Airborne Janitor Posted October 13 Share Posted October 13 1989 4.0L currently running a Mishimoto 3 row radiator, new thermostat, new fan clutch, new OEM temp sensor, 97+ electric fan. This is not the original motor but a Renix head on a HO block. Head gasket was leaking so I took it to a shop back in March for repair. Since the repair the truck has had intermittent cooling issues, and I'm not sure if it's coincidence or correlation with the cylinder head rebuild. After the first overheating episode it was found the electric fan was not turning on. We ended up replacing the Renix t-stat housing with an HO one and rigging a temp sensor to it that activates the fan at 185*. The fan now works but did not solve the overheating issue. Then it was found the fan clutch went bad, replaced that and it helped....for a bit. The truck is currently running a new 180* thermostat, it did have a 195* in there but was changed by the shop. The motor does not overheat everytime you drive it, it almost feels random. Sometimes the temp gauge sticks right between 185-210 and stays there. Other times it goes to the 210-230 mark and stays there. Yesterday on the freeway going 80 MPH on a 60* day I watched the gauge slowly creep into the 230 range after 30 mins of driving. Took the next exit and watched the gauge creep towards the redzone at a stop light. As soon as the light turned green and I applied gas I watched the gauge drop to around 220. It seems temps drop whenever you apply gas. I've watched the gauge move 30* cooler while driving than when stopped. I had never seen the temp gauge move around so much until after the rebuild. I've also noticed that the motor continues to get quite hot after turning it off. If I turn off the truck, then come back in 5 minutes and turn it on the temp gauge is a good 20-30* hotter than when I parked it. There was a time the truck was creeping into the redzone, so I stopped and parked it, only for coolant to be pissing out the overflow bottle minutes later. I want to say it seems to overheat more at highway speeds, but I have not confirmed this 100%. It overheats while driving, it overheats while sitting. Sometimes it doesn't overhead at all and stays below 210*. I feel like I'm playing Russian roulette with this truck when I drive it, and each time I've taken it to the same shop I feel like they're offering nothing but bandaid fixes. My MJ NEVER overheated prior to the head work. I'm hoping this is just correlation and not causation. Any ideas? Thanks. Link to comment Share on other sites More sharing options...
Airborne Janitor Posted October 13 Author Share Posted October 13 Here's a pic of the temp gauge when I was stopped yesterday after pulling off the freeway. Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 13 Share Posted October 13 What’s the condition of everything else? Water pump? Hoses? Heater control valve? Temp rising while driving says water pump of obstruction temp rising while stopped says fans and blockage of the cooling fins to the radiator. Link to comment Share on other sites More sharing options...
Airborne Janitor Posted October 13 Author Share Posted October 13 Just now, eaglescout526 said: What’s the condition of everything else? Water pump? Hoses? Heater control valve? Temp rising while driving says water pump of obstruction temp rising while stopped says fans and blockage of the cooling fins to the radiator. Water pump was replaced 7 years ago with a Gates unit, has less than 15K miles on it. Hoses are good, no heater control valve. Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 13 Share Posted October 13 Anything obstructing airflow? Plugged condenser maybe if you have AC equipped? Do you hear the electric fan kick on during these events? Link to comment Share on other sites More sharing options...
Airborne Janitor Posted October 13 Author Share Posted October 13 19 minutes ago, eaglescout526 said: Anything obstructing airflow? Plugged condenser maybe if you have AC equipped? Do you hear the electric fan kick on during these events? Truck has no AC so no condenser, there's a tranny cooler but those are small and not hurting anything. E-fan kicks on at 185* and stays on until temps drop. Link to comment Share on other sites More sharing options...
TurboedMJ Posted October 13 Share Posted October 13 Sounds like your running an e fan and a mechanical fan? They are placed in different spots on the radiator correct? A mech fan pulling air through an e fan is going to really inefficient. Link to comment Share on other sites More sharing options...
Airborne Janitor Posted October 13 Author Share Posted October 13 14 minutes ago, TurboedMJ said: Sounds like your running an e fan and a mechanical fan? They are placed in different spots on the radiator correct? A mech fan pulling air through an e fan is going to really inefficient. No I'm running the stock fan clutch and a stock 97+ e-fan (instead of the renix fan) that's been rigged up to come on at 185* at the t-stat housing instead of the radiator. I have a Renix motor converted to an open cooling system, should have mentioned that too. Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 13 Share Posted October 13 Is the e fan spinning the right way? Last thing you want is the fans fighting each other creating a neutral space in which air is not going to flow Link to comment Share on other sites More sharing options...
