'96 GMC 2500 overheating problems...or not?

Bought this to pull my racecar with and maybe drive here and there. It's a '96 GMC 2500 ext cab, has 5.7 Vortec 350 V8 w/ 4L80e transmission, and heavy duty everything from trans cooler , to bigger radiator to you name it. The guy I bought it from had just replaced the heater core, and said he had the upper intake seals replaced a few months before, and the water pump was also leaking ever so slightly. Also the temp gauge was not working. I installed a new water pump, new hoses including the bypass hoses, thermostat(195), and also both coolant sensors. Ran the engine till thermostat opened and finished filling to get all the air out. Engine runs fine, but temp rises to around 210-215, then will creep up to 220-225, then cool back down to the 210-215 range. It cycles like this pretty regularly when driving. I thought this to be way to hot, so I pulled the radiator and had it flushed and reinstalled, no change same temps, changed out thermostat to a failsafe (195) version, no change in temps, fan clutch seemed week so replaced it with a new severe duty version, same temp ranges and same cycling of the temp ranges. It seems to run ok, doesn't seem to be overheating, can't smell any coolant, no valve rattling or valve knock on acceleration. I thought maybe a blown head gasket but I have checked for water in the oil repeatadly and found all ok and compression checks Aok. Also I'm not loosing any coolant either. I'm frustrated with this and don't know where else to look for a problem???

Bought this to pull my racecar with and maybe drive here and there. It's a '96 GMC 2500 ext cab, has 5.7 Vortec 350 V8 w/ 4L80e transmission, and heavy duty everything from trans cooler , to bigger radiator to you name it. The guy I bought it from had just replaced the heater core, and said he had the upper intake seals replaced a few months before, and the water pump was also leaking ever so slightly. Also the temp gauge was not working. I installed a new water pump, new hoses including the bypass hoses, thermostat(195), and also both coolant sensors. Ran the engine till thermostat opened and finished filling to get all the air out. Engine runs fine, but temp rises to around 210-215, then will creep up to 220-225, then cool back down to the 210-215 range. It cycles like this pretty regularly when driving. I thought this to be way to hot, so I pulled the radiator and had it flushed and reinstalled, no change same temps, changed out thermostat to a failsafe (195) version, no change in temps, fan clutch seemed week so replaced it with a new severe duty version, same temp ranges and same cycling of the temp ranges. It seems to run ok, doesn't seem to be overheating, can't smell any coolant, no valve rattling or valve knock on acceleration. I thought maybe a blown head gasket but I have checked for water in the oil repeatadly and found all ok and compression checks Aok. Also I'm not loosing any coolant either. I'm frustrated with this and don't know where else to look for a problem???

this with the complete coolant system componet failures, to ME appears to be an engine that did not have the proper amount of coolant[antifreeze ] in it ...with the use of tap/well water and low amounts of antifreeze these componets failed ...it is possible that the internal passageways are corroded..

using an acid flush , draining the engine using the block drain plugs...then see if the acid removed the corroded metals ..

when the radiator was flushed was it dirty ? should be shinny looking not dull with particles ...

It appears many that live in the warm areas do not use the proper amounts of anti freeze because it in their minds is not required since it does not get to freezing that often...

Actually, the truck spent alot of it's life up north. The owner had moved down here last year. It was used as a work truck and daily driver for him and probably neglected.

The cooant system was very dirty. The inside of the radiator was all brown and icky looking. The radiator looked pretty nice after it was cleaned. Also, before having the radiator cleaned, I added a box of baking soda to the system and let it run for a few hours to try and flush out the engine and stuff.

As far as down here in the south neglecting our antifreze, most everybody I know is really carefull about it. Where I live in the winter time it still gets down into the 10-20 degrees range, and heck in spring and fall it can be 70 degrees in the day and then 30 degrees at night so you have to really watch it. Of course in the race car I can't use antifreeze, so it gets drained regularly just in case.

With the temp gauge fluctuating as you described I might suspect there is still air in the system. Keep an eye on the coolant reservoir and keep it filled to the cold line in the AM. Takes a while to purge all of the air out.

