LT4 LT4 Overheating Question

[dlt4]

My LT4, with 96K miles, has an overheating problem that I can't figure out. The water pump was replaced about 25K miles ago and the heater and radiator hoses were replaced about 9K miles ago. Back in May the temps started fluctuating, up and down from 200 to the 230s. I replaced the thermostat with a 180 and everything was fine until a couple weeks ago. Now it's behaving in a similar manner, temps moving up and down, with a max of 240.

This morning I took the cap off and topped the antifreeze off to just below the filler cap. Started the engine, and when the temp reached somewhere around 140 the coolant started moving up and with some flowing into the overflow tank. The level moved up and down until somewhere in the 160-170 range, then it started bubbling out like crazy. Does this sound like a blockage somewhere? I suppose the thermostat could be bad already. Any ideas would be appreciated.

 

[gmjunkie]

My LT4, with 96K miles, has an overheating problem that I can't figure out. The water pump was replaced about 25K miles ago and the heater and radiator hoses were replaced about 9K miles ago. Back in May the temps started fluctuating, up and down from 200 to the 230s. I replaced the thermostat with a 180 and everything was fine until a couple weeks ago. Now it's behaving in a similar manner, temps moving up and down, with a max of 240.

This morning I took the cap off and topped the antifreeze off to just below the filler cap. Started the engine, and when the temp reached somewhere around 140 the coolant started moving up and with some flowing into the overflow tank. The level moved up and down until somewhere in the 160-170 range, then it started bubbling out like crazy. Does this sound like a blockage somewhere? I suppose the thermostat could be bad already. Any ideas would be appreciated.

No,It sounds like a BLOWN Head Gasket or Cracked Head!!hnoes

 

[dlt4]

No,It sounds like a BLOWN Head Gasket or Cracked Head!!hnoes

Yuck! Is there a quick and easy way to verify either of those possibilities?

 

[Tuna]

Pressure check the system - if it doesn't hold pressure but it doesn't leak, there's a good chance it's the head or head gasket.

Also look for water in the oil.

 

[Hib Halverson]

water in the oil, means a head gasket leak from the cooling jacket into the valley.

With all the bubbling in the high-fill bottle, if that's a head gasket problem, it's a leak from a combustion chamber to the cooling jacket.

I'm with Tuna...pressure check will tell you much.

But...let's not forget other cooling "service issues":
Make sure the front air dam is in place and functional.
Make sure there is no debris stuck on the front of the radiator or between the HVAC and the radiator.
Make sure the cooling fans are working properly.

 

[dlt4]

water in the oil, means a head gasket leak from the cooling jacket into the valley.

With all the bubbling in the high-fill bottle, if that's a head gasket problem, it's a leak from a combustion chamber to the cooling jacket.

I'm with Tuna...pressure check will tell you much.

But...let's not forget other cooling "service issues":
Make sure the front air dam is in place and functional.
Make sure there is no debris stuck on the front of the radiator or between the HVAC and the radiator.
Make sure the cooling fans are working properly.

I've looked at the dipstick and the oil looks fine.
All of the "service issues" are OK except I'm not sure about the fans.
As for compression checking, I'll have to take it to my mechanic to get that done.
Would a blown gasket cause the temps to vary by 20-30 degrees like I've been seeing?

Thanks for the suggestions from everybody.

 

[Hib Halverson]

Not "compression checking," but pressure checking of the cooling system is what I believe you need to try first. But, a compression check can be of value if the head gasket failure is between two cylinders. However, based on the symptoms listed to date, ie: the bubbling in the high-fill bottle, DvI (diagnosis via Internet) indicates possibiilty of a blown head gasket.

As for failed a failed head gasket causing higher coolent temps...it can.

 

[Bobsvette]

My LT4, with 96K miles, has an overheating problem that I can't figure out. The water pump was replaced about 25K miles ago and the heater and radiator hoses were replaced about 9K miles ago. Back in May the temps started fluctuating, up and down from 200 to the 230s. I replaced the thermostat with a 180 and everything was fine until a couple weeks ago. Now it's behaving in a similar manner, temps moving up and down, with a max of 240.

This morning I took the cap off and topped the antifreeze off to just below the filler cap. Started the engine, and when the temp reached somewhere around 140 the coolant started moving up and with some flowing into the overflow tank. The level moved up and down until somewhere in the 160-170 range, then it started bubbling out like crazy. Does this sound like a blockage somewhere? I suppose the thermostat could be bad already. Any ideas would be appreciated.

