Mercruiser 5.0L EFI - TBI Low compression all 8 cylinders

[CaptainMak]

Trying to hunt down a low acceleration problem, I decided to check the compression in the block. It is a 2001 EFI with TBI 5.0L.

All cylinders are between 60 and 75 psi. Maybe a bad compression gauge ? What else would cause ALL cylinders to be low like this ?
Maybe something with the timing chain ?

Also, it was a cold engine, with throttle in the "neutral" position. Would that affect it somehow?

 

[Alumarine]

Might be a bad gauge.
Some auto parts stores have loaner programs.

Motor should be warm, throttle open, all plugs out, crank 4-6 revolutions until needle steadies.

 

[CaptainMak]

I'll try a different gauge at a different store tomorrow. This was one from an Oreilly store. Seemed like cheap quality.

 

[bruceb58]

You want the throttle full open too.

 

[CaptainMak]

You want the throttle full open too.

Just tried opening the throttle all the way to WOT, same reading of 75 psi

 

[Bt Doctur]

Then reconfirm with another gauge . Then make an adaptor from a old spark plug , remove all the plugs and put air into each cylinder and listen for the escaping air.
intake =intake valves
exaust = exaust valves
crankcase= rings
a ajacent plug hole = headgasket

 

[CaptainMak]

I believe the battery was the problem. It was cranking slow. Also warmed the engine up a bit. Charged the battery fully, now getting about 125-130 psi between all cylinders.

 

[bruceb58]

How many revolutions were you cranking when you were measuring?

 

[CaptainMak]

How many revolutions were you cranking when you were measuring?

I'd guess 3 or 4. Read somewhere that it needs to be close to 10 on a fully charged battery. Maybe that was my problem.
Boat running a lot better with new NGK plugs in it.