502 MPI - Rich, sloppy idle. Fuel Pressure?
06-02-2018 | 09:50 AM
#1
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From: Virginia Beach
502 MPI - Rich, sloppy idle. Fuel Pressure?
Happy Saturday! Hope most of you are out somewhere on the water today.
I’ve got a problem with one of my 502’s, and I’m thinkng it may be fuel pressure related.
My port engine runs super strong. It’s idle sounds great, throttle is crisp, and when doing my spring tune-up, the plugs looked perfect. No complaints. Starboard engine, not so much. The idle is rough and choppy, throttle response is lazy, and almost sounds like it has a miss until you rev it hard to clean it out. This symptom seems to show up right before winterization.
I did a tune up (cap rotor wires plugs). There were a couple of bad wires (routed incorrectly and burned) and the cap/rotor needed replacement so I was hoping that would help. The plugs for the most part came out black and slightly wet (fuel). Got her all back together, and no change.
Hooking up my scan tool, I only found one fault for low oil pressure (stalled). Looking at the running parameters, readings looked normal with exception of the IAC which was 95 at idle. This leads me to think that there’s a pressure problem, and that the IAC is trying to let in more air to solve for a rich condition (keeping idle at 600).
I have not gone through the manual yet, but to me this sounds like a fuel pressure issue. I’m going to hook up a gauge to it today if I get some time. If the regulator is bad, how hard is it to replace?
Another note is that fuel pump for that engine sounds different at startup (higer pitched).
On the right track?
I’ve got a problem with one of my 502’s, and I’m thinkng it may be fuel pressure related.
My port engine runs super strong. It’s idle sounds great, throttle is crisp, and when doing my spring tune-up, the plugs looked perfect. No complaints. Starboard engine, not so much. The idle is rough and choppy, throttle response is lazy, and almost sounds like it has a miss until you rev it hard to clean it out. This symptom seems to show up right before winterization.
I did a tune up (cap rotor wires plugs). There were a couple of bad wires (routed incorrectly and burned) and the cap/rotor needed replacement so I was hoping that would help. The plugs for the most part came out black and slightly wet (fuel). Got her all back together, and no change.
Hooking up my scan tool, I only found one fault for low oil pressure (stalled). Looking at the running parameters, readings looked normal with exception of the IAC which was 95 at idle. This leads me to think that there’s a pressure problem, and that the IAC is trying to let in more air to solve for a rich condition (keeping idle at 600).
I have not gone through the manual yet, but to me this sounds like a fuel pressure issue. I’m going to hook up a gauge to it today if I get some time. If the regulator is bad, how hard is it to replace?
Another note is that fuel pump for that engine sounds different at startup (higer pitched).
On the right track?
Last edited by TomZ; 06-02-2018 at 09:53 AM. Reason: GD autocorrects
06-02-2018 | 03:41 PM
#3
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From: Virginia Beach
Stumped now.
Had to remove the valve so that I could hook directly to the fuel rail (not enough fiver strength to tighten up the line). About 33 pounds running. Clipping closed the vacuum line to the regulator makes no change. To verify operating, I did the same on the port engine and got just a tick under 30 pounds at idle. No change when clipping the vacuum line on that side either. I probably need to rig up the tester differently.
Port is great. You can gag it, go back and forth on the throttle rapidly, and it recovers with no issues. Starboard get pissed off and tries to stall.
About to order a set of manifolds and carbs and be done with this finicky set-up (too many horror stores for me).
Had to remove the valve so that I could hook directly to the fuel rail (not enough fiver strength to tighten up the line). About 33 pounds running. Clipping closed the vacuum line to the regulator makes no change. To verify operating, I did the same on the port engine and got just a tick under 30 pounds at idle. No change when clipping the vacuum line on that side either. I probably need to rig up the tester differently.
Port is great. You can gag it, go back and forth on the throttle rapidly, and it recovers with no issues. Starboard get pissed off and tries to stall.
About to order a set of manifolds and carbs and be done with this finicky set-up (too many horror stores for me).
06-02-2018 | 03:55 PM
#4
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Joined: Feb 2001
Posts: 4,290
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From: Virginia Beach
Any thoughts on ignition? Maybe the module is going out?
Left it running on the hose just above idle, engine sped up, then went down. Then sped up, then died. When I tried to restart, but I had to give gas to keep running. Then it stalled. Then restarting again, giving gas caused it to stall like it was out of gas. Then it started again, and now runs.
After all that, if I keep rpm at 700 (on the tach), throttle stays crisp and everything seems fine.
What.
The.
F.
?
Left it running on the hose just above idle, engine sped up, then went down. Then sped up, then died. When I tried to restart, but I had to give gas to keep running. Then it stalled. Then restarting again, giving gas caused it to stall like it was out of gas. Then it started again, and now runs.
After all that, if I keep rpm at 700 (on the tach), throttle stays crisp and everything seems fine.
What.
The.
F.
?
06-02-2018 | 04:34 PM
#6
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From: Virginia Beach
I think the reading has to do with the gauge. Per above they were both doing about the same. The port engine posted numbers just a touch lower than the bad one.
Neither respond to closing off the vacuum.
Neither respond to closing off the vacuum.
06-02-2018 | 09:11 PM
#10
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You have an injector or VST issue. IF, and I say IF you have the VST it is venting fuel into the manifold causing the issue you describe. This is due to a ban diaphragm in the top of of the VST. IF you don't have the VST check the injectors. One is likely hung open. Sloppy idles conditions on these motors is nearly always fuel from my experience.
