Ok, 496 Merc Mechanics : Fuel pressure?

[bigkatboat]

The ECM puts out the information, and this is what the DDT sees. The ECM may be putting out the correct signal but if the IAC motor is not operational, the motor won't idle. Put the DDT back on the motor in a static test mode. Run the IAC jog test and put your hand on it while doing so. You will 'feel' it moving (or not), and this will lead you to replace the IAC motor. The "new style" (remote) IAC motors don't have great track record. When the ECM puts out the correct signal, but the IAC motor does not respond, you will NOT get a failure code. The DDT sees the signal, not the actual actions or results. Good luck, and let us know what happens.

[shadow]

No, I didn't. I mainly wanted to see if it was fluctuating or not. I have heard of plugged fuel systems causing the FP to "search" and have erratic readings all over the scale. It was rock steady. It jumped up each time I romped on it, showing the regulator was responding to vacuum readings.
I will try that next. I disconnected the fuel gauge, as I was going to take it to the lake tomorrow. Looks like my plans may change, due to some illnesses in the family members that were supposed to go. Anyway, next plan is to change plugs, read the old ones, and put the FP gauge on again. After all that, I will disconnect the vac line and go from there. I would
hook up a gauge,disconnect the vaccum line and get a good reading before chasing or guessing.May be a simple fact of the pressure is too high.It happened on mine.only mine would load up after long idle,but did get the soot,Turned the pressure down and problem was gone.good luck.

[Beer-30]

I do know that on the 502 EFI if the idle air motor setting or counts are off it will cause this. I am not up to speed on the 496 and how to set though sorry. Check out the link it has more info. Problems are with a procharged 500 EFI but you could pick up something.
http://www.offshoreonly.com/forums/s...ht=setting+IAC
Thanks Paradigm. All the readings will probably be similar, but the IAC motor is on the back of the intake on 496s. It only has a clean air tube (hose) running from the TB back to the motor itself.
I should, however, probably look at the TB blade set screws. If it or them has or have come loose, this would cause similar situation. Just one more thing to check. I really like trying to trouble shoot these things myself as it get one in depth with their motor.
TPS volts were .53 at idle when I scanned it last.

[Beer-30]

I would
hook up a gauge,disconnect the vaccum line and get a good reading before chasing or guessing.May be a simple fact of the pressure is too high.It happened on mine.only mine would load up after long idle,but did get the soot,Turned the pressure down and problem was gone.good luck.
Will do. I saw the adj screw we were talking about at the river. It actually looks like it has been tampered with, as the paint is boogared already.
I assume one would disconnect BOTH vac lines? the one on the upper (factory) reg and also the Merc one down below?

[Beer-30]

The ECM puts out the information, and this is what the DDT sees. The ECM may be putting out the correct signal but if the IAC motor is not operational, the motor won't idle. Put the DDT back on the motor in a static test mode. Run the IAC jog test and put your hand on it while doing so. You will 'feel' it moving (or not), and this will lead you to replace the IAC motor. The "new style" (remote) IAC motors don't have great track record. When the ECM puts out the correct signal, but the IAC motor does not respond, you will NOT get a failure code. The DDT sees the signal, not the actual actions or results. Good luck, and let us know what happens.
Cool, thanks. That sounds real promising. This surging has come on slowly and is pretty constant now. I was waiting for whatever was causing it to fail, so it would set a code. So far, it has not done that. It has gone from doing it every now and then, usually after running at cruise - coming down to idle. To pretty much every time out of cruise, but would stabilize at idle. To now, not stabilizing at idle at all. That led me to believe that some moving part was not moving anymore - like the IAC motor - since it is idle specific.

[shadow]

Will do. I saw the adj screw we were talking about at the river. It actually looks like it has been tampered with, as the paint is boogared already.
I assume one would disconnect BOTH vac lines? the one on the upper (factory) reg and also the Merc one down below?
you just need to disconnect the one at the upper regulator.to adjust it it is a small tamper proof torx bit.I think it's a t15.I've got the bit at work i'll look at it tomorrow to be sure.sonds like maybe when you had it in the shop before maybe someone has been in there messing with it.if they made an adjustment with the vaccum line connected,the setting will be wrong.Hopefully it's that simple,very well could be!with the line connected it can make as much as a 10lb difference which would load up yor plugs and cause runability concerns.You might want to pull one plug and see if it looks fuel fouled.

[Beer-30]

you just need to disconnect the one at the upper regulator.to adjust it it is a small tamper proof torx bit.I think it's a t15.I've got the bit at work i'll look at it tomorrow to be sure.sonds like maybe when you had it in the shop before maybe someone has been in there messing with it.if they made an adjustment with the vaccum line connected,the setting will be wrong.Hopefully it's that simple,very well could be!with the line connected it can make as much as a 10lb difference which would load up yor plugs and cause runability concerns.You might want to pull one plug and see if it looks fuel fouled.
If it was tampered with, it was before I owned it. But that has only been since Jan / this year. And I am with ya on the adj. Every time an adjustment is made, it has to be done w/o vacuum signal. I left the cover off, so it is easy to continue my shooting. If we don't end up at the lake tomorrow, I will slap the gauge back on it and put the water to it. I'll let ya know.
I checked one plug shortly after I got it, when I started looking at the soot issue. It was black around the outer thread area and on the ground "hook". The electrode itself was clean and white - as was the initial area of the porcelein around it. The rest of the porc toward the "inside" of the plug was black. I'll pullem all and look, keeping them in order incase one or two look worse than others.
If it was a Ford, you would probably have more options for me, eh?

[Beer-30]

Ok, changed all plugs to Denso Irridium. All looked pretty good. Still white on the porcelain, dk brown or black on rest of areas. All were firing. None were wet. Doesn't appear any injectors are stuck open.
Taking the vacuum line off of the top reg while running put FP from 36 to 40. Finger over the unplugged line puts it back to 36 - obviously the lower reg is working also.
Going to change fuel filter and check screen at inlet of boost pump next. After that, if still not right, it goes to dealer.

[shadow]

Won't help with your soot problem but you might want to check this for your throttle position concern/idle.Take off your air filter,look in the throttle body and check to make sure it is just not a matter of adjustment of the cable.Check both ends of the cable all the way open and closed.

[Beer-30]

Ok. Fuel pressure good.
Cable, throttle assembly good. Full open, full closed.
Changed fuel filter.
Cleaned inlet screen at inlet to boost pump. Some small metal shavings from the tanks, but nowhere near blocked. No other contaminants found.
Irridium plugs smoothed things out a little bit. During the surge at idle, it doesn't die anymore. It used to die on the down side after a few up/down cycles. Now, it just keeps cycling a little smoother. 4-500 RPM instead of 7-800. Still, crack the throttle forward about 3/4 and it smoothes out to steady 800 or so. Just weird. I still suspect IAC. I am at the limits of home tinkering - off to Merc dealer.
Will keep updating.