GLMoore

My 85 928 has been slowly losing coolant. There is no oil in the coolant and no coolant in the oil. There is no smoke from the exhaust and no coolant leaks. The engine doesn't run hot.

My mechanic feels certain there is a failure of the head gasket at a point between the coolant flow and piston, causing some coolant to escape from the exhaust during the compression stroke of the piston. But there is not enough loss per piston stroke to create visible exhaust smoke.

The mechanic states that the compression stroke of the piston is also forcing pressure back into the coolant system, causing the coolant system pressure to be 19 psi, when it should be about 13 psi. This unacceptably high pressure is the basis of his conclusion of a damaged gasket. The high pressure will eventually cause a failure of the cooling system, probably blown hoses.

Before proceeding with the engine pull and replacement of gaskets, etc., I'd like some opinions from 928 owners about this problem, as it's going to be very expensive to do this.

The mechanic states that this damaged head gasket will not materially affect a compression test of the piston in question. The spark plugs have not been removed for inspection.

The mechanic is certain that the high pressure in the coolant system, in conjunction with the other facts above, clearly shows that a damaged head gasket is the problem.

Should other tests be performed before concluding it's a head gasket failure?

The motor has 118k miles and no prior gasket work.

The Deputy

I'd say your mechanic is on the right track. A small anti-freeze leak, especially a head gasket can be somewhat difficult to diagnose or pin down. A compression test or cylinder leak down test could be preformed...and that may add even more witness testimony to the case. But, considering the miles and unless you have done anti-freeze flushes regularly or added inhibitor packages to the coolant or have ELC (Extended Life Coolant)…a head gasket would be my guess, too.

ALSO, ANOTHER REASON YOU MAY NOT BE SEEING ANY COOLANT/SMOKE COMING FROM YOUR EXHAUST ON START-UP..WOULD BE...THE COOLANT ISN'T UNDER PRESSURE YET AND IF THE LEAK IS TINNY...YOU ARE ONLY GETTING A SMALL PORTION INTO THE CYLINDER DURING WARM-RUNNING CONDITIONS. (and I wasn't yelling, just hit the Cap's key by mistake...and I'm not going to re-type...lol)

Good luck, with your endeavor.

Brian.

BRB-83-911SC

I would pull the plugs and inspect to see if any are "steam cleaned". If so, then that would support your mechanic's diagnosis. If not, it doesn't rule out his diagnosis, it just doesn't confirm it. You could also test the cooling system for combustion gasses with a combustion leak detector. This test could also be inconclusive, but if positive, would support his diagnosis. I suppose a pressure/leak down test of the cooling system would also be useful.

The point of all of this is to try to get some corroborating data to support the theory. There are many places for undetected coolant loss besides the head gasket.

Good luck! I hope you find something simple.

Here is some reading about my head gasket failure.
https://rennlist.com/forums/showthre...ferrerid=34007

dr bob

Coolant in a cylinder, even a small amount, will cause a telltale pattern in the carbon on top of the piston. Brian is spot-on about looking for telltale signs on the plugs, where coolant will clean the plug insulator of any deposits, leaving a pure white to slightly coolant-colored insulator.

Meanwhile, there are a whole lot of places where coolant can leak externally and not be noticed. At very low dribble rates, the coolant evaporates from hot surfaces before it can drip to the ground or pool under the leak. Hidden spots that are most frequent offenders include:

-- The heater control valve, the stub hose and the cylinder head fitting where it connects at the back of the passenger-side cylinder head (on LHD cars).

-- The plastic reservoir, if original, is prone to age-related seam cracking. There's a short hose underneath the reservoir that's almost never serviced.

-- There are heater hoses running across the rear of the engineunder the air cleaner housing between the heater valve and the heater core, then back towards the reservoir, to a steel pipe that runs on the passenger fenderwall, forward to a hose that connects at the water bridge front of engine.

-- You have the two main radiator hoses.

-- There are two galley plugs on the sides of the block, accessible from below. They have aluminum crush-washer seals on them.

-- The plastic radiator drain plug.

