Merc 5.0 Starts fine, stalls when warm, wont restart till cool

[a_edwill]

I have a 2003 Mercruiser 5.0 305hp V8. First time out for the season. I have a fully charged battery that I load tested, it's fine. Start engine it runs great for 3 - 4 minutes then stalls. Won't start again for a few minutes. Then it'll start, run for about 30 seconds then stall again and the cycle continues.

I replaced the plugs, engine oil and filter, fuel filter/water separator just to give it a good tune up. Symptoms continue.

It seems that something is failing once it warms up. Does anyone:

1. Recognize this issue?
2. Have any idea how to test the fuel pump assuming it's the cause
3. know any voltages I can monitor to test anything such as the fuel pump or computer?
4. Where to buy a fuel pump pressure testing device?

Thanks!

 

[Don S]

Re: Merc 5.0 Starts fine, stalls when warm, wont restart till cool

Year?????????

 

[a_edwill]

Re: Merc 5.0 Starts fine, stalls when warm, wont restart till cool

2003. Updated original posting. Picking up a fuel pressure guage and inline spark tester tonight on the way home. Will monitor voltage at the same time. That should provide serious visibility into root cause.

 

[a_edwill]

Re: Merc 5.0 Starts fine, stalls when warm, wont restart till cool

I tested the electric fuel pump pressure at the carburetor, it showed 6.5psi until it started, jumped to 7.5 and maintained until it ran out of fuel then dropped to 3 for a bit, then back to 0psi.

Voltage at the battery was at 12.2v. When turning over the engine dropped to 9.5, but then again I've been starting it for days without any luck, surprised it's not dead. Once the engine started it stayed consistent at 14.2vdc until it died, so alternator is working.

I picked up a spark tester that you hold on the outside of the wire, it showed spark when starting but when the engine was running didn't pick anything up, apparently the spark on a Mercruiser is less than that of an automobile.

Any idea how to monitor the computer or what else to monitor??

 

[freddyray21]

Re: Merc 5.0 Starts fine, stalls when warm, wont restart till cool

the spark should be as hot as on a car. I would suggest taking a look at the wiring going to the hot side of the coil. There should be two wires I believe. One comes from the starter to give it 12v when starting. The other comes from the ign switch possibly through a resistor wire. Sounds like the resistor wire is bad. Hook 12v directly to the + side of the coil and see if it stays running.

 

[Don S]

Re: Merc 5.0 Starts fine, stalls when warm, wont restart till cool

Get a remote fuel tank and hook it directly to the engine to bypass your fuel tank completely. Then see if it will run.

 

[a_edwill]

Re: Merc 5.0 Starts fine, stalls when warm, wont restart till cool

Hooked +12v directly to the coil, but couldn't find a different fuel supply. Didn't work. Still didn't see the spark using the external spark tester. Note that engine will always run for a minute or two just fine when cold.

Note that when sitting on the engine and starting it via shorting the starter solenoid and adjusting throttle manually I see fuel pumping into the carberator fine. When it dies after 60 seconds of running it puffs white smoke back out of the carb.

 

[Coors]

Re: Merc 5.0 Starts fine, stalls when warm, wont restart till cool

Maybe ohm the coil?

 

[CharlieB]

Re: Merc 5.0 Starts fine, stalls when warm, wont restart till cool

I tested the fuel pump pressure at the carburetor, it showed 6.5psi until it started, jumped to 7.5 and maintained until it ran out of fuel then dropped to 3 for a bit, then back to 0psi.

2003 ? Electric fuel pump or manual?

Either one, the fuel pressure should not have fallen off, Period.

Check for restriction in the lines, filter, etc.

Use a test tank to supply the pump and re-test.

Pass or fail?

Dis-arm the ignition by removing the wires from the + side of the coil then disconnect the fuel line from the carb, place it in a suitable container and crank the engine for 15 seconds to test fuel flow. Pass or fail?

 

[a_edwill]

Re: Merc 5.0 Starts fine, stalls when warm, wont restart till cool

"Dis-arm the ignition by removing the wires from the + side of the coil then disconnect the fuel line from the carb, place it in a suitable container and crank the engine for 15 seconds to test fuel flow. Pass or fail?"

Pass. Gas continued to pump.

I got it soon after though. I pulled the top off the carb to monitor the bowl. When the engine began to die I killed the engine and removed the top quickly. Very little gas in the bowl. Knowing that the pump checked out ok I removed the line from the tank.

It appears there is a one-way valve that attaches the hose to the square connector at the tank. When I removed it and examined there was a piece of metal stuck under the ball, causing it to neither open nor close fully. Since it couldn't open all the way the carb was running out of fuel after 30 seconds. The valve allowed enough pressure to refill the bowl after a couple of minutes worth of sitting.

I called Olympic Boats in Bellevue, WA and ordered the carb gasket kit to replace the damaged upper gasket on the carb and the parts person mentioned they've seen this a lot in the Bayliners. Bad gas or debris can lodge under the ball preventing it from opening or closing fully. He knew what it was right off the bat.

Thanks all for your help. At least I managed to give the boat a good tune up.

 

[95yj]

Re: Merc 5.0 Starts fine, stalls when warm, wont restart till cool

Glad you got it going. Those anti siphon valves usually clog up at the absolute worst times. At least it didn't cost you a tow.