94 zj not getting up to temp

JSJJ388

JSJJ388

GREEN GREMLIN
Joined
Aug 31, 2016
Location
HAMPTONVILLE, NORTH CAROLINA
So after my motor swap in my 94 zj I noticed it never seemed to get up to temp. Ran fine, so I figured it was just how that motor ran. Was riding around in the snow yesterday and got a check engine light. One for low alternator output and one for the engine not getting up to temp. Low alt output is because I need a new belt. What would cause the engine to not heat up? Were talking like its running at 100-120. Motor is a 4.0 that has been slightly bored and has an unknown cam. Also started having a problem where after running on the hwy for 15 mins it starts skipping and bogging down. Only when doing 65+, doesnt do it when riding the backroads. Problems connected?
 
Are there separate coolant temp sensors for the temp gauge and the ECU? The ECU is obviously detecting the engine is too cold if it's throwing a code for that, so are you seeing the same thing on the gauge if there are separate sensors? It's pretty easy to check a temp sensor/sender for proper function with a multimeter.

If that's the case, it sounds like the thermostat is stuck open (or is totally missing?). Being overcooled is a pretty short list of suspects, assuming of course that's what is happening instead of just getting false sensor readings.

If the coolant temp sensor for the ECU is bad or mis-connected, and it's actually coming up to temp but the ECU is getting fooled into thinking it's still cold, it will be over fueled and can run badly. It can also change the transition point from open loop to closed loop O2 sensor operation.

I'm not a Jeep expert; those are just my thoughts based on standard-ish ECU operation.
 
Last edited:
Fabrik8 said:
Are there separate coolant temp sensors for the temp gauge and the ECU? The ECU is obviously detecting the engine is too cold if it's throwing a code for that, so are you seeing the same thing on the gauge if there are separate sensors? It's pretty easy to check a temp sensor/sender for proper function with a multimeter.

If that's the case, it sounds like the thermostat is stuck open (or is totally missing?). Being overcooled is a pretty short list of suspects, assuming of course that's what is happening instead of just getting false sensor readings.
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two separate sensors. I have an after market gauge and am reading temps between 100 and 120. I will check the tstat, but when I ran my old motor without a tstat temporarily it still hit 160, just took a while. I ran it for almost 3 hours yesterday and didnt hit 120
 
Okay, so you've got two sensors (that are likely independently wired) that seem to agree. So it would need to be something coolant system related that isn't regulating properly.
Its a pretty short list. Wrong water pump, wrong pulley, along with no thermostat?
Radiator isn't part of that list problem unless its oversized and you're also missing a thermostat, etc.

Fan (constantly on) doesn't matter, because you get a lot more airflow on the highway than what the fan can provide.
 
Thermostat stuck WIDE open
 
Yep, happened to my 4.0, t-stat was bad and when they fail they (thank goodness) default to staying open.
 
Wouldn't a belt that is not spinning the alternator also affect the water pump? but still it has got to be thermostat related.
 
R Q said:
Wouldn't a belt that is not spinning the alternator also affect the water pump? but still it has got to be thermostat related.
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It would, but the belt is a new issue, hasn't gotten up to temp in months.


Ill get a new tstat. Could the temp be my hwy issue?

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Fabrik8 said:
Possibly; see my first post about overfueling or something of that nature. That could be related to your highway problem.
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Ok, so Ill get a new tstat and see about a new ECU coolant temp sensor. thanks for the help yall
 
Fabrik8 said:
I don't think the coolant temp sensor is the problem yet, if your aftermarket gauge is agreeing with the low temp code that the ECU is throwing.
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ok.
 
So when you run your heat in this cold weather it never gets hot, right? When replacing the thermostat be sure to get all the old gasket off.
 
Tradarcher said:
So when you run your heat in this cold weather it never gets hot, right? When replacing the thermostat be sure to get all the old gasket off.
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Nope. Never got hot. I fixed it though
6d3c814c6595a10e91ccdb69e5b6272e.jpg


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