My advice is the same as before; a compression problem and you need to do a wet compression test. You don’t even have to wet test all of the cylinders; just pick a couple of the easy to get holes.
If the wet test shows a problem you’re going to have to go all in anyway.
Those vacuum numbers are a bit low and can also point to compression that is too low.
The fact the gauge wavers between 15 and 16 could be the pressure variations in the cylinders.
I plan to do the wet compression test and intake removal tomorrow. At this point I’m trying to decide if it is worth it to pull just the heads and check valve stem clearance, or just pull the whole engine and rebuild it again.
You might as well put in new valve guides and seals to take them out of the equation.
If your wet compression test shows jumps up to 150, 160, or higher then there’s a ring problem and unfortunately that means going back in whole hog.
Forget a compression test and do a leak-down test.
That’ll tell you exactly what’s goin’ on inside the engine.
Tester
RE: leakdown or compression test. Is the following not true?
If the leakdown shows leakage, he won’t know if it’s from the rings or valves.
If a dry compression test shows low readings that jump when redone with a wet test, then he knows the compression rings are suspect.
Unfortunately, neither will test if the oil rings are working.
The theory behind the leakdown test is that one would hear air hissing from the intake, exhaust, or crankcase which may be debateable.
With the compression test in the wet version and with numbers jumping that does show a compression ring problem.
You are correct in that neither test will reveal anything about the oil wiper rings. It’s quite possible to have 190-200 PSI on every cylinder and still smoke and burn oil like crazy.
The OP apparently has a compression tester and likely is not setup with a leakdown tester so ergo, my suggestion to run a wet compression test.
As I stated previously, just pick a couple of the easy to access cylinders for the test because even one bad one means you’re going back in anyway. Five minutes and done.
My virtual wager of a cold six pack says piston rings because the odds of having valve issues on every cylinder of a rebuilt engine are near zero and the vacuum gauge reading doesn’t point in that direction.
I already did a leakdown test (see my first post). Leakdown was 7%-9%, and one cylinder @ 14%. From what I’ve read those numbers are acceptable. I’m going to re-do the compression test today, both wet & dry (to see if the new oil made any difference).
@Raindem
Please refresh my memory
I seem to recall you stated that you did not replace the guides, because they measured good . . .
That’s correct, db4690. All the guides measured within specs but there were a couple borderline. But it was my first time using a dial gauge so it wouldn’t too unbelievable if a few of the guides had enough slop to allow oil burning.
Just finished up the compression test.
Cyl Dry/Wet
#1 120/130
#2 125/135
#3 130/140
#4 130/140
#5 125/135
#6 130/150
#7 135/145
#8 135/145
So as you can see a consistent 10 psi bump with 10w40 oil squirted into the cylinder. Is that normal, or is it one more indication that the rings are passing oil?
@Raindem
It is considered normal for a wet compression test to be slightly higher than a dry compression test
In my opinion, your problem is top end
That does point to a top end issue as to the compression numbers but that still doesn’t rule out ring problems; especially in regards to the wiper rings.
Are you sure the camshaft is timed correctly? Stock camshaft?
Were the valves and valve seats in the heads reworked?
Just might note that valve guides and seals would have nothing at all to do with the low compression numbers.
Correction - I forgot to block open the throttle on the above test. Re-did the test (dry only) and values went up 5-15 PSI. Dry compression is now 135-155 for all cylinders, which to me puts it solidly in the acceptable range.
I’m sorry. I just can’t resist. Is smoking after rebuild the gear head version of smoking after sex?
I have to respectfully disagree that those numbers are acceptable for 2 reasons; too low and too much of a variation between cylinders.
The general rule of thumb is 20 X the compression ratio for static compression pressures with slight variations based on certain factors.
Anyhoo, I’ll just bow out of this one and best of luck.
Just finished pulling the intake and the gasket looks good. No visible breaks or evidence of oil being pulled in through the intake runners. I tried to look into the top of the valves but I couldn’t see them very well. There is a little moisture in some but I can’t tell if it’s oil or gas from all the cranking.
I can tell this thing is going to fight me to very end. Tomorrow I’ll start on the heads.
I hesitate to make this suggestion but I’ll go out on a limb anyway. This engine has a mechanical/diaphragm type fuel pump driven by the camshaft. I remember the diaphragm would sometimes develop a pinhole allowing oil to be sucked in to mix with the gas. Just a thought.
That’s not a bad suggestion. My first instinct says the smoke would be more consistent if that were happening. Still, it’s worth checking out. How would one go about testing for this?
ok4450, I’ll take your word for it that the compression is low. I’ve been doing a lot of internet research on this and you know how it goes with web-based information. I’ve seen the number “150” thrown out there as acceptable compression so that’s what I went with. Do you still believe the low compression is related to the smoking?
In any event I’ve decided to just suck it up and pull the engine. Trying to solve this layer by layer is not working out well. I also have a leaking RMS that I can fix at the same time. So what are common causes of low compression on a freshly rebuilt engine? Let’s see, 1) ring gap not correct 2) improper boring/honing job 3) incorrect parts 4) cam timing incorrect. What else?
Here’s an update for the benefit of anyone with a similar problem. It turned out to be valve guides.
I pulled the engine and tore it back down. The bottom end looked good (ring gap, piston clearance, etc). The heads looked OK too. There was a little bit of play in the valve guides but I re-measured them with a dial indicator and they were within tolerances. Still, they were the original guides and a couple were borderline so I went ahead and had them replaced (actually had a whole valve job done). Engine is now back together and in the car. I’ve put a couple hundred miles on and, knock on wood, so far no smoke.
Thanks for all the suggestions. It did help narrow the problem down.
Curt