thermostat or water pump?
 

ok guys, i need some opinions about this problem with my 1:1 truck.

when you are driving, and going at a constant speed, the temperature is normal for where it should be (about 210) and the heater blows nice normal heat. BUT, when you let off the gas, the temp goes way up, to the point where a CHECK GAUGES light comes on, and the heat goes cold, but as soon as the gas is applied again, the temp goes way down (around 150-160) and the heat is still cool.

also, if this helps, there isn't that usual smell of overheated coolant, and in fact, all my coolant levels are OK.

so what do you guys think, is it my thermostat or water pump i am having issues with?

thanks for any help!"thumbsup"

BTW, what truck model and year? and miles?

More than likely its a thermostat (good, easy, cheap fix). Check around the water pump to see if there is any fluid leaking. If there is not, and you don't hear a "chirp" or wine at rpm, its more than likely its not the pump. Unless the impellers of the pump are beginning to crack, then it will fail very shortly. That is a very unlikely scenario.

I would swap out the thermostat (< $30) and see if that fixes it.

If it is neither of those, at worst case scenario it is a blown head gasket (NOT FUN, or cheap).

It is likely the t-stat...but I would swap out both. You have to remove the water pump to get to the thermostat...and pumps are fairly cheap.

BTW, when a water pump goes bad, it is usually the bearing that fails...and then you'll see radiator fluid all over the place.

Is the belt on the water pump slipping? just when you say it blows cold air when you let off the gas, that would be the water pump not circulating the coolant through the heater core, I'd say water pump on the verge of seizing or loose belt good luck

oh yeah, i forgot to mention it is a 1992 full size chevy with the 4.3L V-6 in it, with around 220k miles on it.

and i think i will try the thermostat first tonite, it's in a nice, fairly easy place to access on my motor.

I can't speak for that motor, but some Dodge engines have problems getting air pockets in the head. They have a bleeder plug that needs to be removed and anti-freeze poured in the hole. That is exactly how the instruments respond in those vehicles when they have an air bubble in the head. Again, may not be the case with your engine.

On what engines? I don't think I have seen a GM engine that you have to remove the water pump.



Thermostat. We had a similar problem, a new thermostat fixed it.

There are other manufacturers of vehicles besides GM...

I'm an ASE auto technician. (not that the ASE badge really means anything, though)

I think just checking your antifreeze level should help. It sounds like you're about a gallon low or more.

it sounds like what is happening is there is enough coolant in the motor that at speed it's pumping it through the heads and intake (where the temperature sensors are located) but when you let off the gas the water pump slows, the coolant flow stops, and there's a pocket of cold air that the sensors can't read in the coolant passages in the intake.

AND, since the coolant sensors (there are two of them, one is for the computer to run the EFI, the other is for the guage on the dash) can't read air, only liquid, the guages go all screwey and it trips the check engine light.

Try adding coolant first.

AND MAKE SURE YOU ADD THE RIGHT KIND! DO NOT MIX ORANGE AND GREEN COOLANT TOGETHER! If you mix the EXtended-life stuff and the regular stuff together you will get a cooling system that is full of sludge that resembles baby food. And then it'll get REALLY expensive.

In fact, if you have the orange coolant, drain the whole system, flush it with lots of clear water from a hose, and replace it with the old green stuff.

EXTENDED LIFE ANTIFREEZE IS THE WORST THING ANYONE EVER INVENTED! IT DESTROYS GASKETS AND EATS PLASTIC!!!!

Now, once you've determined if the collant level is low or not, then you have to find where the LEAK is. The first thing to do is pull your dipstick after an extended run. Is your oil a light tan and foamy? If it is, I'll bet the intake manifold gaskets are leaking coolant into your motor. (this is a very common problem on ANY GM motor with plastic intake gaskets or a plastic intake manifold. That runs Extended-life antifreeze.)

Normally, this would be a sign of blown headgaskets, but that's really rare on the 4.3's.

Hope this all helps. If you have any other questions feel free to ask!

cool, thanks for the input. i will check that tonite after work.

Hey Steel City,
Thermostat fer sure... I had an s-10 with the 4.3 and ran into similar problems.. If changing the thermostat doesnt work go to advance and use their code reader to see what the trouble codes are, if you have a manual for the truck it will tell you what the codes are.. If it were the water pump you would be loosing fluid all over the place and most likely have your rad hoses expanding and pissing fluid out... You may also want to put a new rad cap on it while your at it, they can get crapped up over time as well...
Good Luck bro!
JAy
Wpac

Psssh... luckyyyy... my xj's out in my driveway right now on jackstands with the tank dropped... and still no juice to the fuel pump:-(

every thread on naxja relating to my symptoms says it's the CPS so i guess i just wasted $125 on the fuel pump...

just did the water pump/t-stat/hoses last month and a new battery last week... i'm hatin' this winter so far.

Good luck"thumbsup"

I've had water pumps (2) that the the impeller broke off the shaft, all the same problems you have, there was enough airflow from the fan and driving to keep the motor cool at speed, then at a stop it would rapidly heat, the AF would fill the overflow and there wouldn't be enough left in the RAD to keep the engine cool anymore:-( and it would eventually overheat and I would have to shut it off

a '92 doesn't have OBDII, no codes.

I vote for pump. The impeller vanes have probably been eaten away, causing loss of flow.

actually there are scan tools for OBD-I, but many cars do not need one, dodge has a sequence of turning the key on and off without starting the car and you watch the check engine light blink. you count the # of blinks and the number you record is the # of blinks before it pauses and blinks out another #. on Gm's there are certain terminals on the computer plug that you link with a jumper wire or paper clip, and then it will blink codes. i do not remember the exact terminals you link but if you want to do it you can probably find info online. and what the guy who said he is ASE certified makes sense, and i would trust his opinion over everbody elses here. he is qualified and probably does this for a living. i am not a mechanic myself but i am taking natef certified programs in school and have learned alot of this stuff. i would not reccomend just replacing what you think the problem is, you should be sure before you make a repair, the thermostat and water pump could both be good, and replacing them would be a waste of money. the thermostat doesn't make much sense because it is operated by heat, so engine speed should have no effect. the water pump would make more sense, because the engine spins it and maybe its ability to move coolant has been reduced. i personally would just add coolant to see if it fixes it. it is the easiest and cheapest fix.

you're rad, or hoses my be blocked also;-)

Fan Clutch!!!! Fan Clutch!!!! Fan Clutch!!!!

Well considering the Engine he is working on is a GM engine.:roll:

And when did he mention this?

That's right....AFTER my first post.

Use your brain kid...:roll:

Whoops.. in that case its easier, just make a jumper (used paper clip) and jam it into the port under the dash, per the hanes manual and see what teh check engine light flashes, the back of the manual will give you a list of what they mean, I had a 92 and one that was earlier, and that method worked for me...