Intermittent starting problem

[Biggmack77]

Hello there,

I have a 1990 5.7 Mercruiser 350 small block with an Alpha 1 (Gen 2) outdrive. It has always been a little hard to start, but would eventually fire up. But this problem is eating away at me because it seems to be getting worse. The engine almost always fires right up at the dock when I first start it up, but after being run for a while it is extremely difficult to start again.

The engine will crank (sometimes faster than others), I get gas at the carb, I get spark as well. Usually when I am doing tests it will start very easily with only a turn of the key. But as soon as I am out on the boat with friends and its time to go home, it will crank and crank and not catch easily at all.

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, is this normal?

I believe I have a fuel problem on the engine side. I would greatly appreciate any advice.

Thank you

 

[beanhauler]

Re: Intermittent starting problem

Check the compressions.
Squirt some oil down the bores and check again.
Needs to over 100 close to 140 if poss but anything lower than 80 is not too good.
I had this problem which was primarily due to worn valve gear.
Check for any rusty looking plugs as tell tale sign of head gasket leaks/riser leaks etc

 

[greiffster]

Re: Intermittent starting problem

Have you checked your timing?

 

[Biggmack77]

Re: Intermittent starting problem

I will do a compression check and post the results. The timing was set to specification about a year ago.

 

[gdombroski]

Re: Intermittent starting problem

I think you may have one of three problems or multiples thereof

sometimes faster than others

This may indicate that the starting battery is not operating at desired cranking amp level. Even though it shows as charged age & use will reduce rated cranking amps. Have it checked at auto parts store.

it will crank and crank and not catch easily at all.

Hope you don't do this for more then about 15 seconds at a time as you will eventually burn up starter. This may indicate a choke adjustment needed. When it does this have you tried inseting a screwdriver in carb to open it up. Mine did this and with a screwdriver inserted it would start right up. The Holley fix was to turn the choke clockwise about two notches off center.

There is often a mist of vapor that comes out of the carb

My fix for that was a new anti-siphon value. When the pumps shut down and valve closed you should not get this.

 

[conk3416]

Re: Intermittent starting problem

IMHO, I would start by getting a marine battery with around 800 to 1000 cranking amps. If that doesn't work i would say the starter is going bad and would need to be replaced.

 

[flargin]

Re: Intermittent starting problem

IMHO, I would start by getting a marine battery with around 800 to 1000 cranking amps. If that doesn't work i would say the starter is going bad and would need to be replaced.

And double check/clean the connections. you may have corrosion or loose connections causing the slow crank.

Sand all connections shiny before re-attaching.

 

[Biggmack77]

Re: Intermittent starting problem

The starter battery is only about 2 weeks old, it works well. The starter is less than a year old. Today I put a new ring connector on the wire going to the starter as it was not looking so good, I think the crank speed problem. But the turning over problem still exists.

I did the compression test today, here are the results:
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

I am worried about the lack of compression in Cylinder 8. This is probably the root of the problem. What should I do to fix this problem?

 

[beanhauler]

Re: Intermittent starting problem

Easiest way is to drop the leg off and remove the engine.
Before doing you can take the heads off and check all the valves
or it may be a head blowthrough but unlikely given the compressions in the other cylinders.
I have taken the heads off before to find duff valves so you may be lucky without removing the engine however depending on the layout it nearly did my back in leaning over it.
Its far better to get it on the bench and investigate more fully.
With zero compression it would lean towards a duff valve/head but in any case where compression is an issue it would be prudent to remove the engine drop the sump and push the con rods to make sure.
It costs one gasket extra on the sump and saves the headache of putting it all back together and finding the compression is better but not much better.
You obviously need the facilities to do this.

 

[searay3]

Re: Intermittent starting problem

Depending on the layout of your boat/engine compartment, you may be able to pull the exhaust/risers and heads without pulling the engine. The white plugs indicate water intrusion. They are being steam cleaned. Probably a manifold riser issue. The zero indicates a head/valve problem. Maybe a hole in that piston. Probably a tuliped valve though. I wouldn't pull the engine just yet unless you want want to rebuild the whole thing anyway. Not a bad idea. But there are other routes you can take if that is what you decide.

 

[Greg Gessler]

Re: Intermittent starting problem

Put compressed air into spark plug hole (weld airhose fitting on old sparkplug case that had the porclian removed) then you can hear where the air is leaking. Intake/exhaust leakage would indicate valves are bent or burnt. Crankcase leakage would be rings/piston.

Try loosening up rocker arms on that cylinder to make sure that the valves are not being held off their seats.

Is compressiion reading in cylinder #8 differernt when engine is cold?

 

[mkast]

Re: Intermittent starting problem

Put compressed air into spark plug hole (weld airhose fitting on old sparkplug case that had the porclian removed) then you can hear where the air is leaking. Intake/exhaust leakage would indicate valves are bent or burnt. Crankcase leakage would be rings/piston.

Try this with the piston at top dead center with both valves closed.
Is this a backyard cylinder leak down test?

 

[Greg Gessler]

Re: Intermittent starting problem

Thank you for noting that both valves need to be closed. This test is not accurate like a leakdown test, but it will identify major problems.