Everything your describing there when you add coolant is normal if you leave the cap off to watch, the thermostat has not opened yet! what you have to do is cap the radiator and catch it just right when the thermostat opens, be careful and release the pressure taking the cap off and you will see the coolant level down and with the help of somebody have them bring the engine up to approx. 3-4000 rpm and you will see the coolant really drop but have them hold the engine rpm constant at 3 to 4000 rpm and you add more coolant to the top and cap it! Then have your helper let off the throttle. DO NOT HAVE YOUR HELPER LET UP ON THE RPM'S UNTIL IT'S CAPPED! Then you should make sure you have good airflow through the radiator and your fan's are working properly. Check the sending unit if the fan's are not kicking in or check the ground's for the fan, they could be bad

 

[dlt4]

Everything your describing there when you add coolant is normal if you leave the cap off to watch, the thermostat has not opened yet! what you have to do is cap the radiator and catch it just right when the thermostat opens, be careful and release the pressure taking the cap off and you will see the coolant level down and with the help of somebody have them bring the engine up to approx. 3-4000 rpm and you will see the coolant really drop but have them hold the engine rpm constant at 3 to 4000 rpm and you add more coolant to the top and cap it! Then have your helper let off the throttle. DO NOT HAVE YOUR HELPER LET UP ON THE RPM'S UNTIL IT'S CAPPED! Then you should make sure you have good airflow through the radiator and your fan's are working properly. Check the sending unit if the fan's are not kicking in or check the ground's for the fan, they could be bad

What do you mean by "catch it just right when the thermostat opens"? Does it matter if I remove the cap after the thermostat has opened, then add the fluid?

 

[dlt4]

Turns out the radiator has a crack in it. I guess it was never big enough to notice until last night when it became real noticeable. I hope that's the only problem. I'll know after tomorrow.

Thanks to all who posted comments.

AND a blown head gasket!

 

[ZRide]

I recently purchased a 1996 LT4 with 46,000 miles that is bone stock. In regards to the coolant tempature I have 2 questions.

1. At constant speed on the highway mine stays right at 205 degrees. Is that normal?
2. Under normal circumstances, how hot should the tempature get before the fans kick in assuming the AC is off?

Thanks,
Travis

 

[dlt4]

I recently purchased a 1996 LT4 with 46,000 miles that is bone stock. In regards to the coolant tempature I have 2 questions.

1. At constant speed on the highway mine stays right at 205 degrees. Is that normal?
2. Under normal circumstances, how hot should the tempature get before the fans kick in assuming the AC is off?

Thanks,
Travis

1. Based on my experience, 205 is very normal.
2. I believe it is 228 degrees.

 

[ZRide]

Thanks for the response dlt4. Glad to hear mine is in the normal temp range on both counts. At some pointm I will probably change the thermostat & adjust the PCM so the fans kick in a littler earlier.

Travis

 

[dlt4]

I would agree that you should take those steps to keep things as cool as possible under the hood. I live in the Phoenix area, and all of that heat over the years is what eventually lead to the blown head gasket.

 

[gmjunkie]

I would agree that you should take those steps to keep things as cool as possible under the hood. I live in the Phoenix area, and all of that heat over the years is what eventually lead to the blown head gasket.

I don't agree!! Lack of Cooling System "Maintenance" is what causes Blown Head Gaskets!! These cars are designed to run hotter for a reason,Less Emissions,Less Sludge and Contaminant build up in the Engine,Longer Exhaust System life to name a few good reasons!!

 

[dlt4]

I don't agree!! Lack of Cooling System "Maintenance" is what causes Blown Head Gaskets!! These cars are designed to run hotter for a reason,Less Emissions,Less Sludge and Contaminant build up in the Engine,Longer Exhaust System life to name a few good reasons!!

I respectfully disagree because I kept the cooling system maintained and still had the blown gasket. I think the aluminum head/iron block combination might have something to do with it too.

 

[dmd]

Thanks for the response dlt4. Glad to hear mine is in the normal temp range on both counts. At some pointm I will probably change the thermostat & adjust the PCM so the fans kick in a littler earlier.

I kept the cooling system maintained and still had the blown gasket. I think the aluminum head/iron block combination might have something to do with it too

Bad idea. Agree with Junkie, the fan temp calibration was NOT chosen by throwing darts at a wall of numbers. Very smart engineers spent hundreds or even thousands of man-hours figuring it out, and they knew what they were doing. You are only sticking a band-aid on a broken arm. Leave the calibration alone and fix what's actually broken. My LT4 has 152,000 miles on it and runs better than new, I can assure you that GM worked out the iron/aluminum relationship long before the LT4 went into production.