Compression Problem

[Biggmack77]

I have a 1990 5.7 Mercruiser 350 small block with an Alpha 1 (Gen 2) outdrive. The boat runs well but it often has trouble starting. I recently did a compression test because I have ruled out most other things? The starter, coil, plugs and wires were replaced last year. The carburetor (Quadrajet) was rebuilt last year, which solved some other problems, but the starting problem did not get any better. It has an electric choke which seems to work well. There is often a mist of vapor that comes out of the carb after turning the engine off, I don?t know if that is a side effect of this problem.
The results of the compression test were not very good. Here they are:
Cyl 1= 95 psi
Cyl 3= 145
Cyl 5= 145
Cyl 7= 150

Cyl 2= 150 psi
Cyl 4= 145
Cyl 6= 70
Cyl 8= ZERO psi

Results after adding oil to the lower cylinders:
Cyl 1= 104 psi
Cyl 6= 85 psi
Cyl 8 still had no compression reading double checked it as well.

The spark plugs all looked OK
1, 5, 7, 2 where white
6, 8, 3 were light brown
4 was black
What should I do to diagnose the lack compression in the number 8 cylinder? Would this be the reason why it is so hard to get the engine started? How expensive could this get? Any advice will be greatly appreciated.

 

[mkast]

Re: Compression Problem

Time to replace or rebuild. $1500 on the best day of your life or $5000 on a bad day, don't ask about the worst day.

 

[bigskiohio]

Re: Compression Problem

if it is mechanical not a crack or the like, bad valves wont be to bad to fix your short block is probably good. no water in oil right. 1500 sounds a bit high for rebuild you can buy a truck long block rebuilt for 900.00 or so and the right mechanic could do it for alot less.

 

[mkast]

Re: Compression Problem

1500 sounds a bit high for rebuild

He's going to spend a tad more than just acquiring another engine.
It's a nineteen year old boat, so what do you think he'll find upon disassembly and inspection?

 

[stonyloam]

Re: Compression Problem

What should I do to diagnose the lack compression in the number 8 cylinder? Would this be the reason why it is so hard to get the engine started?

Pull the head. You got nothing to loose, and you might get lucky and just need some head work. Could be! Good luck.

 

[pocrain]

Re: Compression Problem

Pull the head. You got nothing to loose, and you might get lucky and just need some head work. Could be! Good luck.

It is not a good idea to spend alot of time worrying about anything until you know what is wrong. Just find the problem before you decide. What are the hours on the engine? Just curious, and good luck.

 

[P 0 P E Y E]

Re: Compression Problem

These guys are probably right. The motor is probably spent. But do some problem solving.

Zero compression is interesting.

Do you have lots of blow by?

If the piston is moving up and down and does not have a big hole or is not cracked in half, the compressed air has to be escaping some place, like into the exhaust manifold, intake manifold, cooling system or hyperspace.

Listen for where the air is going during the compression stroke.

 

[stonyloam]

Re: Compression Problem

Ockham's razor states: "entities should not be multiplied unnecessarily." In other words look to the simplest explanation first (make the least number of assumptions). I'm just a shadetree mechanic, but I can only think of a very limited number of ways to get to "0" compression and "boat runs well". The simplest: Valve stuck open.

 

[TilliamWe]

Re: Compression Problem

Zero compression dry and with oil equals a valve issue. Usually nothing more.
The rise in compression on #6 with oil is a little troublesome, but I'll still bet it's a valve issue.
If you are mechanically inclined, remove the valve cover on that side, and lossen the rocker arm nuts on those two cylinders, a little. Recheck compression. If the zero goes up at all, (and the 70psi on #6) bingo you have a tulipped valve.


Yes, the low compression is why the engine is hard to start.

If you just have valve issues, you could spend as little as $500 on machine work and the gaskets to put it all together. Assuming you are doing the labor. If you have piston/cylinder problems, well, then the sky is the limit.
(You probably only have valve issues, though)

 

[TilliamWe]

Re: Compression Problem

I'm just a shadetree mechanic, but I can only think of a very limited number of ways to get to "0" compression and "boat runs well". The simplest: Valve stuck open.

You are almost EXACTLY right. Add "tulipped" valve and a bad valve seat to that statement and then you are 100% right. Of course you could argue that a tulipped valve is stuck open, but it's techinically closed, it's just deformed in shape, not allowing it to seat (which seals the combustion chamber).

 

[bigskiohio]

Re: Compression Problem

a valve job around here would be around 250.00 or less but i guess it matters who you know, and where you live, i could get heads done for less then that
but i run in a mechanical circle so sombody always knows somebody.