and makes sense since the water must be cool enough going into the engine block to do some cooling in order to hold say a 195 to 210 temp in the block.

Also, I suspect that there are different flow rates (equivalent hole size openings) in the various thermostats that we never hear about, that affect the flow rate through the thermostat from one brand to another that may be the other part of the missing performance (thermal control) puzzle.

As many have said before, there may be good reason to stick with certain OEM parts like thermostats. Also a thermostat can open partially and get stuck and then later unstuck, partly open, free itself later, and basically present a varying flow rate if something gets stuck in the one of those tight spaces inside of its travel area.

Jess said:

....If you have a bad injector stuck open in #6 it could cause the temp sended area in the rear of the head to be hotter..

Click to expand...

Actually, a richer air/fuel mix will burn cooler than a lean one.

Kyung

Blaine B. said:

Did you see I already replaced the water pump and thermostat last fall?

The fan is coming on now that I replaced sensors.

Click to expand...

Did you replace the sender for the gauge? They've been known to go bad and give false (high) readings.

Kyung

No, not yet.

Although my temp must have been elevated as the eFan was on as well......

Maybe the increased flow of the coolant past the gauge sender causes it to fubar? Because the last time this happened (this evening) was after a wide open throttle experience That'd be great if that was the problem!

Maybe I'll head to harbor freight for a lazer thermometer too. Is there anyway to use a rectal thermometer temporarily? lol

Good point on the fuel as well...

Although isn't warm up mode actually a rich fuel mixture? To get it up to temperature quicker?

Get one from Sears - costs a touch more than HF, but calibration is likely better. I've got one - fits in the palm of my hand. As I recall, it also takes a single battery - but it's out in the shop, and I've not had to change it recently anyhow (got it a year and a half ago, and the original battery is still in it.)

Flow rate should not have anything to do with the "wear" on the sensor, and there are no internal moving parts. However, it will lose response over time, which is why it wants changing from time to time.

Warmup mode is, as I recall, a rich mix - because the rich mix runs relatively cooler, and is easier to ignite. It doesn't run anywhere near "rich burn limit" - I think it's only something like 14.0-14.2:1 (stoich being 14.7:1, higher numbers being leaner.) WOT will run a "best power" mix of about 13.5:1, and cruise runs a little lean at about 15.0-15.2:1, as I recall (I really should look this stuff up, I keep too much in my hat.)

Running rich will reduce combustion chamber temperature slightly, while running lean will increase it somewhat (the increase runs up faster than the decrease runs down, for similar variations from the "ideal.")

Curious, you said you got a code reading. Did you use a reader, or did you get an engine light? I'm having similiar issues with high heat after my engine change.

I have OBD 1. I didn't use a code reader. The code I got was a "soft code" meaning it won't trigger a check engine light, but will show up if you manually check for codes. I check codes by doing the "on-off-on-off-on" key trick every so often, and this is how I discovered it.

I believe my temp gauge is accurately showing my temperature, at least.....the eFan I believe comes on somewhere in the 216-218 degree range......My gauge was showing somewhere in that region when I heard the eFan kick on this evening.