FredR

Did you listen through the exhaust pipe, the inlet tract and the expansion tank? Not really likely but you could also be losing some air through these tracts as well.

Are your 16V heads fastened on with studs or bolts?

thepurpleblob

The only place I can detect air coming out is from the dipstick tube (although very gently) and from the oil filler cap (presumably via the breather). That's louder than the dipstick. There's nothing from the inlet, the exhaust or the expansion tank.

There's not a great rush of air from anywhere. It quite noisy going in... so I'm a bit unsure if I've missed something.

There's a row of nuts along the top edge, so presumably studs.

thepurpleblob

I just rechecked a 'good' cylinder and there's no air escaping anywhere that I can detect. The tester is still quite 'hissy' (although less so).

Unless I've badly missed something my conclusion would be that the air is getting past the rings on that cylinder. Which isn't the news I wanted.

thepurpleblob

Non update.... I took some very bad pictures with an endoscope (the spark plug hole points in the wrong direction). I'm still completely unsure what I'm doing next. I think the oil was probably me adding some during the compression test.

Which reminds me... 'oil' test made no difference so thay says NOT rings. Leak down test says rings. Sigh....

FredR

Before the purple rain descends on the purple blob parade time to take stock of the situation. Your photos limited as they are in terms of quality show no signs I can see of mechanical damage. There appears to be only one leak path possibility - the piston rings and give or take a little scope for interpretation there is only one cylinder seriously impacted so this asks the question why? The fact is that at 100 psig compression pressure that cylinder is still compressing the air - just not to the extent that one expects. No signs of bore scratches so that reduces the chances of a damaged ring so for whatever reason maybe you simply have debris behind the rings leaving them reduced capability to seal..

Before you start thinking about tearing down the motor maybe it would be prudent to consider some form of proprietary treatment. A single shot of your favourite brew can do no harm other than empty your wallet of 20 quid or whatever. One of my 928 friends used to swear by a product made by Rislone- never used them myself or another one that is often mentioned is Sweepco 502 or something like that. Not sure what is or is not available in the UK but I am sure you can fathom that one out. We are not talking about fuel treatments here rather stuff that is mixed with the oil and run for whatever mileage before ditching the oil type of thing. Liqui Moly have something along those lines- Pro line 2037. Their products are generally excellent. See:
May be worth a try?

Petza914

I ran SeaFoam in my 79 engine to free a stuck lifter. The ratio is 1 Oz of SeaFoam per quart of oil in the crankcase, and only run it for 100-200 miles before renewing the engine oil. I did 2 cycles of this. It's also supposed to desludge a motor from old accumulated oil which is the other reason I went this route as it wasn't as clean as I'd like when I had the pan off to replace the OPG.

thepurpleblob

My current plan is to chuck some Sea Foam down the cylinder. Being too lazy to go out, I've ordered some. I may well give the Liqui Moly in the oil a go too. I've found their products to be good.

Yeh - I'm struggling to know what to do for the best. Getting the engine crane out just seems a bit drastic.

honerboys

just letting my imaginaton run here so bear with me - i've never torn down a 928 engine but i did a couple of 944's - conventional wisdom would suggest that since these cylinders are completely surrounded by coolant jackets that a leaking head gasket would leak into those jackets - but what if the head gasket doesn't leak between the block and the gasket but between the head and the gasket - in other words the compressed air would be escaping the cylinder on the top side of the gasket - could it travel between the head and the gasket and find one of the oil passages? - or would it have to hit a coolant passage first - this would explain low compression, poor leak down, inability of added oil to increase compression, and leakage into crankcase - would this be possible? - those of you who have way more experience on these would know

Petza914

I think in that case you'd be getting combustion gasses into the coolant. They make test kits that will look for exactly that, which would be another good test for the OP.

honerboys

hey petza - that's what i was wondering - is it possible for escaping air/combustion gasses to NOT find a coolant passage but instead find an oil passage and vent directly into crankcase?

Petza914

I don't believe so via a head gasket issue, but excessive crankcase pressure is most often caused by poor ring seal, so when the piston goes up and the combustion occurs, some of the pressure from that explosion gets past the rings and then adds pressure in the crankcase. Poor valve seal will allow that same pressure to escape either through the intake valve (back into tine intake manifold) or via the exhaust valve (out the exhaust pipe). A little crankcase pressure or vacuum depending on the situation is normal and why people install AOS systems and catch cans.

For example the supercharged motor in my 79 was creating enough excessive crankcase pressure due to blowby that it was actually moving oil through my crankcase ventilation system to the point that my vent hose that should have just allowed pressure (gas) to escape was actually carrying oil with it. It was getting all over the bottom of my car so I ran a hose to the back, but soon found out when a buddy was following me that within 5 miles I had covered his windshield with oil film. Compression was good in all cylinders though and the car ran great. I ended up installing a Verus dual chamber AOS and vented the crankcase to one port and the two heads to the other port and now zero oil comes out the vent line. It's also designed so that any accumulated oil runs back into the engine sump once the car is shut off and the one way valve opens when the crankcase pressure stops pushing on it.

Not sure if that's helpful to you or not.

Pete

GregBBRD

Lots of carbon in that cylinder....

That certainly appears to be a burned valve, in the 2nd picture...
You can see where the edge of the valve is flat....and you can see the edge of the valve seat past the flat area.

My guess is that no chemical is going to fix this...

honerboys

oops...i thought we were looking at the top of the piston - that area does look suspect - i really can't see what's going on there - and it could be that oil is hanging on to rough area on cylinder wall?

thepurpleblob

We're looking at the top of a piston. Unless I'm missing something.

My endoscope is only a cheap one and has no 'rear view'. It's also difficult to position usefully as the spark plug hole is at 45% (ish?) to the line of the cylinder.

I'm not really in a position to write a blank cheque to myself, so does anybody more knowledgeable than me want to suggest what they would do next? I'm going to try chemicals because, why not? A couple of extra days is not an issue.

If I was reasonably confident that it was a valve I'd just pull the head. It's relatively straightforward....

Shark2626

I suppose you really have nothing to lose by trying! Why not?

I’m not a fan of many brands, only the Top Tier ones. Chevron Techron is the only fuel system cleaner recommended by Mercedes and BMW (although BMW slaps their own label on the bottle), and it used to be recommended by Porsche until they released their own product, but Techron is not easily available. Liqui Moly has been around since the 50’s and manufactures motor oils approved by Mercedes, Porsche, etc…. In other words, they are Top Tier. I don’t have experience with their additives but if Amazon reviews mean anything the stuff really works! As per the video that Fred posted, I would start with the Liqui Moly oil additive, use as recommended and then change the oil and take another compression reading. I would then add their Valve Cleaner to your gasoline - using the recommended dosage, more doesn’t equate to better when it comes to these products. And then drive the car until the tank is nearly empty. I wish you good luck! 😀

ETA. Try to get their Pro Line products, as shown in the video. The Pro Line products have Black on the can labels, the Consumer Line products have silver on the can, I believe.