Overheating at low RPM

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Overheating at low RPM

Postby DieselSharker » Wed Mar 10, 2010 6:08 pm

This has been going on about 6 weeks or so. Back in December I replaced the water pump because the weep hole was leaking and threw a new thermostat in there while I was at it. It ran fine for 2 months and then all of a sudden got hot on me one day and has been doing it ever since.

Since then I have replaced the thermostat twice and the fan clutch but still no dice. The last thermostat I put in I drilled a bigger weep hole (1/8") to see what it would do. I've also flushed it several times, including running a bottle of Peak radiator cleaner through it. It runs fine down the highway but in town, especially when I'm just getting off the highway, from a 15 minute or more highway drive, it starts heating up and will peg out if I don't keep the RPMs up. The last couple times I flushed the radiator I noticed small black chunks coming out of it but I run the water through til it comes out clean. You can see the corrosion on the internal fins when you look in the cap. There is no oil in the coolant and no coolant in the oil so I think I'm safe from the head gasket as long as I keep the temps down (knock on wood). I also took the water pump back out to inspect it and everything looked good.

Today I took the thermostat out altogether to see how it runs and it is keeping steady temps below the normal line whether on the highway or in town, so I think it is a flow problem. My theory is the chunks are coming from build-up inside the radiator and causing the flow to slow down too much when the water pump is turning slower. I've flushed it backward and forward through the plug in the head out through the radiator cap and vice versa and also pulled the bottom hose and flushed the radiator top to bottom. I was going to get it rodded but I think having a 20 yr-old radiator rodded might cause leaks.

Any other theories? Should I try getting this thing rodded or drop an extra $130 and get a new one from Napa? I can't afford to get it rodded, have it start leaking, and then have to buy a new once since I'm newly unemployed. I don't think it sounds like a head gasket issue, but I've never blown one before. Thanks in advance.
1991 W250 727, 6" Skyjacker, 37" Dick Cepeks, Gear Vendor O/D, SDX 5x.014s, BHAF, home-made 4" intake, VE tuning & DennyT II pin, GDS 60mm with 16 cm, CoolerTubz 3" crossover
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Re: Overheating at low RPM

Postby ellis93 » Wed Mar 10, 2010 6:45 pm

U check the fan clutch. I had a problem like that but only when pulling somethin slow.
93 D250 ,5 speed,4.11s,k&n autometer tach pyro trans boost guages,GDS 60mm h1c 14cm,honed 5x10,hplp/reg,1/8 timing,M+H M2 fuel pin, tims cooler tubz
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Re: Overheating at low RPM

Postby DieselSharker » Wed Mar 10, 2010 6:51 pm

ellis93 wrote:U check the fan clutch. I had a problem like that but only when pulling somethin slow.


Yep, that was one of the first things I replaced and it was a PITA, but still hot. I thought the same thing since it was running cold at high RPM and hot at low RPM. Thanks, though.
1991 W250 727, 6" Skyjacker, 37" Dick Cepeks, Gear Vendor O/D, SDX 5x.014s, BHAF, home-made 4" intake, VE tuning & DennyT II pin, GDS 60mm with 16 cm, CoolerTubz 3" crossover
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Re: Overheating at low RPM

Postby PToombs » Wed Mar 10, 2010 6:53 pm

You don't have a piece of cardboard stuck in the front from cold weather use do you? Is the fan shroud on it?
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Re: Overheating at low RPM

Postby DieselSharker » Wed Mar 10, 2010 7:19 pm

Yep, shroud is on it and I've never put cardboard up in front since it doesn't really get cold enough down here.
1991 W250 727, 6" Skyjacker, 37" Dick Cepeks, Gear Vendor O/D, SDX 5x.014s, BHAF, home-made 4" intake, VE tuning & DennyT II pin, GDS 60mm with 16 cm, CoolerTubz 3" crossover
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Re: Overheating at low RPM

Postby ellis93 » Wed Mar 10, 2010 7:40 pm

If the radiator was stopped up it would stay hot. Might be the thermo even though you've replaced it twice. U took it out and it ran cold right. Buy one from cummins or dodge they have them
93 D250 ,5 speed,4.11s,k&n autometer tach pyro trans boost guages,GDS 60mm h1c 14cm,honed 5x10,hplp/reg,1/8 timing,M+H M2 fuel pin, tims cooler tubz
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Re: Overheating at low RPM

Postby DieselSharker » Wed Mar 10, 2010 8:21 pm

Yeah I just did it this afternoon so I haven't driven it much yet but I'll see how it does driving up to Ft Worth this weekend. I may just order that t-stat and wait til Monday or Tuesday to order a new radiator. Anybody have the part #?
1991 W250 727, 6" Skyjacker, 37" Dick Cepeks, Gear Vendor O/D, SDX 5x.014s, BHAF, home-made 4" intake, VE tuning & DennyT II pin, GDS 60mm with 16 cm, CoolerTubz 3" crossover
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Re: Overheating at low RPM

