'95 Merc 502 dying on throttle up after idling for awhile

[thunder550]

Boat in my sig, '95 Merc 502 mag. Mechanical fuel pump and water separating spin-on fuel filter. Boat had a problem a few months ago when I first got it where if I idled for awhile, like going through a long no-wake zone, then left the no wake zone and tried to throttle up, it would just about come on plane then quit like I pulled the throttle back, run really rough for a few seconds, then die. When I tried to immediately restart, it would catch for a second then die again. If I let it sit for 2-3 minutes or so then restart, it fires up and runs fine like nothing was wrong. No other issues, it'll cruise and run wide open just fine, even throttles up ok off idle as long as it wasn't idling for a long time.

I changed the water separating fuel filter and the problem went away for about 3 months and probably 30ish engine hours. This weekend it started dying again. I changed the fuel filter again this afternoon but haven't had a chance to idle it for a long time so not sure whether it's temporarily fixed.

One weird thing, when I pulled the old filter off this time, it wasn't full all the way. Looking down inside, seemed like it was only about 2/3 full. Is that right?

I bought the boat from a dealer on Deep Creek Lake in Maryland. I'm not sure what type of fuel they have there. Since I brought it to AZ, I have been using 10% ethanol fuel since that's all we have here. Could that be an issue? Any other ideas?

 

[Rick Stephens]

Carbureted? If so, time to kit it. And make sure all your fuel line connections are tight.


Merc doesn't list a Bravo1 502 Mag for '95. Typical, that'd be too easy. You'll probably have to post a serial number for better answers.

 

[thunder550]

It's EFI, I should have mentioned that in the first post. Sorry about that.

 

[thunder550]

Engine serial is 0F593768

 

[Maclin]

You posted "manual pump" which would lead most to think you have a carbureted setup. Moving on to the problem, sounds a lot like vaporlock.

Anyone know if there is a TSB out on that model...

 

[thunder550]

The early EFI motors have the low pressure mechanical pump driven off of the raw water pump shaft. That's what I meant by manual pump, probably using the wrong terminology.

I just put a pressure gauge on it. It's showing 34 psi KOEO and 30-32 with the engine idling on cold start. Seems low. Looking for specs but IIRC it's supposed to be low 40s at idle.

 

[thunder550]

Found it, factory spec is 37 with engine running.

 

[alldodge]

One weird thing, when I pulled the old filter off this time, it wasn't full all the way. Looking down inside, seemed like it was only about 2/3 full

I see a bad fuel supply issue. Need to find out if its the low pressure pump, anti siphon valve, fuel line or tank pick up.

I would start by removing the plug on the side of the belt driven water pump. The fuel pump is mounted on top, and when the plug is pulled there should be only very little lube come out. If it comes gushing out and is thin, then your mechanical fuel pump is leaking.

Next remove the fuel line off the tank and run the motor from a gas can. Just put the fuel line inside the can, and probably need a longer piece of line.

Next put a fuel pressure gauge between the mechanical pump and the VST (vapor separation tank)

 

[thunder550]

I re-ran the pressure test at idle with the engine hot. With the vacuum line on the pressure regulator it was 31 again. With the line off, it was 37, which matches the mfg spec. Is that spec value with the FPR under vacuum or unplugged?

​​​​​​When I first bought the boat I changed the mechanical fuel pump oil. It was black, thin, and smelled like gas. Refilled with Merc gear lube. Have not checked since then, will check again though.

 

[alldodge]

I re-ran the pressure test at idle with the engine hot. With the vacuum line on the pressure regulator it was 31 again. With the line off, it was 37, which matches the mfg spec. Is that spec value with the FPR under vacuum or unplugged?

With the fuel filter not being full, what your seeing on the rail can be suspect. The fuel pressure should be close to 37 psi and increase above it with vacuum line removed

 

[thunder550]

Reading through the service manual, 37 is correct with no vacuum on the FPR. Should be lower with vacuum applied, which is it.

Pulled the fill plug off the lift pump, this is what came out. This was fresh about 40 hours ago (and not overfull like it is now). Smells really strong like gas. Time for a new lift pump? If so, should I ditch the mechanical pump and go with the electric kit specified in the vapor lock TSB?

 

[alldodge]

Fuel pressure should be about 37 psi with vacuum applied, and a tad more with it removed from the FPR

I would stick with the mechanical pump, they are still in use today with Merc's higher HP motors. They do fail like most other things, but is takes a long time in most cases. Look for Airtex 60932.

Airtex makes the pumps for Merc

 

[thunder550]

Here's the troubleshooting section from the service manual (#16, page 5C-61).

I will check the Airtex pumps, thank you for the recommendation

 

[alldodge]

My manual shows the same thing but woldn't be the first time that the manual was printed incorrectly. No other EFI works where the fuel pressure drops once the motor is running

Let see if Fun Times may have some thoughts

 

[211libwtfo]

I see a bad fuel supply issue. Need to find out if its the low pressure pump, anti siphon valve, fuel line or tank pick up.

I would start by removing the plug on the side of the belt driven water pump. The fuel pump is mounted on top, and when the plug is pulled there should be only very little lube come out. If it comes gushing out and is thin, then your mechanical fuel pump is leaking.

Next remove the fuel line off the tank and run the motor from a gas can. Just put the fuel line inside the can, and probably need a longer piece of line.

Next put a fuel pressure gauge between the mechanical pump and the VST (vapor separation tank)

It’s the anti siphon valve and it’s probably right at the barb that comes out of the tank get it off run it, but I think most will say replace it.

 

[thunder550]

I pulled the VST apart today since several threads I've read have referenced a dirty pump screen being part of the problem. Mine looked ok. It's gotta be the low pressure pump. Went to Oreilly a little bit ago to get some fittings to allow me to test pressure off the low pressure pump, but get home and none of them will seal correctly. Ugh. Gotta figure out plan B now. I found the Merc retrofit kit that adds a low pressure electric pump. Seems like it was designed for the cool fuel systems, but I'm getting mixed information on whether or not it will work for my VST system. My preference would be to ditch the mechanical lift pump and switch over to the electric lift pump, but I don't want to spend $400 on the kit if it's not going to work.

 

[thunder550]

Took the mechanical pump apart too since the fuel in the lube reservoir means it needs to be replaced anyway. No surprise given what came out of the fill hole yesterday, but when I separated the diaphragm from the pump side there was quite a bit of gas that came out. Checked the elliptical inside the seawater pump, it looks ok to me. I'm going to go ahead and replace just the lift pump part for now and see if that helps out. Still need to check anti siphon valve too.

 

[alldodge]

You can install a straight fitting for testing with the new pump. If all is good, reinstall antisiphon valve after cleaning and see if it stays that way

 

[thunder550]

Anti siphon valve is clean. Fuel line was empty when I removed it from the tank pickup, nothing leaked out. Normal?

 

[alldodge]

Anti siphon valve is clean. Fuel line was empty when I removed it from the tank pickup, nothing leaked out. Normal?

Yes, that is normal, fuel will drain back once pump stops pumping. Take something small and push in on the ball, it shouldn't take much to push it in. So far everything is telling me its just the mechanical pump that is causing the problem.