Stumped: poor compression on 4.3L Merc

[CAPTahab7]

I have a Merc 4.3 L on a 2003 Stingray. Ran a compression check and get values between 185,130,180, 95, 20, 120. The 20 and 95 psi are the #2 and 4 cylinders. I took apart, found water in 1 cylinder and in oil. I had heads rebuilt (milled, valves replaced/reconditioned and reinstalled. I torque rockers to the 22 ft-lbs specified in manual. Now I get compression values of 60 psi consistent in all cylinders: what the heck is going on? The pistons looked OK. What could be causing this issue?

 

[Bondo]

Re: Stumped: poor compression on 4.3L Merc

Ayuh,... Re-do the lifters,...

I'm guessin' they're too tight...

 

[CAPTahab7]

Re: Stumped: poor compression on 4.3L Merc

Yes, I had thought of that too. I readjusted 1 cylinder to be looser, but did not come up on pressure. I can go back and do all, but the manual says if I torque to 22 ft-lbs, no lash adjustment required. Should I NOT beleive the engine manual? If piston rings good, valves resurfaced/replaced, Good head cylinder and gasket, springs/rockers OK, is there anything else that could cause poor compression other than lash adjustment?

 

[solar7647]

Re: Stumped: poor compression on 4.3L Merc

Yes, I had thought of that too. I readjusted 1 cylinder to be looser, but did not come up on pressure. I can go back and do all, but the manual says if I torque to 22 ft-lbs, no lash adjustment required. Should I NOT beleive the engine manual? If piston rings good, valves resurfaced/replaced, Good head cylinder and gasket, springs/rockers OK, is there anything else that could cause poor compression other than lash adjustment?

Re adjust so that you get them just tight enough to remove the lash and then one more half turn or so i wouldnt worry about the torque setting they should be lock nuts and wont untread or anything. You also may need shorter push rods since the head was machined, they could be to long and not allowing your valve to close all the way.

I had an issue with my 4.3l and found that the by doing it by the torque setting alone was way to tight, when I adjusted them as said above i found the the nut was twice as deep on the stud if you just go by the troque setting. In my case they were so tight the lifters were not getting the travile needed to pump oil to the head. Also when you adjust them make sure the engine is at TDC and adjust them in sequance as manule says.

 

[littlebookworm]

Re: Stumped: poor compression on 4.3L Merc

The heads were milled; push rods are original; tightening to 22 ft-lb brings the valve lifters down too much. I suggest you let the lifters pump up with oil and then adjust them with the engine running. It's messier, but does work. remember to have muffs on the outdrive and water flowing. Warm the engine. Lossen each lifter until you hear it tap. Then tighten slowly until tapping stops. Then tighten 1/2 revolution more. Hopefully, that should do it. Let us know the result, please. Hy

 

[Don S]

Re: Stumped: poor compression on 4.3L Merc

Re adjust so that you get them just tight enough to remove the lash and then ...........................

His valves are not adjustable guys. They just torque down to 22 ft lbs. It's a V6, not a V8

CAPT, run the engine first before doing a compression check, if you did, you might want to check your compression gauge before you start looking for engine problems.

 

[CAPTahab7]

Re: Stumped: poor compression on 4.3L Merc

Thanks guys for chiming in. Once I got the engine running (and used a different compression gauge), the compression returned back to normal values. I did not re-adjust the valves, but left at the factory specs.