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Stalling 502

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Old 06-13-2010, 08:29 AM
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Default Stalling 502

I have a 94 SR271 with a 502 and I recently have to replace the diaphragm in the VST tank and since that the engine will start and idle for a few minutes then sort of stall. If it is running and I give it gas in neutral it does nothing and wants to stall. I took it to the house and connected a water hose to the inlet and tried to run it in the driveway and I could get it to rev up but then without cause it suddenly went to idle and stalled. I then replaced all the plugs and the fuel/water separator. I then started it again and it only wanted to rough idle. Before the diaphragm went bad it ran fine but when the diaphragm went it flooded the engine and fouled the plugs. I thought when I replaced the defective parts it would run as usual but it isn't. Because the tank was empty did the rails get full of air that needs to be purged? Or could it have caused a new problem in the MAP sensor or PCM? I do not have access to a scan tool so I am at at a loss as to what my problem could be. I cracked the fuel lines and have plenty of pressure or at least it appears so. Any help would be greatly appreciated.
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Old 06-13-2010, 08:32 AM
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I have a 94 SR271 with a 502 and I recently have to replace the diaphragm in the VST tank and since that the engine will start and idle for a few minutes then sort of stall. If it is running and I give it gas in neutral it does nothing and wants to stall. I took it to the house and connected a water hose to the inlet and tried to run it in the driveway and I could get it to rev up but then without cause it suddenly went to idle and stalled. I then replaced all the plugs and the fuel/water separator. I then started it again and it only wanted to rough idle. Before the diaphragm went bad it ran fine but when the diaphragm went it flooded the engine and fouled the plugs. I thought when I replaced the defective parts it would run as usual but it isn't. Because the tank was empty did the rails get full of air that needs to be purged? Or could it have caused a new problem in the MAP sensor or PCM? I do not have access to a scan tool so I am at at a loss as to what my problem could be. I cracked the fuel lines and have plenty of pressure or at least it appears so. Any help would be greatly appreciated.
is it only at low rpm's?
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Old 06-13-2010, 09:58 AM
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so far yes, once I replace the VST tank parts I had to crank it a lot to get fuel in the rails, it fired then ran rough, smoothed out and I idled for 5 minutes then tried to go out for a cruise, it would not come off idle. I made it back to the dock and could not get the engine to rev normally. That's when I took it home and replace plugs and fuel filter. BTW the filter was clean and no water, the plugs were fouled.
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Old 06-14-2010, 01:27 PM
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Is it idleing smooth or rough?
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Old 06-14-2010, 09:11 PM
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it idles smooth but when I give it more throttle it bogs and then sort of dies. It really has me confused, it ran fine before I replaced the parts. I have been searching every technical bit of info I can find. A problem is I do not have a scan tool and I am sure that would guide me in the right direction. Looks like they cost around $1300 and it would be nice to have. Can the MAP sensor be tested with a VO meter?
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Old 06-15-2010, 07:36 AM
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The scanners are too expensive for my budget, but your problem sounds a lot like what I had shortly after I bought my 27Fever. It's running a carborated (4150 holly) 502 with mild cam setup. The second time I had it out it lost power, started backfiring and missing on serveral cylinders. After I shut it down and waited a 1/2 hour for a buddy to get there, we decided to fire it up before towing me to the ramp. It started and ran just fine so I decided to head back while I had some power. Before I got half way it did the same thing again. Back at the dock I fired it up again, but this time idleing rough and wouldn't accelerate at all. Too make a long story short (kinda late for that) I did a total rebuild on the holly, changed out the plugs, plug wires, distributer cap, rotor/trigger, and the control module because they all looked suspect. It fired-up and ran really good in my drive so I took to the lake. Its been running smooth as silk and stronger than goats milk ever since. I know this stuff is pretty basic and you've probably already checked it out, but just wanted to throw it out there in case it may help. I don't know if a vo meter can test the MAP. Good luck...
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Old 06-15-2010, 08:27 AM
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it idles smooth but when I give it more throttle it bogs and then sort of dies. It really has me confused, it ran fine before I replaced the parts. I have been searching every technical bit of info I can find. A problem is I do not have a scan tool and I am sure that would guide me in the right direction. Looks like they cost around $1300 and it would be nice to have. Can the MAP sensor be tested with a VO meter?
Bogging and dying off idle is normally caused by low fuel pressure/lean condition. It is possible you have not purged all the air yet or that your pressure regulator is stuck open and dropping pressure. You should check at the schrader valve on the fuel rail and see around 30 psi at idle. You also need to see this thread :

http://www.offshoreonly.com/forums/d...an-1-00-a.html

I have actually checked codes using a paperclip and a test light also. If you have any codes let us know and we can help you through it.
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Old 06-15-2010, 07:49 PM
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Thanks guys everything is great input. We are getting our daily thunderstorm so hopefully tomorrow I can get back at it. I'll post my findings. Thanks again.
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Old 06-15-2010, 07:56 PM
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it idles smooth but when I give it more throttle it bogs and then sort of dies. It really has me confused, it ran fine before I replaced the parts. I have been searching every technical bit of info I can find. A problem is I do not have a scan tool and I am sure that would guide me in the right direction. Looks like they cost around $1300 and it would be nice to have. Can the MAP sensor be tested with a VO meter?
I have this problem some times with one of my 502's on my cruiser, sometimes after a long idle on a hot day. Don't know what makes it do it, but I can cruise on 1 engine for a while and then after about 5 minutes or so, I can give it around half throttle, turn the key, it will spit and cough and then catch and I rev it a little and then It is back to normal.

I am out of answers for this one, i am just listning so that I can maybe figure a few things on mine.
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Old 06-15-2010, 09:30 PM
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You need to check your fuel pressure when its ding the spitting and spuddering. i just went through this with my 97 502 in my sonic. was fine at idle but when it came to a load and more fuel needed the pump couldnt hang and pressure would fall off. NEEDED PUMP. just my two cents.
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