coletrain777 Posted August 6, 2009 Share Posted August 6, 2009 hey guys, This should be an easy question... I have a L28 in my shop that I pulled out of a parts car. I decided to do a compression check on it before I installed it in my 280z. Anyway, I have good compression on 5 of the 6 cylinders (between 148 and 158 psi) but #3 reads less than 20 psi (checked it 5 times now). From what I understand this probably means I have a burnt exhaust valve on that cylinder. Does this sound right, what else could it be (blown head gasket etc...). Just wondering what steps I should take first in order to diagnose the problem. I don't mind tearing into it as it is on the floor in my shop, so disassembly should be pretty simple. Anyway, what do you guys think? Quote Link to comment Share on other sites More sharing options...
Challenger Posted August 6, 2009 Share Posted August 6, 2009 How are you doing the compression test? Just cranking it over by hand? How long since it ran? Least you could do is pull the valve cover to check if all the drivetrain is intact, functional. Quote Link to comment Share on other sites More sharing options...
coletrain777 Posted August 6, 2009 Author Share Posted August 6, 2009 No, I am doing the compression test with the starter (just hooking up cables to battery and jumping the solenoid, pretty simple). I have the valve cover off and the valve train looks perfectly normal, springs look fine, cam looks great, rockers as they should be. What do you think it might be? Quote Link to comment Share on other sites More sharing options...
Challenger Posted August 6, 2009 Share Posted August 6, 2009 Well I was thinking maybe a valve might have been stuck partially open, dont know how to check that neccesarily. Is the intake manifold on? Quote Link to comment Share on other sites More sharing options...
MAG58 Posted August 6, 2009 Share Posted August 6, 2009 How long has the car you pulled the motor out of been sitting? If the valve train looks to be in order my best guess is stuck rings. When I pulled my Motor it was something like 144 125 32 150 152 141. I used a feeler gauge under the valves to see if there was a stuck valve (there would be more gap when the lobe is 180 deg out) and figured out all was good, pulled the head, no burned valves, dropped out the piston and saw that father time glued those things in there snug. Quote Link to comment Share on other sites More sharing options...
coletrain777 Posted August 6, 2009 Author Share Posted August 6, 2009 So if a valve is stuck partially open, then the valve clearance on that cylinder would be much bigger or "open" than normal correct??? That seems to make sense. I am not sure how long the engine sat without being run, but according to the PO it should have been run in the last 2 years (and it remained in the car protected from the elements during that time). I guess I will go out and check valve clearance... Keep the ideas coming, I really don't want to invest any money into this engine that I don't have to, as it is just a temp engine while I either build up an l28et or something else. Quote Link to comment Share on other sites More sharing options...
coletrain777 Posted August 6, 2009 Author Share Posted August 6, 2009 OK, So I just ran out to the shop and checked the valve movement and roughly checked clearances... and everything looked OK. So I guess my only option is to pull the head??? Do you think it would be a safer bet to just pull the head and go that route, or should I pull the pan and pull the suspect piston/rings? Quote Link to comment Share on other sites More sharing options...
big-phil Posted August 6, 2009 Share Posted August 6, 2009 spray some WD-40 in #3's spark plug hole, then check compression again. If you get a good over 100 reading it could be rings, rings land? Quote Link to comment Share on other sites More sharing options...
kolonelklink87 Posted August 7, 2009 Share Posted August 7, 2009 if the head was milled recently shavings can get caught in annoying places and jam the valves... I had this happen to me recently, I'm never going back to THAT machinist lol:shock: Quote Link to comment Share on other sites More sharing options...
Challenger Posted August 7, 2009 Share Posted August 7, 2009 You didnt CLEAN the head before you put it together?? Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted August 7, 2009 Administrators Share Posted August 7, 2009 A leak down test will narrow down if it intake valve, exhaust valve, or rings/piston.. Easier than pulling the head. Quote Link to comment Share on other sites More sharing options...
coletrain777 Posted August 7, 2009 Author Share Posted August 7, 2009 Good idea, I will do a leak down test tomorrow. How will I know if it's the valve or the rings (I guess if it immediately looses pressure it's a valve, and if it leaks slower it rings)??? Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted August 7, 2009 Administrators Share Posted August 7, 2009 (edited) Good idea, I will do a leak down test tomorrow. How will I know if it's the valve or the rings (I guess if it immediately looses pressure it's a valve, and if it leaks slower it rings)??? You don't really need leak down tester for this, just some way to get pressurized air into that cylinder, (Leak down testers due just that, requiring an air compressor to work.) A leak down tester is pushing air into the cylinder, so wherever the leak is, you'll hear it! Just be sure the cylinder you are testing is at TDC of the compression stroke. With the valve cover off, the cam lobes for that cylinder will be pointed mostly up, at the same amount, just opposite each other. Now, with the leak down tester pumping air in the cylinder, put your ear to the following places. Where the issue resides is where the air will be leaking from. You'll hear the distinct HISSS of air escaping. 1) Intake/air filter. 2) Exhaust pipe. 3) Radiator, (remove the cap, maybe the sound of burbling or hissing). 4) Dipstick tube or oil filler hole. 5) Remove #2 and #4 spark plugs and listen in those cylinders. Blown head gasket would allow the air to escape into the cooling system, or sometimes to the adjacent cylinder even though the adjacent "could" still pressurize fine, (hence the removal of #2 and #4 spark bolts.) Hope that helps. Paul Edited August 7, 2009 by BRAAP Quote Link to comment Share on other sites More sharing options...
m4xwellmurd3r Posted August 7, 2009 Share Posted August 7, 2009 Man that sounds exactly like my Z's engine when I had to get the head rebuilt. #3's exhaust was leaking pretty nasty, and at idle it basically was a totally dead cylinder. Weird thing was it seemed to light up at higher RPM. Do what BRAAP said a leak down tester is only needed to find out what percentage of compression is being lost to leaking, or something of that nature (you already KNOW you're bleeding totally, you just need to find out where it's bleeding to now hah) Quote Link to comment Share on other sites More sharing options...
coletrain777 Posted August 7, 2009 Author Share Posted August 7, 2009 Soooo.... I found the reason for the low compression on #3 cylinder... a broken exhaust valve. The face of the valve had a 1/8 by 1/8 chip out of the edge, so of course the cylinder wouldn't have hardly any compression. The good news is that the cylinder walls of every cylinder look awesome (even #3) so wherever the piece went it didn't tear up the cylinder walls. So now the questions are: 1. What would have cause the valve to break like that? 2. I am just trying to get this together in an economical fashion so that the car it's going into will be able to move around and drive for a while (not a long distance DD or anything). With that being the case, would my best course of action be to just pick up a new exhaust valve, clean up the head and lightly lap the valves, put new valve stems in and call it good??? 3. All of my pistons have the numbers 3 and a little 4 except this number three cylinder. It has a 4 and a little 5. Whats the deal with the numbering on the pistons? It's not like aftermarket pistons that have the overbore sizing on them. Anybody know ??? Quote Link to comment Share on other sites More sharing options...
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