Cloud9...68

I have a 92 968 (yes, I know this is the 944 turbo forum, but there seem to be a lot more mechanically-inclined people here than on the 968 forum) which I've converted to a track car, and for awhile now, it's losing a fair bit of coolant during track events, paricularly on hot days. I measured the loss today, and it's an alarming 2.5 cups. I only lose coolant during very hard driving, like during track sessions - I don't lose a bit during the 19-mile drive to and from the track, even on very hot days. I smell antifreeze as I'm doing my session, and the engine is recently rebuilt, so I doubt it's the head gasket, but I don't see any coolant on the ground. So it seems to be disappearing in vapor form. The oil level is very constant, and there isn't any sign of coolant in the oil.

I've had the belt covers off, and can't see a trace of doolant anywhere near the water pump, or any hoses anywhere that I can see. This may be an impossible question to answer, but I'm kind of at my wits end, and was wondering if anyone has any expereince with a problem like this who could recommend where to look. Thanks.

Tom M'Guinn

I'd start by pressure testing the system for external leaks. You can get a pressure tester at Sears or go online and get a Stant kit. If you are smelling antifreeze then it's most likely misting out a leak or out the over flow tube. If pressure testing shows no leaks then put the overflow tube in a plastic bottle to monitor. If it only overflows on hard throttle but not when driving gently then that can be a sign of a bad head gasket, especially if it's accompanied by a repeated need to vent air. Also check your oil cap for signs of mixing -- milkshake-like goo....

thomasmryan

drain the system and pressurize it with 7 psi of air. spray every joint with a 50/50 mix of dishsoap and water, including your radiator seams.

this method will find even the smallest leaks.

Cloud9...68

Thanks; I should have thought of the pressure tester, but I figured given the rate of loss, signs of where the coolant is leaking should be pretty easy to see. I'll try this first.

I don't think it's the head gasket, because it's a brand new Wide Fire, and the head gaskets on these engines seem to be able to seal even when they're visually very worn looking. I had the head's flatness checked (it was perfect) as part of the rebuild. The fact that I can smell antifreeze tells me it's leaking externally. And the oil cap looks perfect - no sign of any frothy stuff.

thomasmryan

you will be surprised how many leaks you will find just using air...even the throttle heater lines.

you might want to pull the fan assembly back a little to see the radiator seams.

in my opinion....if the system doesn't hold to the 150kPa (for the '68) design pressure, the steam pocket party begins!

Tom M'Guinn

Personally I'd leave the coolant in the system and pressurize up to your cap's limit. The leaks will raise their hands and announce themselves. Virtually impossible to miss an external leak that way, whereas if you are testing potential leaks with soapy water and air pressure it seems inevitable that you'd miss places where leaks might be hiding. It's also very quick and simple to test with coolant full. Just my two cents. Test the cap too to make sure it's holding pressure up to its rating. Newly replaced head gaskets have been known to leak -- ask me how I know -- but I agree your signs point more to an external leak.

Cloud9...68

Given the huge mess draining the coolant makes on these cars, I think I'll try that as a last resort. I've tightened every hose in the system using a nut driver, to no effect, so I'm wondering if it could be the radiator. We'll see...

Tedro951

+1 on pressure testing the system. Although I'd consider getting a free loaner from Autozone or Advance Auto before buying one, especially if this isn't something you do often.

Cloud9...68

Yeah, that's exactly what I plan to do.

Dave W.

Check the heater core and related hoses. Do you see any leaks that look like A/C condensation? Also, hot semi-dry coolant can resemble an oil leak.

KevinGross

Go to your local auto parts store and ask for a block checker. It is a simple kit that tests coolant for exhaust gas content, and will give you the good / bad news as to your head gasket.

Do you smell anti-freeze in the exhaust, is it white-ish in color? Or is the smell ambient?

Cloud9...68

OK, on the second try, I found a cooling system pressure tester that fits the car. So, I first tested the cap, and got an interesting result. The cap (a Stant Racing cap) is rated at 21-25 psi. When I pressurize it, two pumps brings it quickly to about 20 psi, but then pumping it many more times barely increases the pressure reading - it barely makes it to 21-22 psi, nowhere near 25. It slowly bleeds back into the 20-21 psi range after I stop pumping.

Before I declare victory and assume it's a weak cap (although the car's behavior, with coolant disappearing only during hard use seems to support that it could be the cap), I have a few questions. First, what pressure do our cars' cooling systems run at normally? Is it possible that they could exceed 20 psi under extreme conditions (Texas summer temperatures, extended running at full throttle, with numerous trips to the redline per lap over a 25-minute session)? And what should value I pressurize the system to in order to check the rest of the system for leaks? Thanks.

solidude

I have been chasing the same sort of thing for the last year in my 88 928 S4. I am loosing coolant at a distubing rate( a quart in 100 miles) and have yet to see where it is going. There is no steam out the exhaust,no hard starting (hydraulic lock-up), no drips anywhere,anytime,it just seems to disappear!
I bought a combustion leak detector and have found no trace of CO2 in the coolant. I bypassed the heater core...made no difference. I bought an inspection camera,took out the plugs and had a peek at the pistons..all the same...slight carbon build-up,none washed,plugs were in the same shape. I installed an aftermarket temp gauge (stock is unreliable due to dash problems) and the car runs at 165 degrees consistenly. The only hint is an occaisional whif of coolant in the car when you slow down,or stop.
I'm becoming a bit flustered at the whole thing,anybody got any suggestions?

Tom M'Guinn

I usually test at 15psi, but stock caps all seem to be rated even higher. The tester itself could be leaking where it seals to the cap, but regardless if your cap is holding 21-22 psi, I'd say your cap is not the problem. Did you test for leaks? Pump it up to 15psi and wait at least 5 - 10 minutes while looking under the car for leaks. It helps to be on a clean cement surface, or have a drip pan, so you can confirm nothing is coming out...

Cloud9...68

Tom,

Thanks for the info - OK, so it's probably not the cap. I was kind of afraid of that, since 21 psi seems to be a pretty healthy pressure retension value. Also, if the cap's spring were weak, the coolant would flow out the overflow, probably creating a puddle, which isn't happening.

I haven't pressurized the system, because I wanted some input as to what value to which to pressurize it. I will probably do that tomorrow after work. I did look under in the passenger side footwell for signs of wetness on the carpet, but it's dry. I also looked under the dash on the passenger side, and didn't see any sign of wetness near the heater core area. And I'm not seeing signs of a white color to the exhaust, but then again, since the problem only happens under the stress of a track session, I wouldn't notice it. Other people at the track haven't pointed this out to me, so I'm going to assume it isn't the happening. I'm going to rule everything out before I even consider that a brand new head gasket is the problem, as this seems like an extremely remote possibility, especially since I can smell antifreeze.