2001 225VX shuts off at WOT

[Aggiedave]

Inherited 2001 225VX OX66. Likely no real maintenance done other than gear lube. I have performed some additional basic maintenance. Replaced water pump, gear lube, spark plugs, thermostats, low pressure/high pressure fuel filter. Three new low pressure fuel pumps on the way.

Engine runs OK but not great. Frequently stalls at idle. Seems to idle a bit rougher than I would hope...but all in all, not terrible. Dig out of the hole, planes and runs well until about 5100 RPM. The motor shuts down like someone pulled the emergency lanyard. Sometimes it has to be restarted. Sometimes not.

From research, this sounds an awful lot like a fuel issue. GIven the expense, I'm really hoping it's not the high pressure fuel pump. I also want to test the O2 sensor. Pretty sure it's never been tested or cleaned. However, I'm willing to test. That said, I'm not sure how. I have a fuel pressure gauge. I have a good Fluke multimeter. However, I don't know what testing harnesses I need to test the right systems to diagnose this problem. Any advice would be greatly appreciated.

[99yam40]

I would install the 3 new lift pumps and see if that solves the problem.
sounds like the old ones could not pump enough fuel

[DennisG01]

With a pressure/vac tester, you can verify the LP pumps - but they're a maintenance item so it's probably not bad to go and do it, anyways. I don't remember the spec offhand - but it's in the service manual (which I don't have where I am right now)... probably good to get one if you're going to do your own wrenching. Same goes for testing O2 sensor - although I don't think that's your problem. Quick test when it shuts off... drain the fuel from the VST tank into a jar - if it's a lack of fuel then there's going to be very little draining out.

[Aggiedave]

I have the service manual with all the specs and procedures. I'm square on the 3 low pressure fuel pump. Its testing the O2 sensor. The test light is on the way as well, but research would indicate that it doesn't monitor the O2 sensor. I've seen other people refer to a testing harness for the O2 sensor, but nobody ever mentions a part number or shares a link. Where might I find an O2 test harness?

[DennisG01]

There are tests listed in the service manual that can be done with your DMM (those are the only ones I've ever done). You need to remove it - but you should do that, anyways, to check for it being carbon'd up and clean it.

[Aggiedave]

I saw the test in the shop manual...but its the flame test. I'm told that is a garbage test and I shouldn't do it. I'm told that test is worthless and could damage the O2 sensor. So how else should I test the O2 sensor?

[DennisG01]

It's probably not the "best" test, but it does check to see that it's operating properly - by seeing the quickly changing values on the DMM. I did that test when I first bought this engine about 6 years ago and again about 2 or 3 years ago. So far so good.