low compression and blue exhaust smoke

[haveacamper]

Hi Guys,

I have an 81 SC 3.0L engine that was apparently rebuilt about 1000miles ago albeit its sat unused for about 18months. i have eventually got the engine running on a test bench and appear to some blue smoke coming from my exhaust.

order of events.
-tested compression after 18months of not running 1,3,4,5,6 120psi no.2 90psi
- i filled the oil tank with oil
- fired the bus up for 20-30secs
- turned off
- fired up again for 20-30secs
- oil was coming out of the tank breather. worked out there was too much oil, so dropped oil and refilled with 10L to run on bench (spec says 13L but for bench testing i put 10L
- ran engine for 10-15mins on high idle (1500-2000rpm)
- smoke coming out of the exhaust continuously.
- compression tested cold 1,3,4,5,6 155-158psi and 2 105
- inspection under engine shows oil on the fins so some oil leaking form somewhere.

i am not quite sure whether the low compression is due to piston rings or valve not closing properly. i am surprised as the engine was runnin g fine before the stint of not runnins - i saw this with my own eyes. compression was low on engine before i over filled it with oil.

any ideas guys?

pic of engine bench setup (photo taken before rig was finished):

[Bob Kontak]

Howdy haveacamper.

Looks like you are making progress. The first thing that comes to my mind is that the "new" rings have not seated in #2. That is my assumption that the rings are new from the rebuild 1000 miles ago ONLY because of the oil from the exhaust. There are others who may know more about the symptoms of over filling and that may be your smoking problem. From your previous threads I don't think you mentioned smoke.

You should have put this post in the old thread since a group of us were following your efforts on getting the engine to run. Please let us know the resolution on that.

911 SC K Jetronic engine wont even fire up

[johnsjmc]

Redo your compression test with about 1/2 teaspoon (1or2 squirts) of oil in each cyl / Oil will tempoarily seal leaking rings and raise the compression. If the leakage is a valve then the comp won,t increase. Reason to do more than the low cyl is all will rise some even with good rings and you need to see the difference. A leakdown test will also locate a valve leak. I would also confirm the valve adjustment on your low cylinder. There may also be oil pooled in your muffler which needs to be burned off to stop the oil burning.

[haveacamper]

Bob, i have posted an update to the thread. i didnt post this on that thread as its a different issue now. point taken on updating thread though.

i intend to check the valves next time i'm working on engine. and i will redo the compression test with some oil in the combustion chamber. i will record the increases in pressure and report back, proabbly wont be until next tuesday though as i'm away the back end of this week.

edit: john you may be right there may be oil in exhaust system, but you would agree that there is an issue with the pressure in no.2

[Quicksilver]

I keep coming back to the question of how was the oil filled? Was the oil level checked with the engine running at idle when warm?

[Bob Kontak]

Ahhhhh - closure on the other thread. That one was a hair puller.

Cool test Johnsjmc noted about the oil for the rings before compression testing. Anybody that diagnoses engines regularly would know this but the DIY guy can learn something everyday in this forum.

At least you have the engine out if you have to pull a bank of heads. To save work I do not think there is a requirement to remove all the heads from the cam tower. I think you can slide the whole assembly off. If it is rings you may not even have to pull one head from the cam tower assembly.

I have a dial indicator and the cam tools if you have to go this route. I will send them over if you cover shipping and return.

[haveacamper]

when i removed engine from vehicle i drained the oil from the tank, and plugged the hoses to the engine. once on the test rig i forgot that i left the oil in the engine. i filled the oil tank with 8-9litres as a precaution (not remembering there was oil in the engine) then when i ran the engine for the 20 sec stint the oil squirted out of the oil tank breather onto the floor.

when i drained the oil i drained 15L so had about 2L too much in the engine. please not, that the compression was measured to be low before had over filled it and ran it for 20secs.

i have subsequently filled with 10L of oil and run for at least 10mins at 1500-2000rpm.

NaFe

[johnsjmc]

I just found this thread If previously overfilled I suspect the muffler is still filled with unburned oil. I have seen it take a day or two of hot running to clean them out after a fillup. Also a dose of seafoam and some running will probably bring compression up to near normal in the low cylinder.(still worth checking the valve lash) With the engine out of the car it,s easy to do now than later.

