I have been tasked with diagnosing and fixing an 801 Ford with a 172 diesel that has just been rebuilt by another local mechanic. It has new sleeves and pistons, reworked head, rebuilt injection pump and tested injectors. Problem is it won't build the 365-400 psi called for. Cylinder 1= 260, 2= 300, 3=300, 4=270. Tonight I verified timing and then pulled #1 piston. The sleeve looks good with new crosshatches, rings aren't damaged and piston is correct. Camshaft is original. The oil was thin and likely was diluted with diesel but not much thinner than the break in oil I use for overhauls. I've never had a compression issue on a back to standard rebuild but could the rings be leaking by enough from not being seated to loose that much pressure? Unless I can find something definite, I will reassemble and load it to its rated power on my dyno and see if it improves. Maybe dope it with a little Bon Ami. What I don't understand is the inconsistency in the compression test readings from front to back.
Recheck cam timing. Compressed air to each cylinder on TDC compression stroke and see where it's coming out.
Check the ring gaps and that the rings are positioned in the correct order. The injection pumps are bad about the shaft seal leaking fuel into the crankcase.
Ring gap is good on #1 top chrome ring. Too late for a leak check and timing is spot on.
Air check all cylinders. If good, degree the camshaft, at or at least do a "good enough" degreeing so that you know for sure the valves are opening and closing at the right times. What else could it possibly be?
Cam shaft is keyed with no indication of breakage as valve timing matches stroke and it also drives hydraulic pump at rear of engine. I pulled the gear and checked key. I'm as far as I can tell down to something wrong in the holes (rings not seated, wrong pistons, etc).
Well if the valves are opening and closing when they should be then it is down to wrong piston or leakage from somewhere. Wrong piston seems far fetched but who knows? I guess you could use some clay to measure chamber volume like they do on race cars and calculate compression ratio from there to tell if it is a wrong piston.
If I had access to a dyno I would drop it on there first.. get it warm, recheck the valves, then run it in and recheck the compression. THEN if theres a problem, go into the engine.. That way, all you have in it is dyno time and only acouple of hrs of labor
Engine is already in pieces. Head off, #1 piston out, timing cover off. I need to decide on a plan before I go back together with it.
check the valve head recession into the head. may have been machined too far mark
The two things I could think of is making sure the ring gaps aren't lined up and valve lash, like Mark says. Maybe do a leakdown test. ISZ
The piston I removed had the top and second ring gaps lined up. Will that really allow for more of a leak? I have thought that the leakby will merely chase around the piston to the next gap and then go down. Valve lash was good between all cylinders before teardown and during compression testing.
Ring gaps MUST BE staggered..
I would be checking to make sure the pistons are right. I have also seen the wrong crankshaft put in an engine which had a slightly shorter stroke causing low compression. Rings being lined up is not right but it won't cause low compression. We had cylinder packs coming from cat reman with rings lined up which we would disassemble and reposition the rings . When we talked to Cat reman Center about it they were adamant that it made no difference.
Did you check overhead? make sure valves weren't too tight maybe holding open? Also make sure ring gaps are staggered, maybe even make sure theres not one cracked or broken from bad installation.
If I remember correctly, there are different sized head gaskets for that engine.. if the block was decked and the head surfaced, who knows whats going to happen. Screw-it.. pull it down all the way and start measuring and verifying piston and crank #'s.. Make sure the rings are in the correct position. Then on your way back up, find & verify TDC.
Very common (on this 172) for the fuel return lines under the valve cover to leak fuel into the motor oil. Brass tubes leak at one or more banjo fittings.
just because it was rebuilt doesnt mean it was rebuilt correctly.....
Crank is original and a diesel, head was resurfaced but no work done to deck. Had new liners installed, pistons are correct. The owner and I have for now decided to replace the rings and ball hone the bores, load it immediately on the dyno to near rated output, get it hot and see if it will seat. I haven't ever worked on a gas to diesel conversion as this engine is but instead have rebuilt many dedicated diesels that call for much higher pressure and I've never had a compression issue. My postulation now is that those engines may have had a lower than spec pre break in pressure but it was still plenty high enough for good starting and a low compression issue to never show up, but this engine is nominal in its spec to begin with and before ring seating, it is nearly too low to run as a typical diesel should.
There are different head gaskets for these 172 engines. Also different heads due to a mid production change. The heads and blocks must be matched. If a late gasket is used on a early engine or vice versa, it will cover part of the water passages between the head and block and will result in major overheating.
And the problem was
Good question.. I had forgotten about this..
Wow. So had I. As I recall, I reringed it, had the valves checked over, and burned it in on my dyno. It was still a bit hard starting in cold weather but seemed to be good otherwise. Had a bit of an overrule issue we also had to deal with.