Cold compression test

I developed a bad external coolant leak, presumably head gasket, on the MK10 that I noticed just after I dumped the oil for a 100 mile change. Did a cold compression test with 135-150psi on the 9:1 engine, came up to 145-160 wet. The engine ran smoothly with ok power before. Anyways, it came with the filthiest oil you’ve ever seen, but cleaned up to almost smokeless with fresh oil. Hard to tell about oil consumption, because I’ve been trying to get it drivable enough to see about that but didn’t use a significant amount. You can tell how that is going. Oil pressure was 40-45psi in the road, 15psi on the gauge hot at idle. Plugs a touch rich looking, not oily. I’m going to pull the engine to change the transmission, and I’m thinking the bottom of the engine is probably fine. I’m inclined not to tear it down past the head job. Any opinions?

How many miles are on this engine?

Odometer shows 14K, but the speedo cable was broken. Very little pedal wear, so goes along with the story of low mileage from 3rd party seller, but that story has proven unreliable. So, essentially unknown, but I’ll guess 50K from a barely worn replacement front brake rotor on one side and a nearly worn out original looking one on the other.

Given that, I can’t see not doing an engine refresh while it’s out of the car.

Reason to do it:
because it’s out - saves the effort of taking it back out
Reasons not to do it.
might not need it
expense 1-5K based on my own experience. Already way more $s in this Mk10 than it’s worth
time - I could be painting my xk120
ties up my lift for another 1-3 months depending on machine shop
the last machine shop was so bad that I’ll be trying a new one, 5 hour round trip. No mail-order machine shops for me.

If I had nothing else to do, sure I’d go through it for the fun of it. I don’t find anything in the facts that necessarily condemns the engine, so I’m not sure which “that” you refer to. Thanks for the opinion, but I need a better reason than the fact that it’s out. When I have the head off, I can inspect the bores for wear. Significant taper would push me to a full rebuild.

I just don’t trust cold compression checks. I’ve had very low and uneven results in the 150s on good engines that tested very high from 190-200 psi on a warm engine.

And have had results go up from 185-195 after running MMO thru the oil.

I agree, and that is part of my thinking. This engine has already cleaned up a lot since I got it. I think it has more potential and is why I was changing the oil after only 100 miles.

The converse of your statement is also true - normal compression doesn’t necessarily mean the engine is good. I had an e-type 8:1 engine with 170psi warm compression that burned oil like a diesel locomotive. When I tore it down, the rings were completely loose from wear in the grooves. I don’t consider those cold compression numbers too low. I’m just reluctant to run it any more - would have to dump fresh oil in it also. Maybe I should though. Would no change in hot compression figures convince me to tear it down? Probably not.

Follow-up for those interested. As was obvious, I was trying to talk myself into keeping the bottom of the engine, but, as I got into it, it was clear that a tear-down was needed. The forensics are interesting, if depressing.

I pulled the bonnet, stripped the engine of ancillaries, removed the front suspension on its subfame, and lowered the engine and transmission to the floor under the lift.

First, I noticed there was another engine number under the Mk10 number that starts with 7E. Probably the only time an e-type head went to a mk10 rather than the other way around. Broken damper laying in the chain box. Mismatched and poorly torqued head-bolts, but a pretty decent looking head although with plugged cooling passages from rusty debris. I’ll have it gone through.

The bores were .020 oversized, no taper, but there was a rust line in #1. So much for possible low mileage. On the bottom, more mayhem. Mains in wrong places, no locking tabs used on anything, rods in wrong holes, some with illegible numbers, crank already 30/30 under with debris in the bearings. Good thing I have a better crank and some rods, although I’m going to looks for some xj6 rods. Anybody with an extra set for sale?

The +20 pistons look pretty good, so I might be able to use them if #1 bore cleans up. There are still hone marks on the cylinders.

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