jeepdude89 Posted June 11, 2017 Share Posted June 11, 2017 I know a few guys on here have or have had some of the 5.9 ZJ's so I thought I'd ask here since ComancheClub is much more professional than other Jeep forums I've been on. My 98 5.9 has started running hotter than it normally has over the past year and I'm getting stumped on it. It never overheats-just likes to run right around 220 when driving-if you first start it and let it idle-don't drive it at all it holds at or a little under 210 just fine. But it's only when you drive or have been driving it gets to 220 and stays there. It's got a completely brand new cooling system-radiator-cap-all hoses-water pump-heater core-thermostat and I've replaced the electric fan as well and even have it jumped on high to run all the time to see if that made a difference to make sure I didn't have failing temp sensors and no change at all-the same thing. Only things I can think of are maybe a cracked head(s) or a catalytic converter issue-although the oil is nice and clean with no signs of coolant in it and it isn't lacking power any which if the cat was clogged I figure I'd be having some power loss-but maybe not. Anybody got any other ideas? I just don't like to see it running that hot. Thank you! Link to comment Share on other sites More sharing options...
DirtyComanche Posted June 11, 2017 Share Posted June 11, 2017 Gauge error? Is it actually running hot, or has the gauge or sender started to fail? I changed the tstat in my truck because it was running cold, I naturally figured somebody had put the wrong tstat in it to try to mask an overheating problem. The temperature was the same afterwards. So, obviously it was the gauge or sender at that point. Turns out the temp sender was no good, I threw it in a cup of boiling water to check it and the gauge, then did the same with a known good sender. If it isn't an indicating problem, then your cats plugging would be a reasonable thing to check. Link to comment Share on other sites More sharing options...
jeepdude89 Posted June 11, 2017 Author Share Posted June 11, 2017 Yes-I have verified that it is actually running that hot/gauge is reading right. I've even swapped in another sending unit as well just to triple check/go over everything. I've been kinda thinking it might be the cat slightly getting clogged. I am driving it today and was idiling in a sun soaked parking lot with the AC blowing 100% for 10mins and it never went over 210 until I got on the freeway-went up to 220 in less than one mile. I'll replace the cat soon and see what it does then. After 265,000 miles it might be shot seeing how it is the original cat I'm pretty sure. Link to comment Share on other sites More sharing options...
Jeep Driver Posted June 11, 2017 Share Posted June 11, 2017 If you're not moving coolant above idle............water pump. Link to comment Share on other sites More sharing options...
jeepdude89 Posted June 11, 2017 Author Share Posted June 11, 2017 If you're not moving coolant above idle............water pump. It's already been replaced-is a new one and from my experience a bad or failing water pump will overheat more so at idle-not at speed. My issue is the opposite. Link to comment Share on other sites More sharing options...
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