One of my Father's Day events involved having my kid help me do a compression test on the 1970 240Z. Unless you have one of those handy under-hood starter thingies, it's a two-person operation and there just hasn't been the right opportunity until today.

The rebuilt L-24 showed consistent 180 psi readings from cylinders 1-2-3 and 5. Cylinder #4 read 185 ~ not terribly far off from the others.

But Cylinder #6 read 195. Twice.

Not trusting that, I re-did the whole test, with identical results.

All plugs were out, we used three power strokes per cylinder, and the throttle was left alone.

Previous tests have read consistent numbers across the board. Why should one cylinder suddenly read 15psi higher than the others?

Additionally, plug #6 showed marginal signs of being too hot ~ cleaner than the others (immaculate, in fact), with white ash just beginning to form around the electrode.

Guesses?

Frank

_________________
1970 240Z

If your rocker arm lash is too loose on that cylinder and can't open each valves fully it 'could' read as higher compression because there is less overlap between valves. The rockers on that cylinder would be noisy due to the extra clearance though.

White plug means lean mixture but high compression means no air leak. Puzzling.

_________________
Bryan Little
Datsunzgarage.com

1970 240Z - enhanced F54 L28 w/P90, Weber DCOEs, 4:11 R200, Nissan T5 5-speed

-Carbon build up.

-Worn cam lobe/rocker/incorrect lash/incorrect wipe pattern

-A Bit of oil in the cylinder from head gasket, or valve stem seal beginning to fail.

Repeat the test with a squirt of motor oil in each cylinder to see if they even up.

_________________
-Dave
76Z Turbo'd
72 In Progress---DONE!

Ding Ding Ding! The valve guide seal suggestion sets off alarm bells.

Altho my engine has fewer than 7,000 miles (est) on it since rebuild, that rebuild was more than 21 yrs ago. It had abt 2,000 miles on it when I bought the car back (three years ago). I've put about 5,000 miles on it since then.

Valve guide seals seldom fail with only 7,000 miles on them, but like Phrog once said in another Post, 'old is old'. Within the past 1,000 miles I have begun seeing blue smoke (burning oil) at an idle (high vacuum condition). That's a sure sign of oil being sucked down from the cam area, past the valve guide seal and into the combustion chamber(s).

Without the sparkplugs in, the engine would have no vacuum or compression, but the oil could (could) run past a badly worn seal just the same. Stuffing each cylinder with the compression checker would give the vacuum back to the cylinders one by one.

I didn't conduct a "wet test" (squirting oil in each cylinder), but you're right ~ if I got enough oil past the seal(s), that would seal the cylinder (rings) enough to show an elevated reading.

THANKS! That makes perfect sense.

By the way, Vinny Bedini did NOT do this engine. It was rebuilt by the PO all those many years ago. He was a young Mercedes mechanic and I feel he did a commendable job. But old is old, and I guess it's time for new guide seals.

Frank

_________________
1970 240Z