88swampedmj Posted November 18, 2007 Share Posted November 18, 2007 ive had a tech thread before about my red 91 overheating, but it never got solved i guess ill say what ive done and see if anyones done anything different or knows something i don't.. the truck slowly heats up like normal but just keeps on going, i have a new radiator cap, just put in new coolant, flushed the radiator and motor.. i have a newish waterpump and new thermostat.. iam not mechanically disinclined ive done all the tricks i know to do but the damn thing just keeps overheating.. there are absolutely no leaks at all.. the head gasket is just fine.. intake manifold gasket is fine as well.. its just a red hunk of poo... :headpop: :nuts: :mad: :eek: Link to comment Share on other sites More sharing options...
jtdesigns Posted November 18, 2007 Share Posted November 18, 2007 I read somewhere that you could bleed the coolant from somewhere on the block I believe. Its a pressurized system maybe you could take it somewhere to get a pressure test done..... Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 18, 2007 Share Posted November 18, 2007 Did ya check for a clogged cat? Caused an overheating problem for me big time a couple of years back............. Link to comment Share on other sites More sharing options...
86FUBAR Posted November 18, 2007 Share Posted November 18, 2007 ya id say uv got air bubbles , my manch did the same thing over and over until i drilled a 1/6 hole in the thermostat to help let the bubbles escape . i believe that there is an air blead on the back of the block maby try that ? Link to comment Share on other sites More sharing options...
Eagle Posted November 18, 2007 Share Posted November 18, 2007 I'll go out on a limb and say you need a radiator. They don't last forever, you know. A 91 is now 16 years old. I had to replace the radiator in my '88 Cherokee in 1999, which was only 11 years. The replacement was a GDI 3-core. That one lasted until this Spring, and had to be replaced again. Flushing after 16 years doesn't accomplish much. Link to comment Share on other sites More sharing options...
88swampedmj Posted November 18, 2007 Author Share Posted November 18, 2007 well i flushed the radiator and it seems fine, i think ill check the cat that does sound plausible.. Link to comment Share on other sites More sharing options...
jeepinjay Posted November 18, 2007 Share Posted November 18, 2007 Could be a bad clutch on fan. Link to comment Share on other sites More sharing options...
CWLONGSHOT Posted November 18, 2007 Share Posted November 18, 2007 Lots of simple overlooked things can cause overheating. These are problematic in closed systems like our manche's have. Air in the system as others have suggested was my first thought. ESPECIALLY, if the overheating is a new thing since the water-pump and thermostat. Eagle is the voice of reason and is spot on that Radiators do go bad. One other thing, Although the MJs' don't have a true pressure cap, like an open system. If the cap doesn't seal, the system cannot pressurize and this could also cause the overheating issue. Check this stuff and get back to us. CW Link to comment Share on other sites More sharing options...
Eagle Posted November 18, 2007 Share Posted November 18, 2007 well i flushed the radiator and it seems fine, i think ill check the cat that does sound plausible.. :???: You just wrote that flushing didn't help it. the truck slowly heats up like normal but just keeps on going, i have a new radiator cap, just put in new coolant, flushed the radiator and motor.. i have a newish waterpump and new thermostat.. iam not mechanically disinclined ive done all the tricks i know to do but the damn thing just keeps overheating.. Link to comment Share on other sites More sharing options...
Eagle Posted November 18, 2007 Share Posted November 18, 2007 One other thing, Although the MJs' don't have a true pressure cap, like an open system. If the cap doesn't seal, the system cannot pressurize and this could also cause the overheating issue. His original post specified that he was asking about the '91. That has a conventional radiator cap, which he said has been replaced. How badly does it overheat? There is one last-ditch thing to consider, and that's an incorrect water pump. The 4.0L and the 4.2L engines use the same bolt pattern on the front of the engine for the water pump mounting. The old 4.2L engines with conventional vee belts rotated one direction, the 4.0L engines with the serpentine belt have the water pump rotating in the opposite direction. This requires that the vanes on the impeller be reversed, and if you look at a 4.0L water pump out of the vehicle, most will have a big 'R' stamped on the impeller. Dunno who installed the new water pump or where it came from, but if all else fails to resolve this, I would suggest removing the water pump to confirm that you have the correct one for a 4.0L engine. Link to comment Share on other sites More sharing options...
TNT Posted November 18, 2007 Share Posted November 18, 2007 I have also seen the vanes on the waterpump rust out, causing low coolant flow and overheating. If you have the t-stat out port it out. That combined with a Robertshaw thermostat made a big difference. It warms up faster and once the t-stat opens it mantains a very constant temperature(within 2-3 degrees). I think you can get the same t-stat from Moroso now. Link to comment Share on other sites More sharing options...
88swampedmj Posted November 19, 2007 Author Share Posted November 19, 2007 well eagle your comment got me thinking and i think that could be the case, the truck ran hot on the way home from buying it, that guy screwd me over alot not telling me alot of problems it had, so i wouldnt doubt he bought the wrong waterpump and created the problem. Link to comment Share on other sites More sharing options...
carnuck Posted November 19, 2007 Share Posted November 19, 2007 I ran the "wrong" waterpump to Ouray and back. It only affected low RPM temps. '91s are infamous for cracking heads on the first of the HO series. (putting air pressure into each cylinder with the valves closed will cause air bubbles to percolate in the rad) Unscrewing the temp sender allows you to "burp" the system. A can of Redline Water Wetter drops temps drastically (that's the only reason why I didn't lose my headgasket at Ouray) BTW, if your rad says Blackstone on it, it's jammed full of crud because the tubes are too small diameter. Link to comment Share on other sites More sharing options...
88swampedmj Posted November 19, 2007 Author Share Posted November 19, 2007 it does get the hottest at redlights so iam thinkin now it may be the water pump Link to comment Share on other sites More sharing options...
Kenosha Warrior Posted November 19, 2007 Share Posted November 19, 2007 yeah, check that WP remans are cheap, if that fixes it, then go get a new one and keep it handy. Link to comment Share on other sites More sharing options...
Eagle Posted November 20, 2007 Share Posted November 20, 2007 it does get the hottest at redlights so iam thinkin now it may be the water pump Another "Aha!" That's a symptom of a bad fan clutch. When you're not moving, the only air through the radiator is pulled through by the fan. If the fan clucth is slipping, it doesn't pull enough air. Once you start moving, vehicle motion pushes air through the radiator. If it heats up at lights and cools back down when you start moving, the first suspect is the fan clutch. Link to comment Share on other sites More sharing options...
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