Hey all,

I'll do my best to keep this post from ending up as a short story

'77 Corvette. Non-original rebuilt 350 / mild street build with about 500 miles on it. Cast iron heads - simple rebuild with new hardened seats, 3 angle valve job, new springs and guides.

Here is the situation:

• Noticed overflow tank was low, topped it off - this has happened twice
• Engine not running hot (about 195 - 200 max with IR gun), but I have noticed a little bit of coolant out of the overflow tube from the tank
• Slight bit of coolant around the overflow tank cap (push on cap - not twist on)
• Checked plugs and all look correct except #4 plug is too clean - like it just came out of the box. Appears to be "steam cleaned"
• No apparent leaks under car - radiator is clean and was replaced by the PO
• Oil is clean and at normal "full" mark on stick
• No white smoke / steam from exhaust
• No sweet smell from tailpipe(s)
• Engine runs great - good power, quick to start etc. (but parked since I noticed these symptoms)

Given the symptoms, head gasket or cracked head is my concern

My plan is to remove the radiator cap and run a compression check. My gauge doubles as a leak-down tester. If I am seeing a low reading, or coolant is being pushed out of the radiator, my next step would be to pull the top-end of the motor.

Am I on the right track here, or is there something I'm not thinking of? Any advice would be greatly appreciated.

Thanks!
Adam

 

I am in a similar situation except all the spark plugs look tan...a compression check was good....I didn't have a single problem with losing coolant until I installed a new brand-name thermostat....then the temp would climb over 200* and the aluminum coolant reservoir would be very low.

I haven't done a radiator pressure test yet.......

I'll post here and in my thread if I find the problem

 

Hey Doorgunner - thanks for the response. Hope your tests come out OK.

Hoping to get a little more feedback from the members here before I start testing things. Sadly, if this is something major, I'm afraid it could be a game changer for us continuing this project.

 

Sounds like you're on the right track. Post the results.

 

If you're sure coolant isn't leaking but coolant is disappearing, its got to be going through the engine somewhere. The oil looks normal but one plug is unusually clean - I think your diagnosis is correct. Sorry.

 

Please go punch whoever told you to do this in the throat. No Corvette engine needs hardened seats and many many heads have been ruined by machining through the seat into the coolant passages.

Find out in you are actually loosing coolant or not, and from where. Do you have a stock expansion tank set up?

 

Thanks for the response - If it's a head gasket or worse case, a head, I'm sure I can figure out a way to get it taken care of. If it turns out to be an issue with the block, well... let's just say that wouldn't be good for us at all.

Yes - stock tank and cap. The tank is in good shape and the cap seems to snap on tight.

 

I'd say very unlikely to be a block problem. Head gasket most likely.

 

Look for bubbles in rad. The clean plug might not be firing. The small "leak" could be other things like a faulty rad cap. If it is a small leak try retorquing the heads and intake in the proper sequence first. This should be done after a couple heat/cool cycles anyway and could very well be the only issue. Barr's Stop Leak is great for very small, minor leaks, the Chevy dealer uses it on many applications (standard in many new engines, Northstar) and they sell a repackaged version of it at the parts counter. http://www.amazon.com/Bars-Leaks-HDC...RankDescending

 

I'd run a leak down/ compression test on that cylinder as well as compression tests on the remaining cylinders. If you truly are loosing coolant then that would be the most probable place. They make a kit that takes a certain fluid and sucks air in out of the radiators free space. If the fluid turns a certain color then you have hydrocarbons in the air in the radiator indicating a bad gasket or cracked head, I think those kits are cheap and believe you can get them from oriellys or summit. If you do have a leak, it's either the gasket or the head. To be leaking at the rate your describing one would think that it would be apparent when you take it apart. I'm not for sure which heads you have, but if there 882s their notorious for cracking, I'd say the chance would be bigger on a leaky head gasket. Like i said, Start out with the leak down test. When you say the rest of plugs look correct, can you post pictures of one correct one and the suspect plug? Good luck and curiously awaiting what you find.

 

Good luck Adam, seriously hope it's something minor.

 

Hey all,

Did a compression test this morning. I'm showing 150, 155, 160 across the board.

Plugs: As I mentioned before, I had one plug (#4) that looked very lean. Since that time I have driven the car some (before I noticed the coolant being low etc.). When I pulled the plugs today, they are all much more consistent, although still appearing to be lean in general (very slight tan color, but on a few this is only in a spot or two and the rest of the plug is white).

Coolant: When I pulled the radiator cap, I got a release of pressure into the overflow tank (heard bubbling in the tank). The motor was cold at this stage and had not been run in about two weeks except this morning to move the car to the center of the garage, which only took about 30 - 45 seconds. I also found that the coolant is still bright green, but I had a some small brownish colored bubbles floating on top of the coolant.

