Porsche Enthusiasts Club Forum

I envy a man with an engine bay like that. Brilliant.

Swirl pot installed tonight. If it works the install will be tidied up

You may also notice that I have put some clear hose into the header tank because some muppet didn't like the yellowness of it and thought spraying it black would be better. Thats because even I could't justify the £200+ for a new one. Clear reinforced hose, 75p!



Took it for a quick spin and boost and it didn't spit its coolant out on the two occasions I switched off whilst hot, previously it would. Will check it over again once cold in the morning and then take it for a proper blast. I hope to reconnect the knock sensor headphones and give it full beans on full boost

I remember somebody asked you about the black header tank when you painted it. Oh well, at least it matches and everything else now appears to work.

The tank was that yellow you would have struggled to see the coolant clearly anyhow, so the muppet who painted it black was me! At least with the clear pipe I can see whats happening inside. This morning when I shut down I could see air bubbles returning up the pipe back into the header tank.

Hi Steve, I have only just been able to check out your photos of the head off, in particular the combustion chambers and the piston crowns.

Are you using oil? How much oil per 1000 miles do you think you are consuming?

I have a few concerns

Hi Jon, thanks for checking back. What you thinking?

Yes I do need to top up with oil on occasion. I've used about 3 litres in the last 4000 or so miles since rebuild. I appeared to use more pre head gasket change (first 3000 miles) than I have since in the last 1000 miles. You can see some deformation in the copper oil ring in the old head gasket which I think failed when the centre studs were loosening (may have been damaged in fitting as there is also marks in the metal surfaces). I had oil and vapour spitting out the oil separator for the last 500 - 1000 miles pre head gasket change. No oil or water mixing pre HG change, just some water in no.1 & 2 bores.

I think I need to rebuild my spare head over winter as I think its causing a few problems. Fancy the work + further development? Car will be off the road after the NEC Classic Show next month for the winter. Need it back for March.

Last night I checked the plugs and bores due to the system being pressurised when I shut down as I had been driving quite hard and fast (hard to squeeze silicone coolant pipes) and then very soft once cooled. The pressure had bled away. All bores were dry apart from #1 which was lightly damp either from coolant (likely) or oil (unlikely). The moisture was black carbonised when putting a cotton bud into the bores and wiping around. Ideally I could use a borescope.

I believe my head is either cracked, porous or warped and letting combustion gasses into the coolant under boost/high rpm as per issues found before the last HG failure. Need to look into this further as I fear the this could be the end to my driving this Autumn. I am hoping its only a head issue and not block/piston related. A spare head needs to be rebuilt over Winter I guess! I am assuming it to be head related although I do recall Bass GT having all sorts of HG failures during his development. I'm running Goetze this time, previously a Reinz. Not sure a MLS one would be the cure? Especially if the head is buggered.

If you believe the coolant system is pressurising under boost you can try doing what I did the last time I went out in mine.
Run a hose from the coolant overflow inside the car into a clear container. Take the car out for a spirited drive on boost, and observe to see if any coolant is pushed out on boost. It's easier with a passenger.
Mine was pushing coolant from 0.5bar upwards from what turned out to be a tiny pinhole leak on the head gasket.
It's at least worth a try. I feel your frustration. Bloody annoying problem.

Looking at head temps, it does make me wonder if it would be worth reversing coolant flow and going to the head first with a completely revised system and a swirl pot.

Hi Wes, coolant is being lost due to pressurisation after shutdown not during running. I think my head is scrap and causing my issues.

Sounds like you are getting localised coolant boiling on shutdown, which I would not point the finger of doom at the head gasket or the head, as long as the pressurisation is just happening at shutdown and not during running.

Localised boiling could be happening in your turbo if you are running a watercooled turbo. I can't remember what hybrid type you had made... But if it is watercooled, then you might be best off running a 944 turbo electric waterpump which is set to keep circulating coolant through the turbo on shut down.

It can also be caused by poor oil cooling. If your oil cooling is marginal, then the coolant will have to absorb all the heat of the engine in shutdown, as the oil will already be at a higher temperature, possibly to hot to allow a thermal pathway from the engines deep internal parts to the oil, which could litterally be glowing, leaving the coolant as the only thermal release path... The ideal engine oil temperature, should be around 100 degrees C when running, and the coolant at about 85 degrees C, however the coolant will boil at 100 degrees C at atmospheric pressure, higher with a good pressure cap, where as the oil will not boil until much higher temperatures, meaning the oil can be a good heat soak for the block, especially at drawing heat from the block and sump heat sink combination when the temperature soaking out of the bores etc exceed 100 degrees.. as long as the oil is not hotter than that already.

