gbgastowers

I rented a compression gauge tonight. The conditions were cold engine,all plugs removed,WOT,and coil wire grounded,battery is weak from trying to start today and on battery maintainer. I checked 3 cylinders and got 30,30 and 60. I will verify tomorrow or the next day but it looks like my decision not to change the rings out was a bad one. The good thing is everything is clean now if I have to take it back out. Gunar

Landseer

Maybe.

But what about the valves?
Was this set of heads with the replacement engine or the original?
Could the valves be the issue, not the rings?

gbgastowers

decision on the rings made here after taking the heads off the 2nd engine and posting these pics. Original engine had a big gouge in cylinder wall from a piece of valve spring stuck in ring. https://rennlist.com/forums/928-foru...let-it-be.html

Landseer

I remember that.

But what about the heads? If they weren't disassembled and valves checked, or at least fluid tested before disassembly,maybe they could be the issue, not the pistons/rings.

gbgastowers

These were the heads off the replacement engine(M28/12) which were rebuilt and 4 slightly bent exhaust valves replaced. Original engine was a M28/11. I think I can add oil to the cylinder for a compression test and if it goes up it verifies a ring,piston,bore problem.

Hilton

Did you put a teaspoon of oil into each cylinder prior to the test? On an engine that hasn't run, been washed out with fuel, etc., there's a good chance the rings aren't sealing properly.

I'd repeat the compression test with a teaspoon (5ml) of oil in each cylinder and see what numbers you get.

Were the two cylinders that showed lower on the same bank, or on opposite banks from each other? If on the same bank, could be cam timing is off.

Before jumping in there, did you double-check the ignition timing is correct? (i.e. plug wires on correct cylinder for for the firing order, distributor timed correctly). As each thing is eliminated it'll make troubleshooting a little easier.

docmirror

Houston, we have a problem. The engine won't run with that kind of compression. Plus the backfire would indicate you've got an intake valve staying open for some reason.

With three jugs that low, the engine is going to need to come apart. At least get the heads back off. If you want to do a leakdown test first, it'll tell you where the problem is. As Hilton said, if they are in the same bank, it could be a badly timed camshaft.

So, take the valve covers off, do the cam timing per the book, and then setup the ignition while you can see the valve position and determine the firing stroke. Once back together we can revisit the CIS issue.

gbgastowers

Thanks- I'll recheck when the battery is fully charged and follow your suggestions.

gbgastowers

I only tested 3 cylinders and 2 were on 1 bank (30 and 60) and 1 on the other was 30. These numbers are lower than original low #s (100) given to me by the PO. I thought the original low #s were due to bent exhaust valves that were found during head rebuild. I used oil on 1 of the cylinders that read a 30 and it went to 60 but my battery is low so I'll get some more accurate #s with oil in cylinders later. I feel confident the basic timing settings are in spec (tdc,rotor on 1, cam gears on their marks,harmonic balancer on correct and cam gears not on backward). But I also felt pretty confident about the rings being good.

Landseer

Don't feel bad, I just broke the CPS off in the block and failed at every attempt to remove it. Car has MSDS with very rusty bolts and I dont think I can manipulate it apart in-situ. Might be pull time here too. Hope not.

Podguy

The distributor is out. There should be a little line for the initial alignment. The initial setting is about in the middle.

Podguy

You can not do a compression check on the 928 engine after it has set idle for a while - more than a couple of days. To get acurate readings you need to put some light oil down each cylinder. it is not abnormal to see readings this low on a cold dry engine. Many bogus valve jobs have been sold on this basis.

Oil the engine and do the test over. You should see readings between 120 to 150. Even with a bit of a range the engine is still good. After you have the engine running a hot engine will read very consistent.

Podguy

Doc,

How do you set the valve time on a 16 valve engine. What valve covers are you going to take off?

gbgastowers

How am I going to get enough compression to start or does it matter?

gbgastowers

I am beginning to think this whole compression thing is a wild tangent I'm on. I rented another gauge and put 3 pumps of oil from a squirt can in a cylinder and got up to 240. Time to get back on track with the timing. Gunar