As for the temp being that high. It could be real or not. Two ways to check it are a good mechanical gauge in the driver side head or an engine capable scan tool that can read ECT temp going to the computer.

Good luck.

most likely the gm approved dexcool was mixed with non GM approved dexcool also the use of water other than distilled causes this black goop to GROW inside the cooling system ...acid cleaner then complete engine draining with the baking soda flush will need be done at least once ...maybe 2X ,,,,MY 1996 LT1 engine the interior coolant system metals are still clean ...65% dexcool to 35 % distilled ...same on my 2000 5.3L ....

I have had to do this flush on vehicles that were abused as descibed ,,it works ...dont forget to blow out the heater core every time also ..

As far as down here in the south neglecting our antifreze, most everybody I know is really carefull about it. Where I live in the winter time it still gets down into the 10-20 degrees range, and heck in spring and fall it can be 70 degrees in the day and then 30 degrees at night so you have to really watch it. Of course in the race car I can't use antifreeze, so it gets drained regularly just in case.

what they usually do is have leaks ,,,,when the warm weather comes in they just put tap water in it ,,,,then if it does not call for cold weather they leave the water only in the system .maybe for 2 years ..believe me this is how they act ...dexcool costs money ....


then they have volts in the radiator ,,,and the complete system is corroded leaks everywhere ...

what they usually do is have leaks ,,,,when the warm weather comes in they just put tap water in it ,,,,then if it does not call for cold weather they leave the water only in the system .maybe for 2 years ..believe me this is how they act ...dexcool costs money ....


then they have volts in the radiator ,,,and the complete system is corroded leaks everywhere ...


Although I do appreciate your replies, I am asking for help, not insults and criticism. The bastard reject that owned this truck for quite a few years actually was from up around your area, not mine, so enough with all of the negative posting about us here in the south.

I don't really care about what everybody does or doesn't do with their vehicles, I just want help fixing mine. I have been working on my cars, restoring cars, and racing cars for over 25 years but I don't know everything and don't pretend to. I also know when to ask for another opinion on a problem, and that's what I'm here doing!

As far as dexcool, I wouldn't put it in my worst enemy's vehicle for fear of the same kind of terrible problems associated with it. I don't use it ever, I always flush it out and put in real antifreeze , the green stuff. As far as I'm concerned the dexcool is great for GM cause about the time the warranty runs out on their product the coolant system is so damaged by the dexcool coolant that is has to be replaced on the customer's dime.

Although I do appreciate your replies, I am asking for help, not insults and criticism. The bastard reject that owned this truck for quite a few years actually was from up around your area, not mine, so enough with all of the negative posting about us here in the south.

I don't really care about what everybody does or doesn't do with their vehicles, I just want help fixing mine. I have been working on my cars, restoring cars, and racing cars for over 25 years but I don't know everything and don't pretend to. I also know when to ask for another opinion on a problem, and that's what I'm here doing!

As far as dexcool, I wouldn't put it in my worst enemy's vehicle for fear of the same kind of terrible problems associated with it. I don't use it ever, I always flush it out and put in real antifreeze , the green stuff. As far as I'm concerned the dexcool is great for GM cause about the time the warranty runs out on their product the coolant system is so damaged by the dexcool coolant that is has to be replaced on the customer's dime.


I did not attack you or your southern citizens ...

I said vehicle owners in warm climates !

I buy new vehicles ,,,when this occurs your in control. especially when all repairs and servicing is done buy ME ...

I have never had any of the issues you believe DEXCOOL is reponsible for ...I don't get vehicles every few years in 40 yrs of owning vehicles I am working on number 4....

distilled water must be used , dexcool with the approved label is dexcool ...compatible coolant is NOT DEXCOOL !

so obviously do what ever you want ....but now you know one vehicle owner however , that has not had any coolant issues with this method of coolant maintenance....

with tap water you will continue to get organic material growing within your system ....

acid must be used ,,,thats the only way to clean this all out... I'm sure your aware that your coolant system has a total capacity of 4.25 gallons ...for me that means 2.5 gallons of coolant [antifreeze]..