-- There are two vapor vent hoses that run between the radiator vent nipple and the reservoir, and the water bridge and the reservoir, respectively.

-- There are seals where the water bridge connects to the heads.

-- There's a seal at the thermostat housing.

-- There's a blank-off plate at the rear of the driver's side cylinder head.

-- The heater core itself.

-- The radiator itself.

-- The reservoir cap.

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There are a few ways to find leaks. For the leak your mechanic suspects cylinder into the cooling system, there are telltale solutions that detect combustion gasses in the coolant. This is a definitive detection process.

Your mechanic has, or you can free-rent a tool to pressurize the cooling system cold. This lets you find leaks by looking for coolant without worry about it evaporating. Fill the reservoir to the top. Pump the system to no more than the 1 bar (about 14.5 PSI) cap rating, and start looking in those places I described above. Let it sit a while, and watch for drips on the floor, puddling in the engine V near the water bridge, and seepage around and under the plastic reservoir. You'll be able to watch the gauge on the tool to see if pressure is bleeding down. Since the system is mostly hydraulic, changes in coolant volume show up pretty quickly on the gauge.

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Most folks forget about all those hoses and the reservoir until there's a problem. For me that problem will show up on a very hot day on a lonely highway someplace, as a burst hose or similar. I hate getting stranded by Stupid Stuff, so the hoses and related parts are on the PM schedule. Every other timing belt change, ALL the hoses are replaced at the same time. So far so good on not getting stranded on that hot desert highway.

davek9

Yep, stop guessing and pull the plugs, do a compression test while there out.
Head gaskets do and can go south depends on type of antifreeze used and other factors.
But you should be 80% sure as gaskets, new studs, and all the other bits add up fast, and don't forget the labor too.
Not an inexpensive job if do it right.

The Deputy

You can sometimes smell a rich, almost sweet aroma in the exhaust...if there is a head gasket issue. Run the car to operating temps in a well ventilated area and sniff around at the tailpipe location. Sure, don't get to crazy...since the exhaust can be toxic/deadly, if taken in large amounts. But, just let your old-sniffer see what it finds.

There use to be a clear glass tool, that you then filled with fluid and mounted on top of your reservoir/cap opening…and while doing a cylinder leak down test…bubbles would surface inside the glass bottle. Or you could run the engine and bubbles would appear, since cylinder compression over rides coolant pressures. Only problem with running the car is…you don’t know which cylinder is leaking. However, odds are…you’d do both sides if you’re going through all of the bother. Granted, I’ve done plenty of one-side jobs…at the customers’ request. As weird as that seems...sometimes a few hundred dollars more for the other side is a deal breaker for them.

dr bob

Look at for an example of the tester you'd use to detect combustion gases in the coolant. This kit has enough to do your car and a lot of your friends' cars too.

M. Requin

To add to dr bob's list of things to check, include the coolant pressure switch. That little booger was the source of a leak that took me forever to find.

And although your mechanic could be right, I would not want to pay him to find out. I would much rather have the results of a definitive test before authorizing replacement of a head gasket.

GLMoore

Thank you very much for these comments.

I will present them to the mechanic for consideration before authorizing removal of the heads.

It seems that the kit from Amazon to confirm combustion gasses in the cooling system would be a good test to perform now. The mechanic has confirmed a build up of pressure while the motor is running, but hasn't confirmed that it is due to combustion gasses.

A lack of combustion gasses in the coolant system would seem to indicate that something besides a failed gasket is causing the pressure to rise dangerously as the motor runs.



Gary

dr bob

The Lisle combustion gas detector kits or similar are available from better local parts places, and the 'tool truck' guys have them for sure, albeit at slightly higher prices. These kits are a lot more elegant than the one I last used a few decades ago. That one I used a turkey baster to grab a coolant sample, a few drops of the magic reagent in the sample, and a color change to indicate gas presence. Fancier turkey basters these days, with the safety of not having to handle hot coolant directly.

