Won't Idle

[bassattacker]

1998 3.0 L mercruiser, alpha one out drive. I changed the oil and put 5 gal of gas in the boat. Initially it ran fine, but lost power once on plane. I came back to the dock, it seemed fine so I went back out. Same thing happened it got up to speed and then lost power. Thinking I had a fuel problem I siphoned all the fuel out of the tank, replaced both filters and new gas. Now it won't idle, it will start, though with considerable effort, (pump the gas a few times and hold the throttle wide open) it will run but you have to keep the rpms above 1000. I've since replaced the distributor cap, rotor button, plugs and wires and rebuilt the card. I've tried swapping coils but the same problem. Any ideas?

 

[bassattacker]

Engine ser# OL089228, model# MCM 3.0L

 

[Eric 3.7LX Alpha 1]

Checked the fuel filter in the carb inlet?

 

[bassattacker]

Yes. Both the filter in the fuel pump and the one in the carb are new.

I changed the oil using an oil extractor, which was a first for me. I followed all the directions, but I wonder if I didn't get all the oil out and perhaps put too much back in? I've read these engines have some type of oil overfill sensor that might be causing a problem?

Aside from the 5 gal of fuel (which I initially suspected was bad) The oil was the only other change prior to the engine performance decline.

 

[Bondo]

Now it won't idle, it will start, though with considerable effort, (pump the gas a few times and hold the throttle wide open) it will run but you have to keep the rpms above 1000

Ayuh,.... Welcome Aboard,... Sounds like some of that crud in the gas got to the carb, 'n the idle circuit is now plugged,.....


Rebuild, 'n clean the carb,....

 

[pro-crastinator]

get a nice clean 4 gal of gas in a plastic gas can.
Get a few feet of fuel line and connect one end to the fuel pump nipple on the tank side. (mines 3/8")
Stick the other end of the tube in the (presumably red) gas can sitting on the deck of the boat.
I would bypass the fuel filter to keep it simple.

Now you have a perfectly clean fuel supply from which to test your motor.

Will best case scenario be that the motor runs perfectly ? - or will the symptoms persist?
Either way, you have to know in no uncertain terms that the fuel tank supply is perfect.
The suspected "clog" could be in the tank.

 

[bassattacker]

This is my plan for Saturday. I'm getting fuel sprayed back at me each time I remove the fuel line so I can't believe it's a bad pump or clogged line, but we'lll soon find out.

 

[bassattacker]

It took me awhile to get back to it but I tried the fuel can trick and got no change. Actually before I tried the fuel, I found the oil to be over full and removed what I'm guessing was a quart of excess oil, which I don't understand. I'm wondering if I have a four quart pan. I extracted what I thought was all the oil and added 5 qts. So either I didn't get it all extracted or it doesn't have a 5 qt capacity. Regardless, with the boats fuel tank out of the picture, I'm getting the same reaction with a couple feet of fuel line and the fresh gas. It is extremely difficult to start and sounds like it's flooding and once started will not idle below about 1500 Rpms. If while your cranking the boat and it starts to fire you add fuel to the carb via bottle or ether, it loses fire which tells me it's too rich. Vacuum leak? Timiming problem?

 

[Fastatv]

It took me awhile to get back to it but I tried the fuel can trick and got no change. Actually before I tried the fuel, I found the oil to be over full and removed what I'm guessing was a quart of excess oil, which I don't understand. I'm wondering if I have a four quart pan. I extracted what I thought was all the oil and added 5 qts. So either I didn't get it all extracted or it doesn't have a 5 qt capacity. Regardless, with the boats fuel tank out of the picture, I'm getting the same reaction with a couple feet of fuel line and the fresh gas. It is extremely difficult to start and sounds like it's flooding and once started will not idle below about 1500 Rpms. If while your cranking the boat and it starts to fire you add fuel to the carb via bottle or ether, it loses fire which tells me it's too rich. Vacuum leak? Timiming problem?

As Bondo had mentioned, sounds like "crud" has gotten into the carb....maybe when you changed the carb fuel filter. I would recommend pulling the carb, give it a thorough cleaning, put her back on and give it a shot.

 

[bassattacker]

Carb has been rebuilt since the problem started with no change in performance. Few questions.

1) Would a vacuum leak create a situation where it runs rich? My plugs are fouling, not with oil but rich fuel mixture.

2) Would low compression cause it to run rich? This thing sat for so long, I'm wondering if maybe the rings are gone, though if that were an issue I'd expect to see some burning oil or excessive blow by from the PCV.

3) What could I have screwed up by having too much oil in it? I troubleshot this for a week before I realized there was at least an extra quart of oil in it. For the life of me I can't figure out how that extra oil got in there, other than perhaps I have a 4 qt pan.

4) Is there any truth to the assertion that there is an oil over full sensor on this boat?

5) Could timing cause a situation where it's running rich?

I'm planning a compression check next. I can' get it to idle anywhere near 800 RPMs, so I'm not sure a timing light is going to tell me anything.

 

[skippy2235]

when running, pull a spark plug wires off. One at a time, find the cylinder that makes no difference in running.
Meaning a rough idle can be caused by a dead cylinder, pulling off the wire should make the engine die, or run worse. When you find the cylinder that makes no change then you know where to focus.

 

[Bondo]

Carb has been rebuilt since the problem started with no change in performance.

Ayuh,..... By Whom,..?? A Rich condition is caused by dumpin' too much gas into it,....

A vacuum leak would cause a Lean condition,...

Low compression is a problem all it's own, but will not cause a rich condition,.....

The oil level shouldn't have hurt anything,... Ya got it Right now, Right,..??

#4, No,...

#5, No,...

 

[bassattacker]

Compression test complete. 30 psi in 1,2,3 130 Psi in 4. I'm suspecting jumped timing, blown head gasket, burnt valves, or burned Pistons.

Going to perform a reverse compression test next. Any suggestions?

 

[Bondo]

Ayuh,.... Pull the motor, it's got problems,....

 

[flipbro]

Ya pull the head and get ready for a rebuild..

 

[skippy2235]

I would not pull heads yet. I would pull valve covers, and then make sure valves are closed and do a cylinder leak down test, on each cylinder, This will tell you if heads or valves or rings... If you pull the heads and it looks good, put it back together only to find out it was the rings, you would be a little pissed.

 

[flipbro]

Pull the head and inspect. With only 30 psi in 3 cylinders its valves or wrings either way it has to come apart. And when its apart the problem will be obvious. But a leak down test is a good start. Seaman

 

[skippy2235]

Cause it also could be a jumped timing gear.

 

[flipbro]

Fair enough.

 

[flipbro]

He claims nothing about back fire through carb so makes me think it Hasn't skipped a tooth. If you want to test it put a couple tablespoons of oil in the cyliders and recheck compresion if it increases consiterably then its rings if no change then valves.