engine runs too cool-expert advice needed
CF Veteran
Joined: Nov 2009
Posts: 8,172
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From: The Republic of TEXAS
Year: 1998
Model: Cherokee
Engine: 4.0L HO
The "Temperture Reference" chart in the FSM says minimum (heater) temp at floor outlet should be 144F at 60F ambient. That's presuming motor is at normal operating temp. If your vent temp (heat) is 143F, then it sounds like the heater is working fine and the motor is at actual normal operating temp BUT the temp gauge is incorrect at 150.
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From: Prescott, Az
Year: 1990
Model: Cherokee (XJ)
Engine: 4.0
no i did not. i had it in my head that the odds of getting a bad stat are low. surely i wouldn't get 3 bad ones in a row! i should have tested them before i chunked them. i did another test when i got home tonight. i let it idle till it quit gaining temp on the dash gauge. the gauge planed at about 150 (this is after 20 minutes of mixed driving and 10 minutes of idling). i then turned the heat all the way up, the blower on high and blowing out of the vents. i stuck a pen thermometer in the vent and it planed out at 143. that is using fresh air not recirculated. i think this is another confirmation that the dash temp gauge is telling the truth.
i am not worried that the tstat isn't opening. the exact opposite is what i am thinking (if the tstat is the culprit). i think it is opening at a very low temp. the stats i have been through have been in the closed position when i removed them. if they were stuck closed i would have the opposite problem, overheating.
the map sensor theory sounds promising. i have decided to take it to the dealer. i read on another forum last night that performance cooling additives can cause thermostats to act funny. the PO may have used some of this. i am planning on having the dealer flush the cooling system and replace the thermostat with one of theirs. i will have them check the MAP system connections if i haven't done it already. that's the kind of insight i am looking for. all of my mechanical experience is with non computer diesel and gas engines, so when it comes to o2 sensors, map sensors, bad injectors, etc i am pretty much lost.
thanks again!
i am not worried that the tstat isn't opening. the exact opposite is what i am thinking (if the tstat is the culprit). i think it is opening at a very low temp. the stats i have been through have been in the closed position when i removed them. if they were stuck closed i would have the opposite problem, overheating.
the map sensor theory sounds promising. i have decided to take it to the dealer. i read on another forum last night that performance cooling additives can cause thermostats to act funny. the PO may have used some of this. i am planning on having the dealer flush the cooling system and replace the thermostat with one of theirs. i will have them check the MAP system connections if i haven't done it already. that's the kind of insight i am looking for. all of my mechanical experience is with non computer diesel and gas engines, so when it comes to o2 sensors, map sensors, bad injectors, etc i am pretty much lost.
thanks again!
CF Veteran
Joined: Nov 2009
Posts: 8,172
Likes: 17
From: The Republic of TEXAS
Year: 1998
Model: Cherokee
Engine: 4.0L HO
......but wouldn't u agree that if the OP's heater vent temp is 143 and the FSM says 144 at 60 ambient, that the actual coolant temp is most likely +/-195 or normal?
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Joined: Aug 2011
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From: Prescott, Az
Year: 1990
Model: Cherokee (XJ)
Engine: 4.0
But, that is a "minimum" figure. Think there would be a 50* drop from the coolant temp to air flowing through the heater core?
CF Veteran
Joined: Nov 2009
Posts: 8,172
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From: The Republic of TEXAS
Year: 1998
Model: Cherokee
Engine: 4.0L HO
Just stating the FSM says "minimum air temp at floor outlet" is 144F at 60F ambient and motor "idling at normal operating temp". Presuming normal operating temp is +/-195 and the OP is getting 143 (like the book says) then his actual coolant temp is closer to 195, not 150 or less. Looks like there could be a 50* degree diff in coolant temp and vent temp.
Last edited by djb383; Oct 22, 2012 at 09:38 PM.
Thread Starter
Member
Joined: Jun 2011
Posts: 199
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Year: 2005
Model: Grand Cherokee
Engine: 4.8L
i see where you are coming from. keep it simple stupid is usually a good motto. the fact that i am concerned with is the oil pressure coincides with the temp. i have been around enough vehicles to know that oil pressure will be on the high side until normal operating temp is reached. but i have never (until now) had my oil pressure decrease 20 psi while waiting in line at mcdonalds after driving twenty minutes to get there in the first place. when i sit still and idle temp goes up to 150 and pressure goes down to 40. when i take off and drive down the road the temp fall to 130 until i stop. when i stop and idle the pressure reads 60+ and falls as the temp rises. i have verified the temp with an infrared thermometer. i have not verified the the infrared thermometer's calibration, although i have shot everything in sight with it and it always reads the temp you expect it should. it is a really fun device.
this is an old vehicle. all sorts of sensors and vacuum lines could be deteriorating. another symptom i did not mention is that it idles high. it doesn't have that small rpm drop after it runs for a bit.
if it is impossible for over-fueling to decrease my running temp to the extremes i am seeing please tell me now. before i throw money in that direction.
this is an old vehicle. all sorts of sensors and vacuum lines could be deteriorating. another symptom i did not mention is that it idles high. it doesn't have that small rpm drop after it runs for a bit.
if it is impossible for over-fueling to decrease my running temp to the extremes i am seeing please tell me now. before i throw money in that direction.
