The lamest cooling problem

My 94XJ 4.0L has been notorious in the past for cooling systems but has always managed to scrape by with a new thermostat/fan clutch, etc. Recently, I was running a little warmer on the highway than I would like, so I garden hose flushed it meticulously, refilled it. Ran great in the city, needle shot up continuously under heavy acceleration to get to freeway speeds and never could recover; I noticed the temp dropping and rising so I assumed a thermo starting to go. Bought a 195 at napa and took it to a mechanic I love, had them power flush the whole cooling system, put in the thermo (along with some purple ice). Now, unbelievably, the exact opposite is happening. It can't keep itself cool in the city (doesn't overheat in 25min of 85degree driving, but probably gets to 235 or so), but the second I get on the freeway (even with a/c) the temp goes back down; the EXACT opposite of what was happening before. When I brake hard or make a turn the temp probably moves 15 degrees up, then back down when I start going again. And once warmed up, even with the new therm/coolant, it NEVER goes under 210. Could this possibly be attributed to a DOA thermostat? The mechanic tested out the whole system, no leaks, good coolant flow, new pressure cap, etc, and my it isn't like any of my fans just quit spontaneously. If anyone has any idea why my symptoms would switch around please let me know. I'm tempted to just get a mr. gasket thermostat, either that or the temp sender is off (which I have a hard time believing it would fail that badly so quickly). Thanks for the help

fan clutch.

Rev

I would agree if the problem developed over time, but before I took the jeep in it cooled well in the city and overheated on the highway. I can't imagine it would coincidentally fail within 1 hour. Does a concentration too high in antifreeze/water manifest itself more obviously when there is less airflow (i.e. the city?) I asked them to do 66/33 water to antifreeze and realized later I was charged for 2 gallons of antifreeze in a 12 quart system... I would be a bit disgusted if the mechanic mixed that up.

I had to change through three t-stats (including one mr gasket) before I got one at the dealer (OEM) that actually worked right on my XJ. So reasoning from that, my money is on the t-stat. Really curious what you find out.

Engine ground good? Temp sensor good? Fluctuating rapidly like that leads me to think of these things. Especially since the sensor was removed then put back on in the process of changing the thermostat.

when in doubt on the cooling system, go oem T-stat

Thanks for the advice. I'm going to pick up a Mopar thermostat from the jeep dealer this afternoon and put it on either tonight or tomorrow sometime. I'll let everyone know how it goes.

So I put in a 180 balanced thermostat... let it idle for over 30 minutes and the temperature stayed RIGHT at 180, never budged with a/c on. This was great, as after I had the thermostat changed at the mechanic, it got too hot just idling. I took it for a little city drive, stop and go... temp still barely budged. So the big test came on the highway... and sure enough, the temp started creeping up, albeit slower. I am pulling hair out over this. After the mechanic flushed it and changed the stat, it overheated at idle but ran steady on the highway. The ONLY, and I mean ONLY variable I can think of that is different between my work and the mechanic's work is they have a machine that leaves no air in the system and I don't. Could air pockets seriously be the cause of this unfathomable inconsistency? I have no idea what else it could be.

Why the 180 degrees t-stat? Why not the 195 degrees? That's what the engine was designed for.

I believe that for the open system air entrapment is not such a huge deal. Maybe inititially, but if you burp it, or after some driving most of that should be out.

Pull the cross brace on top of the radiator so you can look straight down.I had an 82 S10 with the 2.8L V6 and A/C. Over time leaves and stuff built up between the rad and the condensor. Around town it was fine, when I got up to hiway speeds it would start to heat up. Had a bunch of leaves and crap in there that would get sucked up into the front of the radiator at hiway speeds then fall back down when it was stopped or running around town. Drove me crazy for a week or two. Finally picked up a new radiator and was going to replace it, when I tilted the old one back and saw all the stuff in there I used a shop vac with a long cardboard tube and vac'd all the crud out. Ran fine after that, returned the modine, and left the old one in there that one that was less than a year old.

All of last summer I ran a 180 thermostat after the 195 in there went bad and wouldn't open until about 220 or so; there was no difference in gas mileage and the temp never went above 210. Dutchjeep, what you're saying regarding the open cooling system is what I thought, that it eventually gets rid of its air. This is why I have absolutely no clue how flushing it makes it run hotter on the highway, getting it machine flushed makes it overheat idling, and then changing the thermostat makes it overheat on the highway again (I know this new thermostat is working as it should). The engine is in fantastic shape, gets good mileage, no oil burning, etc. I've scoured these forums left and right and don't think I've overlooked anything obvious, but have never had anything this frustrating. Is there anything I could possibly be overlooking that would lead to this unpredictable behavior?

