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MCX 240 Lean AFR Under Boost

MatthewW14

Veteran
Joined
Mar 20, 2016
Messages
31
Age
35
Location
Nova Scotia
Country
Canada
Snowmobile
2010 Nytro MTX converted to XTX
I have a 2010 Nytro with a 240 MCX kit on it with the head shim. I had it out yesterday and it started running lean. Idle was fine 14, then cruising along up to 0 boost was 12-12, then as soon as it started building boost it leaned out. As the boost built it leaned down up to 17-18 @ 10 psi.

At first it seemed like one of two things

1) extra injectors were not opening when they should
2) Fuel pump couldn't supply fuel demand when under boost.

The troubling part is that is didn't miss a beat. Didn't bog, run rough, cut out. It ran flawlessly even when AFR were in the 17-18 range. I am now thinking leak in exhaust before the AFR sensor.

I'll pull the exhaust off tonight but I wanted to check first where I start to trouble shooting the first 2- injectors or fuel pressure?
 

Wow 17-18 on AFR is dangerously high. Is the 240 kit new to the sled? Have you run it before without leaning out?

I don't believe the 240 kit required a new fuel pump, just the 270 kit. It might be labouring though.

Have you checked vacuum line that runs to the MCX fuel controller (EFI box) from the stock Map sensor vacuum line? If that has a leak it might not be sensing pressure so it's not adding more fuel.
 
No the kit has been on the sled since new in 2010 and has 5000 km on it. I put lots of miles on it with it running consistently in the 12-13 range under 10psi boost. (full throttle)

The 240 kit uses the stock pump bit it has additional injectors in the plenum.

I started stripping it down last night so I will check the vaccuum lines and electrical connectors. Where is had been running flawlessly for so long hopefully its just a simple thing like a loose hose or connector.
 
Hummmm you say it’s fine until
It gets into boost, sounds the the FPR isn’t working or the pressure/vacuum line is leaking. Check all vacuum lines teed off that one also, a leak anywhere will be a problem.
 
17-18 is usually maxing the gauge out. I would think it would fall on its face if it was truly that lean?? Are you anywhere near the revlimiter? Anything that causes the least bit of a misfire will send the AFR reading lean also.
 
That was my thought as well that if it was that lean it wouldn't run normal. It wasn't near the rev limiter as it was only reving 8000. I put a new set of primary weights in afew runs ago and I am still getting them dialed in so I am slowly bringing the rpm up. It wasn't pulling over 8200 in all the runs I tried it.

I will go over all the electrical, lines, and exhaust and look for leaks.

It must not be common for the injectors or fuel pumps to mess up on these?
 
If it were an exhaust leak it would affect idle afr’s not full throttle readings, there’s too much pressure in the exhaust for atmospheric air to enter.

The factory map sensor is on its own dedicated line, that’s working fine it sounds like. The mcx controller takes over once your into boost and it’s usually tee’d into a bunch of other stuff like the BOV. If there’s a leak anywhere in the system you won’t get a full boost reading too the mcx map sensor and this is exactly what happens. If your tee’d in off all three throttle bodies for your vac/boost signal and one of the hoses comes off just one of the throttle bodies it will also do this same exact thing. Hoses tend to get condensation on them also, blow them out and pressure test you vacuum lines.
Hope ya get it ciphered out, CM
 
If running that lean,pull plugs to see what they look like.If all looks good then maybe o2 sensor .When is last time it was replaced.
 
If running that lean,pull plugs to see what they look like.If all looks good then maybe o2 sensor .When is last time it was replaced.

This would be my first place to check if it is truly running that lean it would run like crap. I usually replace mine every 2 years and usually it starts to give me odd readings. Also if you are running any av gas or race fuel it is hard on them.
 
Not sure I would want to actually get it into boost to check it, could hurt the motor. You actually need to get it running into boost then shut it down quickly for a plug read. Wouldn’t it just be easier to check all you vacuum/boost lines first?

It won’t necessarily run like crap, the combustion chamber is so good on these motors they won’t lean misfire even up into the 17’s.

Leaded fuel will ruin them after a while but it takes time.

If your running regular pump fuel, no additives then there’s no reason to replace the O2 sensor ever 2 years. ZX4ever must be putting something in his gas that’s damaging the sensor. MCX 240 kit doesn’t need anything other than pump premium.

Be careful or you could blow it up!
 
Checking the plugs at this point won't tell me anything as I drove it back to the truck about 1/4 throttle so it wasn't into boost so the plugs would look great.

I tore it apart last night and noticed the boost line going to the MCX controller could be pulled off easily. Every other line was held on tight with the zip tie but that one could be easily pulled off and on. I am thinking that may be the culprit. I am going to re-zip tie it tonight and see if that fixes it.

Is their any clamp that people use on the small vacuum lines instead of a small zip tie?

I run pump 91 octane with a small amount of VP C14 mixed in just for safety precaution but I may be able to cut that out if its not needed and hard on the Sensor
 
Did you check you Blow off valve? I use a regulated air supply and blow like 35 psi into mine, if it’s leaking you will hear it.
 
Yea C14 will kill sensors but if your only mixing 1/4 ratio it will take some time too die. If you’ve been mixing it for 6 years then I’m concerned about you sensor.
 
They are cheap enough, you should just replace it and if the problem doesn’t go away atleast you have a spare.
 
Who’s Wideband are you using?
 


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