Again, please check my thinking on this, but given the coolant pressure and brown floating bubbles, it's looking like a head gasket / head issue to me.

Is there something else I should consider or do which might possibly help with the diagnosis?

Thanks in advance for any help,
Adam

Edit - shot of plugs

 

Sounds like a head gasket to me also.

 

Retorque heads and intake. A bad head gasket between the cylinder and cooling jacket should show up on your compression test. A leakdown on suspect cylinder will show bubbles in the radiator. Sounds like a very small leak somewhere but you always HAVE to retorque head bolts after a couple heat/ cool cycles. See post 9.

 

Curious - how did factory motors go through heat / cool cycles and not require the heads to be torqued again?

 

I believe it's cause they used steel shims that weren't compressible, but i could be wrong.

 

I don't know about that. All I do know is I always retorque them. If I run them in on the dyno it is done before it comes off. If the cam gets broken in it gets done after break in. If I replace heads it is done after a couple cycles. Guess I been doing it wrong because everyone says after 1st heat cycle. SBC is particular because the two center exhaust ports are side by side creating a hotter area in the center of the head that expands more than the outside requiring retorque. Notice your issue seems to be a center cylinder.
I have blown head gaskets in the past and it has always been 2,3,4 or 5 that I had an issue with. Did you have the heads resurfaced? They usually need a pass or two to flatten them out.

From AFR:
All high compression, supercharged, turbocharged & nitrous motors should receive a retorque after the first run-in and complete cool down.

From Brodix for ALL SBC compatible heads:
A re-torque after initial warm up is desired. Allow engine to
cool for twelve hours before re-torquing.
http://www.brodix.com/wp-content/upl...structions.pdf

From Edelbrock:
A re-torque
is recommended after initial start-up and cool-down (allow 2-3 hours for
adequate cooling).
http://www.edelbrock.com/automotive/...6000/60909.pdf

 

My set-up has the expansion tank with the rad cap on the tank. This sounds like the pressure cap is on the rad and just a vent cap is on the overflow. Is that right. I've never had one of these set-ups, so please excuse the dumb questions to follow. When you say you "topped off" the overflow, what do you mean? You filled it to some hot or cold fill mark on the tank or you actually filled it to the top of the tank?

Since the oil doesn't look like a tea and milk combo and no white smoke or sweet smell in the exhaust, it doesn't seem that the 2 fluids are mixing in the engine. The compression sounds good. No wet carpet on the interior (heater core leak)? The only evidence of a leak is the loss of fluid level and actual observed loss of fluid from the overflow hose and some green around the vent. This sounds so much like another recent thread, where the owner was overfilling the expansion tank and, of course, when the car got hot and was shut off, it burped fluid out of the tube because there was no empty volume in the tank to take the expansion. I know this sounds simple. Don't shoot me.

 

If you leave the plugs out and pressurize the cooling system bet you see a drop in pressure

A blown gasket wont always smoke or make milky oil
Had that happen in a L98 by the time it DID stink or smoke it was too late the little coolant that did make its way through took the bearings out with it, motor was done. Hard to pinpoint but at a point parking the thing and having the lower radiator hose blow off the radiator gave me a clue.
May as well magnaflux the heads too you just never know.

Agree with retorque being a good practice. With iron heads seemed to always need it, alum so far they were all where they were supposed to be.

 

Hey all,

Thanks for all the excellent feedback and ideas - it is GREATLY appreciated.

Super 6 - you are correct. Pressure cap on the radiator and snap on plastic cap on the overflow tank.

Today's update: I rented a block tester. If you're not familiar with them, here is a link that describes how it works.



I ran the car up to full operating temp (shows 190 - 195 on the gauge) Actual temp with an IR gun is 185. Tested it off of the radiator directly, and found NO exhaust gases in the cooling system.

With that said, I still have an issue, but this test eliminated the head gasket / cracked head from the equation.

I did discover another problem in that when I started the car (cold) with the radiator cap off, I immediately had coolant flow in the radiator. This tells me my fail-safe thermostat is stuck open, which is what it should do if it fails. Will have to replace that for sure. I'm also going to flush the cooling system and put a new radiator cap on it at the same time. Both the cap and thermostat were replaced when we put the new motor in at the beginning of 2012, but that doesn't mean they are still OK (obviously).

The other part I'll replace is the coolant overflow cap. That cap is part of the recovery of coolant as the motor heats up and cools down (fills up as the coolant expands and then the radiator draws coolant back out as is cools off).

While I'm doing this I'm going to crawl under the car and inspect the radiator for leaks just in case.

Something isn't quite right, but there was zero change in color of the chemical in the block tester, so that's a very good sign.

Adam