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Chaplain Kent 08-13-2007 06:04 AM

We recently took two short 300 mile trips on both the engine overheated. The only thing I have done was to close off the heater vents coming into the cabin. I opened up the vent on the driver's side and the temp immediately went to normal and stayed there for the rest of the trip. There is plenty of coolant and I have a fancy low coolant buzzer warning system which does work. Could the closed off heater vent be the cause of the increased temp?

moosetags 08-13-2007 06:57 AM

Based upon my experience with old cars, the answer to your question is yes. When you allow coolant to run through the heater, you are in effect lengthening its trip through the cooling system, allowing it to cool further before returning to the engine. In an old car you can turn the heater on and watch the temperature gauge go down.

When you have to use this trick to lower the temperature, it is usually because of having to idle in traffic for an extensive period. Sometimes this type of overheating is the result of low coolant level or a malfunctioning mechanical thermostat.

bobkelly 08-13-2007 07:35 AM

Start by checking/replacing the thermostat. Take the old one out and test in a pot of water heated on the stove. Use a thermometer to check the opening temp. If this doesn't resolve the problem, troubleshoot the rest of the cooling system, but if this is not your cup of tea, have a specialist do it.

Overheating engines controlled by pumping heat out thru the heater core is just a stop gap measure. Repairing a heat damaged engine is major bucks.

swebster 08-13-2007 07:54 AM

Yes, when you think about it the heater core is a little "mini radiator". I agree on the could be stuck closed (or partially closed) and the heater loop is acting as your bypass.

If the thermostat is OK then you may have a radiator blockage. When they replaced the engine last year did the update the cooling system too?

safari57 08-13-2007 09:41 AM

Mr Gasket Co has a high performance thermostat. Far less prone to malfunction, and those of our group who have installed them are very pleased with the results. They are not the cheapest at around $18C but they sure do work (and when compared to the original you can see the difference in quality).


FreshAir 08-13-2007 10:21 AM

Vacuum advance
I have had several overheating experiences directly caused by a malfunctioning vacuum ingnition advance, The first clue would be a lack of uphill power even when everything appears to be running fine.

Neil Holman

guy99 08-13-2007 10:49 AM

Couple of thoughts:

A cooling system pressure check is always a good idea although since you don't seem to be loosing coolant, it doesn't sound like you have a coolant leak. But loss of pressure will cause it to run hot - although I doubt think opening the heater valves would modify this.

I understand that it is possible to get a 'high flow' thermostat - if you replace yours this might be worthwhile.

Is it possible that your fan clutch is malfunctioning? When your rig heats up, can you hear the fan come on?

Is there any operating malfunction which will make a diesel run hot?

Beginner 08-13-2007 12:09 PM

Chaplin Kent
Turning on the heater is an old towing trick. It just adds radiator capacity as previously alluded to.
You said you were on two short trips. I have to assume that the unit overheated whils traveling down the highway(45 MPH+), then you turned the heater on.
1. I would suspect the thermostat first, probally stuck, a little open. If it were completely closed the radiator would boil over quickly and violently (very little water circulating through the engine). This is indicated by the engine taking a long time to warm up when its cold outside. Go and get a good thermostat. The ones Advance/Autozone carry might last a year of two (in my experience) so go to the dealer (Chevy Chassis?) or NAPA and get the most expensive one they have (unless of course you enjoy changing the thermostat). There was one suggested on one of the other posts. I have had a lot of thermostats go bad over the last few years (most were Robertshaw, some Stant).
2. Fans have very little effect over 30 MPH, however, they can impede air flow at higher speeds. Does your unit have electric fans or a "clutch fan" to pull air through the radiator?
Electric Fan: Does it/they come on at upper temperature? Can you turn it freely?
GM Clutch fan: When you start the engine and hold the idle around 1200 RPMs do you hear the fan roar then get quiet after 20 to 60 seconds? If it does it could be OK.

Turn the engine off (very important for the next step).

Grab the fan blade tip. Using no more force than it takes to pick up a large coffee from 7-11, try and make the fan blade move forward and backward, not left and right. If it feels loose, have an experienced mech look at it. Rotate the fan. Does the clutch (big round finned thing that the fan is bolted to) make dragging noises or does the rotation feel rough or is it leaking some sort of fluid where the shaft from the water pump goes into it? If so take it and have it looked at, it could be time for a new fan clutch.
That is the best I can do. I know little about diesels but have worked a few overheating problems in gas engines before.

Chaplain Kent 08-15-2007 03:43 PM

And the winner is. Fan clutch or strangely enough the fan was not turning at proper rpm. And yes, I do need to allow the hot air from the heater to exhaust even when the heater is turned off there will be a small amount of hot air escaping. My wife felt that it was like the fires of hell so I stuffed rags into the vent on her side and while I was at it stuffed them in my side as well. When I pulled out the rags out on my side it allowed just enough hot air to escape to keep the engine from overheating. The mechanic will reroute the heat on the passenger side. I have moved one of the dash air vents to blow cold air down to my feet. I still wish there was some way I could install an outside air vent. Seldom do we even need AC all we need is good old fresh air other than the windows.
Thanks for the help guys.

jcanavera 08-15-2007 04:09 PM


Originally Posted by Chaplain Kent
And the winner is. Fan clutch or strangely enough the fan was not turning at proper rpm. Thanks for the help guys.

Not only does a bad fan clutch potentially invite overheating, but it can also cause your air conditioner compressor to blow a seal due to high head pressure. It happened to me once and obviously required two repairs. The tell tale indicator was the spray of oil that came out of the compressor when the seal blew. It left a tell tale oil trail on the inside of the hood. The mechanic leaned over and spun the fan blade easily. He knew what the cause of the A/C failure was....


safari57 08-15-2007 05:44 PM

Great news! Problem solved means camping once again.

Glad that's all it turned out to be. The next time you have your antifreeze changed get them to install a high performance thermostat at the same time. It's well worth it.


Beginner 08-15-2007 06:50 PM

Hot Air
Why not install a hot water cut off in the heater hose that feeds the heater core.
Turn off the water in summer, On in the winter.

A/C compressor: most GM compressors have a high pressure pop off that vents the freon out if the head pressure gets too high. If that is the case the compressor might well be ok. A quick check with dry nitrogen (150 PSI or so) will tell the tale.


vswingfield 08-15-2007 07:01 PM


Originally Posted by Beginner
Why not install a hot water cut off in the heater hose that feeds the heater core.
Turn off the water in summer, On in the winter.


Hi Beginner,

The whole point was using the heater as an extra radiator to combat the overheating. He could have just turned the temp down and stopped the heat from entering the cabin, but the engine overheating would have increased.

Glad it was a cheap, simple repair Chaplain Kent.


sunset19 08-15-2007 07:17 PM

Antifreeze concentration
If all the above suggestions don't solve your problem, check your antifreeze to water ratio. Too much antifreeze and not enough water as a coolant is very inefficeint and will cause overheating. If you for some reason topped off the system with straight antifreeze or mixed it to rich to start with, this could contribute to your problem.
I had a customer pull that one on me once and it took me a month to find out what was causing his problem.

Follow the mixing instuctions on the can.
Just a thought,,..??

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