LT1-LT4 Modifications 1993-97 Gen II Small Block V8

They tell me it's an external head gasket leak

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Old 12-17-2009 | 12:37 PM
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Default They tell me it's an external head gasket leak

1994 Z28, Auto Tran, All stock, 176k miles

Had water pump, t-stat and radiator and low coolant sensor replaced two months ago after mysterious coolant loss lead to them finding faulty WP (leaking out of weep hole). Prior to replacement, they ran pressure test and exhaust gas in coolant test...I passed both, so probably not head gasket.

But I still was getting low coolant warnings within a week or two of having both items replaced. One day after driving to work (25 miles on highway), I popped the hood and saw/heard the radiator cap release a tiny amount of coolant in what looked like a pressure blow off, which I now know it isn't designed to do...lol

So I'm driving back to the shop one morning and after driving less than one mile it is almost in the red zone (can't believe it got that hot that quickly). Had it towed the rest of the way, and they end up telling me it's probably an external head gasket leak which caused air pocket to form...so it sits for four weeks while I decide whether or not to junk it (can't decide), so then I drive it home the nine miles to my house with no problems...engine temp is perfect the whole way.

I start the car up a few days later just to test it out (before starting I checked the overflow tank and it read a wee bit below normal, but not too bad..checked the radiator level and it was low...added coolant to radiator).

I drove it around after letting it warm up a minute or two, and I noticed the temp gauge was once again rising faster than normal less than a mile into my trip (again, this is after sitting for several days)...once it hit about 185-190 degrees, the temp gauge plunged back to under the first tick mark....I assume the thermostat opened at that point as it should...I drove around for a little while longer and the temp remained within normal operating range.

I then started it up again the next day and did the same test...only this time, the temp gauge took a lot longer to get to 185...not like the way it quickly rose the day before.

I noticed that about one mile into my trip, the temp was continuing slowly to rise, until it almost hit the red zone at which point I turned the car off...let it sit for about 15 minutes...then drove another .5 mile when it almost went into the red again....let it sit 15 minutes again...then started driving, but this time I cranked the heat...and then I noticed...NO HEAT!!! But then, after cranking the heat for about a minute, I notice the temp gauge quickly plunging back DOWN into the normal range, and as it is doing so, HEAT started coming out of the vents!!

All these recent, varying symptoms have me scratching my head big time (not to mention I'm kind of clueless when it comes to these sorts of things...I wish I knew more, or knew of someone trustworthy here on Long Island that could give me a definitive diagnosis).

Any thoughts? Does it really sound like a head gasket problem (no white smoke coming out of tail pipe)?
Old 12-17-2009 | 12:44 PM
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WTF is a external HG leak???? Are you sure they used the right t-stat? LT1s need a special style stat. I would also make sure all the hoses are conected to the correct locations, that will def cause issues.
Old 12-17-2009 | 12:49 PM
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MAybe it wasnt bled properly, they can be a pain in the *** to get all the air out of the line
Old 12-17-2009 | 12:51 PM
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also do you smell coolant ? check the carpet on the passenger side and make sure your heater core isnt leaking
Old 12-17-2009 | 01:09 PM
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Originally Posted by djjab57
MAybe it wasnt bled properly, they can be a pain in the *** to get all the air out of the line
^^^^^^ This. Most shops either don't know or have forgotten that the cooling systems on these cars is reverse flow. As a result, you end up with lots of air left over.
Old 12-17-2009 | 01:16 PM
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It was a GM dealer that I had been taking it to for 10 years...they at one point even used what they termed a special vacuum designed to make sure all of the air is out of the system...

They say an external head gasket leak is when the coolant doesn't actually mix inside with the oil, but I guess is leaking outside...and can allow air inside...but then you would think you'd get serious smoking off the engine block as it leaks down the side...I don't really know wtf is going on, which is why I posted here...

Like I said, it drove fine for one week after the WP was replaced, and then the next week, I had the radiator replaced, and it drove for another week without overheating...and then boom, these problems...

