DI Air Pressure Troubleshooting

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Active Member
I know the forums are quiet this time of year but I am back at work on my DI rebuild that started in Sept 2019. Long story short, installed a new engine and was breaking it in and things were going well. It had an occasional miss but ran good otherwise. On the first WOT run it went like crazy but as soon as I let off the throttle, it started running really rough at low RPM. Tons of troubleshooting later, I think the issue is low/fluctuating air pressure at idle. This same fluctuation is showing up in the fuel pressure. Whatever the cause, it results in the ski not idling well and it will not idle on the PTO cylinder only. MAG only idle is fine. At higher RPM's, the ski will run on PTO only. Everything has been swapped between the cylinders (spark plug, fuel injector, air injector, ignition coil), with no change. Compression is 120 on both cylinders, which seems consistent with SES rebuilds.

Video of air pressure at idle and varying RPM's:

The air pump puts out >110 psi dead head. I used a tire inflator to put air to the fuel rail and it regulated at 78 psi (correct), but the engine still would not idle on PTO. It *might* have sounded slightly better at idle but really unsure. My latest attempt to fix was replacing the air pump head (part w/ reeds) and gasket... no change.

My next thought is to replace the fuel rail/air regulator. Perhaps it is correctly regulating at higher flow rates but for some reason not at lower RPM's.

Any suggestions from folks based on the video or thoughts before I continue throwing money at it?


Well-Known Member
If fuel pressure is not 107 at all rpms, it won't run right. The air pressure won't affect the fuel pressure and air pressure looks fine. Sounds like a fuel pump problem.
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