Need advice - Leak-down great, Compression test not so much

hcoles · 09-19-2021, 05:44 AM

Hello Porsche engine experts,

Baseline - plugs look good, low oil consumption, starts with no smoke, runs fine.
Maybe 40k miles after top end rebuild, rings were replaced and cylinders properly honed by Jerry Woods shop. The cylinders are Nikasil. Cross hatching very visible at the time of re-ringing.

Well I need expert advice.

Here is what I did...
Engine was warmed up - hot. At the beginning a bit hot to work in there.
Yesterday I did a compression test and then a leakdown test.
The compression was low e.g. 90 across a few cylinders on different banks. I got a different (new) gauge - same result. The "hold" valve at the gauge on my gauge was leaking.
I got my leak-down devices and did the tests. I tested at 80 psi and all the second readings were very close to 77 or 78. I heard no air moving. Taken at TDC and crank held with transmission and brake. Crank didn't move.

Now what do I try other than to take the engine apart? PEA cleaner?

Thanks,
-Henry

hcoles · 09-19-2021, 06:27 AM

Calling Henry Schmidt? Henry this is Henry. :-) I've posted on a mod you made to chain tensioner part. You made the pivot bushing longer. That was a while back. Thanks.

snbush67 · snbush67's Garage

Just throwing this out there because it happens.Did you rotate the engine to be at TDC for each cylinder?

hcoles · 09-19-2021, 07:32 AM

Quote:

Originally Posted by snbush67

Just throwing this out there because it happens.Did you rotate the engine to be at TDC for each cylinder?

Yes, rotate to TDC per the marks on the pulley and watching the rotor. I can hear the air rush out just prior to TDC - this confirms I'm on the compression stroke.

This brings up a question.... maybe do a leak down at other than TDC in an attempt to find the location of the compression leak. I see this discussed here and there.

Thanks for the question.

Mr Beau · 09-19-2021, 08:21 AM

Sounds like it runs fine. Any other issues other than low compression on the gauge? Was the throttle open when you tested compression?

hcoles · 09-19-2021, 08:47 AM

Car runs fine. Throttle open but we tried it both ways and didn't make much difference. The question is what to do now. Repeat compression test, repeat after running it hard, try some sort of ring cleaning juice, etc. , etc. I can take the cat off and put on the test pipe if running something through it will possibly help buy may cause cat damage.

dannobee · 09-19-2021, 11:10 AM

If you can hear the air rush out just prior to TDC, that sounds more like the overlap part of the cycle, not the compression stroke.

If the compression is low, try fogging some oil in the cylinder then retesting. If the compression comes up, the rings aren't sealing. But if the leakdown was ok, you probably already know that. 78/80 on the leakdown is fine.

Incorrect cam timing can throw off compression readings, too. How confident are you that it's correct?

hcoles · 09-19-2021, 11:37 AM

Update after better sealing of the Stomski attachment and using a better gauge valve.

Cylinder 1 - 135psi (had a bit of oil put in yesterday)
Cylinder 2 - 125psi
Cylinder 3 - 131psi
Cylinder 4 - 132psi (had a bit of oil put in yesteday)
Cylinder 5 - 122psi
Cylinder 6 - 129psi

Viewed 2 cylinders with borescope some carbon is noted at the bottom on the cylinder walls. Places where we looked on the cylinder still had cross hatching.

@dannobee - Places I read indicate the air rushing out is TDC on the firing stroke. I'm pretty sure of the cam timing - I used a dial indicator when I built the top end and followed the Wayne book. I put oil in two cylinders yesterday and the numbers came way up. Today the numbers on those cylinders are down a bit.

I'm thinking to get advise from Henry or ? on something to run in it to clean some amount of carbon and then retest to see what happens. Maybe leave well enough alone at this point.

hcoles · 09-19-2021, 11:49 AM

Those numbers were taken this morning on a cold engine - I should have noted that.

snbush67 · snbush67's Garage

Can you tell if air is leaking by the rings or through the valves? Maybe just double check your valves and adjust if needed.

hcoles · 09-19-2021, 12:17 PM

The valves will be on dwell during a leak down test. Adjustment won't change that.

snbush67 · snbush67's Garage

They should be if they are adjusted correctly, but if they are too tight won’t that prevent them from closing completely?

hcoles · 09-19-2021, 12:35 PM

Quote:

Originally Posted by snbush67

They should be if they are adjusted correctly, but if they are too tight won’t that prevent them from closing completely?

Well I guess yes but during the 20 years I've been adjusting the valves on this car the gap hasn't changed much and not near zero lash. I guess it could of happened but I adjusted the valves about 2 months ago and there was one valve that was maybe 1/2 mil tight.

snbush67 · snbush67's Garage

Probably not the issue then. I have nothing further. I’ll page Henry for you, wait here.

hcoles · 09-19-2021, 01:07 PM

Thanks much.

Henry Schmidt · Henry Schmidt's Garage

My ears were ringing. I'll give it a try. If the cylinders were "properly honed" years ago you may be dealing with excesses wear. Back in the day, honing Nikasil was a little more of a mystery than it is now. RA factor was at times ignored.
If the RA factor of the cylinder was in excess of 10, lets say 12-18, with 40,000 miles on a set of rings, the rings are worn out.
If you're dealing with excessive carbon, you may have stuck rings. The way to clear the carbon is to introduce water into the fuel mixture. With carbs, MFI or ITBs that is pretty easy. Poor a small amount of water [1 or 2 ounces] into each cylinder. The steam created by the water will break the carbon free. CAUTION! TOO MUCH WATER WILL HYDROPHILIC THE ENGINE, POSSIBLY BENDING THE RODS!
If that frightens you [why wouldn't ?] you could try some Supertec Bearing Life.
It's designed to reduce friction on contact parts but it has been known to free up sticky rings.
For now that's all I have. Good luck.

hcoles · 09-19-2021, 01:51 PM

Henry, Thanks!
I'll ask Jerry Woods re. RA that they might have used.
For the water..... pour in water. Crank engine to get excess out to avoid hydro lock.
Then install plugs and go for it? Is that the process? I have Motronic.
Supertec Bearing Life - where does this go .. I guess the crankcase? I didn't see this on your website.

hcoles · 09-19-2021, 01:59 PM

Henry.... maybe spray water in the air cleaner housing with the engine running? This should prevent hydrolock. Thanks.

hcoles · 09-19-2021, 02:09 PM

If the rings were worn out - would that show up on the leak-down test?

dannobee · 09-19-2021, 02:20 PM

Introduce the water while the engine is running (fast idle). A little bit at a time. You might end up running a pint or two over a few minutes time. Just a little at a time. The more "new school" way is using GM Top Engine Cleaner as specified on the label and toward the end, let it back down to an idle and let it run rich enough until it dies. Then let it sit for a couple of hours and let the stuff work its magic.

With respect to leak down testing, check which cylinder the ignition rotor is pointing to and look at the crank pulley to set that cylinder to TDC. Use 100psi and the second gauge will read the percentage directly. There should be no big whoosh of air leaking anywhere.