Ok, I only had time to do # 1 & 2 cylinders due to their easy accessibility with the headders. Anyway, on my 50k orig. mile lb9 305, they were both at approx 142 PSI. I verified the guage is working properly by checking my friends fairly new Jasper L98, which came out to approx 162. I know I SHOULD do all the cylinders, but with seeing that 2 are this low, I think I have a good idea of whats goin on.

Do these results sound low considering the mielage of the engine? It has had a constant unfixable rough idle problem, and is 3-4 thenths slower in the 1/4 mile. Help!

 

I don’t know what the optimum cylinder pressure is for your engine, but 142 PSI is not low by any means. I had a stock ’79 Chevy 350 that ran great, with 135-PSI cylinder pressure. Still, I would recommend checking the other cylinders as well. If they are all within 10 PSI of each other, you’re in good shape.

 

The bad idle and slow thing could be a result of bad compression, does it burn oil? You really need to check the other cylinders for comparison.

 

Well, no oil burning that I can see, or know of. The reason I didnt do the other cylinders is becasue I dont have a 90* fitting for the tester yet, and with the headders, its almost impossible to use a straight one. Im basically trying to get an idea of what the average compression is of these engines... I searched alot, but didnt really find any concrete answers.

My other problems include: Getting major amounts of knock retard up top, not sure why. Ignition is fresh, basically Ive gone over the car with a fine tooth comb, but havent come up with anything. I did the valve lash a few weeks ago, just to do it. What I found was that most of the valves were overtightened about 3-4 turns. Im not quite sure how this happenned because the engine is orig. with 50k, and the valve covers have never been lifted.

Im gonna go ahead and hopefully finish the comp test tomorrow... another thing Ill do is to squirt some oil into the cylinder, and see if that bumps it up at all. If it does, bye bye rings... Sadly, Im hoping this is the problem... gives me a reason to get the warranty company to shell out the dough for a new 350 instead of this 305!

 

A compression test can be missleading if not done properly.
You should only let the piston compress three times then take
your reading. You can crank the engine a bunch of times and
build alot of pressure in the guage which will be incorrect.

Cylinder pressure at cranking speed is much greater than cylinder
pressure in a running engine. I have seen multiple hits on a
guage read up to like 175lbs. Then three hits read 145lbs and
a running compression of 125. This would be typical of a "good"
engine.

These three readings are very important when compared to one
another. They will uncover burned valves, burned valve seats,
sucked valve seats, bad guides, bad guide seals, bad lifters, bad
valve springs and even bad valve springs.

The other day I had a 302 Ford come in with a dead miss and the
number two cylinder ran 150 on miltiple hits then 120 on three
hits but only 35 running.

The 150 and 120 were decent for a 130K engine but the 35 made
me look for a sticky guide or a sucked seat. I poped the valve
cover and rockers and laid a straight edge across the top of the
valves and found the exhaust valve on that cylinder quite a bit
higher than the rest. Diagnosis, sucked seat, The seat overheated
and shrunk up into the head. The 150 and 120 readings were
with no oil pressure. Once the engine was running the lifters
would pump up and cause that one valve to be held open off the
seat causing the low compression.

Ok, ok, I'll stop rambling now.

 

Don't squirt oil in the cylinders. Run the test dry. Crank the engine
untill the guage is toped out and jot down the reading. Then hit
the guage with three compression strokes and jot down the
reading. Then start the car, release the pressure from the guage
and see what it pumps back up to and jot down the reading.

Do this to all the cylinders and sit back with a few beers and
compare them cylinder to cylinder, test to test.

Post back anything that is not average and we will see where to
go from there.

 

Oh yea, pull the injector fuse when you do the cranking speed
tests so the injectors don't spray gas and wash the oil off the
cylinder walls. Put the fuse in to do the running test but pull it
again when you switch cylinders.

 

What's with the formatting of your posts Swapmaster? Did you paste that from somewhere or something?

 

No, just hitting CR when I get to the end of the window on the
post screen. It is only 52 charaters wide while the viewing page
is like 90 characters wide.

I don't know, I'm just goofy.

 

Thanks for the advice! Im gonna finish up the test as soon as I get a chance... then Ill post the results and go from there!

 

As for the compression test, you first run all the cylinders in the normal fashion (all plugs out, coil disabled, choke and throttle held full open, crank each cylinder until max pressure reached, record each cylinder's pressure, etc.). That's the dry test. Then, you put a tablespoon or so of oil in the cylinders through the spark plug holes, turn the engine over a few times to spread it around the cylinder, and then repeat the compression test. If the rings are worn, the oil will tend to enhance the seal and raise the compression pressure significantly from the dry test. You have to do both in order for the wet test to have any meaning. If the dry test numbers are close cylinder-to-cylinder and where they should be (say 150 psi), and no significant change to the dry test, then everything's fine. If the dry test is low and the wet test is low, then you have valve sealing problems.

Thanks 5-7kid...

 

Hold up. You said you ran the valves a few weeks ago and found that they were overtightened by 3-4 FULL TURNS????? That's more adjustment range than the hydraulic lifter has in it! Did the motor even run before that?

Also- while removing the plugs make sure you pay attention to them as you take them out. Do any look "different" from the rest? If so, you can concentrate on that cylinder as the likely offender.

Shooting some oil in the plug hole will raise conpression if the rings are shot, but use it only after you do the dry test as a diagnostic procedure.

 

Yeah, I had to back the bolts off anyhwere from 2-4 turns before the valves started taping, then adjusted them accordingly with the engine running. Dont ask me how the hell they got that tight though.
Yeah, it ran pretty well acually, but seemed to run better after I adjusted them... now, a few weeks later, its back to running like crap again. Im thinking of pulling the covers and re-adjusting them again. But then I got to thinking that being that tight was alot of stress on the springs, so maybe I should change those bad boys too. Hopefully Ill be able to finish the comp. test on Wednesday, then well go from there.
Thanks all.