cooling ?

I have a 90 4.0 and i have a cooling questions for you guys.

I just changed the radiator to an aluminum one and changed the thermostat to a 185. I noticed the other day when i was driving that my temp would only get up to about 165 or 170 and no more then that when i am driving. It will get up to about 190 when i am stopped and then the thermostat will open and cool it back down. befroe i change the thermostat it would be at 190 on the road.

Is it normal to be running that cool when i am driving?

What temps are you guys running at while driving?

You should have just used a normal 195 degree thermostat... you probably are getting lousy gas mileage with how it is now, and may not pass smog as a result.

Normally I run at about 210 unless it's cold and I'm traveling quickly while lightly loaded.

poomero said:

What temps are you guys running at while driving?

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200-210F
Hot day/cold day.

A 180 t-stat will NOT control max temps, that's not how they work. They control the MINIMUM temp at which your engine begins to receive coolant.

X2 with Kastein, you'll run too rich et al.

yeah gas mileage isnt that great right now with it. i will change it back to a 195 on thursday.

why does it make the gas mileage shit?

The ECU doesn't enter "closed loop" (fuel injection pulse duration calculated off sensor readings) mode until it sees the engine get to 170 degrees or so. So if your engine remains cooler than that, or in that range, the ECU will simply continue hosing your cylinders down with as much gas as it thinks it will ever need to get the engine running. If you smell your exhaust you'll be able to tell. This can probably also reduce the lifespan of your catalytic converter.

wow, thanks for the info. i am going to change it for sure.

I suspect something wrong with your temp gauge. The XJ cooling system is not big enough to run much less than 200F this time of year. What are your outside temps? We run a 180F t-stat winter/summer in our '98 XJ and gas mileage/performance is fine. Ours will run 180 in the winter but again, not this time of year. Does your gauge look something like this.

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no mine is the older style gauge. the outside temps are maybe 80 but no hotter right now. When i had the stock thermostat in it would run at 200ish and i just changed it and now its not that hot at all. i cant even get it that hot if i just let it sit in the driveway.

WOW! It's 102F here as I post.

poomero said:

I have a 90 4.0 and i have a cooling questions for you guys.

I just changed the radiator to an aluminum one and changed the thermostat to a 185. I noticed the other day when i was driving that my temp would only get up to about 165 or 170 and no more then that when i am driving. It will get up to about 190 when i am stopped and then the thermostat will open and cool it back down. befroe i change the thermostat it would be at 190 on the road.

1/... Is it normal to be running that cool when i am driving?

2/... What temps are you guys running at while driving?

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Congratulations .... arty:
You must have the worlds best XJ cooling system ...


1/... Its typical for a "stock" XJ to run at 200 - 210 with a factory style operation thermostat and cooling components.

2/... 195'f ... at ambient temps of 40' through to 110' ... on & offroad ... with a proper 195' thermostat ... and a few other minor changes.

Your vehicle ????

a)... Auto with in-tank radiator heat exchanger for the gearbox ?
b)... Aircon condenser fitted in front of the radiator ?
c)... Fan system still stock ?

This new aluminium radiator ????

A piddly little stock type, factory replacement - or some "special" heavy duty unit ?

kastein said:

You should have just used a normal 195 degree thermostat...

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X 2 ...... Until you found what temps you were running at with the new radiator .... and then change the thermostat if required for your driving conditions.

kastein said:

The ECU doesn't enter "closed loop" (fuel injection pulse duration calculated off sensor readings) mode until it sees the engine get to 170 degrees or so. So if your engine remains cooler than that, or in that range, the ECU will simply continue hosing your cylinders down with as much gas as it thinks it will ever need to get the engine running. If you smell your exhaust you'll be able to tell. This can probably also reduce the lifespan of your catalytic converter.

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Whilst I agree that 170' is a fairly minimum temp suitable for a general purpose engine to operate at ...

... how does this washing of the cylinder walls work without causing flooding symptoms with the engine ???

All those extra gallons dont cause fouling of the plugs or anything ???

Apart from START & WOT .... Isnt open loop the "WARM UP" cycle ??? ... any engine is "warmed up" well before 170' I would think.

In fact the fault code for a "too cold" engine relies on a temp of about 20'f I believe .... ???

Is there any verifiable tech data to show that OBDI systems dont change from open to close loop until 170' ....

.... something like a chrysler tech bulletin ... or pic similiar to ... an OBDII scangauge showing OBDII closed loop at 65'f would be much appreciated ..... :cheers:

This pic shows "closed loop" on our '98 OBD-II XJ at 94F coolant temp, when I snapped the pic. It went into "closed loop" well before 94F.


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djb383 said:

This pic shows "closed loop" on our '98 OBD-II XJ at 94F coolant temp, when I snapped the pic. It went into "closed loop" well before 94F.

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Yea thanks djb383 ..... By "similiar to" ....... I meant some OBDI scan gauge or computer printout info ...

......... As we OBDIIers have it easy, disproving the ..... must have 195' thermostats ... and XJs must run hot ..... folklore

I've got plenty of OBDII gauge pics showing 60'-70' loop change .... and I watch it happen every day ....

