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Help Chasing Erratic Idle Please

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Old 01-07-2019, 08:24 AM
  #41  
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Originally Posted by Tractionless
I've been working on this for over the past year with scans etc. Upon the advice of another member today I pulled the vac. line to the fuel pressure regulator while the 1k rpm surging was taking place and it immediately stopped the surging. Even though fuel pressure reads good with the vac line attached at all times, including during surging I may be leaning toward replacing the FPR. I will update the thread with my findings.

Keith
I was having some hard start/surging idle issues on my 502 MPI although it is a Mefi3/cool fuel model. Chased TPS and IAC first. Checked Fuel Pressure and found to be only 30 psi, regulator was bypassing and pump wasn't building any static pressure and bleeding off quickly. Replaced regulator and instantly idled and started cold with touch of the key. Didn't miss another beat all season.

I think you are on the right track.
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Old 01-07-2019, 08:07 PM
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Originally Posted by Gimme Fuel
I was having some hard start/surging idle issues on my 502 MPI although it is a Mefi3/cool fuel model. Chased TPS and IAC first. Checked Fuel Pressure and found to be only 30 psi, regulator was bypassing and pump wasn't building any static pressure and bleeding off quickly. Replaced regulator and instantly idled and started cold with touch of the key. Didn't miss another beat all season.

I think you are on the right track.
Nice, thanks it's been a long road switching parts from side to side, getting ECU's to agree with logging software which required removing them from the boat, shipping and dropping $200 for modifications to them, throwing new parts at it etc. My fuel pressure has always been per the manual there's obviously an issue with that regulator.

Keith
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Old 02-17-2019, 02:01 PM
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Replaced the fuel pressure regulator with a new Mercruiser unit and the issue persists.

Keith
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Old 02-17-2019, 02:50 PM
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The VST plenum flooding comes into play after a warm engine has sat for a period of time allowing fuel to percolate out of the aged VST internal diaphragm thru the yellow hose and into the plenum.
Don't agree, at no time should fuel be in the yellow hose.
My prior build used the VST and I removed it with current build. The VST is prone to vapor lock, and mine did VL at times, but no time was fuel present in the line

Your pressure is 37 psi, and when black smoke comes there is no fuel in the over flow line?
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Old 02-17-2019, 10:54 PM
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Originally Posted by AllDodge
Don't agree, at no time should fuel be in the yellow hose.
My prior build used the VST and I removed it with current build. The VST is prone to vapor lock, and mine did VL at times, but no time was fuel present in the line

Your pressure is 37 psi, and when black smoke comes there is no fuel in the over flow line?
We DO agree. Note I said 'aged', or deteriorating internal VST diaphragm. Under normal condition there should be NO fluid in the yellow hose. But when the internal diaphragm within the VST fails that fuel will percolated through the yellow hose into the manifold. That is how it is designed. This bleeds down pressure and floods the motor. The starting symptoms with this condition are identical to the vapor lock condition. In fact you could have both conditions going on which compounds the trouble shooting.
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Old 02-23-2019, 11:13 AM
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Not sure how the VST came back into play but it was fully rebuilt less than 60 hrs. ago which is about 2 years. I also use non-ethanol fuel and the issue will happen prior to shutting the engine off. I have new yellow hose and zero liquid in it.

Keith
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Old 02-23-2019, 11:38 AM
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Only reason to ask about the VST is because I don't remember ever seeing what the pressure is, only that it was rebuilt and pressure is ok
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Old 02-27-2019, 02:48 PM
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Originally Posted by AllDodge
Only reason to ask about the VST is because I don't remember ever seeing what the pressure is, only that it was rebuilt and pressure is ok
Gotcha. In spec with the manual and fluctuates with the surging. Sorry I don't recall the spec. Value anymore, it's been awhile. When the FPR vac line is pulled the issue stabilizes. That's why I changed the FPR, issue persists. Remember this isn't exactly at idle but rather at partial throttle around 800 rpm and the tach bounces in a 200 rpm window when it is at its worst.

Keith
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Old 02-27-2019, 03:35 PM
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With it subsiding when vac is pulled from FPR, keeps me wondering about the fuel pressure. Pressure increases when line is removed and a small increase in air due to leak

VST should be at 37 psi
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Old 04-19-2019, 08:44 PM
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Originally Posted by AllDodge
With it subsiding when vac is pulled from FPR, keeps me wondering about the fuel pressure. Pressure increases when line is removed and a small increase in air due to leak

VST should be at 37 psi
Idle fuel pressure is 28 psi with the vac line on and 30 with it off. Brand new 805227A1 Merc. Regulator which per Crowley Marine's fiche is the correct unit for a VST equipped engine.

I see in the manual where the VST equipped engines are to be between 34-36 and cool fuel 28-32. For what it's worth the other engine is at 30 psi and runs fine.

So I'm guessing the thinking now is both VST electric fuel pumps are on their way out?


Last edited by Tractionless; 04-19-2019 at 08:52 PM.
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