Thanks for the suggestion RichP, I'm going to go check that out right now.

pauldo39 said:

All of last summer I ran a 180 thermostat after the 195 in there went bad and wouldn't open until about 220 or so; there was no difference in gas mileage and the temp never went above 210. Dutchjeep, what you're saying regarding the open cooling system is what I thought, that it eventually gets rid of its air. This is why I have absolutely no clue how flushing it makes it run hotter on the highway, getting it machine flushed makes it overheat idling, and then changing the thermostat makes it overheat on the highway again (I know this new thermostat is working as it should). The engine is in fantastic shape, gets good mileage, no oil burning, etc. I've scoured these forums left and right and don't think I've overlooked anything obvious, but have never had anything this frustrating. Is there anything I could possibly be overlooking that would lead to this unpredictable behavior?

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What pressure is the radiator cap ? Should be 16lbs, not the 13 that the AA and such parts books say.

It is a 16lb pressure cap; almost got a 13 on accident last year and happened to look at it after the guy grabbed it from the back of the store. Now, I'm just going to lay this out here. The radiator is as old as the jeep... nearing 14 years. However, it has no pressure leaks, no leaks in general (nor does the whole system). It seems way too much of a coincidence to me that it was running fine before I initially flushed it, then starting highway overheating when I refilled it, then city overheating after mechanic flushes it, now back to highway overheating after I change the thermostat. I'm not saying the radiator is perfect, I'm sure there is substantial room for improvement. BUT... it worked fine 2 weeks ago and the temperature has been constant. Which is why I just don't understand how this is happening. Time to get back to removing the cross brace...

RichP, I looked in there and it wasn't too bad... at least no buildup of anything that would change between city/highway. I usually try to keep the condenser clean, but it was worth looking. This is the only possible scenario I could possibly think of.. someone please let me know if this has any merit. Before I flushed, the system was fine (just doing the 2 year change). I flushed by myself... introduced air in the system, couldn't keep up on the highway. Had it professionally flushed, so no air; but the morons put in about 70-80% antifreeze instead of the other way around (after draining out a gallon my worry was confirmed) so the system could keep up with good airflow on the radiator on the highway but not enough air in the city made the too concentrated fluid unable to transfer enough heat. So I changed the thermostat but again introduced air in the system that is still there, which is why my situation is now back to where it originally was after the first flush (hot highway, cool city). To my understanding this is the only variable. Can anyone vouch that this is at the very least a possibility?

It could be that the tubes in the radiator are clogged or after flushing you loosened up crud that was in the block and it clogged the radiator.

Most new thermostats have a hole in one side. When I put a new one in, I always drill a hole 180 degrees opposite then install it with the holes up and down. Any air trapped will rise to the top. The bottom hole will then allow the system to burp itself.

crasy1_69 said:

It could be that the tubes in the radiator are clogged or after flushing you loosened up crud that was in the block and it clogged the radiator.

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Ding ding ding, we have a winner

Or that an upper or lower hose is failing internally. I would kinda shoot for bad wiring if your temp changes when you take sharp turns and such.

Radiators can go bad a couple of ways that I have had happen to me. First was clogging from build up when I used a 50/50 mix of anti-freeze and tap water before I knew better. The tap water up here is very high in minerals even with the particulate filters I have in the water line as soon as it comes into the house from the 500+ft deep well we have. Result is after a couple of years my radiator inside looks like a lab experiment with pretty colors of blue and green growths. To fix this I used a 8 hour prestone flush, this stuff was scary to me, by hour 7 on a run from the poconos to syracuse ny and back the inside was bright and shiny like new, I was afraid it would eat the solder. The fast flushes, 15 min ones that you pour in, have always had bad results for me, usually the water pump goes in short order, I think the stuff sandblasts the crud out. The 8 or 7 hour flush uses a chemical reaction to put the stuff back into suspension. When I drained my S10's 2.8 I drained the coolant into a bucket then used an automotive paint type filter to dump into another bucket and the stuff that was in that filter was nasty.
The second one was the fins being rotted, the radiator guy in town that does new rads and also does recores showed me a trick. He ran his fingernail backwards against the fins with a enough pressure to deflect the fins, the bad spots or rotted fins were obvious as they would not 'spring back' after being deflected. In his opinion and knowledge world he blamed it on the constant heating and cooling along with the road treatments that the radiator fins get exposed to on a daily basis.
I guess the third way which I have only had happen on my TJ's and on my 69 willys is the factory radiator separating from the top tank, happened to all three, 4 counting the YJ we had last year. Both current TJ's have new radiators put in in the last 2 years. I'm still running my original 98's radiator with over 330,000mi on it and I know it needs to be replaced as it has been running a tad on the warm side this summer, I may even get to it this weekend, had a new one on the shelf for the last year or so. Thats if I get the lift done on the blue 97 TJ which I plan on starting in a couple of hours.
Once I started using distilled water instead of tap water the build up problems went away, also my yearly coolant change helped alot.