What I'm interested in .... Is why the same vehicle - the XJ ... with the same 4.0l i6 engine, has two totally different engine "warm up" temperature requirements between year models ....

carves said:

Yea thanks djb383 ..... By "similiar to" ....... I meant some OBDI scan gauge or computer printout info ...

......... As we OBDIIers have it easy, disproving the ..... must have 195' thermostats ... and XJs must run hot ..... folklore

I've got plenty of OBDII gauge pics showing 60'-70' loop change .... and I watch it happen every day ....

What I'm interested in .... Is why the same vehicle - the XJ ... with the same 4.0l i6 engine, has two totally different engine "warm up" temperature requirements between year models ....

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dont know the answer, but the original poster has 90 xj which is the renix(87-90) fuel injection and is pre obd 1.

89xj said:

dont know the answer, but the original poster has 90 xj which is the renix(87-90) fuel injection and is pre obd 1.

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Cheers 89xj .... Keep forgetting about the Renix ... We never saw them down here.

That would make three different "warm up" temps for the same engine then ... as there are posts here on that state the Renix system has a lower temp requirement than the OBDI.

Tis an interesting little puzzle ....

ok so this is what i have done so far to the cooling system.

new radiator. Stock style aluminum.
new water pump.
new hoses.
new thermostat housing.
new fan clutch.
new thermostat

so basically a whole new system. I did the water pump and the radiator at the same time and i changed the thermostat to a 195 when i did all of this. When i did this the xj was running at 200ish under any driving conditions.

I changed the thermostat yesterday because i had a leak in the housing and i bought a 180 temp thermostat and just replaced the whole thing housing and all.

89xj your right it is a renix and i should have mentioned that and it is pre obd-1

sounds like i should just throw a 195 back in and see what happens. if it goes back to normal then i will be good.

If i am pre obd-1 will it matter if i run that cool?

poomero said:

ok so this is what i have done so far to the cooling system.

new radiator. Stock style aluminum.
new water pump.
new hoses.
new thermostat housing.
new fan clutch.
new thermostat

so basically a whole new system. I did the water pump and the radiator at the same time and i changed the thermostat to a 195 when i did all of this. When i did this the xj was running at 200ish under any driving conditions.

I changed the thermostat yesterday because i had a leak in the housing and i bought a 180 temp thermostat and just replaced the whole thing housing and all.

89xj your right it is a renix and i should have mentioned that and it is pre obd-1

sounds like i should just throw a 195 back in and see what happens. if it goes back to normal then i will be good.

If i am pre obd-1 will it matter if i run that cool?

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Amazing what a difference a new fanclutch AND radiator will make ....

I suspect that the new radiator may be of better cooling quality e.g. tube sizes etc ... as I agree with djb383 that in high temp areas - the stock type is highly unlikely to cool to the temps you originally mentioned ...

Mine never did with a 180 thermostat .... running temp of 190 with 210 under load/hills ... but never 170 unless in 40' temps at night.

Since you now have an effective cooling system ... the 195' heating device (thermostat) would seem most appropriate ... as a 200' constant temp is good IMHO.

Running temp of 190 - 210 ... and you wont notice any fuel use issues ... unless you measure your fuel use with a teaspoon.

cool thanks for all the info guys. i will put it back to stock and let you know how it does. i am going to pismo this weekend and want to dial everything in for that trip.

The computer has several ways to see if the engine is cold (coolant temp is one, O2 is another). However all it really cares about is if the catalytic converter is hot enough to do its magic, so it pretty much does the same thing for all of the sensors that report "cold" temperatures, which is to dump fuel so that post-ignition combustion heats up the cat converter. Check the FSM if you want more info, there is a lot of information on the engine management in the fuel injection chapter.

ok so i changed the lower hose because i noticed a small leak in it and i changed the thermostat back to stock and it runs great now. we hit up pismo all weekend and it didnt get hot at all. it was about 110 out side on the way home and we hit some trafifc and we were still good. thanks for the help with this guys.

carves said:

Yea thanks djb383 ..... By "similiar to" ....... I meant some OBDI scan gauge or computer printout info ...

......... As we OBDIIers have it easy, disproving the ..... must have 195' thermostats ... and XJs must run hot ..... folklore

I've got plenty of OBDII gauge pics showing 60'-70' loop change .... and I watch it happen every day ....

What I'm interested in .... Is why the same vehicle - the XJ ... with the same 4.0l i6 engine, has two totally different engine "warm up" temperature requirements between year models ....

Click to expand...

OBD modles define open and closed loop differently, and it's less dependent on coolant temps, and more dependent on O2 sensor temps/readings.

Also in a Renix the ECU gets' it's temperature from the block sensor, not the one that runs the gauge in the head.

trust me, it needs to warm up properly to enter clsed loop, Renix is not anything like OBD

damnit, I don't have my renix PDF on my flash drive.

I'll post up the criteria for the Renix ECU to enter closed loop when I get home.

Also, even though the motor enters closed loop, it needs to run at 195-210 degrees for proper emissions, and to burn off the contaminants in the oil/block. Running the motor too cold will cause some nasty things to build up in the block, as it's not getting hot enough to evaporate moisture/combustion by products.