Low compression on one pot.

[Mal B]

Due to budget blowout on 10 x 5m worknexe for the side of my bus, and the rather inconvenient demise of my hws, I've had a lot of time to think about why the S1 VN V6 in my D21 has lost compression on No. 5, 75psi compared to 150psi for 1 + 3. No's 2,4 and 6 are still in the too hard basket (heater hoses going to front of motor & bottom radiator pipe going to rear all conspire to make spark plug access pert near impossible).
I'm thinking overheating or lack of oil would affect all cylinders, so that was discounted.
The only real variable, with the cylinders, that I found, was the three different brands of spark plugs, one of which was heat rated(colder) for gas and was in No. 5.

Could it be, the cold plug has clagged up the rings on No. 5? Or am I just trying to convince myself it's not a broken ring, which could $$damage$$ the bore?

 

[ericcs]

does it feel like it's running on 5 cylinders?
if so, could be as simple as a blown head gasket, or a burnt valve!

 

[Mal B]

does it feel like it's running on 5 cylinders?
if so, could be as simple as a blown head gasket, or a burnt valve!

Definitely rings or piston, oil down the hole bumped compression up to 100psi.

 

[Old.Tony]

I'd suggest that if putting a little oil gave you more compression, it's just stuck rings.

There are additives that claim to free up stuck rings. I can't vouch for their effectiveness but they're not expensive so that's something you could try. Otherwise you're going to be busy with the spanners.

 

[Paulsw55]

Doing an air check will give you a more positive result. need to have an air fitting to put compressed air pressure down the spark hole with the piston top dead centre compression stroke . need to hold in this position maybe top gear . listen for air leakage exhaust for exhaust valve ,inlet for inlet valve , crankcase for rings and piston from oil cap .

 

[Mal B]

There are additives that claim to free up stuck rings. I can't vouch for their effectiveness but they're not expensive so that's something you could try. Otherwise you're going to be busy with the spanners.

That's what I was wondering about Tony, Americans reckon a soak with Sea Foam will do the trick, but it's a bit thin on the ground here. With a bit of DuckDuckGoing I found SF is roughly 4 parts diesel, 2 parts naphtha and one part isopropanol. The diesel's easy, brake cleaners and shellite are naphtha and isopropanol mix and easy to get.
The spanners will be pretty busy for a fair while on the rest of the neglect, so plenty of time to let it soak. And besides, what's the worst that can happen, it doesn't work, engine out for rebuild, I do more damage, engine out for rebuild, it does work.

 

[Mal B]

Doing an air check will give you a more positive result. need to have an air fitting to put compressed air pressure down the spark hole with the piston top dead centre compression stroke . need to hold in this position maybe top gear . listen for air leakage exhaust for exhaust valve ,inlet for inlet valve , crankcase for rings and piston from oil cap .

Compression test, with oil down the hole, has pretty much proven a piston/rings problem Paul. But speaking of tdc, do these computer controlled engines still have a timing mark somewhere?