502 bogging badly for first 20 or so

[Ssva13]

Hey guys,
I have a merc 502 magnum with multi port injection. For the last month or so it's been bogging down badly (not even enough power to get on plane) for the first 15-20 minutes of leaving the dock. It'll rev no prob but once back under load it bogs again. After about 20 or so minutes it'll start running fine. To make it more annoying, once I'm out on the water, I can shut off for a few hours and it'll run fine when started up again. It only does this after sitting for couple days. I've spent years building and tracking big and small block Chevy motors and the closest thing to resemble this is a vapor locked carb which it obviously isn't. Can't figure it out...any suggestions are greatly appreciated!

 

[tpenfield]

:welcome:
to iBoats . . .

You will probably have to hook the computer diagnostics up to it and check for any codes, as well as all of the sensor inputs while running and even under load. Since it seems to be temperature dependent, it could be either the ECT or the IAT sensor, but it gets frustrating just taking a shot in the dark with sensor replacement.

Another thing that you can check is the fuel rail pressure to see if somehow that is not up to spec when the engine is cold. You will want to post the serial number of your engine, so additional advice can be given more precisely per your engine, as there were a few changes along the way for the Merc engines.

Of course most folks don't have the computer program or even the fuel pressure gauge in their tool box, but paying a mechanic to do these things can quickly become greater than the cost of getting them.

MPI and EFI engines run great, until they don't. Then there is hell to pay.

 

[Scott Danforth]

my guess is the IAC is not moving, however like Ted mentioned, without plugging in and seeing the diagnostics, its a shot in the dark.

 

[Ssva13]

Thanks for the input. You're right...the only diagnostic readers and programs I have are for autos. I'll post the serial number as soon as I get out to the dock.

 

[Ssva13]

I might be onto something....a couple weeks ago the batteries where so drained it took 2 boost packs to start her up. Once running the juice gauge read 13 but after shutting it off after running for a bit in my slip the batteries were dead again. OK, so they were a few years old so I pulled one at put in a new one. No battery issues for next couple trips until yesterday. Boat started up and I went to gas dock where I needed a boost. The battery gauge hit a high of 11 then rapidly dropped below 10. Obviously my alternator went and now I'm wondering if it was starting to fail and going in and out for a few weeks. It's my understanding that marine fuel pumps run directly off the alternator...could this cause the low power I described in my original post? Guess I should have kept a closer eye on my gauges!
Oh, the engine serial # is 0k173437

 

[tpenfield]

The fuel pump(s) run off of the engine power system, not specifically the alternator. But, if your alternator is not charging the batteries, then they will die after a while of running. Another thing to check would be it you have something that is draining the batteries - short circuit or something left on.

​I don't think that the electrical issue is the cause of your engine performance problem, unless the engine runs well with freshly charged batteries and does not otherwise.

 

[bentle]

I have this same problem with my 502 magnum same motor. What I have been dealing with is vapor locking issues in my fuel rail as well as the canister where my electronic fuel pump resides. Right on top of my canister I can open what looks like a nut or bolt and bleed off that pressure to get her running and running smoothly again or at least until it locks up again. I've read there is a diaphragm that needs to be turned over in these because they were put in upside down and will not allow pressure to bleed off normally. I'm also going to check for water in my fuel but these motors are in fact famous for the vapor lock issues.

 

[Ssva13]

Interesting. Does your issue go away after about 15-20 minutes of run time too? This is driving me nuts! Any idea where this diaphragm is?

 

[bentle]

Yes I get the same issues your having. If I'm running at a lake such as Lake Powell where it gets 100+degrees outside it will shut down completely until I either bleed off the pressure or let it cool down. I will try to get a photo of the part but if you can locate your electronic fuel pumps housing it resides in there.

 

[bentle]

No Title

 

[alldodge]

That's a VST and if your running and it gets where the tank has vapor in it, then I think you have a problem with your low pressure pump. Its pushing some air with the fuel due to a leak or drawing a vacuum. The isn't a seal which needs to be turned over, its just a bowl with a needle and seat like a carb which the high pressure pump sits in.

