924Board.org

[SilverGhost]

A couple of months ago my 924 started missing on #4 for 20 seconds or so after startup from cold, and losing perhaps a litre of coolant each 1000 km. Once running she fires evenly and runs well.

If the last run was brief and with the coolant header tank cap off, then she starts on all four.

There's no unexpected pressure build up or bubbling in the coolant header tank. There's a slight oil film on the header tank contents, but I used grease on the recently fitted new water pump gasket so it could be from that.

If I remove the header tank cap the day after a good run, air is sucked in to the tank as I remove the cap.

If I remove plug #4 the day after a good run, its wet, and judging by the slow evaporation rate its wet with coolant. The resistance from the centre electrode to the body is less than 1 MOhm, while the other three plugs are dry and have a resistance greater than 10 MOhm. Immediately after a run, all are more than 10 MOhm.

The car has new plugs, plug leads, dizzy cap and injectors. System pressure and control pressure (at 15C and operating temp) are in range. I checked the head bolt tensions and all were close to correct.

My guess is that there's a small leak from the coolant jacket into #4 cylinder, and that the head will have to come off.

Has anyone else had these symptoms and determined the cause?
Does my diagnosis seem sensible?
Are 924s prone to cracks in the head, or is this more likely to be the head gasket?
_________________
"Priscilla" 1981 924 NA RoW auto, metallic blue

[Ozzie]

HG failure is most likely.
Do a compression check
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[emoore924]

+1 on the HG. I'd also run it up to normal temp. Wait. Then remove the #4 plug, crank the engine and see of water shoots out...

Experiencing any hydrolock? (when the engine starts to crank, then it stops suddenly, then starts cranking again after a few seconds... If so, STOP doing that, you'll bend a rod. And you'll definately need to determine what's failed....

[SilverGhost]

Thank you gentlemen.
No water shoots out emoore924, nor hydraulic lock, but thanks for the warning.
To paraphrase the Queen of Hearts, off with its head.
_________________
"Priscilla" 1981 924 NA RoW auto, metallic blue

[SilverGhost]

Priscilla is back from the mechanic, with a new head gasket and new coolant hoses. The missing-on-start problem is no more.
I supplied the gasket kit and hose kit, and he charged 16 hours labour. I inspected frequently during the work and was pleased with the care being taken.
She purrs like a kitten now, and no longer loses water.
Is it still recommended to recheck head bolt tension 5000 km or so after a head gasket change, or are modern gaskets different to those of the old days? Along the same vein, should exhaust manifold bolts be checked after 5000 km (its quite awkward to access some of them)?
_________________
"Priscilla" 1981 924 NA RoW auto, metallic blue