Coolant system pressure spikes are rare, but casual overheating from a failed water pump, radiator, thermostat etc, or even just an air bubble in the water bridge are the most common causes. If there are no combustion gases in the coolant, grab an IR thermometer and start looking for hot and cold spots. These will help you ID the places where coolant isn't flowing correctly.

Thanks to Martin for reminding us about that presure switch. It's in one of the vent hoses just forward of the coolant reservoir. There's also the coolant level switch in the top of the reservoir, and the temperature transducer or switch for the fans in the front bottom driver's side (on LHD cars) of the radiator. The radiator end tanks and their o-ring seals are popular failure points. In the tanks there are oil and ATF coolers depending on year and transmission, with o-rings where the fittings pass through the tanks. Lots of places to look!

FredR

The warning sign here is "pressure in the cooling system". Plenty of places coolant can leak from but not many that can over pressure the cooling system that normally should see little if any pressure [see Bill Ball's thread on his cooling system problem].

If you are truly seeing a pressure of 19 psig then I am wondering how this can be as the cap let's go at 15 psig if I remember correctly -unless it is blocked/not functioning correctly or the gas release volume overwhelms the cap capacity!

Remember combustion pressure is rather high and will find small leak paths due to head gasketting issues should they exist the same in reverse is not necessarily true [coolant into the combustion chamber] but logic suggests it will be when it gets "bad enough". Once a leak path develops there is also the chance such may well etch the mating surface so if this is what is happening I would suggest you need to get after it quickly and find out for sure what is causing the "overpressure" problem- leaks from the cooling circuit to the exterior do not cause over pressurisation.

Good luck

Fred R

The Deputy

Yes, that was my thinking from the get-go/original post. A rise in coolant system pressure is almost enough of a "smoking gun" to say you have a head gasket issue. Usually, compressed air (compression stroke) can cause air to travel through a much smaller cavity/weak head gasket material than coolant, and only with the intake stroke can a small portion of coolant be drawn/produced in the cylinder. As things get worse...then you will find coolant being forced into the cylinder by coolant system pressures after the engine is shutdown and steam/smoke will be normally visible on start up.

Of course, I'm new to these engines, and here, and there may be other factors that I am unaware of...so where did I put that "grain of salt" I usually keep handy...lol

Plus, were talking about thirty year old gasket material here (since an 85 year model was probably produce in 84) and the material is inherently subject to corrosion/fatigue issues over time. I'd be interested to know how many folks here have the original head gaskets in there thirty year old 928, and what type of extensive maintenance they did to achieve this.

I hope the original poster checks back in with his findings.

Brian.

SteveG

I think you have to be a member, but a feature exists for you to post a poll, a non-scientific survey, but a survey just the same. I will generalize and say the answer to how many is "lots." Another generalization is changing the coolant every two years will help extend the gasket. Coolant deteriorates and so on. There are threads on brand vs. brand. That's not "extensive" but I believe it is about the only thing you can do. Maybe, don't let it sit.

The Deputy

This is good news, since I bought an 84...and it had the original water pump...and I don't believe the gentlemen I purchased it from was very mechanically inclined.

This is a concern for me...since the car sat for two years without much action. The anti-freeze came out looking pretty good (dark green), but I did not test its hibitor-protection values, yet. We have test strips at work for doing such...and I'm almost afraid what I will find...lol. But, it is what it is...and I will just move forward regardless.

Thank you,

Brian.

GLMoore

Well, my 30 year-old motor has again declared that it needs attention.

A couple of days ago, my Belt Tension light came on. A little surprising since it was installed 30,000 miles ago; but not so much since it is almost 8 years old.

It's time to pull the heads, do a valve job, replace the belts and gaskets, and take care of the other things that need fixing. (And now I can finally paint the intake manifold).

I'm guessing about $5,000. I bought the car in 2001 for $10k and have spent $28k in repairs and maintenance since then ( about $2k per year, a lot of it labor).

The car is beautiful and well worth the money to keep it in like-new condition.

Again, thank you for sharing your expertise about these matters. It was very informative and will be very useful when the repairs are made.

Gary