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Perplexing; 2006 Sierra 1500 5.3 cooling system issues


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I have a perplexing problem with the cooling system on my 2006 Sierra 1500 with a 5.3.


It recently started losing fluid, but I'll be damned if I can find the source of the leak. It sometimes leaks a whole bunch of fluid out underneath the radiator, but not always.

It sometimes overheats but not other times.


It's possible it is also losing fluid at times other than when there is visible leaking coming from under the front of the truck.

If the fluid is coming out from under the rad, where would it be coming from? And why would it leak only sometimes but not others.

I'm pretty sure it doesn't have any head gasket issues, as it is simply not exhibiting any of the symptoms typical of a blown head gasket.


There is no visible evidence of leaks anywhere else on the vehicle.


Twice now I have attempted to get it to leak and been unsuccessful. It only seems to leak when it feels like it, and when it does, I've been on the road and I simply can't seem to source where it's coming from. It does seem to puddle in the lower rad cradle and leak on the ground from there on both sides of the truck. That is the closest I can get to where it is sourcing from.

I can understand if the rad was cracked, but then it would leak all of the time. Not just sometimes.


There had been some threads a few years ago about a bad batch of cylinder heads from the years 2003 to 2006. However, I don't believe this is part of the problem as it would have nothing to do with the fluid coming out from underneath the rad sometimes.Also, there was a switch done midyear in 2006 on the 5.3 engines. My truck has a 'T' in the VIN and was built in the latter half of the production year. I suspect it may not have the problematic Castech heads from earlier years.

I have also suspected that the rad cap may be a culprit in this, however, if any fluid is making it out of the overflow hose, it is only doing it sometimes. This doesn't explain how the fluid is getting over to the left side of the truck underneath the rad.


This one has been driving me nuts, as well as perplexing several of my mechanically minded buddies. Does anyone have any idea what the heck is going on here?

Keep in mind the rad, condenser and trans cooler were all replaced with new items 1.5 years ago. This problem only first showed up about 4 weeks ago. Truck currently has 270,000km on it and everything else is original. I've owned it since new.

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The both sides of the truck thing is a distraction. If I recall correctly that lower radiator cradle is kind like a little c-channel job that holds the radiator. So, anything that leaks onto that will probably come out on both sides of the truck. I think the only way to go here is to pull out the pressure tester and go at it. Let the thing sit with pressure on it overnight and see what's up. As an aside, it's not overheating is it?

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It sometimes overheats, but not always. When it overheats, it seems to do it after the fluid level has dropped....

 

Yes, the rad cradle can channel fluid, but I haven't been able to find out how the fluid is getting into the cradle in the first place. Everytime I try to replicate the problem with the truck at home, it doesn't leak or overheat.

 

When it goes above 110 celcius while driving, that's when it seems to loose fluid visually; When I park it immediately or soon after, fluid will run from the cradle, or from the Front right corner.

 

I may have to try to force it to overheat.....

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I'm thinking the overheat is the problem and what you're seeing is the overflow from the coolant bottle. Where does the overflow hose from the coolant bottle run, I can't remember. My bet is that if you take the overflow hose from that bottle and stick it somewhere then the "leak" will be at that somewhere and not in the unknown. My concern is that the overheat may not be related to low coolant, but rather, the coolant is as a result of the overheat.

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I'm thinking the overheat is the problem and what you're seeing is the overflow from the coolant bottle. Where does the overflow hose from the coolant bottle run, I can't remember. My bet is that if you take the overflow hose from that bottle and stick it somewhere then the "leak" will be at that somewhere and not in the unknown. My concern is that the overheat may not be related to low coolant, but rather, the coolant is as a result of the overheat.

The overflow line has no physical connection to the rad cradle. It is a short piece that is attached to the side of the surge tank. It directs overflow to the inside of the inner fender, above the wheel well. It doesn't appear that the fluid could get to the rad cradle from there.

My best guess is that there are a few things going on here:

1. There is a hidden leak somewhere causing fluid levels to drop.

2. Low fluid levels cause other symptoms such as the heater to not work, as no fluid can get to the heater core. It then causes overheating. It doesn't overheat if fluid levels are up.

3. Overheating then causes another leak, possibly in the rad somewhere, or as suggested in the post above - the rad tank gaskets? (Could the rad cap be faulty allowing too much pressure?)

I dumped some Prestone stop leak in the system the other day and it seems to have stopped, or dramatically slowed the hidden leak. It did not overheat Saturday evening in over an hour of very slow stop & go traffic and fluid levels were still up when I got home, after several hours of engine runtime.

 

The question is: what are the culprits here & how do I find the hidden leak?

 

4. All this fluid leaking from the rad cradle seems to have damaged my front air bag sensors. I replaced one yesterday that was clearly damaged and had fluid inside it when I broke it apart to examine it.

Probably the tank gaskets

Would these only leak sometimes?
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Possibly a intermittent thermostat sticking partially closed? If it is that would possibly cause extra pressure in the system from the overheat and pushing fluid out wherever it can?

Chatting with a friend who is a former tech, he suggested we may have to test a whole bunch of the system parts individually to trace this one...

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  • 2 months later...

Update-

 

The rad is definitely leaking down on the bottom somewhere. I replaced the water pump this past wknd and it showed signs of some leakage on the mounting surfaces/gaskets as well as the thermostat housing interface & gasket.

When I filled everthing back up and fired it up to properly fill the system, the rad started leaking once the operating temp was reached. I hadn't seen it leak in a while.

 

This must have been a bunch of issues all rearing their head at once.

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  • 1 year later...

I have a 99 Silverado with the 4.8 with cooling system issues. Bought it used with 227,000 on it. Engine runs strong and sounds great with the exceptions; 1. Lifter tick on start up for several seconds to a minute. 2. Excessive pressure in cooling system-new belt and all new hoses, new thermostat and housing, new radiator, new pressure cap on reservoir and flushed and filled with proper Dex-cool/water mix. Losing coolant-1 pint every 200 miles or so. I have re-tightened hose clamps several times but pressure is forcing small leaks at hose connections. Heater temp is barely warm. Return hose warm, hose to heater normal(hot). I believe previous owner used stop leak as there is telltale signs of crud formations. My belief is head gasket or small crack in head/block. System holds pressure overnight which kind of perplexes me. No sign of coolant in oil or vice verse. I have read several similar posts on symptoms but none with actual diagnosis and successful repair. Any help is appreciated. Vick.

 

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I've seen a few times a small crack in radiator that opens after engine is turned off. The engine usually warms a little builds a little pressure opens the crack expels water. Usually at the seams. Sometimes the thermostat is sticking at the same time. I had this happen just two wks ago.

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