I will say though, that during the two weeks it was "driving fine" after WP and radiator replacement, I still kept getting low coolant light warnings every now and then....couldn't brush it off as simply a bad sensor as THAT was new too...
Old 12-17-2009 | 01:19 PM
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the low coolant light is a common problem with these cars. most of the time its a false positive
Old 12-17-2009 | 01:58 PM
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Originally Posted by Badazz 97 TA
the low coolant light is a common problem with these cars. most of the time its a false positive
That I know (along with opti woes)...but when the radiator was replaced, the low coolant sensor was also replaced...plus, everytime I check the radiator, it is low...

What would cause the car to be on the verge of overheating the other day, yet the air blowing out of the vents when the heater was turned on was COOL...and then a minute or two later, the engine temp suddenly starts to drop rapidly back towards normal, and then heat starts to pump out of the air vents?

Could that be due to trapped air? Or does it sound more like a thermostat issue?

Almost overheating with no heat blowing out the vents, and then sudden rapid engine cooling out of the blue with heat returning to the vents sort of sounds like thermostat to me, but I'm kind of clueless like I said...would a stuck thermostat block heat from coming out of the vents even though the temp gauge reads near the red zone?
Old 12-17-2009 | 02:03 PM
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I would also think if the HG was leaking on the outside of the block you would see smoke smell it and find wet spots.
Old 12-17-2009 | 02:19 PM
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air being cool then blowing hot again could be trapped air, do you smell coolant
Old 12-17-2009 | 02:20 PM
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Coolant has to be going somewhere. If you can't smell or see it then it's being released internally which means you should start pulling plugs to see if there is one or more that are far cleaner than the rest...
Wondering also if the hoses from the reservoir and steam line into radiator have been mixed up.
Old 12-17-2009 | 02:25 PM
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What you have definately sounds like an air related problem in your coolant. An air pocket in the right place will cause virtually no flow in a cooling system.

As for the head gasket, I don't know about LT1s in this respect, but the newer 3.4 cars and minivans like to blow their head gasket in just such a way that just exhaust will get into the coolant, and cause air pockets to form resulting in basically your exact symptoms. Not saying that's your problem, just saying head gaskets can do weird things.
Old 12-17-2009 | 02:27 PM
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i had a similar probem once. not exactly all the symptoms but i would loose alot of coolant at once. i would fill it up and the next day would still be fine. then all of the sudden, im missing almost a gallon.

long story short, it was my head gasket. a small tear that let coolant go straight into the combustion chamber caused all that. gave me a good excuse to put on thinner head gaskets and mildly port the heads.
Old 12-17-2009 | 02:30 PM
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also i have heard of dexcool eating the head gaskets of cars not designed to run it and thats what i think caused my problem. if u take it to the dealer, they might have put dexcool in urs
Old 12-17-2009 | 02:59 PM
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Originally Posted by SS RRR
Wondering also if the hoses from the reservoir and steam line into radiator have been mixed up.
I read up on which hose was supposed to be going to the reservoir, checked it, and it appears to be correct.
Old 12-17-2009 | 02:59 PM
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Originally Posted by ElkySS
also i have heard of dexcool eating the head gaskets of cars not designed to run it and thats what i think caused my problem. if u take it to the dealer, they might have put dexcool in urs
always looked green to me whenever I checked it...
Old 12-17-2009 | 03:39 PM
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Originally Posted by ElkySS
also i have heard of dexcool eating the head gaskets of cars not designed to run it and thats what i think caused my problem. if u take it to the dealer, they might have put dexcool in urs
Dexcool has been GM factory fill since about '96.
Old 12-17-2009 | 05:49 PM
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Topped off the radiator last night...let it sit...started her up today and drove her around the neighborhood for 10-15 minutes...everything seemed normal...no overheating issues...I will say that at first, I thought the temp gauge was rising a little too fast as soon as it started lifting off the bottom, but it seemed to slow down its rise soon after...or maybe I'm way too paranoid at this point...

Will check the level in the radiator tomorrow and report back...
Old 12-17-2009 | 08:11 PM
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Have you opened the bleeder screw when you topped it up?

After I put my engine back together, I let the engine run and warm up with the radiator cap off. Gave it the opportunity to have the thermostat open and allow any air trapped in the heads and water jackets out of the system, and top up if it the coolant went down.

Give that a go, see if any bubbles surface up?
Old 12-17-2009 | 09:43 PM
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Originally Posted by fleetmgr
Dexcool has been GM factory fill since about '96.
his car is a 94


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