Causes of low compression

[Tractorboy88]

Ford 3000 diesel.
Owner bought it used several years ago. Has had to use ether since day 1 to start it.

Fuel system checked out, has visible white smoke (unburnt fuel) coming through exhaust, fresh fuel, new filter, and the valve is open.

Checked compression with all injectors removed. All cylinders 225-250 psi. Spec is 420-510 with 50 psi variation between cylinders.

In addition to low compression, I hear air escaping through exhaust and blow by.

A valve and ring job later, I'm back to square one.

Tried a spare set of injectors with no change.

Removed injectors and checked compression.
275 psi.

Measured bore is 4.2.
Measured stroke is 4.2.
Piston crown to deck is within spec at .012.

What am I missing here? Could timing gear alignment be off causing low compression?
What other possibilities for low compression are there? I'm pulling my hair out.

 

[davesl708]

Timing could cause a valve to be open when in compression.

 

[Egon]

Rings may not help for worn cylinder bores.

 

[Tractorboy88]

Rings may not help for worn cylinder bores.

Surprisingly the bores were in decent shape. Within taper and out of round spec.

 

[Tractorboy88]

My manual does not list an intake/exhaust lift measurement, only event timing.

Does anyone have a manual that gives a lift spec? If so does .310" sound close for intake lift?

 

[Tractorboy88]

Timing could cause a valve to be open when in compression.

That's what I'm thinking. Have to pull front axle to remove crank pulley and timing cover to check. That will have to wait till tomorrow.

 

[KYErik]

When the rings were replaced, were the cylinders honed to break the glaze? If so, it may just take some time for them to wear in.

As was mentioned, if the valve timing jumped, it would affect compression.

 

[Qapla]

How many rings are on each piston? Did they get "lined up" when they were installed so that the gap where each ring comes together creates a passage for compression to escape? (I have seen this done before) They should be offset so that the seal is complete.

When the valves and rings were done, was the head planed or trued? Were any valve guides replaced?

 

[Tractorboy88]

When the rings were replaced, were the cylinders honed to break the glaze? If so, it may just take some time for them to wear in.

As was mentioned, if the valve timing jumped, it would affect compression.

All 3 cylinders were honed as well.

 

[KYErik]

Are valves adjusted with the correct lash? A valve with no lash would leak compression- especially as the engine warmed up.

 

[Tractorboy88]

How many rings are on each piston? Did they get "lined up" when they were installed so that the gap where each ring comes together creates a passage for compression to escape? (I have seen this done before) They should be offset so that the seal is complete.

When the valves and rings were done, was the head planed or trued? Were any valve guides replaced?

3 compression rings and the oil ring. All three sets of ring gaps were offset as described in the manual, 120 degrees apart. Ring gap was measured before being installed on pistons and was within specification.

The head was trued and i believe all valve guides were replaced.

As far as i can tell, i'm not losing compression by the rings, as i have no leak by through the valve cover.

 

[Tractorboy88]

Are valves adjusted with the correct lash? A valve with no lash would leak compression- especially as the engine warmed up.

Set lash after torquing head down. Rechecked and verified valve lash after running.

 

[Tractorboy88]

Thanks for your quick responses. Please continue to offer any ideas, suggestions you have.

 

[KYErik]

How many hours have you ran it since the rebuild? Maybe run it under a moderate load for an hour or two and see if compression improves due to rings wearing in/sealing better. Might be worth a try before you tear off the front end to check the valve timing.

 

[Tractorboy88]

Its ran maybe an hour. I'd like to think it was the rings needing to seat, but everyone i've talked to has said that its not that hard to start one unless the rings are completely worn.

I pulled the head back off tonight to recheck measurements. 8 bolts and 2 tie rods and the front end can roll forward. Then its just a matter of pulling the crank pulley.

 

[skyhook]

Just a thought, have you tried a "wet" compression test, squirt some oil in the cylinder before testing. This will tell you if the rings are bad (if compression rises in this test).

 

[Tractorboy88]

Before I replaced rings and pulled head, I performed a wet test and saw roughly 50 psi improvement.

I replaced rings because I did not trust old rings with a fresh valve job.

 

[Qapla]

On many newer cars you must replace the head bolts when you remove the head due to the bolts not really torquing correctly when they are reused (seems they "stretch"). Just a thought.

 

[RBLapham]

Where you take the compression readings - connect an air hose from your air compressor(adapter needed) then with the piston at top dead center and locking the crank if possible. Slowly add air pressure and check where the air escapes. Out the carb bad intake valve. Out the exhaust - bad exhaust valve. Bubbles in cooling system - blown head gasket or cracked block. Out oil filler - blow by on rings. Have fun.

 

[kencunningham]

One thing that hasn't been mention yet, Has the engine ever been bored out? If it was, are you sure you rec the correct rings for it? The leakdown test is what should give you a good pointer as to the true problem. Good luck.