F115 won't rev past 4,000 WOT

[fourvalv]

2005 F115TLR
Low Hours-Low Usage
Starts fine, idles fine
Will NOT pull past 4,000 WOT
Can 'sneak up on it' and get it to about 4,000, maybe a bump over. If I 'throttle down' to WOT, it'll typically just seem like it is tied to an anchor and hold at about 3800.
Neutral revs, can easily blip past 5,200.

To date:
-Replaced small screen filter in VST
-Then replaced entire VST, high pressure pump, etc. with new part (YAM) (Ran exactly the same with new VST as old)
-New Plugs
-New Filters
-(also new fuel pickup in fuel tank as well as new line from tank to engine connection)

39psi at fuel rail, idle and WOT

The current 'lean' (based purely on forums) is 'injectors'. new pieces seem extremely expensive, but remans/refurbs are at least attainable, but no need to just throw even more parts on it unless others think it may be the issue.

I was hoping I was going to get a fuel pressure drop at WOT to indicate it might be the low pressure pump, but seems to stay steady at about 39

Injectors?

Other?

Thanks for any guidance.

[fairdeal]

if you have measured rail pressure of 39 psi at 4,000 rpms
then the only "fuel" thing that can be holding back is injectors.

You can get them tested & cleaned for ~$20 each

(personally I like to know how they tested, before cleaning-
if they test at 100%, that wasn't your problem)

Alternatively,
I wouldn't hesitate to buy 4 "Ebay" injectors
for $40 if I could find them.

[fourvalv]

Thanks for quick response. I'm ordering a set of remans. And if I can find a place to test and clean the originals, I'll do so and let you know the results if possible. I'm open to injector 'rebuilders' if there any recommended. Again, thanks for quick response.

[fourvalv]

Original Poster:

I am using same post, rather than starting new. if this gets no response, perhaps I'll start new thread.

1) I replaced injectors with cleaned injectors, no difference. Actually maybe a little worse. Will NOT REV UP at WOT under load.
2) CORRECTION TO EARLIER STATEMENT: I got a different fuel pressure gauge and fuel pressure is NOT holding firm at WOT. it is dropping, directly in correlation to the RPM at WOT. It starts and idles around 38-39PSI. Roll throttle up a bit, maybe 1500-1800, still stable. If I throttle down, FP psi starts dropping. It seems like it is just 'bogging down'. But FP is dropping significantly. It'll drop all the way to around 10-15psi.

The VST and all internals are brand new YAMAHA parts.

The Injectors are 'OEM cleaned/rebuilt' from a company that had a good reputation/rating.

Could this be a bad low pressure fuel pump? I don't want to just throw parts at it, but something is causing a drop in FP and the usual culprit (VST) should be fine since it is totally new.

Thanks for any help.


(Fuel tank was drained and new fuel added prior to the initial process of first post last weekend.)

[99yam40]

I would think the pressure drops because the VST is running out of fuel.
bad lift pump, restriction, sucking air, pressure relief valve recirculating too much fuel, are some things I can think of right off hand

test what you can before buying parts

[fairdeal]

Agreed,
if the starvation of the rail is not the from the screen in the VST,
then the next most likely cause, is fuel level inside the VST running low.

(Although the F115 - and other engines with mechanical lift pumps-
doesn't use a recirculating pressure relief valve).

Using the primer bulb as a diagnostic tool can be very informative,
although it requires another pair of hands:
someone to drive the boat while the troubleshooter concentrates on the bulb.

It should be possible, by squeezing the bulb, to assure the VST is kept full.

If the VST is kept full, and the engine runs fine,
that confirms the problem is not 'in' the VST or high pressure system.

If the VST is kept full, and the engine still runs poorly,
that eliminates the entire low pressure system.

If unable to keep the VS full,
then the behavior, and sensation, of the bulb
points to the location, and nature, of the problem.

[99yam40]

I would think the pressure drops because the VST is running out of fuel.
bad lift pump, restriction, sucking air, pressure relief valve recirculating too much fuel, are some things I can think of right off hand

test what you can before buying parts

well I tried to figure out how to strike a line thru that recirculation valve statement but cannot figure it out.
thanks fairdeal to bringing up that the F115 does not have one
apparently I was not thinking too well when I put that in

[Back2bowhunting]

I’m following this as I’m having similar problem with 225 ox66. I can’t seem to get above 4800 rpms.

based on what I was suggested by mechanic, I replaced the low pressure pumps and gaskets in addition to fillers etc like you’ve done. My LP pump gaskets were in bad shape and that helped me get more rpms. Still not working like it should though.

[pstephens46]

(Although the F115 - and other engines with mechanical lift pumps-
doesn't use a recirculating pressure relief valve).

But the cam driven fuel pumps still have too “loop” the fuel as the VST float pin closes. So this system does not use a pressure relief valve to do this? I always assumed there was one on my motor.

[fairdeal]

But the cam driven fuel pumps still have to "loop" the fuel as the VST float pin closes.

No, no looping.
The diaphragm just takes the strain of "deadheading"

[pstephens46]

No, no looping.
The diaphragm just takes the strain of "deadheading"

I did not realize that. Thanks

[gtreanto]

We had the same issue with our F115. ultrasonic cleaning of the injectors and new VST filter fixed it. We have twin F115's and both will go over 5K RPMs now. Max before was 4.8K.

[fourvalv]

I'm original post. Thought I'd do an update.

Replaced inline Filter to no avail, so now all filters changed. Fuel pressure holds solid at 40psi at 4,000, runs smooth. as I move lever forward, fuel pressure starts to drop and it starts to bog. If I romp it at lower rpms, it bogs and will drop to about 20 psi, even if running only 3,000.

Diagnostics shows Throttle Position Sensor (TPS) at "75%" when throttle is wide open.

I rotated TPS as far as I could and got it to 85.5% on diagnostics. I took it back out and it got to about 4,700 rpm, runs smooth. If I move it further, it starts to bog.

Point being, that rotating TPS, even if only showing 85.5% on diagnostic, it DID pick up 600-700 rpm, with no other change. The steady RPM was easily replicated, as was the bog if I go beyond that point on throttle lever.

Is it feasible that the ECU is seeing "75%" (or 85%) throttle position, but the fully open intake is giving full blast of air?

Would it make sense to try a different TPS on it?

Thoughts? Thank you,

[99yam40]

you need to find out why the rail pressure is dropping.

[Ferris11]

I've been following this thread as I had a similar problem with my 2006 f90TLR years ago which turned out to be bad fuel and the clogging of the vst filter. I see that you changed that filter as well as the VST tank, pump, fuel line etc. When you did that did you also run the motor from a clean external tank that had new uncontaminated fuel ? If you didn't and used the fuel from your original tank, you may have re-contaminated your fuel which could have plugged up that VST filter again.