deuce_roadster
Well-known member
Any chance this engine got overheated and affected all the rings in a bad way?
40 V8 compression change all cylinders same time
- Thread starter L78CHEVELLE
- Start date
philipswanson
Well-known member
That's fine if you know where TDC is exactly but his method is probably much better if you don't.
L78CHEVELLE
Senior Member
Engine block and heads were baked, steamed, and run through a cleaning machine to get all the scale out. No over heating since the rebuild. I have a cylinder leak down tool coming and will run the test and post the results.
philipswanson
Well-known member
So what's the latest on this interesting story?
I have been following this discussion and there have been a lot of great suggestions so I thought I would throw in my 2 cents worth. My 36 Cabriolet shows 60 PSI in 6 cylinders and 50 PSI in the other 2 cylinders, (needs a valve job) but it starts and runs nice on 6 volts. Others have suggested it so I will add my vote with ignition or fuel.
L78CHEVELLE
Senior Member
Got the leak tester and hooked it up to each cylinder at top dead center. I put 100 psi into each cylinder and got the following results:
#1 cylinder held 90 psi
#2 cylinder held 65 psi
#3 cylinder held 60 psi
#4 cylinder held 65 psi
#5 cylinder held 75 psi
#6 cylinder held 90 psi
#7 cylinder held 75 psi
#8 cylinder held 75 psi
Each cylinder tested lost the air pressure through the intake valve with air coming out of the block on the intake valve opening. My questions are: 1. Why would a new engine that was good for 2,000 miles develope an intake valve leak on 6 of 8 cylinders in such a short time? 2. Will new intake valves from a different manufacturer correct this issue? 3. Since cylinder 1 and 6 are the simplest to find top dead center on and both of them have acceptable readings have I missed TDC on the other cylinders? I did do a compression test with the engine turning over with the starter on some cylinders with the same readings as the leak down test on the chosen cylinders. Any thoughts or comments? I think I have found the problem, but again WHY??
L78CHEVELLE
Senior Member
Yes, the engine does have adjustable lifters.
glad you got some data to study. 1st, i am not a motor wizard, but lucky for you we have some here at the barn. next, i am confused, you say "air out the block whe intake valve is opened" well, intake air should come out the carburater, not the block. how, and why would you open the valve? the point is to see if its leaking, then record the data and move on to the next one. you have the intake manifold off, so its not going to be so easy and obvious to hear air out of the block as it would be with just the breather pipe, but air out of that area is ring leakage. if we have it narrowed down to burnt valves, only two possibility's come to mind, you set the lash with the cam not in the correct location, or your adjustable lifters moved. at any rate, to proceed on you cant just drop new valves in and have it rite. first of course, pull the old ones out and try to see if it burned the seat or the valve, or both. then, you will need to set up the new valves in the car, if you choose. to do it rite you will need a valve machine to atleast check the new valves, and a seat cutter to do the seats in the car. it can be done, its messy and the utmost effort to clean your mess up is essential. well, thats my backyard opinion, lets see what real motor guys say....best wishes
Tinker
Well-known member
Had a burnt valve on my 36coupe. Looked like this... Still had 30psi on that cylinder.
Replaced it with a used/pre-run nos assemble from someone here for 7$. Lapped it and installed. 90psi. Been driving it for yrs now. The 36 is not equipped with a "barn burner" engine, mind you.
These were original valves, Hard to believe your valves are burnt in 2000 miles. But just a reference on what a burnt valve looks like. Intake valve, most likely due to running lean for a very long time.
Replaced it with a used/pre-run nos assemble from someone here for 7$. Lapped it and installed. 90psi. Been driving it for yrs now. The 36 is not equipped with a "barn burner" engine, mind you.
These were original valves, Hard to believe your valves are burnt in 2000 miles. But just a reference on what a burnt valve looks like. Intake valve, most likely due to running lean for a very long time.
Attachments
Last edited:
Russ/40
Well-known member
As I said before, the symptoms scream burnt valves. Its time to pull the motor. It all needs to be inspected to see what needs to go and what you can keep. In jeopardy I would say: Valves, seats, lifters, cam, and rings. Be prepared for the worst and maybe you'll be lucky. Don't try to do it in the car, that's crazy. I wish you were closer as I'd love to help. Send me your valves and I'll check them and resurface them for you gratis. PM me. Good Luck,
L78CHEVELLE
Senior Member
As I stated earlier the intake is off the engine. The exhaust and intake valves on each individual cylinder are closed when I put the 100 psi of air pressure into each one of them for testing. When I put the air pressure into the cylinders the air escapes out of the intake opening in the block on every cylinder tested. I would have thought the hot exhaust valve would have been the problem, but the intake valves are the problem?? Does not seem like the intake valve would be damaged and the exhaust not damaged on a new rebuilt engine. Next step is for me to pull the heads and look at the valves. Maybe the valves are sticking on the intake side. Every piece in this valve train was new when rebuilt.
Russ/40
Well-known member
I suggest you remove the 2000 miles since build from the equation. Its coloring your perspective of what can be happening. A lot of bad things can happen in far less than 2000 miles.
I have to compliment you on your stick to it ness. Getting the heads off is a major step to resolving your problem. Your going to beat this. You really don't want to do any valve seat work in the car. I would make an exception if the condition suggests lapping only. Wet lapping controlls the mess well. My valve reconditioning offer stands if your inclined. I'd like to help, no cost offer.
I have to compliment you on your stick to it ness. Getting the heads off is a major step to resolving your problem. Your going to beat this. You really don't want to do any valve seat work in the car. I would make an exception if the condition suggests lapping only. Wet lapping controlls the mess well. My valve reconditioning offer stands if your inclined. I'd like to help, no cost offer.
rotorwrench
Well-known member
A leak down test is the best way to find out where the compression is going. Carboned up valve seats can leak, stuck valves, and gooped up compression rings can all cause leakage. MMO down the throat will loosen the sticky stuff up. Low time since overhaul engines shouldn't have a lot of ring wear yet unless it's been huffing dirty air or running too rich for too long.
Tinker
Well-known member
I think the op is well beyond a mmo lube now. I mentioned it 3 pages back, not sure if it was tried. My 38 truck rebuilt engine stuck real bad with new guides/valves after a yr. mmo cleared it up. Flatheads were designed with leaded gas in mind (upper cylinder lubrication).
Probably not the issue though. Motor would have bucked under load with sticky valves. Which would have been seen easily with a vacuum gauge too. Think the poor performance is due to compression loss. Hard to say....
.
Probably not the issue though. Motor would have bucked under load with sticky valves. Which would have been seen easily with a vacuum gauge too. Think the poor performance is due to compression loss. Hard to say....
.
L78CHEVELLE
Senior Member
Russ, your offer to help has given me encouragement to continue even with my limited knowledge and resources. I will take the heads off next week after my legs and back recover from the work and testing I did last Friday. I will post what I find when I get the heads off.
Russ/40
Well-known member
I bet your back hurts. It gets old real fast.
Another possibility occurred to me today. A few years back there were some bad valve guides on the market. They wore real fast. When your back stops killing you, try selecting an intake valve in the worst cylinder, completely seated ( some lash to know its fully seated) and grab the bottom of the valve stem and see if you can move it back and forth. A severley worn guide will prevent the valve from seating fully and compression will be lost.
Your going to get this figured out, and when you do we all will have learned a lot.
Last edited: