InTacosWeTrust Posted June 7 Share Posted June 7 My 87 is still overheating at idle, need advice, I’ve replaced my thermostat to a new 195 with jiggle pin, new water pump heavy duty, new csf rad, and I also changed my clutch fan a couple months ago I let the truck idle for 20 minutes or so and it stayed around 210 until I checked it at around the30 minute mark and it was pushing close to the red. Any advice? Link to comment Share on other sites More sharing options...
Pete M Posted June 8 Share Posted June 8 have you verified the temp with an inferred thermometer? have you tried bleeding air out the temp sensor yet? Link to comment Share on other sites More sharing options...
InTacosWeTrust Posted June 8 Author Share Posted June 8 24 minutes ago, Pete M said: have you verified the temp with an inferred thermometer? have you tried bleeding air out the temp sensor yet? I have not, where would I check with the infrared thermo? Also how do you bleed the temp sensor? I have an open cooling system as well Link to comment Share on other sites More sharing options...
ghetdjc320 Posted June 8 Share Posted June 8 36 minutes ago, InTacosWeTrust said: I have not, where would I check with the infrared thermo? Also how do you bleed the temp sensor? I have an open cooling system as well You can check the hot side of the radiator (coolant return side). That will be pretty much the hottest coolant temp reading you can get. Make sure all of your hose clamps are tight with zero leaks. A small leak can cause an overheat as the system can’t pressurize to raise the boiling temp. Is your water pump a stock replacement? Have you changed the radiator cap? Are you using the oe fan shroud? Do you have an auxiliary e fan? Link to comment Share on other sites More sharing options...
89 MJ Posted June 8 Share Posted June 8 What does the inside of the engine block look like? It could be that the cooling passages are rusty and partially obstructed. Is your fan clutch in good condition? Link to comment Share on other sites More sharing options...
Gojira94 Posted June 8 Share Posted June 8 11 hours ago, 89 MJ said: What does the inside of the engine block look like? It could be that the cooling passages are rusty and partially obstructed. Is your fan clutch in good condition? This video shows what a block with sediment in it looks like. It's a HO example but the plugged hole where our Renix temp sensor would go is an eye-opener. 1:05 Preview of the issue 35:01 - 35:52 Overview of where the issue is 38:33 - 43:37 Expansion plugs/ temp sensor plug removal and the full reveal of rust sediment 52:52 - 53:33 Blowing it all out with a pressure washer Dex Browder and his machinist brother, Justin. A fun and informative channel... Link to comment Share on other sites More sharing options...
87MJTIM Posted June 8 Share Posted June 8 How old is the lower radiator hose? Is it collapsing at high RPMs? Link to comment Share on other sites More sharing options...
89 MJ Posted June 9 Share Posted June 9 12 hours ago, Gojira94 said: This video shows what a block with sediment in it looks like. It's a HO example but the plugged hole where our Renix temp sensor would go is an eye-opener. 1:05 Preview of the issue 35:01 - 35:52 Overview of where the issue is 38:33 - 43:37 Expansion plugs/ temp sensor plug removal and the full reveal of rust sediment 52:52 - 53:33 Blowing it all out with a pressure washer Dex Browder and his machinist brother, Justin. A fun and informative channel... Yes! I love that channel! Link to comment Share on other sites More sharing options...
InTacosWeTrust Posted June 10 Author Share Posted June 10 I have replaced everything within the past few months, so everything except for looking in the engine block is new, before i thought I had fixed the issue a while back within the year before, but it would still run slightly above 210. I decided to upgrade the radiator and do a high flow water pump and at idle the first check it started to over heat. but I said screw it let me drive it around and get air flow to see if it’s that. I haven’t tried to let it just idle alone like I did before. But as of now it’s staying well below like way below 210 which is great especially on these hot summer days while driving. Link to comment Share on other sites More sharing options...
pizzaman09 Posted June 11 Share Posted June 11 I had this issue for a while, I had a small pressure leak that didn't allow it to build enough pressure to prevent the coolant from boiling. In my case I had a bad pressure bottle cap that had come with the new coolant bottle. I put the factory cap back on and it fixed the problem as it held a higher pressure. Link to comment Share on other sites More sharing options...
