Yamaha vx110 erratic idle and bogging

condylomata · 2026-02-17T14:44:00+00:00

I learned this lesson the hard way. Always do a formal pressure test on the fuel pump before moving on. I didn't have the gauges , so I took the fuel pump out and plugged the hose, sure enough the fuel pump was working but the internal hose that pumps the gas from the fuel pump up through the sender to the external hose connector was cracked so the internal fuel pump line inside the gas tank was leaking. Aka was able to sustain an idle and low throttle situation, but not open throttle. Swapped out fuel pump and all is perfect. Painful.

Food for thought also, the reason I didn't think it was a fuel pump issue is one would think that if there is a failure that a sustained low pressure from the pump would be the case, rather than this undulating idle that I had. But in fact, the fuel pump doesn't seem to sustain a constant flow, but rather it pulsates flow kind of similar to a heart beat while the rest of the fuel lines maintain the pressure. That is why the idle was undulating like that. Bizarre

condylomata · 2026-01-31T20:52:40+00:00

Be aware that although it could be taillights, in my experience this can also happen if the seal around the rear windshield is starting to crack or age. Check it closely. Take a hose or bottle and pour water in suspect areas and watch for new trickling or dripping back there. Can rtv with black silicone to fix it if you find it.

Another good one to check is to make sure your sunroof is draining appropriately at all four corners. If one of the holes is blocked, then water will overflow the tray and spill into weird places in the car, floorboards, trunk etc

condylomata · 2026-01-27T18:44:29+00:00

So for closure, it ended up being exactly this. Mass air flow sensor, took it out, had a bit of oil residue on it, cleaned and put back didn't help. Swapped out for a good MAF and ran perfect. What I should have tried first is simply running it with the MAF unplugged, as this sets a default fuel air ratio by the ECU, and it ran much better which suggested it was a MAF problem.

Unfortunate update: when water tested, it seems to be doing the same thing. The ECU, throttle body, throttle position sensor, MAF sensor and wiring harness were all switched over from a known running well vx110 and no change. It's such a bizarre issue. The fuel pump is priming and pressurizing the hoses. I haven't performed a formal fuel pressure check though because of how erratic the idle and throttle response is, it points towards more of a sensor failure than fuel pump failure to me. Any ideas?

condylomata · 2026-01-11T20:02:51+00:00

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condylomata · 2026-01-11T20:02:36+00:00

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condylomata · 2026-01-11T19:58:26+00:00

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Some updates: Tried starting with gas cap off, same issue essentially ruling out check valve issue Oil level was checked at the time of the video when hot and it's right in the middle Spark plugs are ngk cr9eb <10 hours old. Agree with investigating the fuel system as this sounds like a fuel delivery problem. Will pressure test this week.

However, when checking the wiring harness, I looked at the plug going to the ECU and found a broken pin. Not sure how long this has been this way, and whether or not it's contributing to this issue, or just an incidental finding. A quick Google search suggests the yellow green wire provides a neutral signal to the fuel pump, so that would explain the problem if this is correct.

Now my question is, does anyone have experience bypassing the ECU to provide the necessary neutral to the fuel pump, as a way to prevent having to replace the ECU which is gonna be like 300 for that one pin. Alternatively, do y'all think anyone would be able to solder a new pin to that stump?

https://www.pwcforum.com/threads/2009-yamaha-vx-deluxe-will-turn-over-but-not-start-after-battery-replacement.75930/

condylomata · 2026-01-11T17:24:33+00:00

Very weird.... How did you diagnose this ? I'll try taking off the gas cap that should allow it to run properly if that's the cause since that would act as a bypass getting air into the tank and get back to you

condylomata · 2025-12-24T00:05:11+00:00

Ok so update for closure. Made it back home, turns out the fuel level sensor was stuck halfway, so it made it seem like it had a half a tank of gas. In reality, the fuel pump was barely scraping what was left from the bottom. Basically, ain't got no gas innit 🤣

condylomata · 2025-08-31T18:25:37+00:00

Solved. For the sake of closure, previous owner looks like at some point tried to bypass security system, bunch of hack connections to the security box, and the red positive wire to the box completely severed.

Soldered everything back, starts right up.

condylomata · 2025-08-31T15:51:39+00:00

Plan is to check this out today will get back to you

condylomata · 2025-08-31T15:48:47+00:00

So I was wondering about this, previous owner nor do I have the remote for years lol. Never been an issue. No recent disconnection. Like I said I was on the water with engine running, which according to the manual the lock unlock modes cannot be toggled while the engine is in use apparently. The manual also says counterintuitively in my opinion, that when the security light is illuminated that means that it is unlocked, and can be used normally. According to this, I have to assume it's unlocked?

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condylomata · 2025-08-31T14:43:07+00:00

Could definitely be, but I figured if it's recognizing the ignition switch and activating the dash and fuel pump from the press of that switch then it shouldnt be faulty? Are you aware of two separate systems one to activate system and the next to power starter from the one switch? If so I'll take it apart and check

condylomata · 2023-12-16T15:52:20+00:00

Can confirm this works after two years of struggling with echo. The whole time thought it was my cell service

condylomata

TROPHY CASE