If you stop after a run and it heat soaks then you can get vapor lock and while there is also a fix for that, none should start running out of gas just running down the lake in 100 degree heat

 

[bentle]

Only locks up after it's been sitting in the heat or after a hard run on the lake. If we try to go at that point then either it's real sluggish for a bit then take off or takes off then shuts down completely. Either way it won't run right until the pressure is down or cooled down, way down I might add. There is a diaphragm under that cover which is suppose to bleed off the pressure.

 

[alldodge]

Only locks up after sitting for awhile. If we try to go at that point then either it's real sluggish for a bit then take off or takes off then shuts down completely. Either way it won't run right until the pressure is down or cooled down, way down I might add.

OK, that's vapor lock and there is a fix, but since this isn't along the OP issue I'm going to leave it unless the OP is ok with it. Otherwise if you post another thread I can explain

 

[bentle]

Hey guy's there was a service bullitin on VST tanks in 97 that fixed this same problem. The only reason I know this, is I had to apply the fix on my boat when I bought it used. I got a good deal on my boat because it had this problem and no one could fix it. Of course the seller denied any knowledge of the problem.
Anyway, Mercruiser changed VST tank top in 97 and the air passage lines were different, so the spring was installed on the wrong side of the vent diaphram. All I had to do is get a o-ring seal kit and pull the top off, then pull the little plate holding the vent diaphram on the upper housing. After inspection, the spring was holding the diaphram closed all the time, so it would never vent, and in effect vapor lock. Put the spring on the opposite side of the diaphram and reassemble, and never had a problem since then.
The spring must be on the side of the diaphram, that has the vacuum hose orifice. When the motor starts, the vacuum opens the vent port in the VST tank, to release the pressure buildup due to heat soak in the tank.
If your boat has the incorrect setup from the factory and nobody has fixed it, then it could happen to both motors at the exact same time. I eventually found the fix in the VST overhaul section of my mercruiser manual.

BTW mine did the exact same thing as yours. I figured it out to begin with, by rigging up a fuel pressure gauge on the inlet fuel line. As the motor sat, the fuel pressure started increasing to about 10 psi inside the VST tank. Then after starting the motor the pressure stayed there and after the electric fuel pump sucked the VST tank dry then the motor died. The mechanical pump only pumps like 5 psi, so it can't pump against the 10 psi in the tank.
Sorry for the long post, but I'm pretty sure this is what your problem is. I spent countless hours figuring this out.
Hopefully you get it figured out, because the hotter it gets outside, the more this will happen.
OH, a simple test would be to let it set after running to build up pressure and then take out the inspection plug on the top of one of VST tanks to release the pressure and then put it back on. Then drive it and that motor will not die, but the other one will.
(This was copied and pasted here, I stand corrected about the diaphragm needing turned. It's the spring itself that needs moved.)

 

[alldodge]

I might be onto something....a couple weeks ago the batteries where so drained it took 2 boost packs to start her up. Once running the juice gauge read 13 but after shutting it off after running for a bit in my slip the batteries were dead again. OK, so they were a few years old so I pulled one at put in a new one. No battery issues for next couple trips until yesterday. Boat started up and I went to gas dock where I needed a boost. The battery gauge hit a high of 11 then rapidly dropped below 10. Obviously my alternator went and now I'm wondering if it was starting to fail and going in and out for a few weeks. It's my understanding that marine fuel pumps run directly off the alternator...could this cause the low power I described in my original post? Guess I should have kept a closer eye on my gauges!
Oh, the engine serial # is 0k173437

You might be on to something, if your bats are weak they may be having issue running the fuel pump and ECM. after it has run long enough it may build enough charge to keep things going until it has sit for a long period like over night. Suggest checking further into ALT and battery load test

bentle your having the trouble after it has been running and it sits for a while. The OP has a problem when its first fired up (cold engine) and does not have an issue after he has shut it down and restarts a couple hours later.

 

[Ssva13]

Just got the new alt and (weather permitting) will be installing today. Hopefully that'll work as it seems the easiest issue to fix. If not I'll be looking into the diaphragm and spring issue. I'll post when done. Thanks again for the guidance!

 

[Ssva13]

Finally got the alternator in and installed and took a test run. No lagging or "vapor lock" symptoms. Looks like it was alternator/insufficient power issues.

 

[bentle]

Interesting, guess I'll be testing mine as well just to rule that one out. Glad your up and running.