I finally got around to running a compression test on my rebuilt, overheating L98 engine.

The results...

DRIVER'S SIDE
1. 173 PSI
3. 180 PSI
5. 185 PSI
7. 165 PSI

PASSENGER'S SIDE
2. 200 PSI
4. 200 PSI
6. 197 PSI
8. 185 PSI

The only thing that I think might be "disturbing" the results is the fact that, on the driver's side, I only cranked through four compression strokes for each cylinder (per the instructions), whereas on the passenger's side I cranked it until I got the higest reading (~6 to 8 compression strokes). Also, the instructions said that I should get the engine to operating temperature before running the test, but I couldn't do that because I have all of the intake off.

Also, some of you might have seen my post on "wet sparkplugs". Well, while I had all of the plugs out to do the compression test, the same cylinders that those plugs came out of (especially #6) were spraying out oil as I cranked the engine (hence the plugs were covered in clean oil). I've attached a picture of a piece of an originally clean piece of paper that my dad held to the plug hole as I cranked it.

I've never done a compression test before, so I don't really know what to make of the results. Anyone here have any interpretations?

Also, what do you think about the oil comming out of the cylinders? I should note that this engine has not been run for several weeks, and even when it was running, it did not appear to be burning a lot of oil (it would shoot out some smoke if I "punched" the accelerator, but remember that this is a rebuilt engine and the rings probably haven't seated yet). Also, since the intake and distributor were out when I cranked the engine, the oil pump would not have been pressurizing the oil...

Any ideas?

I appreciate any and all help!

 

i've never saw an engine blow oil out the plug hole when you crank it. new engine right? i'd leak it and see what i get and check the valve guides for wear. you should do the compression test with the engine warmed up, all the plugs out and the throttle blades open. you want all the numbers within 10% of each other, the actual number isn't that important.

 

Having liquid of any kind (oil, anti-freeze, etc.) in the cylinder while doing a cranking compression test will skew the results dramatically. GO do it again and make sure you block the thorttle wide open when you do it. Those results look pretty bad.

If you've got stuff spraying out a plug hole or two consistently, then something's pretty badly wrong- read: head gasket or crack in the block/head. Even if the rings are GONE you still won't have oil coming out consistently- it's laying in the pan and won't make it past the rings at cranking speeds becuase it just doesn't get thrown up there fast enough by the crank. If there's consistent fluid squirting out it IS antifreeze, even if it looks dark.

 

Thanks for the replies!

ede: By "leak it" do you mean do a leak-down test? How can I do that, and will I need any new tools?

Damon: I don't know if it makes a difference or not, but I can tell you that the intake manifold was off for the test, hence you really can't get any more "wide open throttle" than that.

As far as what's comming out of the hole, I'm 95% sure that it is oil. Notice in the picture how the paper is nearly transparent where the fluid is; as far as I know, only oil does that to paper.

Here's the thought that I had: We originally did this rebuild because the valve guides in the heads were so bad that oil would leak down overnight and wet the plugs, making the engine unable to start. So we had the heads rebuilt (along with the rest of the rebuild stuff). I'm wondering if mabye my machine shop either forgot or did a bad job on that head, and the oil that's comming into the cylinder is just leaking through the valve guides. That's the only thing that I can think of because the oil looks clean, and the engine has not been run for a while.

Anyone agree/disagree?


Either way, I think I'll probably re-test the driver's side, as the results on that side are really all over the place. (I don't know what happened on #7...)

 

By the way... If I have to go and start pulling heads, can that be done without pulling the whole engine out? Seems like the passenger's side would *really* be a pain, what with all of the A/C equipment in the way.

I'd just go ahead and skip all of this guesswork if yanking the heads looked easier, but I want to try to have a better idea of what could be going wrong before I start doing anything that invasive...

 

I don't want to put words in Ed's mouth, but, yes. I would try a cylinder leakage test. To perform that, you'll need a reliable source of compressed air, a pressure regulator, and a cylinder leakage tester.

The compressor will have to maintain at least 100 PSIG for the test, and is the most expensive component needed. If you don't have and didn't plan to get one, you can either have someone else perform the test (which would probably require you to reassemble the engine and drive it somewhere) of borrow/rent a compressor.

You can purchase a cylinder leakage tester for right around $120, or build your own for about $30. If you already own a compression tester, hopefully it is NOT the type that is simply held against the spark plug opening, but has a hose/fitting that threads into the spark plug hole. The first type is notoriously innaccurate, mostly from poor sealing and leakage. If you have the type with a hose/fitting, you already have the most difficult/uncommon part in your hands - the adpater hose. Find a quick conneect fitting that will mate with the one on your compresion tester. Assemble a manifold from two ¼" NPT "Tees", a couple of mating quick-connect fittings, and one specially modified ¼" NPT nipple between the two "Tees" as such:



The nipple needs to be plugged with a solid material that can either be brazed or silver soldered in place. A small section of bolt or other plug would be suitable. Through this plug you will drill a 1.0mm hole (0.040") using a 1.0mm or #60 drill. This is the "secret" controlled orifice that makes the leakage testers work. The orifice regulates the flow of air to the cylinder, and in that way creates a pressure differential between the two gauges. The greater the flow (cylinder leakage), the greater the differential.

Set the regulator at 100 PSI for the incoming (line) gauge and monitor the leakage (cylinder) gauge to compare the two readings. Using 100 PSIG on the line gauge makes the percentage calculations direct - 100 PSIG in and 85 PSIG out = 15% leakage. It's that easy.

 

I'll see if I can whip-up a leak-down tester and see what it tells me.

In the meantime... Does anybody else have any ideas as to why I might be getting oil out of the cylinders (some of them) like that?