Airborne Janitor Posted October 15 Author Share Posted October 15 On 10/13/2024 at 11:14 AM, eaglescout526 said: Is the e fan spinning the right way? Last thing you want is the fans fighting each other creating a neutral space in which air is not going to flow Currently sitting in a parking lot with the motor off waiting for it to cool off, temps around 240*. Drove her on the freeway for a little bit getting up to speeds of 75 to see if it would cause the motor to run warm. Probably after 15 mins the needle went to 210 then started to creep past it. Pulled over at the next exit where the needle continued to rise until almost red. Efan is currently running and spinning clockwise pulling air from the radiator to the motor. Temps are starting to cool, no coolant spraying out the overflow... Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 15 Share Posted October 15 Silly question but the cooling system has been bled right? Link to comment Share on other sites More sharing options...
Airborne Janitor Posted October 15 Author Share Posted October 15 5 minutes ago, eaglescout526 said: Silly question but the cooling system has been bled right? Presumably the shop who has done the work did it, especially after the head rebuild and thermostat replacement..... So the efan just shut off, even tho temps are at 210 right now. If I turn on the defroster it will start up the efan again...now temps are under 210 and the efan is completely off.... Link to comment Share on other sites More sharing options...
Airborne Janitor Posted October 15 Author Share Posted October 15 Just started her up to drive home and now the gauge says 180..... Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 15 Share Posted October 15 Hmmm. I would do this. Try to burp the rest if any of the air out and then maybe we can see if the temp sensor is going bad. Link to comment Share on other sites More sharing options...
Airborne Janitor Posted October 15 Author Share Posted October 15 31 minutes ago, eaglescout526 said: Hmmm. I would do this. Try to burp the rest if any of the air out and then maybe we can see if the temp sensor is going bad. I'm assuming this means popping the cap off the radiator, letting it run with the heater on full blast until it reaches operating temps? Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 15 Share Posted October 15 24 minutes ago, Airborne Janitor said: I'm assuming this means popping the cap off the radiator, letting it run with the heater on full blast until it reaches operating temps? No there is a much easier way. If you unscrew the temp sensor in the back of the head and get the engine tilted forward enough to purge the air out, it should come out via the sensor. Link to comment Share on other sites More sharing options...
Pete M Posted October 15 Share Posted October 15 just don't bleed the air out when anything is hot. Link to comment Share on other sites More sharing options...
Airborne Janitor Posted October 15 Author Share Posted October 15 1 hour ago, eaglescout526 said: No there is a much easier way. If you unscrew the temp sensor in the back of the head and get the engine tilted forward enough to purge the air out, it should come out via the sensor. My truck sits 2" higher in the back, I parked it on a decline in my sloped driveway, and it still made a huge mess lol. How high does the motor have to be for this to work without losing half a gallon of coolant? Link to comment Share on other sites More sharing options...
Pete M Posted October 15 Share Posted October 15 you only need to unscrew the sensor juuuuust enough to bleed out the air. Link to comment Share on other sites More sharing options...
Airborne Janitor Posted October 15 Author Share Posted October 15 3 hours ago, Pete M said: you only need to unscrew the sensor juuuuust enough to bleed out the air. Gotcha. Ended up burping the system last night by popping the radiator cap and letting her run for 30 mins, no issues. 50 min commute this morning, temps stayed at around 190 then after 30 mins started to rise. I stayed off the freeway and opted to do the stop and go drive through town. Temps would go up to the 210 range when stopped at lights, then drop to 190 once I hit the gas. Made it to my destination and the needle is pegged right around the 210 mark when parked. The gauge seems to be very sensitive to RPM changes, the lower the revs the more the temps want to rise it seems. Water pump possibly? Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 15 Share Posted October 15 Water pump could be an issue. When was that replaced? Link to comment Share on other sites More sharing options...
acfortier Posted October 15 Share Posted October 15 Isn't 210/215 normal operating temp for these? Curious, did you confirm that the gauge is accurate by measuring temp another way? Link to comment Share on other sites More sharing options...
89 MJ Posted October 15 Share Posted October 15 This sounds a lot like my issue. The temp stays fine until it’s not suddenly. What gearing do you have? I suspect my issue is that with lower gears and highway speeds the water pump starts cavitating. I think I'm going to try a flowcooler water pump and a high flow thermostat. Link to comment Share on other sites More sharing options...
derf Posted October 15 Share Posted October 15 42 minutes ago, acfortier said: Isn't 210/215 normal operating temp for these? Curious, did you confirm that the gauge is accurate by measuring temp another way? He put in a lower temp thermostat so in theory it shouldn't run that hot. Link to comment Share on other sites More sharing options...
Pete M Posted October 15 Share Posted October 15 210 is normal. sudden rises in temp can definitely be indicative of an air bubble appearing at the back of the head. 27 minutes ago, 89 MJ said: This sounds a lot like my issue. The temp stays fine until it’s not suddenly. What gearing do you have? I suspect my issue is that with lower gears and highway speeds the water pump starts cavitating. I think I'm going to try a flowcooler water pump and a high flow thermostat. back in the day you could get a 2.5L with 4.56 gears and tiny tires from the factory. plus I regularly drive around without using overdrive. higher rpms won't directly cause cavitation. Link to comment Share on other sites More sharing options...
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