Postby 12valve350 » Thu Mar 11, 2010 12:49 am

mabey time for a new radiator?
Six in a row makes 'er go, Eight in a v, theres nothin' too see!
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Re: Overheating at low RPM

Postby dieseldawg604 » Sun Apr 18, 2010 10:03 pm

sorry to say this but that sounds like exactally what i had going on !!!
it was my head gasket!! :cry:
whats your overflow doing?? take a look before you take her out then go for a good rip and take a look!!
1990 w250 auto 1400stall tc, 100gph electric pump, dual filters, 1/2" lines to ip,
he351cw, 3"crossover, stainless waterjet airhorn, 4"dp to 5" dual miters
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Re: Overheating at low RPM

Postby bnorth » Mon Apr 19, 2010 8:30 am

Are you putting in a cummins t-stat or a jobber?
92W250 club cab, a518, d60/d70lsd, front leveled 2", indexed fuel pin, ground washer, 4" straight pipe, K&N, high idle screw MIA, 9" pro comp lights, OD on a potentiometer, 366 spring waiting to go in.
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Re: Overheating at low RPM

Postby DieselSharker » Mon Apr 19, 2010 6:05 pm

The overflow is filling up a lot. It can be at normal level when I leave and when I come back it is bubbling. Of course now my truck hasn't moved in 3 weeks because of the rear end but I'll be done with that this weekend. I don't know how I could have blown the head gasket with the stock turbo and never really romping on the truck too much.

I had been putting O'Reilly t-stats in before but before I blew my rear end I was driving around for a week with the thermostat out and it was still heating up at low RPM, but mainly after I had been on the highway and then come into town. It is about normal on the highway without the thermostat.
1991 W250 727, 6" Skyjacker, 37" Dick Cepeks, Gear Vendor O/D, SDX 5x.014s, BHAF, home-made 4" intake, VE tuning & DennyT II pin, GDS 60mm with 16 cm, CoolerTubz 3" crossover
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Re: Overheating at low RPM

Postby dieseldawg604 » Mon Apr 19, 2010 6:40 pm

91StrokerSmoker wrote:The overflow is filling up a lot. It can be at normal level when I leave and when I come back it is bubbling. Of course now my truck hasn't moved in 3 weeks because of the rear end but I'll be done with that this weekend. I don't know how I could have blown the head gasket with the stock turbo and never really romping on the truck too much.

I had been putting O'Reilly t-stats in before but before I blew my rear end I was driving around for a week with the thermostat out and it was still heating up at low RPM, but mainly after I had been on the highway and then come into town. It is about normal on the highway without the thermostat.


my friend you most definitly have a blown headgasket!!!
my overflow did the same thing!!
when you replace it use a cummins gasket mabey even a .020 over size
and be sure to retorque, retorque and then retourque again!!
sorry about the bad news man!!!
1990 w250 auto 1400stall tc, 100gph electric pump, dual filters, 1/2" lines to ip,
he351cw, 3"crossover, stainless waterjet airhorn, 4"dp to 5" dual miters
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Re: Overheating at low RPM

Postby DieselSharker » Tue Apr 20, 2010 4:59 am

Some time this week I'm going to attempt to borrow a pressure tester from vatozone or something and see if I can locate a leak somewhere. The thing that gets me is that it runs at normal temp on the highway but jumps up when I'm driving around town after it has heated up. I'm really hoping it's not the head gasket but in all actuality the head gasket would be cheaper for me to replace than the radiator.
1991 W250 727, 6" Skyjacker, 37" Dick Cepeks, Gear Vendor O/D, SDX 5x.014s, BHAF, home-made 4" intake, VE tuning & DennyT II pin, GDS 60mm with 16 cm, CoolerTubz 3" crossover
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Re: Overheating at low RPM

Postby JBradley500 » Tue Apr 20, 2010 2:50 pm

this gives you a great opertunity to put in head studs! you lucky dog.
92 W250- K&N Filter, Smoke Screw Adjusted, Synthetic Transmission Fluid.
340hp/662tq (8/24/12)
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Re: Overheating at low RPM

Postby DieselSharker » Tue Apr 20, 2010 11:21 pm

If I had a job I would be studding it but for right now I can only afford to replace them. I got quoted $130 for the .020 gasket and $130 for the allen head bolts shipped.
1991 W250 727, 6" Skyjacker, 37" Dick Cepeks, Gear Vendor O/D, SDX 5x.014s, BHAF, home-made 4" intake, VE tuning & DennyT II pin, GDS 60mm with 16 cm, CoolerTubz 3" crossover
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