Herp Derp Jerp
Joined: Nov 2011
Posts: 18,251
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From: Parham, ON
Year: 1999
Model: Cherokee
Engine: 4.0L OBD-II
Just stating the FSM says "minimum air temp at floor outlet" is 144F at 60F ambient and motor "idling at normal operating temp". Presuming normal operating temp is +/-195 and the OP is getting 143 (like the book says) then his actual coolant temp is closer to 195, not 150 or less. Looks like there could be a 50* degree diff in coolant temp and vent temp.
It also sounds like a fuelling issue to me, although with such exaggerated symptoms I can't help but think something else is going on. Regardless fuel consumption is horrible, and it can cause cold running, so I'd start there.
CF Veteran
Joined: Nov 2009
Posts: 8,172
Likes: 17
From: The Republic of TEXAS
Year: 1998
Model: Cherokee
Engine: 4.0L HO
Let's just "suppose" for a moment and say the OP is taking his XJ to the dealership for a heater performance evaluation (even though the OP has already said heater works well). The dealership service dept measures heater vent temp and gets a reading of 143F (same as the OP) and tells the OP, "the heater vent output temp of 143F is within the FSM spec chart".
I can easily see vent temp being 50* less than coolant temp....so does the Factory.
One must presume the vent temps stated in the chart are also presuming +195 actual coolant temp......not 130-150 actual coolant temp.
I can easily see vent temp being 50* less than coolant temp....so does the Factory. One must presume the vent temps stated in the chart are also presuming +195 actual coolant temp......not 130-150 actual coolant temp.
Last edited by djb383; Oct 23, 2012 at 01:10 PM.
CF Veteran
Joined: Mar 2011
Posts: 2,570
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From: California
Year: 1992
Model: Cherokee
Engine: 4.0 HO
An easy way to test this would be for someone else to take a reading at operating temp. 50* drop seems a bit extreme. I'll run out in a bit and get mine up to temp and take a reading. I bet it will be closer to 180* at 200ish engine temp.
To the OP: as stated you need to verify your oil pressure with a mech gauge. Even though you've replaced the sending unit the gauge itself or the wiring could be bad, forget about the oil pressure until you verify it.
To the OP: as stated you need to verify your oil pressure with a mech gauge. Even though you've replaced the sending unit the gauge itself or the wiring could be bad, forget about the oil pressure until you verify it.
CF Veteran
Joined: Nov 2009
Posts: 8,172
Likes: 17
From: The Republic of TEXAS
Year: 1998
Model: Cherokee
Engine: 4.0L HO
50* drop may seem a bit extreme but the Factory math, 195-144, is 51*......and my money says any dealership service dept that sees within 51*, or less obviously, it going to tell the customer "heater vent temp is within spec, nothing else we can do?.
Tell us your vent temp when coolant temp is 130-150? The OP is saying heater works well at 130-150 indicated gauge temp. Do u get 144 vent temp when the gauge is indicating below 1/4 mark?
Tell us your vent temp when coolant temp is 130-150? The OP is saying heater works well at 130-150 indicated gauge temp. Do u get 144 vent temp when the gauge is indicating below 1/4 mark?
Last edited by djb383; Oct 23, 2012 at 01:44 PM.
CF Veteran
Joined: Mar 2011
Posts: 2,570
Likes: 4
From: California
Year: 1992
Model: Cherokee
Engine: 4.0 HO
50* drop may seem a bit extreme but the Factory math, 195-144, is 51*......and my money says any dealership service dept that sees within 51*, or less obviously, it going to tell the customer "heater vent temp is within spec, nothing else we can do?.
Tell us your vent temp when coolant temp is 130-150? The OP is saying heater works well at 130-150 indicated gauge temp. Do u get 144 vent temp when the gauge is indicating below 1/4 mark?
Tell us your vent temp when coolant temp is 130-150? The OP is saying heater works well at 130-150 indicated gauge temp. Do u get 144 vent temp when the gauge is indicating below 1/4 mark?
CF Veteran
Joined: Nov 2009
Posts: 8,172
Likes: 17
From: The Republic of TEXAS
Year: 1998
Model: Cherokee
Engine: 4.0L HO
Something tells me the FSM/chart will NOT be rewritten. How accurate is aftermarket gauge/where is the sending unit mounted? May be some apples and oranges coming into play. I would think the factory gives a minimum number because any reading above the minimum would be extra gravy, no?
Last edited by djb383; Oct 23, 2012 at 02:52 PM.
CF Veteran
Joined: Mar 2011
Posts: 2,570
Likes: 4
From: California
Year: 1992
Model: Cherokee
Engine: 4.0 HO
Peaked at about 172 with engine temps at 200. Aftermarket gauge was a $90 gauge so it better be accurate. IR gun readings at thermostat show it to be +/- only a few degrees.