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Discussion Starter · #1 ·
My truck has been running cool ever since I got it couple years ago. Last year we had a mild winter and it didn't bother me much. But since then, I've replaced the glow plugs (60G), Injectors (Bosch OEM) and two new thermostats (ACDelco OEM). Its been cold the past couple mornings, but even on the warm days the themperature doesn't rise above the 180'ish mark (Second big mark). You can bairly call it "warm" air coming out of the vents, and when I turn the defrost on, it gets cold quick.

Truck doesn't heat in summer either. Usually hangs around the same mark, slightly higher while pulling a load up hills, but still never over the middle mark.

Possibly stuck thermostats, even though they new? Sure was a waste of $60 if they are. Any other ideas?
 

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Check for proper water flow through your heater core.

Also, the servo motors on the heater doors tend to break and quit working.
 

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When on defrost the AC is on which might make feel cool as well.
 

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I'd run it until it gets to the normal running temp, then start feeling the heater hoses with the heater on hot. If one is feeling hot/warm and the other is cold then you have a clogged heater core and need to back flush it. If they are both about the same temp, then your problem is elsewhere.
Good luck
Don
 

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My truck seems to blow warmish air with the dash gauge on or above 160ish F. It get warmer once its at 180F but again it blows enough heat before to make a difference. When gauge reaches 180F heater air should be toasty (if its not really cold out say above 25F).

Do you have a vent leak? Sometimes my stepper motors are slow changing air ducts over to defrost and I get a delay and eventual thump in the dash then air rushes out defrost vent. You may have something going on similar thats allowing cold air in and chilling the heater core air. What do you think? Does the climate temp control knob make appreciable difference in air temp?
 

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The A/C is only on in little 'bursts' to give a shot of cool air every so often. Sounds stupid, but have you had the system flushed? Didn't know about the motor thing, but it sounds like it would only control the direction of heat, not the amount. And yes the stats can be bad, even new from OEM. Pull and test them and if under warrenty, try new ones OR go with ones from site vendor(s).

Hope this helps,
 

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Discussion Starter · #7 ·
Ok, I did peak behind the glove-box towards the middle of the dash and watch that actuator move back and forth while the temperature control was being moved. So I recken that part of the system is working, now weather the door is broke or not, I can't tell.

As for the heater core, I'll check that in a little bit and see if both hoses are hot. I'm hoping that might be the problem, although I hate having to think about replacing it.
 

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It sounds like heater core to me. As they said, check the hoses for temp, a sure sign what's goin' on. You don't need to remove it to flush it. Just pull the 2 hoses off at the firewall and flush till it's clean and good flow.
 

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Discussion Starter · #9 ·
Thanks for the reply's. I finally did get to check the heater hoses, and sure enough, the pipe coming from the thermostats are a lot warmer than the hose going to the radiator. SO, I recken the heater core is one problem anyways.

What's a good cleaner to use to flush out the core? Purple Power or dish detergent? Its gotta be sludge stopping up the core, right?
 

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I think I would take both hoses loose and pipe in fresh water to the hose that is cold and see what comes out of the other hose, maybe catch it in a container.
 

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In the past, I would add a box (from laundromat) or two of POWDERED tide to cooling system. Run it for about a week and drain it good. Prestone used to make a kit that would splice into one of the heater core hoses that would allow you to attach a garden hose and back flush the system. The tide will find the weak spot as it is abrasive, so don't go more than about a week, or just use a commercial flush from store.

Hope this helps,
 

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Something else to check is how the heater core is situated in the housing. I had problems with mine until I found that the core had dropped and was laying on the bottom of the housing. The air was just flowing over it, not through it. Instead of pulling the entire housing out and fiddling with it I was able to just prop up the inboard end and brace the whole thing in place with a little block of styrofoam in the bottom of the box on the outboard side.
 

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When I first bought my truck and it was totally stock it's normal operating tempurature was between 195 and 205 the heater worked great and it got 3-4 more mpg than it does now.

Then 3 years ago I made a bunch of performance mods within a short time frame and it wasn't winter so I didn't notice the heater issue.

Somewhere in all the mods in my sig. My normal operating temp dropped to 180 fuel, economy dropped to @13mpg and my heater seems to take a very long time to start blowing hot air.

I want my temp back up so I can start getting warm and hopefully regain some of my fuel economy.

I have posted this issue several times over the years and no one seemed to be experiencing the same issues until this post came along.

I haven't concerned myself too much with this issue because my truck hauls a$$ and has been running strong without any other issues for the last 25K miles.

I still keep looking to see if someone finds the reason for the lack of desired operating temp.

Even with new 205degree Robertshaw t-stats the temp will not go above 180 degrees.

The temp sensor is working I have one on each head and both give the same reading.

I was going to try running without my fan for a couple of days since I only have a 4 mile commute, just to see what happens.
 

