CTCarguy Posted April 11, 2011 Report Posted April 11, 2011 I bought a used engine for my P12 since the original had a spun bearing. The seller told me it was running when it was removed. It's a 1947 230. After installing it, it would not start. Checked spark, fuel and timing. All OK. Did a compression test cold. All but 1 cylinder was 30 psi. The one was 50 psi. I put oil in and no change so I assumed it was valves. I have a professional valve job done. I pulled out and installed the valves and he machined them and then lapped them in. After putting the valves in (what a pain those keepers are!), I reinstalled the head and manifolds without the carb. I decided to do a compression test cold before trying to start it. Damn! the reading are the same. I even tried putting oil in check the rings. Any ideas?? I may try to start it tomorrow. The problem before was the compression was so low, it was not sucking fuel into the cylinders. HELP! Quote
randroid Posted April 11, 2011 Report Posted April 11, 2011 CT, Howdy, and welcome to the forum. Did you personally do the valve job or have it done? I ask because if you did it you probably would have checked the lift and dwell on each valve whereas a hired hand might not. I'm also assuming the valves were checked for true. What I'm getting at is those compression numbers could be duplicated if the cam lobes were all worn to a nubbin and I can't think of anything else that would do it after a good valve job. I also can't think to take my car keys with me sometimes when I'm going for a drive so my suggestion isn't necessarily the definitive answer. I hope the solution you will eventually find is as painless as it can be. -Randy Quote
norrism1 Posted April 11, 2011 Report Posted April 11, 2011 CT, Have you squirted 30 weight oil into a cylinder then check compression? If PSI goes up then bad rings. If not, then timing or valves not sealing completely. Worth a try. Quote
CTCarguy Posted April 11, 2011 Author Report Posted April 11, 2011 The valve job was done by a professional machine shop that is very familiar with these engines. While rotating the engine by hand, the valves open and close cleanly with what looks like about 1/4" so I don't think the cam is rounded off. I also checked that the valves closed tight and set the valve lash at .009 intake, .014 exhaust which is what my manual states for a cold engine. I think a new head gasket might help. The replacement I got was wrong. I checked the copper gasket carefully and there is no breaks crossovers. Quote
CTCarguy Posted April 11, 2011 Author Report Posted April 11, 2011 Yep. sure did. No change. Question about the head gasket, does the sealing ridge go up or down. I wonder if the gasket was put in wrong. I put it in the way I found it which was up I think. The shop manual does not show which way. Quote
41/53dodges Posted April 12, 2011 Report Posted April 12, 2011 this may sound stupid, but has this engine been sitting awhile? throw a little starting fluid at it and see what happens! BTW: DID YOU USE AN EXTENSION ON THE COMPRESSION TESTER? YOU VERY WELL MAY HAVE BENT THE VALVES WITH AN EXTENDER. trust me, it sucks, ALOT! Quote
martybose Posted April 12, 2011 Report Posted April 12, 2011 this may sound stupid, but has this engine been sitting awhile? throw a little starting fluid at it and see what happens!BTW: DID YOU USE AN EXTENSION ON THE COMPRESSION TESTER? YOU VERY WELL MAY HAVE BENT THE VALVES WITH AN EXTENDER. trust me, it sucks, ALOT! Yep, that's what I did too. I just got the car running with a new set of valves a couple of weeks ago, but I've got an issue with the new front brakes to deal with before I drive it much. Marty Quote
Cpt.Fred Posted April 12, 2011 Report Posted April 12, 2011 I'm going to do the same thing, pull the head, get the valves out, grind seats and have the valves done by a pro... i got between 90 and 95 psi on all but one cylinder, which has about 35psi cold. 30 psi on all cyls sounds awful! but while we're at it: how much should it be when everything's alright? about 130-140 psi? CTcarguy, good luck with your engine! i'll have you in mind when i get to the spring retainer locks... fred Quote
41/53dodges Posted April 12, 2011 Report Posted April 12, 2011 i think anything over 70 and it runs is pretty much a pass, unless you are aiming for performance of some sort. Quote
Cpt.Fred Posted April 12, 2011 Report Posted April 12, 2011 the engine ran smooth, at low idle, good starting hot and cold. the mpg wasn't that good but the car acted quite lively and drove very well... there was a little puffing once in a while which i now know was most likely the 3rd cylinder pumping raw gas into the exhaust. now that it has headers and dual exhaust, the small puff became a fairly loud detonation. i can't wait to get it fixed, i wonder how it will run afterwards. sorry, btt... Quote
Jim Yergin Posted April 12, 2011 Report Posted April 12, 2011 Just a thought, is it possible that your compression tester is defective? Would it be worthwhile to check compression with a different tester? Jim Yergin Quote
Don Coatney Posted April 12, 2011 Report Posted April 12, 2011 how much should it be when everything's alright? fred That depends on your compression ratio. For an indication of your current compression ratio take a look at your compression readings and compare them to the following chart. 6.5/1 = 110 7.0/1 = 120 7.5/1 = 130 8.0/1 = 140 These numbers represent a perfect engine and few of us have a perfect engine. Quote
