Phil Reinsch Posted February 17, 2022 Share Posted February 17, 2022 New to me pampered truck started overheating 100 miles into 200 mile trip. Limped it along staying in the green at mostly sub 55. Private mechanic shop checked it out but gave up short of throwing parts at it. Small town Jeep dealer wanted to replace near everything (radiator, thermostat, overflow tank, etc.). Nothing leaking, though there’s a crack on top of overflow tank, recently installed water pump and thermostat. I am told the electric fan works. Any ideas? Could the new pump be bad? Will now overheat in traffic, never noticed that prior to trip. Not looking forward to a long hot slow slog back home. Link to comment Share on other sites More sharing options...
Phil Reinsch Posted February 17, 2022 Author Share Posted February 17, 2022 Is this common with this engine in Comanches and Cherokees? Link to comment Share on other sites More sharing options...
Phil Reinsch Posted February 17, 2022 Author Share Posted February 17, 2022 Edited out it’s a 4.0 liter Link to comment Share on other sites More sharing options...
Smokeyyank Posted February 17, 2022 Share Posted February 17, 2022 If your overflow tank is cracked you need to replace that. You can swap in a volvo one IIRC that is a slight improvement. Since it's a closed system any leaks will cause issues. Link to comment Share on other sites More sharing options...
Pete M Posted February 17, 2022 Share Posted February 17, 2022 is the radiator original? normally I don't advise throwing parts at a problem, but there's nothing wrong with brand new cooling system components. Link to comment Share on other sites More sharing options...
89 MJ Posted February 17, 2022 Share Posted February 17, 2022 I put a new water pump, radiator hoses, and coolant pressure tank in my 89 4.0 and it still overheated. The radiator was not allowing enough flow. It is probably corroded inside and flow is restricted. Link to comment Share on other sites More sharing options...
JMO413 Posted February 17, 2022 Share Posted February 17, 2022 If the tank is cracked your chilling system won't build pressure. Replace the tank and retest. Link to comment Share on other sites More sharing options...
AZJeff Posted February 17, 2022 Share Posted February 17, 2022 The 4.0 engine in the MJ and XJ has what I call a "intolerant" cooling system. When all the parts of the system are in "as new" condition, the system can keep the vehicle temperature within design limits just barely. Because of this, the vehicle cannot tolerate any serious degradation of ANY part of the overall system. If any parts of the system starts to degrade (pressure tank, water pump, thermostat, cooling fans, or radiator), now the system can no long do the job, and it starts overheating. Start with the pressure tank and its' cap. If that isn't holding pressure, it needs to be fixed so it does. I am thinking that, since you report you are overheating at road speeds, your cooling fans are not the culprit, because enough air enters the radiator without the fans under those conditions. Then look at the radiator itself. Is the coolant nice and clean, or does it look grungy? Grungy coolant tends to imply poor cooling system maintenance, and that leads to clogged radiators. I am thinking this is where your issue is, but I hate firing the parts cannon without further examination. Link to comment Share on other sites More sharing options...
gogmorgo Posted February 17, 2022 Share Posted February 17, 2022 What year truck are we dealing with? The closed system used from '87-'90 does indeed rely on the coolant reservoir to hold pressure, but the '91+ open cooling system doesn't pressurize the overflow reservoir, and won't care if the top of the jug is cracked. It's also very common for people to upgrade to the newer open system. The open pressure system is more "tolerant" but arguably closed cooling systems are more effective at cooling. Something else people struggle with is getting the system properly bled of air. Slowly losing coolant over time could create air pockets that may not refill completely when it's topped up. Overheating could also loosen up some crud inside the engine block and transport it around and plug up the radiator. My strategy here would be to replace the tank if it's a closed system, and while you've got the system open, pop the hoses off the radiator and give it a good flush. Link to comment Share on other sites More sharing options...
Phil Reinsch Posted February 18, 2022 Author Share Posted February 18, 2022 Thanks everyone for the input. I’ll let you know. See attached for my truck. Link to comment Share on other sites More sharing options...
87MJTIM Posted February 18, 2022 Share Posted February 18, 2022 Fix that grill! It’s upside down. Link to comment Share on other sites More sharing options...
rokinn Posted February 18, 2022 Share Posted February 18, 2022 Is that a Triumph Spitfire next to it? If so, what year? Link to comment Share on other sites More sharing options...
Phil Reinsch Posted February 18, 2022 Author Share Posted February 18, 2022 Dang, the grill is upside down - I'll get right on it, after I locate a new coolant reservoir tank. That said, "Have Duct Tape, Will Travel!" With the sage advice above, I taped up the tank and made the 200 mile trip without incident, doing a leisurely 55-60 most of the way off the Interstate, cranking it up to Texas Interstate speeds as I got close to the DFW metro area, in order to see if I ran hotter (It didn't). I now know some differences between an open vs. closed system and its use of a coolant reservoir tank to pressurize the system. I appreciate everyone's input and look forward to fun times with this 1988 truck. And the car in the background is a 1965 Sunbeam Tiger, wanna buy it? There's a new girl in town... Link to comment Share on other sites More sharing options...
lippetd Posted February 28, 2022 Share Posted February 28, 2022 The overflow tank is part of the pressurized cooling system. If it has a crack, your system will not build pressure (Approx. 15 p.s.i). Your coolant will boil at a much lower temperature if it is not pressurized. ( Straight water boils at 212 degrees at 0 psi versus 257 degrees at 15 psi.) Link to comment Share on other sites More sharing options...
Phil Reinsch Posted March 7, 2022 Author Share Posted March 7, 2022 Replaced tank with new one from RockAuto. Seems to have solved problem although yet to take a road trip. BTW, the tank came with a cap which seems to be working fine; bought a new one separately that hopefully I won't need. Again, thanks for helping out the uninformed! Phil Link to comment Share on other sites More sharing options...
Noendnsight Posted March 7, 2022 Share Posted March 7, 2022 1 hour ago, Phil Reinsch said: Replaced tank with new one from RockAuto. Seems to have solved problem although yet to take a road trip. BTW, the tank came with a cap which seems to be working fine; bought a new one separately that hopefully I won't need. Again, thanks for helping out the uninformed! Phil Is it a Renix 4.0 closed coolant system(factory setup for 87-90)? If so and you replace 1, multiple, all or anything in the coolant system you'll most likely have air trapped in the head. The problem will come back. The air will need purged for it to ever work properly. There is several ways one can go about doing so. There should be several on here. I should mention the heater core can too depending on what's been replaced and in what order. It usually takes several times following any procedure to remove all the air. It will include opening up the heater vents to cycle coolant through heater core at operating temps so the thermostat is open. It helps to know what tstat you have and follow your temp guage on cluster if you have one. All mine typically jump after hitting around 210 after idling a bit with air trapped. The air can give false readings on the temp gauge which it will suddenly climb and usually boil over at tank if you don't catch it. If your system is fully operational and in good working order, it seems to happen very fast under normal conditions. That's because everything is sealed and the system is closed. The hydrolocking happens faster which makes it seem that the problems worse than before but it's just the opposite. You just need to be patient and take your time to learn by repeating the process and digesting in your mind how the system functions. Most guys will just tell you to swap it to an open system. Link to comment Share on other sites More sharing options...
PIKE Posted March 8, 2022 Share Posted March 8, 2022 This has been said many times. MAC'S Tank + 2.5 Expansion Bottle = Open System It's that simple Don't feel bad. took me a long time to figure this out. Link to comment Share on other sites More sharing options...
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