The next thing would be the pressure cap, make sure its a Berh one, not a cheap copy, your car will need a good pressure cap with a good reliable pressure blow off.. I have seen lots of aftermarket ones release pressure too early, which means your coolant is not running at the correct pressure, even after shutdown, which will lower the boiling point of the coolant.

Ignition timing, even though your problem is happening on shutdown, could be a real issue. If it is too retarded, you could have some seriously glowing exhaust valves, even following a period of idle, which will dump their heat into the valve guide, into the head and into the coolant in the head, causing localised boiling of the coolant. A poorly timed 924 non turbo, can make the exhaust manifold glow red, without even driving it, just having it ticking over at idle.. So, if your ignition is retarded at idle, to provide a little less advance on boost, you could be giving your valves a serious amount of heat, which can eventually also become an ignition source and cause detonation as well as pre-ignition.

What is your idle mixture? If it is set to Porsche tolerances, it can cause a loss of performance as well as too lean an idle, as the original fuel chemistry available in 1981 was much different to what we have today, you need to run their idle mixture a little richer than you would back when the car was new, but also you want to use some of the additional fuel to cool the valves at idle, which would also help them recover after boost.

What is your shutdown proceedure? are you using a turbo timer to let the car idle for a while before shutdown? either an electronic one, or just your own manual methods?

Have you turned your ignition back on after shut down to watch the temperature gauge? Localised boiling may not show up on the instruments, as you might have a problem where the coolant is fizzing around the exhaust ports, but elsewhere in the engine it may be normal, this will cause additional pressure and lots of bubbles in the coolant, which will increase pressure as vapourised coolant takes up more space than when it is in its liquid form.

You could plumb a 944 turbo electric water pump between your heater hoses, with a RC timer and two relays to make a simple device to run the pump for a couple on mins after shutdown, this will cause coolant movement after shutdown, but if you have a watercooled turbo, then placing it there with the same electrical controls would be a good idea, but possibly setting it up to run whenever the engine is running, as well as for 2 mins after shutdown would be good.

The worry I have, is that you can see your engine is burning oil, which would have been ok in the first 1000 miles at most, but after that, the signs of oil burning should have burnt off and your piston crowns and combustion chamber should be looking very different.

I am also concerned that your pistons, do not seem to providing the right squish and swirl I would want to see leaving whitness marks on the combustion surfaces.

If could be you are running a little too rich, which is causing an excessive boundary layer on the combustion surfaces, as well as washing the bores.

Or it could be the current level of tune is bringing on some combustion with poor form or flame front speed or distribution, which could be the onset of detonation (not audable with det cans) which is causing piston ring flutter due to poor cylinder pressure distribution.

Lastly, the oil reminants and consumption could be due to excessive crankcase pressure, which could be an engine build issue, but more likely you have exceeded the capibilities of the current crankcase breathing circuit.

You have a few issues there, and I hope some of this has given you some pointers to the route to sorting them out and improving the car in more ways than just rectifying the current issues you know about.

Hi Jon

Quick answers to your feedback.

Rad cap - brand new 951 Behr. Always go for OEM of better!
Rad is a new 56mm core 951 all ally version
Shut down proceedure - try and stay off boost for last few miles. Switch rad fan on to keep temps as low as possible. Allow to idle for 1-2 minutes before shut down. Allow fan to run on for a minute or two.
Temps on shut down - I have several reversible temp stickers on the engine. Oil cooler = 100deg. Sump = 77 deg, block = 77-85deg, head = 85-90 deg, coolant pipe to rad = 85 deg. At 90 the fan will kick in if I haven't manually switched it on.
Coolant pipes appear to become more pressurised after shut down and coolant is pushed out the rad cap overflow within 30 seconds of shut down, not before. Water temp gauge can go up slightly after shutdown. Once cooled down system is sometimes under low pressure, sometimes no pressure. To avoid coolant being pushed out on shutdown I am now releasing pressure from vent screw, enough to drop pressure, not enough to lead to continual boiling
Turbo is a rebuilt Carrera GT spec K26 2660 6.10 without water cooling so not adding to coolant problem but could be adding heat to the oil. Oil is Fuchs Titan Pro Race S 10w50.
AFR = 14.7 at idle, high 13's to low 14's on cruise. 10.5-11 at WOT. Should I richen it for idle + cruise? WOT is over rich and not changeable without making the WUR adjustable
Ignition has been set as per FSM 25deg at 2000rpm. So circa 9deg at idle
The engine is running the later additional head breather. The oil separator/Purolater is venting to atmosphere. Light vapour can be seen when blipping the throttle but no mist or oil is contaminating the immediate area as happened before the HG failure

So onto an electric water pump. Can this be added in line with the existing pump or is it an either/or? I am not running with a cabin heater, I have bypassed the heater straight to the thermostat housing. I am thinking I could place the electric pump under the inlet manifold pulling coolant out the back of the head and through the thermostat housing.
Any benefits in removing the thermostat for better flow? Any issues when cold? I will not be running the car as a daily in the Winter, in fact it wont go out at all!
At what point would coolant flow reversal be a benefit? Cooler coolant into the head first?

Despite all the issues the engine is currently pulling like a train whether at 0.7 bar or as I tried out briefly on Sunday at 1.0 bar. It feels smooth and now no untoward noises. I did fear some det over the last week or two but found it to be the wastegate line rattling on the clutch line at exactly 3000rpm. Its now fixed.

Whilst on one hand I am enjoying the continual development, on the other I do want to run it and rag it without fear I'm going to turn the internals into a molten mess.

I normally set up the idle CO on a 924 Turbo or CGT to 2.5%, which is usually richer than 14.5:1 on an AFR setup, you need this to cool the valves when not on boost, such as hard driving followed by coming to a stop due to traffic or traffic lights etc.. But also at idle, being as lean as you are going can cause very hot combustion as that thermal energy has nowhere to go.. Its why most cars do not run 14.5:1 off a lambda sensor at idle, even new cars with the latest emissions standards.

With your spec, I would probably recommend going for a completely revised breather setup, as even the updated version with the Montego valve as specified by Porsche is just a bandaid solution.. even if you are not getting oil spitting out of it, if it is allowing a build up of crankcase pressure, the piston rings will not seal properly, which causes oil burning, which in turn causes sticky rings, glazed bores and detonation.

With the turbo auxillary electric waterpump from the 944 turbo, just plumb it in where the heater used to be plumbed in, it will equalise the temperature around the head and help the engine get some natural convection of coolant around the whole circuit, even opening the thermostat to allow this to be effective.

Unless something has seriously gone wrong with your build, you should not be blowing multiple head gaskets, no matter what make you are going for, unless your using "Ho Chow Min technologies" from some dodgy importer! So stay away from the commetic or MLS ones, they will just make things worse.

Richen up your idle and see if it helps.

Is your ignition system S1 or S2? and what have you set the ignition advance to and how did you set it?

S1 igniton. Ignition set at 25deg at 2000rpm with dizzy advance hose disconnected and vacuum hose blanked off as per FSM. This equals just shy of 9deg at idle.

Interesting, I used to bung in a couple of degrees more retard for unleaded 95 RON fuel usage, but normally this would result in a dynamic timing of about 10 degrees, so it looks like you are 1 degree more advanced than I would have been anyway, so I doubt it is too much retard in there.

But then again, if your pistons have any change of crown design from stock, the whole ignition curve needs to be rethought anyway, but you should be in the right ball park for idle anyway, and you are making reasonable power, so it can't be too far off at other loads and rpm.

I would say richen up your fueling at idle a bit though, it could be you are a bit lean, especially for recovery following any boost. This could be enough to cause some hot spots which could be causing problems after shutdown.

Is there any chance that the expansion tank kneck is anything but perfect? it could be that the rubber in the cap is sealing well enough for the pressures of running, but when you shut down it is leaking at a less than normal blow off point due to the seal only being marginal.

Interesting problem you have, but I am sure we can get to the bottom of it.

Good call on the neck of the expansion tank. I had looked at buying a new tank but at £200+ for new it was seen as a cost too far in the scheme of my lengthy and deep spending on the car. Excellent (read white versus yellow) used ones never seem to come up for sale. I originally used a 944 rad cap and it was absolutely fine through 1500 miles of running in, low revs, low boost. It first decided to spit out coolant during driving on my first spirited run over a longer period. So I upped the spec to a used 951 cap. This again is fine until you up the revs/boost, held the coolant in during driving but spat it out during shut down. The current cap is brand new and definitely has a stiffer spring than the used ones.

I'm going to look into a new tank again. I'll also richen up the mixture to low 14 at idle. I need to tick off all possibilities