CTCarguy Posted April 12, 2011 Author Report Posted April 12, 2011 Re: the compression tester extension, I did that as well and had to replace one valve. The tester is fairly new and I used it on a ohv Pontiac engine with correct results. My other tester read zero so I am sure it is bad. That's why I may put the carb on today just to see if it fires. Next step - put in a new head gasket. Since this was the one that was in it when I got it and the readings did not change, it may be bad. It looked OK? Otherwise I will have the head checked to see if it is warped. Thanks for all of the advice. I will post what I find out. Fortunatly, I have the original engine so I may put that head on if nothing else works. I've been working on cars for about 50 years and never ran into this before. CTCarGuy Quote
Cpt.Fred Posted April 12, 2011 Report Posted April 12, 2011 thanks for the numbers, don! where are they from? that would mean i'm not THAT far off... if i remember correctly the 1940 85hp 201 engine had a 6.5 to 1 compression ratio... Quote
suntennis Posted April 12, 2011 Report Posted April 12, 2011 Is it possible that when doing the compression test that you only rotated the engine one revolution and not several as needed? Quote
Plymouthy Adams Posted April 12, 2011 Report Posted April 12, 2011 given that this is an engine purchased from another party..PO statements of condition have already been proven worng thus the entire integrity of the engine is questionable... have you verified that the cam is properly timed with the crank...? should the timing chain have jumped or somehow been disturbed by the PO or whatever no amount of piston ring seal, head gasket intergrity or valve seat seal will net a high compression reading due to the valve openin/closing out of time with TDC of the piston Quote
Don Coatney Posted April 12, 2011 Report Posted April 12, 2011 thanks for the numbers, don! where are they from?... I do not remember where I got these numbers but I have had them in my files for years. Quote
Reg Evans Posted April 12, 2011 Report Posted April 12, 2011 Just a thought, is it possible that your compression tester is defective? Would it be worthwhile to check compression with a different tester?Jim Yergin I was thinking that too. My brother and I have the identical tester and his reads about 10lbs higher than mine. I like his best. Quote
40phil41 Posted April 12, 2011 Report Posted April 12, 2011 FWIW, I think that you need to do a leak-down test in order to determine where the problem is. If you do a search on previous posts from this forum you will find the instructions for this test. Phil Quote
TodFitch Posted April 12, 2011 Report Posted April 12, 2011 That depends on your compression ratio.For an indication of your current compression ratio take a look at your compression readings and compare them to the following chart. 6.5/1 = 110 7.0/1 = 120 7.5/1 = 130 8.0/1 = 140 These numbers represent a perfect engine and few of us have a perfect engine. Been a long time since I've thought about that. First cut would be that the pressure should be 14.7 psi (1 ATM) times the compression ratio. Which would lead you to think that a 6.5 CR should have a 95 psi compression reading. But when you compress air it gets hot. I don't know what the number is for a fuel/air mixture but for straight air you'd need to take the CR to the power of 1.4. So for a 6.5 CR you'd have 202 PSI which seems very high. So then I got confused and did a web search. Turns out, if my search pulled up a good source, that the rule of thumb "in the real world" is that the compression test should be somewhere between 15 and 20 times the CR depending on valve timing, design of the rings, etc. So a 6.5CR should have a compression test (depending on engine design) between 98psi and 130psi. Given the relatively low tech head design and non-racing cut of the cam, it seems that the 110psi given in your table is reasonable. But the other numbers don't match up the same. 6.5CR => 98psi to 130psi versus 110psi 7.0CR => 105psi to 140psi versus 120psi 7.5CR => 112psi to 150psi versus 130psi 8.0CR => 120psi to 160psi versus 140psi Looks like your chart used the formula PSI = (CR*20)-20 or PSI=(CR-1)*20... Probably a reasonable set of numbers but I would not be surprised for our L-head engines using 1930s technology (updated to 1940s in your case) would yield numbers on the low side of this. Quote
Don Coatney Posted April 12, 2011 Report Posted April 12, 2011 Tod; Keep right on thinking! Thanks again for your input. Quote
P-12 Tommy Posted April 12, 2011 Report Posted April 12, 2011 this may sound stupid, but has this engine been sitting awhile? throw a little starting fluid at it and see what happens!BTW: DID YOU USE AN EXTENSION ON THE COMPRESSION TESTER? YOU VERY WELL MAY HAVE BENT THE VALVES WITH AN EXTENDER. trust me, it sucks, ALOT! I can attest to that one!Tom Quote
CTCarguy Posted April 15, 2011 Author Report Posted April 15, 2011 UPDATE! I put in a new head gasket today and guess what..... no change. I'm thinking the valve timing is off. The problem is how to get the big nut off the crank. I have a socket to fit it and tried putting the car in gear to hold the motor steady but the nut was stronger than the cars traction. Any suggestions of how to get it off? This engine is being a pain in the ***. I want to take the car to the AACA meet in Stowe Vermont in May and hope it's running by then. Since the Allstate tires are so old, I will trailer it up and use it while there. I plan to enter it in the HPOF class (Historic Preservation of Features). Quote
JerseyHarold Posted April 16, 2011 Report Posted April 16, 2011 I've loosened crankshaft bolts by putting a socket on the bolt, resting the breaker bar on the ground, and cranking the engine with the starter. It helps to have the radiator out and pressing the socket against the bolt with a piece of 2X4. Just make sure that the breaker bar is in the proper position to be pushed against the ground when the engine spins. Quote
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.