Erratic RPMs when holding throttle still and shuts off

I unloaded my just bought 08 WR250R and went up the street less than a mile and it died like it ran out of fuel. Tried to start it a couple times and it sounded like a bad battery connection for when I hit the starter it just bumped and quit. Being new on the bike I did not know if I was doing something wrong or the bike was acting up. Then it started and had no problems while I drove it about a hundred miles on road and then it died for a fraction of a second and threw on the check engine light, but ran fine until I got to where I was going. Started it up and took it into the woods and it died. Now when it started it would not run at an idle but when holding the throttle at a constant position it would reeve up and down like a car with an extreme cam but with much more fluxuation. When it cools off for a few minutes it will idle and then goes back to its erratic behavior.

The guy I bought the bike from has taken it back to his house to work on. It has a new battery with tight connections and is showing no error codes. It has a FMF programmer but does the same thing when it is disconnected. Fuel pump wirrs like it should, hopefully he is checking the pressure today.

Any ideas?

What can or did the guy you bought it from say about this and when it started doing this?
Or, was it a case of, "It never did that before." ?

"It never did that before."

And I do tend to believe him for now he is trying to rectify the situation.

He said that the only thing he did was put in a new battery right before I bought it.

He is an equipment service technician and specializes in electronics. Him and another mechanic are trying to figure out the problem today.

Vacuum leak? Check the clamps and seating of the intake boot.

Thanks, I will check that out or have him.

I would imagine that if the previous owner who is now looking into the problem is an "electronic's expert", then he would have started at the basic's such as alternator charge rate etc.....

Does seem like a battery voltage supply issue could be possible. ( Aside from the "known" early fuel pump fault. You say that he is checking the fuel pressure, so if it plays up he should be able to diagnose it ).

Cheers.

The first time it shut off, looking back, it reminds me of a battery or connection problem, and also since he never had a problem until he installed the new battery makes me wonder.

He did do a voltage check and it was charging, at what rate, I do not know.

He was suppose to switch out batteries today and check the fuel pressure but I have not heard back from him yet.

I am not sure tho, how a voltage problem would cause the rpms to surge and drop in a consistent rhythm.

I have searched all the post and have not seen anyone having that symptom.

I could see how a fuel pressure regulator malfunction may be the cause, or even a bad fuel pump or injector or even a bad coil, but I know nothing about fuel injection, which is why I left him work on it.

jaydee3403 wrote:

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He did do a voltage check and it was charging, at what rate, I do not know.

I am not sure tho, how a voltage problem would cause the rpms to surge and drop in a consistent rhythm....

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Correct & stable voltage supply to all components - such as sensors, actuators, modules, relays, ECU etc, is absolutely critical for any EFI system - proberly even more so on a small capacity single cylinder.....

With-out a good and stable voltage source from both battery & alternator, all sorts of strange things can and will occur. Erratic idle could very well be a symptom, and i have seen this being the case on many different EFI vehicles that i have worked on over the years.

Fuel pump / pressure does seem more likely to add up on the given symptons - but it does sound maybe more than just coincidental that the problems have occured since fitting a new battery.

Anyway like i've already mentioned, a simple battery load test & alternator charge rate test should clear this possibility up either way.

Cheers and keep us posted on any progress.

They have narrowed it down to the computer module.

The bike would run until it would get warm, then run erratic until the check engine light would come on, and then run fine, but would not throw an error code.

Meaning that when it would get warm the computer module would not allow the bike to run correctly until it went into limp mode. That's the diagnosis anyway.

Bike goes to the dealership next week to check the module.

Thanks for all the input.

Turns out it was the air pressure sensor. They seller stood by it 100%. The shop charged him 3 hours labor at $70 per hour and $110 for the little plastic sensor.

Thanks everyone for your input.

Great news, and thank you for reporting back on the matter. :)

jaydee3403 wrote:

Turns out it was the air pressure sensor. They seller stood by it 100%. The shop charged him 3 hours labor at $70 per hour and $110 for the little plastic sensor.

Thanks everyone for your input.

Awesome job reporting back.   

Any chance you have a picture of the part and/or a picture of where it's located?

x3.....cheers for reporting back with the outcome.....

Did they elaborate any further as to any DTC being logged when the problem was occurring ?

If the repair shop had a plug in diagnostic scan tool, then they may well have seen the incorrect output data reading from that sensor as the fault was happening.

Anyway all good that it's now sorted and that the P/O looked after you with it.

Cheers.

I can't get the pic to upload but it is a black sensor with a plug and a tube about 3/4" that I am thinking goes into the intake.

After the original owner originally said that there were no codes showing, he told me there were 5 codes and the last one was for the air pressure sensor. Why that wasn't a clue for him leaves me clueless.

I have no information from the shop which he took it to.

That problem is solved but now I have a blown fork seal after only putting 500 mile on the bike. He originally said that he had just replaced the seals but know he tells me it was last year.

Seals have been ordered and hopefully will be received next week.

Anyone have a preference as to the best maker of fork seals? OEM or aftermarket?