[haveacamper]

i'll definitely be giving it a service before putting bak in as it's alot easier to work on while its out. i'd be a bit apprehenive putting back in if i havent solved the compression/oil smoke issue tho, as this may result in me having to pull the engine out all over again.

sorry for the stupid question, but whats seafoam?

[5String43]

When my engine was rebuilt last year - well, complete top-end job, at least - it was using some oil, with smoke coming from the exhaust, especially on cold starts. I was being very gentle with it. Wrench sez to me, "You got to drive it like a Porsche! Rev it hard to the rev limit a number of times, the rings will seat." I did. He was right, the rings seated. It's fine now. Could that be what's going on with your engine?

[johnsjmc]

Seafoam is a fuel additive the marine guys use alot in similar conditions. It will free up stuck rings and might help/ might not. You can introduce it thru the intake and add to the fuel.

[haveacamper]

ok guys, after a bit of a break as i went to my first music festival but back on trying to get the engine running smoothly.

this is what i have done. i re-set the valve gaps to ensure the i didnt have a valve staying open due to no valve gap. i then re-tested the compression with the same result. 150+ on all cylinders except no.2 was 105. i then put a squirt of oil into the combustion chamber - this didnt change the pressures. if anything it increased 5psi, but nothing major.

it sounds like i am going to have to pull the head to inspect the valves and piston rings. does anyone know the process of removing the head on a 911SC? ideally i want to keep the engine as complete as possible and the head assembly complete if possible.

NaFe

[johnsjmc]

You should buy Wayne,s book on engine rebuilding first. There are a couple of special tools you need to reset the cam timing.

[911pcars]

Do you know how to perform a cylinder leakage test?

Before you disassemble, I'd suggest a test on no. 2, the low cylinder, to confirm the source of the compression loss.

Sherwood

[haveacamper]

hi John, i do have this book already and have read a few bits of it. i noticed that i will beed a couple of special tools, like the crow wrench etc.

Sherwood, i know how to do this test, but do not have the pressure equipment. i have access to a compressor, but not the necessary gauges - Yet.

after reading Waynes "how to rebuild and modify Porsche 911..." it was difficult to understand what extra this will tell me. i realise it will tell me the % leakage, but i already know that the compression is low, so the air is getting past either the head/cylinder, valves or the piston rings. unless i actually hear the air hissing past these i will be unsure where the low compression is coming from?

your advice and expertise is very welcomed.

[johnsjmc]

The wet/dry compression results have already pinpointed the leakage from valves,gaskets but not rings. (No rise with oil). personally I would be hesitant to pull the heads on an engine with compression above 100psi.A tiny piece of carbon on a valve seat can cause such results. I would want to run it under load and put some miles on it and then redo tests. I have seen engines which sat a long time show similar results and return to normal with some miles. However the engine is already out of the car .

[haveacamper]

That is the dilema I'm dealing with: low compression that could just be that its not had a good run (done any actual miles under load) with it being on the engine stand where accrss is good etc. Compounding the decision further is that I would like to dress the engine and get the tinware etc paintwd so its looking nice for when the vehicle comes back from bodyshop.

Last thing I'd want to do is take it back out because compression is still low after some miles

[johnsjmc]

It only takes a couple of hours to remove the engine if it doesn,t improve when run. There are probably 15-25 hours involved in R&R heads

[Tigerrat]

While the engine is out I would get a leakdown test gauge or at least the adapter where you could pressurize the cylinder. Then you should be able to find out where the leak is...intake valve, exhaust valve or elsewhere. You might be able to determine the problem. I'd spend the $$ for the leakdown test to avoid possibly pulling the engine again. Ken

[haveacamper]

Ken,

i have managed to get hold of a leak down tester with a long rigid spark plug end. i will be testing the cylinder on tuesday. if the result is from a valve, i will be temped to think its a excess bit of carbon as a result of the issues i have had in trying to get it running properly over the last 18months. if so i will probably wait until engine is back in vehicle and take it for a spirited run as stated in the beginning of Waynes rebuild book (heading "Carbon Deposits")

Watch this space