Firstfives.org

Hello

Any advice on interpreting my compression test results would be appreciated.
I don't have much experience with this. .

My car had been running ok (not great) as far as i can tell (no reference, maybe has always had a problem). It misfires a bit at idle. That is why I am checking. Runs fine at high RPM
Also goes through oil (not a leak) out of the engine. It is has a
M30B28 with 250K miles.

Here are the results (on warm motor but cooling by the end because I had a lot of trouble getting the tester into the last two cylinders)
From front to back.
170 psi
172 psi
172 psi
167 psi
145 psi (ouch)
167 psi

On the low one (send from the back), I put in some oil through the plug (saw that this was
a diagnostic for worn rings, walls) ...maybe I put too much oil in hind site,
I cranked and the pressure went up to 280...I should have stopped earlier.
I hope I did not mess that up.

I think this means it is not my head gasket or valves but that the cylinder walls and rings in the second to last cylinder are messed up.

The plugs did not appear unusual (all seemed the same as far as I could tell).

Again, advice would be appreciated, Thanks
Jan

I am now thinking about the test results.
What do you think? Is the one cylinder not that bad and so
I should just leave it alone? At this point it seems that this gives
me slightly rough idle but once driving, not noticeable. How many HP
could I be losing from this?

Jan

280#? I'd be re-checking my setup. It takes a lot of compression to get 280#.
Aside from that, I'd run it like it is. Do you have a lot of blow-by from your crankcase vent?

blow-by? I don't know about this. How do I tell?

onovakind67 wrote:280#? I'd be re-checking my setup. It takes a lot of compression to get 280#.
Aside from that, I'd run it like it is. Do you have a lot of blow-by from your crankcase vent?

Blow-by is combustion gases that escape by the rings into the crankcase. If you observe the crankcase vent while the engine is running you will see vapor escaping. There is always a small amount, which is vented through the PCV system, and faulty rings will increase this.

While not perfect, those are perfectly acceptable compression numbers. Unless you have other problems smile and say good, but not perfect if someone asks. If you were up to 280 or even 180 on wet retest, you squirted too much oil in. Way too much.

I tried testing for blow-by. I am not really sure how to do that
(can't locate the "crankcase vent"). But start, I started the car, the unscrewed
the oil fill cap. I figured I would look for oil or pressure coming out of there if there was a problem. Is there a better repair manual available (I have a chiltin and a Haynes)

Instead the engine sputtered and stalled. Is that normal? Is that connected to the vacuum and so I simple spoiled the vacuum for the car, give such a rough idle that it stalled?

Jan

onovakind67 wrote:Blow-by is combustion gases that escape by the rings into the crankcase. If you observe the crankcase vent while the engine is running you will see vapor escaping. There is always a small amount, which is vented through the PCV system, and faulty rings will increase this.

jkycia wrote: Instead the engine sputtered and stalled. Is that normal? Is that connected to the vacuum and so I simple spoiled the vacuum for the car, give such a rough idle that it stalled?

Jan

Yep, E12s didn't use any kind of a PCV valve, the largish hose off the valve cover is for ventilation and yes, you did just induce a huge vacuum leak. All normal.

You/we can't even expect the engine to run smoothly with the dipstick not being all the way in!
That's why guys sometimes have to fit a small rubber gasket around a worn factory dipstick gasket.
Our cars run a closed system; no outside air at all.
Sounds like you're good to go.
Greg