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I had to replace my evaporator core about 4 months ago, had a leak.
(not a job I recommend) :mad: and I replaced the heater core as well.
I thought I had a fan problem too, not getting much airflow.
The evaporator core was almost completely plugged, dirt, hair, years
of unfiltered air flowing thru it had closed up the fins. I now can feel
the fan blowing on low again.:)
Point I am slowly getting to is, do you have good airflow?
Or do you need the fan on high just to tell it's on? You may have the
same trouble I had.
 

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I'm sure that my heater core could use a cleaning, but I don't think that it is preventing my motor from getting up to the operating temp which should be dictated by my thermostat rating.
 

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My truck has been running cool ever since I got it couple years ago. Last year we had a mild winter and it didn't bother me much. But since then, I've replaced the glow plugs (60G), Injectors (Bosch OEM) and two new thermostats (ACDelco OEM). Its been cold the past couple mornings, but even on the warm days the themperature doesn't rise above the 180'ish mark (Second big mark). You can bairly call it "warm" air coming out of the vents, and when I turn the defrost on, it gets cold quick.

Truck doesn't heat in summer either. Usually hangs around the same mark, slightly higher while pulling a load up hills, but still never over the middle mark.

Possibly stuck thermostats, even though they new? Sure was a waste of $60 if they are. Any other ideas?
It is always a good idea to test your thermostats before installation to see what they are doing. Heat water in a pot, use a piece of string to tie to the thermostat, use a thermometer to see what the temp of the water is. You can see what temp the thermostat starts to open, when it is fully open, etc. It's surprising.

Good luck,

Rob :)
 

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It is always a good idea to test your thermostats before installation to see what they are doing. Heat water in a pot, use a piece of string to tie to the thermostat, use a thermometer to see what the temp of the water is. You can see what temp the thermostat starts to open, when it is fully open, etc. It's surprising.

Good luck,

Rob :)
Does your modified plenum mean you dont have EGR anymore.

My truck ran the same temp @180 with 195 Robertshaws (from Kennedy) then I switched to 205 Robertshaws (from Evans) to try and get the temp up.

I was thinking maybe my fan clutch was a problem, but this doesn't explain why my motor isn't keeping the coolant inside until the t.stats open.

I also thought that since I don't have EGR anymore, maybe my coolant temps just wont go up to 205, but then it would seem that every F motor would be running @180.

Would some folks with F motor's enlighten me as to your average normal operating temps.

Maybe weighing in at a little more than 1/2 the weight of a heavy duty truck my Tahoe just isn't working the motor enough to keep the heat up.

I have heard about blocking the radiator with cardboard or something like it to reduce air flow over the radiator, but this doesn't explain why my t.stats are not stopping coolant flow through the motor until the desired temperature is reached.
 

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MDT, my Tahoe never has any trouble cranking the temps up. The EGR isn't gone, but it is not doing anything. I am using the 180 degree stat and the electric fans. I find that when the thermostat fully-opens, the coolant temp (according to the dash gauge) is about 195-200. (I'm not doing this with sophisticated equipment - I just put my hand on the upper radiator hose to see how much hot coolant is going through it) I have programmed the fans (which are controlled electronically by a controller and a temp probe that inserts into the radiator between the cooling fins) to turn on around the same temp.

Perhaps your fan clutch is engaging too soon, and keeping the temp down too much. It's hard to figure out, since the fan is never 'completely' off, like with the electrics. With the electrics, the fans are TOTALLY off until they hit the temp I set, then things go on to about 60% speed. It's all adjustable. I'm considering turning it up slightly, but I wish the temp control was digital, so that I can just punch in the temp, instead of turning the little potentiometer.

Sincerely,

Rob :)
 

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Thermostats have been known to be defective, right out of the box. Do you have access to one that is known to be working properly?

If you don't have a good thermometer to stick in the boiling water pot, that is the next best thing.

Basically, the only thing I know of that can cause low coolant temps in a set up like ours is a faulty thermostat.

Of course your gage could be off too.
Try checking the temp with an infared thermometer. They are pretty cheap at Harbor Freight,. 20 bucks in the last flyer.
 

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My '94 is a single-thermostat system, but I know that mine needed to have the genuine ACDelco thermostat in it... the one that Bill Heath pulled out had a different-length bypass plunger on it, even though it was the (technically) right one for the truck. The plunger wouldn't even REACH the flange it was supposed to seal on.

Does the 97 system work like that? It seems to me that the big difference between MDT's and Rob's tahoes is MDTs 97 upgrade and robertshaw thermostats.

I agree that the thermostats should hold coolant IN until operating temp is reached, and should maintain that minimum temp level. If that ain't happening, you have thermostat issues.

All I'm suggesting is that those issues might NOT be opening temp...

Then again, it should be obvious that I don't know swuat about the 97 cooling dual-therm setup.

Jim
 
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