Although I do appreciate your replies, I am asking for help, not insults and criticism. The bastard reject that owned this truck for quite a few years actually was from up around your area, not mine, so enough with all of the negative posting about us here in the south.

I don't really care about what everybody does or doesn't do with their vehicles, I just want help fixing mine. I have been working on my cars, restoring cars, and racing cars for over 25 years but I don't know everything and don't pretend to. I also know when to ask for another opinion on a problem, and that's what I'm here doing!

As far as dexcool, I wouldn't put it in my worst enemy's vehicle for fear of the same kind of terrible problems associated with it. I don't use it ever, I always flush it out and put in real antifreeze , the green stuff. As far as I'm concerned the dexcool is great for GM cause about the time the warranty runs out on their product the coolant system is so damaged by the dexcool coolant that is has to be replaced on the customer's dime.

Right on man: The dependent/fanatic needs to have this kind of feed back, although he's got it before it rarely does any good. Hang in there and hope for more constructive advise without the broad criticism he gives out.

Truck runs at 215F still? Without A/C on... that's too hot. It should be running around 195. Truck does have fully functional fan shroud and you can hear the fan clutch kick on when hot?

You might want to install a mechanical temp gauge to verfiy temps...

Yes, still runs about 210 most of the time except when going up and down.Yes, the fan shround and everything else is intact, and the Clutch fan is a new severe duty unit that I installed. Actually seems like it is engaged most of the time.

OK... and you say you are not loosing coolant, you have checked for head gasket issues. you had radiator cleaned and replaced fan clutch, water pump and thermostat.

Assume you are running a 50/50 mix of coolant / water, assume the radiator is not restricted (mud and crap between the radiator and A/C condensor, assume nothing is restricting the air flow to the radiator. Assume the check engine light is not on and you have no trouble codes set. Also its assumed you have the OEM radiator fan.

Normal driving you should be around 195F, going up fairly steep grades with A/C on,it would be normal to reach 210F or so.

I only use GM brand radiator fan clutches as I don't think the aftermarket does a good job on those. If radiator had a lot of sludge in it....the engine block could also have the same crap in it.

One thing came to mind.... Make sure you have the correct thermostat for that engine....they have a little "disk sensor mechanism" on the bottom, they are not like the old school GM 5.7 thermostats. Verfiy that. You can see pics of these on various parts web sites..to inlcude Rock Auto, Autozone and others.

Yes, that is the style thermostat that is installed. I actually thought that the first thermostat that I installed was faulty, so I took it back to the store and double checked that it was the right one and it was. I went ahead and bought a failsafe version and installed it just to make sure. I had a '96 Tahoe a few years back that had this same engine and I never had it to run hot, very seldom even when pulling a trailer did it even approach 200 degrees. I guess I need to do the strong acid flush to clean out the engine block cause the radiator was very dirty before I had it cleaned.

I don't know at this point.... based on what you are saying, apparently you have all the correct physical parts, and none of the other issues raised seem to apply.

As suggusted you might want to get a mechanical temp gauge to check temp for sure.

Also might want to get a ECM scan tool to verfiy engine temp.

You could simply have a bad temp gauge or temp sender.

If radiator looked like crap, you can bet engine block does too.

I sold the 92 K1500 I had. I have a 99 5.7 Vortec in my land cruiser and it runs around 180F in the winter and 190 or so in the summer, but I have electric fans on it with a crude meachanical switch that controls them.

My 01 suburban with 496 runs about 195F all the time...summer , winter, hills.. whatever..

Truck should run around +_5 degrees of the thermostat with OEM setup...with A/C on and under load..210 would be reasonable , with high ambient temp. But truck should run right around 195F with A/C off and general driving.

I appreciate the suggestions. I guess I am going to try the acid flush anyway just to make sure that the engine block is not clogged up, and also I have a mechanic friend that has a Matco scanner he is going to swing by and hook up for me and check the temps. My friend also said that sometimes the catalytic converters get clogged up on the vortec trucks and will make them run hot, but my truck still has plenty of power and no bad exhaust smell or anything, so hopefully that's not the problem. Again, thanks for the help, I will post what I find out.