Engine/Transmission (1998.5 - 2002) Truck is overcooling, Please help.

[ScottyG]

Been lurking since 1999. Quietly absorbing all info. Thanks to all. Now I got a real problem I can't solve.



My truck is over cooling. Towing 12k, at 75mph, in some hills, sometimes 1200egt, truck cools down to 120* after 2 or more hours of towing. For the first 1 hour and 45 minutes, all is fine(190*). after that, starts creeping down. I have spoken to all the "professionals" I know. No one has heard of such a thing. I have tried 2 new thermostats. It not that. I am calling out the big guns now.



Please help.



2000, 4x4QC, 6 sp, 4. 10, DD1's, ddufm, 4" exh, AFE filter, gages, ...

 

[Turboman]

Replace the temp sender. Sure sounds like electrical. Get you a laser point thermometer, when it does it shoot the thermo housing to see what it is.

 

[mike r]

Sending unit is gone. My oil sender did the same thing 2 or 3 years ago, Would loose all pressure then creep back up then die again. First time it did it I thought the oil filter fell off, bout gave me a heart attack. :{

 

[ScottyG]

I have the lazer temp unit. During the week, the temp gage is at 190* Lazer confirms. Fine all week. Only after towing for 2 hrs or more. Last weekend, I dropped the trailer at the camp site, unhooked and went into town for dinner. Got to the resturant and gage was at full cold. Checked with lazer unit, it read 127*. :--) Gages are fine. Lazer confirms. I am really baffelled!





Scott

 

[The Adjuster]

Don't our thermostats go to fully open when they fail? ... might be a thermostat on its way out . . just my $0. 02 worth



I had an old car of mine do the same thing and turned out to the thermostat was stuck open, leave it over night and it was fine the next morning until you drove it about 30 miles then the temp would drop

 

[AKAMAC]

The fan clutch can fail closed. The heat created by a diesel at idle is minimal. With the fan running enguaged at idle the cooling from the fan will cool the engine through the bypass. Try pulling the fan assembly off while not towing and avoid traffic conditions. You can run w/o the fan in place and not over heat. The clutch assembly is not too spendy and can be purchased online.

 

[ScottyG]

I am running on the 2nd new t-stat. I thought the original was sticking open so I replaced it. Then, the replacement had the same problems, I bought another thinking I got a defective one. I appears it is not the t-stat. Also, the clutch fan appears to be operating properly. During the first 1 1/2 hr of towing, the temp is at 190* or slightly higher and the fan will cycle normally. The fan is not cycling when the temp is cooling down. I can hear it when its engaged.



This problem is only when towing or running for more than 1 1/2 hrs. During the week, there is no signs that anything is wrong. The truck warms up and running at 190* all week. Even pulling my tractor around town(10K). I flushed the radiator last year and replaced the coolant. The radiator was removed to clean the junk out of the fins.



Still stumped.



Scott

 

[gbm]

ScottyG,

When you notice the drop in temperature on the gauge, turn on the heater. Does the air temp coming out of the heater feel cooler than it should? Maybe you could tell the difference between 120 and 190 degrees in the heater core? If it feels like normal heat (verify with your infrared heat gun), I would suspect it is gauge related. Maybe try a mechanical gauge? Do you notice any other symptoms when this happens - a change in power, boost, EGT, fuel pressure... ?



I worked on a gasser that did the same thing. After towing heavy for a long period, the fuel pressure regulator would stick and peg the fuel pressure. So much fuel was going through the motor that the temp gauge would drop like a rock. I would have suspected that the owner would have noticed that his fuel mileage dropped to about 2 miles to the gallon - but he didn't. Of course this gasser story does not apply to a diesel, but it shows that strange things happen.



When you find the solution, please share it with us.



Geoff

 

[ScottyG]

Geoff,

Thanks for the reply. The problem has always been while running the a/c also. So on the last couple of trips, I tried running with the a/c off. No difference. The other problem, race season is over. No more towing trips. I don't know how to trouble shoot the problem now because during the week, everything is AOK. The infared heat gun confirmed the low temp at the t-stat housing. The block was almost cold. No problem putting your hand on. The top of the radiator and it hoses were luke warm. Almost cold to the touch. At this point, it looks like I won't have this problem again until next season. I don't think it's gage related as the infared verifies the gage. The power, pressure, exh temps are all fine also. Two trips ago, I was running at 75mph, towing 12K loaded, a/c on, up hills with egt at 1200*, it was 95* out, AND IT WAS STILL RUNNING BETWEEN 140 AND 160*. With all that heat, you would think it would be getting too HOT, not cold.



