Dropping boost in mid climb last season.

08 mcx sent this email off to mcx see what happens.

End of last season I started having some unusual problems. Sled currently has 8000km on it and has basically run flawless since the day I purchased it. I have been through this kit/sled numerous times on basic maintenance so I am very familiar with the set up. Here is the story and hopefully you can give me some feedback or what to look for.

One day while riding deep snow at elevation all of a sudden I was losing boost pressure…..it would build boost for about 2 seconds then dump all the pressure. First off I was thinking the charge tube must be off, checked it it was fine, intercooler was tight, all charge tube side looked good. Then started going through the basics, charge side, everything looked good, exhaust side no leaks. Bov was functioning correctly. Check wire connections everything seemed to be ok. Finally stumbled upon the control wire that runs from the mcx box back to the altitude sensor deal back near my air intake, it had rubbed through and I think was messing with the sensor causing it to dump boost? Not sure if that is a safety setup that if the sensor does not read correctly it dumps boost? Anyway patched it up and seemed to run better. Later that night I patched in a new piece of wire. Next day it seemed like I was building boost and not much trouble.

The next weekend I go out and intermittently it started doing the same thing. This time it would allow me to build boost and run for 3-4 seconds…….then dump boost ………..then build boost again. It did not seem to matter if I gave it full throttle or came on nice and smooth it would eventually loose boost pressure then build back up again. This seems to happen more often under heavier load like climbing a longer hill. It will be pulling very hard and drop pressure so quickly that some times I almost go forward over the bars. I checked very closely and it was not hitting the rev limiter but that is sort of what it felt like just more dramatic/severe.

I do have a datalogger hooked up but did not do any datalogging when this was happening as it was the last day/trip of the year.

Again just wondering what your thoughts are? Could the alt/temp sensor on rear be malfunctioning?

In the next 2 weeks I plan to do a minor breakdown/runthrough of entire sled just not sure what I am looking for that could be causing this.

It has a new fuel pump on it (external fuel pump), maybe 500km on it. I will replace the fuel filter as well.

Wondering if maybe the injectors where not firing fuel for some reason? Any reason the injectors would be failing.

It was like I was loosing fuel, injector spray, dumping boost, hitting rev limiter sensation.

Cruising at low speeds low to mid throttle it never seems to happen more at ¾ to full with load……ie. Climbing or very deep snow.

I have had exhaust leaks, bellow leaks, exhaust gasket leaks, plugged fuel filter before but this is acting different then any of those conditions.

I will be climbing away building boost running smootly and then sputter sputter sputter……then back to building boost.

I understand it can be numerous items just curious at what your thoughts are. If it happens again I will definitely do some data logging to help diagnosis the issue.

Erik's reply:

Hi.
It is hard to say what might be wrong, but is it probably something with the fuel supply.
Start with changing the fuel filter.
It can be something with that the extra injectors don't deliver fuel all of a sudden.
Check all the connectors and the ground wire to the external EFI-box.
Check the wire harness inside the plenum so there are no short cuts.
If you run the gen 3 EFI-box the connector to the box is not that good, so it happens that a terminal has moved backwards inside the connector.
A new EFI-box wire harness can be a good investment.
It can be the EFI-box itself, but usually when it is broken it does not act like this.
Best regards Erik.

NM said:

Any chance you installed a lighter spring in your BOV? I have seen some of these springs overcome by boost and open up the BOV.

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I am wondering about this myself. There is a fairly light spring in there. Would make sense since it happens at extended max boost.

I always through new plugs in at the begging of the season.

Started tear down yesterday.

Will do a better job at replacing wires on the sensor TVC?

Found 1 broken wire in nose cone, I think it the one use to set the CO's.

Everything else so far looks fine.

The kit is one of the original mcx 270 kits with external fuel pump.

No loose pins or frayed wires that I can see.

Dont think it was freezing vacuum lines.

Charge tube and header look like they have no leaks.

Not sure what injectors I am running.

Changed the fuel filter, it was not restricted but you could definitely see discoloration in the paper filter.

My main thoughts all along have been, ground wire, plugs, fuel filter, bov. It only dumps the boost at or near max boost under extended conditions and more consistenty under a heavy load like deep snow or steeper hill. Typically never happens on the flat. Then again I dont usually hold 17lbs boost very long on the flat 5 seconds vs. 25-30 second hard pull.

I will keep digging has to be something simple just hard to find.

Really need to run the datalogger and see what it is saying. Then I could see if its over/underboosting, spiking af or hitting limiter.

I am sure sylvan laker has a set of coils I could test to see if it makes the symptoms go away. I could see how that would get missed, usually a coil works or fails......intermittant coil are more rare.

All good thoughts. Again I convienced it will be a simple fix just a pita to find what is the main cause.

Turbo11T said:

I have heard of the TCV going bad. That is the little black solenoid that the wire you repaired goes to. I would replace that. Fairly inexpensive.

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I am curious how to tell if the tvc/tcv sensor is going bad or failing? I assume you just stay at the lower boost level setting and as you gain alt it does not increase the boost pressure?

What are the symptoms of a failing sensor?

What are the symptoms of the wire that connect to the sensor if its shorting or getting full strength down that wire? The 2 wire control wire to the sensor I am not really sure what its function is? I know the wire was rubbed through but I dont know if that was the cause of my problem? If that wire rubbs through and is shorting does it cause the sled to dump boost?

79.99 for new AF sensor and tvc is 129.99$

Thoughts?

MCX response:
The sensor itselg is very reliable, so the risk that is is bad is very low.
It is higer riks that the wire to the sensor or thet the EFI-box is bad it it does not seems to work.
It is easy to determine if it works or not.
Just turn on the key and simulate turbo pressure into the MCX EFI-box.
Then it shall start clicking when you pass about 110 kPa absolute pressure.
Best regards Erik


I am still curious what riding/driving symptoms appear with failing sensor or control wire. I am not really sure what the control wire is doing?

Anyone?

Called Prince George Yamaha and here is the info I was told.


If TVC fails the sled still runs as normal but as you increase in altitude you actually start to loose boost. Instead of running 13-14lb down low and it increasing as you go up in alt to 16-18lbs you will notice you can run only 10-12lbs boost. As atmosphere pressure drops as you go up absolute pressure will also drop and this is sensed at mcx box. It then sends a signal via control sire to tvc sensor to make changes accordingly to tvc sensor to correct for the altitude change. I run around 208-210 KPA absolute pressure and any altitude.

This will also happen if you unplug the 2 control wires to the tvc. You will be make less boost then even your low altitude setting.

If there is a break or shorting in tvc control wire it can possible fry your main mcx box, blow your main fuse or potentially give my experience, it will dump all boost. He was guessing that under max boost, climbing more vibration thus causing and interruption in signal and that the valve will completely open and drop all boost?

Some of this made sense and some did not. I would assume that unplugging it and rubbed wire would give similar results.........drop in max boost pressure instead of 17lbs I would only make 10lbs? Also can the tvc really open enough to drop all boost like a bov?

I am still vague on symptoms of a damaged control wire to tvc.

I have fixed the wire and maybe try to test it on a dyno to see if its acting up.

This one part of this kit I have never really understood all the parts of it.

My AF sensor was not working when this was happening so I never datalogged.............I should have.

Would like to get this worked out before riding season would hate to be chasing my tail on the hill instead of riding.

Tues