NA Poor Compression, Head Gaskets OK - What have I done?

Hello,

I'm at a bit of a stand still with my engine and I think it is time I asked for advise.

I overheated my engine pretty badly on a summer day 12 months ago on my way to CQ. As I was planning to unregister the car and buy a 4x4, fixing this has been low priority and I was lazy and left the poor zed alone for ages.

When it overheated I pulled over and turned the car off, and I can't remember exactly how but the coolant had been emptied, either through the radiator cap or the fill point, no hoses had come loose. So I slowly undid the radiator cap and released a LOT of steam. Once RACQ arrived we filled up the radiator with water and turned the car on with rad cap off. After 5 seconds or so it would bubble like crazy spitting water out. I took this to mean that gas from the cylinders was escaping into a coolant section and when the thermostat opened it was released through the cooling system to the cap. I had it towed to my new house.. 3 months or so later when I ran it again and did the same test it hardly bubbled at all, just a tiny little bit, at this point I ran Chemiweld through the coolant to see if it would fix the problem - I know, cheap fix but worth a shot.

About another 3 months later I pressure tested the engine, using jumper leads as of course it had a dead battery. The results were 120psi in every cylinder +- 1 or 2 psi. I did not carry out the test again with some oil in the cylinders. I stripped the plugs and hoses ready to remove the plenum but couldn't get the plenum off due to using silicone gasket last time I did a plenum pull.

So a few months later I managed to get the damn thing off and then I didn't stop until I had removed the cylinder heads, thinking that I had blown a head gasket and/or warped/cracked a head.

My gaskets are both ok, heads look good as far as I can tell. Nothing obviously visually wrong with the block or heads..

Some people have told me that I have done the pressure piston rings and need to remove my pistons to replace them - some have said that there's no point doing that and that I must either have a cracked head or block.

I'd really just like to know what my next step should be in order to work out what is wrong and fix it. I'm going to be in Mackay next week so could take the heads into a shop to be dis-assembled and tested, and refurbished if they are ok. But could I have a wrecked block and be wasting my money? Do I need to remove the pistons? Can one cracked head cause equally low pressure in all cylinders (surely not?). Could the low pressure simply have been due to using another car and jump start cables to turn it over for the compression test?

I have accepted that I may well find the engine is rooted and I would be better off buying another complete engine for financial and reliability's sake, but if I can I would rather have a go at fixing this engine.

What do you guys think?

I really miss driving her and it's time I got her running again.

Thanks,

 

Had the same problem during the last couple of months and did the head job twice but it turned out to be a crack in the head that didn't open up until the engine got hot. Around 98 degrees. Compression in all cylinders was good.
Went back to the drawing board and purchased another NA motor, did a 100k service on it. Cam seals, idlers, timing belt, water pump, thermostat, 3 new exhaust studs each side and rear main seal. Pulled the old engine, put in the replacement engine and now its running sweet. Don't forget to flush the radiator while its out. Overall this turned out to be the most efficient and cost effective way around the problem.

 

Well I haven't spent any money on it yet and have had fun taking it apart, but putting it together again will probably suck haha so that sounds like a pretty good option.

 

I rebuilt an NA engine once. It was a mistake. I wish I had just brought another NA motor. Faster and cheaper solution.

 

Nem?

Are you referring to this engine?

http://www.aus300zx.com/forum/showthread.php?t=328175

 

An interesting story....

.....however you have left out the important details.
You mention you pulled over when the engine overheated, it tells us zero.
Why did you pull over?
Did you notice the temperature gauge indicating high temperature?
Did the car lose power?
Did the engine rattle?
Did you check the coolant level prior to departing on your trip?
Was it full?
Where did the coolant escape from the system?
When an engine overheats severely to the point where no coolant remains in the system, it causes severe engine damage.
Your compression tests confirmed this.
You should have pressure tested the cooling system prior to dismantling the engine as a means of identifying the source of the coolant loss.
Whoever told you the piston rings are damaged has probably provided accurate diagnosis, but that is unlikely to be the only damage.
As others have mentioned, forget about rebuilding this engine & purchase another.

 

Appreciate the advise.

It was looking down and seeing the temp gauge being right at the top that made me pull over. Nothing else was out of the ordinary that I noticed. It had coolant prior to overheating. I had already been driving for about 5 hours at 100km/h when it happened.

In future if I am ever dealing with a leaking cooling system again I'll be sure to pressure test the cooling system.

Cheers,