Work Case #19 Honda CRF250R MX

[Thedktor]

Previous #18 TE450


Work Case #19 Honda CRF250R MX
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The Honda MX bikes are fuel injected, and at the time just kick start with no battery, so how exactly did that all work then?! Well, somehow it did and only a kick or two would provide enough electrical power to fire up the bike’s ECU and run the fuel-pump sufficiently to start the bike. Well, apart from this particular machine which took forever to start from cold and the unfortunate owner was not surprisingly pretty fed up with it!

Interestingly it started effortlessly once warm, and initial checks when cold showed a good solid spark and plenty of compression. So what was causing this issue then, the cold-start button? No as it happens, and not everyone is aware that on fuel-injected bikes this “choke” control does not make the mixture richer for cold starts but just raises the idle speed slightly. Bikes, and most modern cars fitted with electronic throttles, automatically adjust the idle speed for cold starts so don’t need a manual control.

CRF250.jpg (83.77 KiB) Viewed 2854 times

So the bike’s ECU monitors the engine coolant temperature, and the outside air temperature, and adjusts the mixture accordingly. Except this one presumably wasn’t richening things up when cold and causing the reluctance to start. OK, so how to check this? I had a Honda engine management computer thing but it needed the bike’s electrical system powered up, and the bike had no battery! Aha, of course, you have to stick a separate 12V power supply onto the bike’s loom and hey presto I could read some data.

The outside temperature was showing 15 degrees on a cold morning, which was bang on, but the engine temperature was over 20 deg, too high. So it must be a faulty sensor then and tried swapping that. Ah, exactly the same - oh! Borrowed another ECU from one of the second-hand bikes in the showroom, and..... all ok.

A quick swap back and forward to double check on the good bike and yes, definitely the ECU. Called Honda who tell me it can’t be reprogrammed, and the only option is to replace it.

The customer is quoted £400 ish for a new ECU but decides that is too much and takes it away as it is!

Oh, ok then...

[Asgard]

I wonder if placing a resistor in the line to the sensor could produce a different (colder) reading?

[Snaf MKII]

I wonder if placing a resistor in the line to the sensor could produce a different (colder) reading?

It would confirm the fault with the ECU which sounds as though it's gone low resistance, but it could then see the upper range as an overheating situation. You could however make the resistor switchable for starting purposes but there is still an ECU fault.

[steve the grease]

Try a company called BBA Reman. They know their ECU stuff.

http://www.bba-reman.com/gb/index.aspx

Give them a call it costs very little if they think they can fix it .....

[Thedktor]

Useful link thanks steve the grease.

I left the company a while back so this particular problem is fortunately not my concern anymore ...

Interestingly some of the more basic "power boost" plugs use a simple resistor/thermistor to fool the bike's ECU into thinking the outside temperature is cooler than it is to richen the mixture slightly, to overcome very lean emission-friendly settings.

I recall some Husky Terra owners being quite annoyed by the bike's tendency to randomly cut-out when approaching a junction as it was set too lean. The fix was one of these booster plug things, but the instrument gauge used the same sensor for ambient temperature which then showed 20 deg out

I hear a lot of problems with modern bikes having snatchy throttles or poor running from overly-lean mixtures for emission purposes - you wouldn't accept it in a car would you?

[Richard Simpson Mark II]

My understanding is that modern vehicles have a 'dry over-run' ie when the vehicle is turning the engine and the throttle is closed the fuel shuts-off completely.
We don't notice it in cars because the vehicle is heavier and the rotating masses are greater. In bikes you get the engine occasionally cutting out altogether, and also a snatchy transition out of over-run. Riders like me (sad old git) who grew up with old British iron have a natural tendency to blip the throttle as the bike slows, so we tend not to notice it so much.

[Ksithumper]

I had a brand new (petrol) car of a certain German brand which wouldn't start below 5degC. If you could somehow manage to get it going it would cough and fart for 30 secs - classic weak mixture - then be absolutely fine all day.
It defeated the dealer, UK importer and the German parent. Over many months they replaced absolutely everything electrical (incl. loom), plus cams, variable cam thingy, complete cylinder head, all injection hardware, - all to no avail. They honestly spent more than the cars value trying to sort it. Problem was, they needed a cold day to test any start, and if successful, then had to wait overnight at least until it was below 5deg again. It took forever..eventually they hired a climate chamber!
Finally [though I had to get fairly snotty, 1 hour from going legal] I sucessfully rejected it and got a full refund, plus a killer deal on a new one. I'd also driven their courtesy car free for a year, so happy days.
My conclusion was that the ECU tuning was so close to the edge for emissions reasons that this particular car went beyond, and the maker was terrified it could be a wider problem, hence their huge efforts to understand it. They have quite a a few issues with emissions I'm told....and to this day the dealer remains in the dark about the conclusion. I heard they scrapped the car.

On two wheels, my 1200 Tenere was also badly mapped, weak just off idle. Lots of people bought ECU flashes and power commanders to sort it but I heard others say that simply moving the temp sensor outside the airbox meant it richened up as it saw colder temps. As a bonus, the temp gauge then also displayed accurately rather than several deg high. (I couldn't be arsed, I just wellied it everywhere) .

Bloody electronics. Life was much better in the days of Amal and Lucas. Oh, hang on...

[Thedktor]

Ah, yes, modern engines do bring some very complicated problems with them - I too had a new German car about 15 years ago that would "hiccup" in certain driving conditions that the dealer could never reproduce, until I managed to demonstrate it to the service manager.

They resolved it eventually with the help of the German HQ, laptops, and much head scratching.

I have had cars with snatchy throttles, in particular a Cavalier 2.0i bought privately many years ago that Vauxhall for some reason couldn't fix, so I sold it as it was quite annoying. Other similar Cavaliers did not have the problem!

It is indeed often related to the complete fuel shut-off on the overrun of injected engines. My friend who develops parts for the Yamaha T7 is trying to work out how to get round this on this bike as it apparently jumps from 0 to 10% fueling when reapplying the throttle, making slow speed riding jerky. Not everyone is complaining however.

My Husky 701 is delightfully smooth on and off the throttle, it has a cat so has some kind of emission standard, but maybe it doesn't shut off completely on the overrun. 2016 model, pre Euro 4.