Here's What's Going On...

[texasz]

Since I kinda hijacked someone else's thread I am starting my own here but this is a link to the background of what I'm talking about. http://www.classiczcars.com/forums/showthread.php?s=&threadid=7875

I went over to another member's place to do some testing on my car and we started with a compression test. Here are the results.

cyl 1 = ~120psi

cyl 2 = ~120psi

cyl 3 = ~120psi

cyl 4 = ~40psi :surprised

cyl 5 = ~120psi

cyl 6 = ~120psi

So it is now clear why my Z will not run well went she is cold (runs good when hot, after being driven a bit) and explains the blowby!

With this info we skipped the leakdown test, which is what we originally started out to do.

In stead of cleaning the plugs we just replaced them. The old ones were kind of a white-ish color. From the books I have that indicates a normal wear.

The hoses we had available were not long enough to make a catch can so that did not get done but I will get some hose and take care of that.

This is kind of a drag as I will not have to devote time, money, and effort to getting the L28 ready to install which will take me some time as I am just learning about them and will be attempting a rebuild on it.

So that's where I'm at. I'm going to bed now...:sleepy:

[Bambikiller240]

Originally posted by texasz

..... compression test. Here are the results.

cyl 1 = ~120psi

cyl 2 = ~120psi

cyl 3 = ~120psi

cyl 4 = ~40psi :surprised

cyl 5 = ~120psi

cyl 6 = ~120psi

Sorry to hear of such low readings on your engine. As you may know Factory spec on the L24 is 170psi in a new engine, and it isn't uncommon for 30 year old engines with under 200K miles to still have compression readings of over 150psi in all cylinders. With readings of 120 on most cylinders it appears that your engine is very tired. It could be valves that are worn or not closing properly, worn valve guides, or piston rings, or all of the above. (a leakdown test would give you an idea of which is more likely) That cylinder with 40psi is of a more immediate concern. It could also have a head gasket leak from the cylinder to a water jacket, an oil passage, or to an adjoining cylinder, or (less likely) to atmosphere.

In any event, at least now you know more of what you are up against and you can try to formulate a plan to address your needs.

Originally posted by texasz

In stead of cleaning the plugs we just replaced them. The old ones were kind of a white-ish color. From the books I have that indicates a normal wear.

Yeah, that does sound like normal wear, though with that low reading for cyl #4, I'm surprised that the plug for that cylinder wasn't significantly different in color from the rest of the plugs. It must still be firing, but losing virtually all pressure, and not getting significant excess oil into the cylinder (like from bad valve stem seals), or you'd have found it to be black.

[texasz]

A tiny bit of additional info that I forgot to put in which may help. Upon discovering the very low cyl we put just a little heavy oil in the cyl through the plug hole and then tried the compression test again and it shot up to 150! The person helping said that this pointed to a bad ring.

[Ben]

Based on those findings, I'd be interested to see what compression the rest of the engine produces when tested 'wet'.

[26th-Z]

My 2 cents:

Sounds exactly like a worn out engine. The low cylinder may indicate a broken ring and may cause further damage to the cylinder wall. Usually, an engine rebuild would require new rings and honing of the cylinder wall, but if it is scored from the broken ring, you may need to bore the cylinders to a larger size and replace the pistons. Of course, a valve job is always recommended simply because you have the head off. Make sure to replace the valve stems and seats with steel for unleaded gas.

Early Z engines are relatively inexpensive and easy to rebuild. You may have an E31 head casting and that makes your engine most desireable as it potentially develops a higher horsepower rating than later types (stock). If it is blowing out enough oil to gunk up your filter, it is time to do something about it.

Good luck and let me know what you do with the engine should you decide to replace it.

[Daniel]

I would think about putting a set of rings in the 240 engine.That would buy you time and you get a chance to develop your skills at Z rebuild.I've seen the oil pan dropped from the car and with the head removed install rings with the engine in the car.I would guess the 240 could be done like that .In the end its cheaper practice than blowing an all fresh 280 engine and you get a taste of the tools required.