Scott

 

[Rman]

man this one is really odd. The overheating problems are fairly easy, but this deal is really strange. Keep us posted on this, i'm anxious (prolly not as much as you) to see what it is!

 

[Teamropin983]

Just a stupid suggestion but have you tried checking the coolant level? Sounds like you might have an air pocket.

 

[ScottyG]

Yes, I have been looking alot at the levels thinking the same thing. But, why during the week (roughly 200 miles a week) would everything be OK. It warms up in the morning at the normal rate. The heat works fine. I never checked the heat when the engine was "cooling off" but the infared confirms the coolant and block is cold.



Please keep the suggestings coming. I open to anything right now.



Scott

 

[Chief USA]

Provided you have ensured all the above mentioned items are in good working order; install a set of Eric Buckel's "Winter Blockers". He sells them in the basement enterprises section of the classified adds on this website. I have a set that I just reinstalled today and they work great.

 

[AKAMAC]

Since you have eliminated the fan clutch I'm going to throw an odd and rare occurrence in the mix. Some times a piece of casting will cycle through the block and not fit through the t-stat. It will jam the t-stat open until it falls back on low flow, high temp.



I'd try a high back-flow flush with the t-stat out and a magnet stuffed in the hole.



This has got me perplexed.

 

[JCamper]

Me too!

I was running a search for thermostat problems, my truck has exactly the symptoms you describe. Noticed it a time or two towing this summer, and then this weekend, it did it again. Only after an hour or so of heavy towing. Last year I replaced the radiator, and then had the system flushed, new hoses installed. My truck will hit 150,000 miles today, I have been thinking it was a thermostat that only stuck open once the truck got warm enough to truly open it all the way, like maybe it catches at the top of its travel. Then when you park it and it cools, it contracts enough to snap closed again, then works fine until a heavy thermal load situation presents itself. I can say, the heater was definitely a little cold when it did it, so I don't think it is a gauge related thing. I am going to put a new Thermostat in, and check it again after. But during normal driving, it seems to work and cycle fine. Jcamper

 

[ScottyG]

JCamper,

Thanks for the reply. It's been a couple of weeks since towing and the truck is functioning perfectly during the week and non heavy towing. The problem only happens after 1 1/2 hrs of sustained towing. I have changed the t-stat twice and it didn't correct the problem. :{ I have not been able to locate anything wrong yet. I will be towing again on 11/6. I'll let you know the out come.

 

[Hohn]

Check your engine ID plate. You may have experimental adiabatic engine



Seriously though, I'd reason this through the following way:



Power comes from heat. If I have no heat, then it's either being transferred away too quickly, or it was never made in the first place.



If your EGT and engine power are normal, then you can assume that the fuel is being burned, so the heat is being made. Or can you? Can you verify that you're actually getting the fuel flow and power when the coolant temp drops? Normal FP? Normal EGT? Same apparent power?



So if the heat's being made, where is it going? Oil, Coolant, Exhaust, others? Every BTU has to be accounted for.



This is pretty baffling. Of course, some Mach 3's would help ensure that the heat was being made



Justin

 

[JCamper]

Hohn, no problem with making heat/power. No difference in the way the truck runs at all when it does it. Could Scotty have gotten some bad thermostats? Where did you get them from Scotty? I thought about the casting issue, but with 150k on this motor, and a few flushes besides, I wouldn't think that would be an issue. But then again, who knows. I think what I'll do is before I change my t-stat, I will put a pretty good magnet on it, and drive it for a couple days, maybe I can trap whatever it is, if that's the problem. I have always been very happy with my t-stat before this, too, none of the fluctuations that people gripe about, mine has always been rock solid. Have a good one, Jcamper.

 

[JCamper]

You know, rereading this thread again, it seems the common denominator between our situations is the radiator removal, and flushing this last year. That lends some more weight to the piece of casting or whatever in my mind. What do you guys think? Jcamper

 

[Hohn]

I don't think a T-stat stuck open would behave as described. I mean, this happens after a long time of heavy towing, right? Wouldn't the t-stat already be open and stay open?



It's hard to think of a reason that the coolant temp would just dive after a while when all other things are the same.



Maybe the IR gun AND the temp gauge are wrong, and off by the same amount? Talk about astronomical odds...