XRT38 Posted Tuesday at 05:31 AM Share Posted Tuesday at 05:31 AM I've been running mine without a thermostat for like years. A tad cool in the winter so you might put one in for that but these trucks just don't have enough cooling to begin with & that high pressure overflow always leaks. Had that thing blow out due to overheating with a thermostat. Frankly I'd just leave it out unless you need heat for winter. Link to comment Share on other sites More sharing options...
ghetdjc320 Posted Tuesday at 12:31 PM Share Posted Tuesday at 12:31 PM On 6/11/2025 at 8:37 AM, InTacosWeTrust said: do a high flow water pump Interesting. What radiator did you use? The high flow pumps have given problems in the XJ/MJ platform. The high flow pumps coupled with certain “upgraded” radiators can move water too quickly for heat dissipation. Many upgrade radiators actual have lower fluid capacity (often a result of multiple thin cores). Coupled with a higher flow water pump, the relatively smaller amount of coolant is moved faster which is a recipe for over heating. There are some but not many improvements to be had in an MJ/XJ stock cooling system. Link to comment Share on other sites More sharing options...
89 MJ Posted Tuesday at 01:06 PM Share Posted Tuesday at 01:06 PM 34 minutes ago, ghetdjc320 said: Interesting. What radiator did you use? The high flow pumps have given problems in the XJ/MJ platform. Not trying to hijack, just throwing my own experience out there. I know you’ve said this time and time again, so I was really skeptical to try the high flow water pump, but I was out of ideas. I’d done the Mishimoto rad, which helped a ton, the block was absolutely perfect in the coolant passages, when I got the engine, it already had a new water pump, and the thermostat was working fine. On long grades at 70+ mph (so I’m turning 2700+ rpm), the truck would still got hot. I swapped in a flowkooler pump and it only got warm once, but it turns out that was because I still had an air pocket in the cooling system. After changing to the flowkooler and changing nothing else, it’ll now sit at 195F running the same route that would make it hit 230 before. And I was trying to make it overheat, so I was running 75-80mph. Link to comment Share on other sites More sharing options...
AZJeff Posted Tuesday at 01:39 PM Share Posted Tuesday at 01:39 PM The 4.0 engine in the MJ/XJ runs hot BY DESIGN. With OEM cooling system parts, normal operating temperatures will be about 200* F, give or take 5 degrees, when using an OEM rated 195* thermostat. One should not trust the guage on the instrument panel (and I don’t, because they are only an “approximate” indicator or actual temperatures.) An infrared temperature measuring tool aimed at the thermostat housing will give you the best measurement of actual engine temperature. Link to comment Share on other sites More sharing options...
AnotherOldJeepGuy Posted Tuesday at 01:41 PM Share Posted Tuesday at 01:41 PM I learn something every time I read stuff here! Until today I always thought all water pumps that fit did pretty much exactly the same thing! Link to comment Share on other sites More sharing options...
ghetdjc320 Posted Tuesday at 08:50 PM Share Posted Tuesday at 08:50 PM 7 hours ago, 89 MJ said: Not trying to hijack, just throwing my own experience out there. I know you’ve said this time and time again, so I was really skeptical to try the high flow water pump, but I was out of ideas. I’d done the Mishimoto rad, which helped a ton, the block was absolutely perfect in the coolant passages, when I got the engine, it already had a new water pump, and the thermostat was working fine. On long grades at 70+ mph (so I’m turning 2700+ rpm), the truck would still got hot. I swapped in a flowkooler pump and it only got warm once, but it turns out that was because I still had an air pocket in the cooling system. After changing to the flowkooler and changing nothing else, it’ll now sit at 195F running the same route that would make it hit 230 before. And I was trying to make it overheat, so I was running 75-80mph. The flowkooler with the billet impeller is a great unit. There are other “high flow” pumps though that use a stamped impeller. You have the right combo though. The mishimoto has a greater capacity than stock. Mishimoto, cold case, Novak and the old Griffith have multiple cores with large passageways and higher overall capacity. I’m sure some other radiators do as well but there are also many that don’t. When you start getting a radiator with small passageways and/or low capacity, the high flow pump can work against you. As stated above, the 4.0 is designed to run warm. But it should hold a relatively steady temp once it gets to normal operating range. Even an inaccurate temp gauge shouldn’t keep skyrocketing into the red given that the gauge sweep is exponential and not linear. It should hold steady at some point with a functional cooling system. But to the OP, I would suggest an IR temp meter as well. If you check the thermostat housing, just know that will typically read a few degrees above the gauge temp. Link to comment Share on other sites More sharing options...
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