WR250X FI: intake air temperature sensor bias mod

[dave_in_michigan]

Seems to me that adding a bias to the the air temp sensor circuit so that it indicates a bit "colder" than reality should be a simple way to fool the ECU into handing out more fuel across the board. Goal is to richen things up to keep up with increased flow due to airbox and exhaust mods. Depending on the circuit requirements, I could even imagine wiring in a switch to select either "economy" or "power" bias levels...

Does this make sense or am I way off base with this? I searched, but didn't find anything...

Looking for advice / comments.

I'm going to try to figure out what the sensor output looks like...

[dave_in_michigan]

Well, scoping out the sensor was easy. Remove the left panel, move some wires out of the way and the sensor pulls out cleanly (it's just press fit into a grommet in the side of the airbox).

Did a couple quick measurements on the sensor itself:

- air conditioned basement: 70 degrees => 2.28 k Ohms

- ice water: 32 degrees => 3.65 k Ohms

- under tongue ? : 98.6 degrees => 1.30 k Ohms

Plots out very linear. So if I want the ECU to think it's 10 degrees colder, I can just add 0.353 k Ohms in series with the sensor. Or add a 1k resistor to drop the temp by 28 degrees. Easy.

Now the hard part. What temperature bias to make?

??

PV=nRT, right? Hmmm... Any of you fuel experts wanna comment?

A dyno and a 1k potentiometer would do the trick, but I'm fresh out...

[LiveSideways]

https://www.thumpertalk.com/forum/showpost.php?p=6569836&postcount=9

[dave_in_michigan]

Thanks. Looks like he just put the 5k pot in place instead of the sensor. I still want the sensor doing its thing, just adjusted a bit.

I'll send MPFab a PM and see if he can give any insights to what resistor values he ended up using.

Thanks for the link.

EDIT: I read his post again, and looks like he did have the 5k pot in series with the sensor. ? Just need some ideas on what the right resistance values are that he ended up with...

[mtb2104]

I was thinking.... maybe the cheapest way is to glue those heat-shields to the air box and make sure it sucks in more cold air? ?

[dave_in_michigan]

Hard to get much cheaper than a resistor... ?

[dave_in_michigan]

Found this in the Programmer Battle Thread, for reference:

...We also shoot for around 13.2:1 for general use onthe WR and somewhere mid 12's for power. It seems to like fuel.

So to try to reach those numbers, here's what I worked out.

In order to go from an air-fuel ratio of 14.7 (stoichiometric, presumably what the stock ECU tries to achieve) down to 13.2, need 11.4% more fuel, all else being equal.

In order to get the ECU to think it has to add that much more fuel, it must think there is 11.4% more air coming in. To get that just with temperature, you need to drop from 70 degrees (for example) all the way down to 16 degrees (keeping PV/R constant). Resistor value in series to do that would be 1.94k.

To go from 14.7 down to 12.5 AFR, the ECU would have to think it's -9 outside (below zero F) when it's really 70. Adding 2.83k of resistance would do this.

I think I'll just wire up a 2k resistor in series with the sensor and see how she runs. Should be a pretty safe mod to try, as far as I can tell. My only concern is whether the ECU does any other strange things when it thinks it's so cold outside.

[dave_in_michigan]

I was thinking.... maybe the cheapest way is to glue those heat-shields to the air box and make sure it sucks in more cold air? ?

Pullling air from the left side instead of the right side would likely be cooler because of the exhaust routing, I'd think... ?

[mtb2104]

My only concern is whether the ECU does any other strange things when it thinks it's so cold outside.

Too cold to ride... so refuse to start? ?

just kidding... I am sorry but I am a total tard on electronics and therefore can only say:

good luck ?

[dave_in_michigan]

Too cold to ride... so refuse to start? ?

Lol! ?

[a16tony]

Dave,

I have had a similar desire to trick the ECU, just didn't know how. I was thinking of a toggle switch labeled "on/off road" at the controls. Just wasn't sure which sensor or how to effectively trick the system. I was thinking temp sensor, but if 'air' works, I all in.

When are you going to start the mod?

[dave_in_michigan]

I may get around to it today even. Only thing is, I'm only going to be able to give a "seat of the pants" review of the results, FWIW.

BTW, a 5k resistor => -71 degrees => 10.8:1 AFR (in theory!) -- too rich.

[bmdtech]

would that ratio change if you were to have airbox lid off and exhaust mod? I am a noob on this stuff.

[dave_in_michigan]

would that ratio change if you were to have airbox lid off and exhaust mod? I am a noob on this stuff.

I'm no expert, so caution advised! In general though, adding a resistor to this sensor should richen things up.

I was assuming the stock ECU is trying for 14.7, and I was trying to calculate what air temperature input would trick it into trying instead to achieve 13.2 (or 12.5, etc), just to estimate a reasonable resistor value to try. I don't know what the actual AFR's are.

I don't know how the stock ECU compensates for intake and exhaust mods, but I think I read that those mods lean things out somewhat, especially with the airbox door fully removed. Mmmm... here it is:

What I have noticed on the dyno is the header/slip-on with the airbox flapper held down was in fact lean in some spots, but not so much that I would worry (high 13's). That being said, when I removed the airbox door during the same session power came up 1.5-2whp but AFR in the 5-8K range was super lean (15+) so I decided not to run it that way until I got some fuel in it. Next go around I made a little device that tricked the EFI into thinking it was super cold outside so it richened things up. This got AFR's into acceptable levels again with the lid off and I felt safe driving it around with in this state. The noise drove me nuts so we started drilling holes in the lid until power was close to no lid, but the noise was much less. I'm going back today with the Power Comander to optimize fueling and hope to pick up some more power and torque in the mid range were the fueling is still a little thin.

So adding a resistor to the intake air temp sensor should richen things up (just like MPFab did), and 2k or 3k seems to be a reasonable value based on my crude calculations (3k would richen things more).

[camlifter]

i'm prety sure the 14.7 afr goal would only be for idle and light throttle modes. on heavy loads it should shoot for around 13:1. i'm demoding an srt4 dodge soon. in the next 3 or 4 weeks so i'll have a wide band afrm i can rig up to the wrx and find out the true afr for the bike.

[2fastRR]

In theory it would certainly allow some adjustment to the overall fuel delivery. My only word of caution are that in the power commander map for airbox cover removed there are still some areas that the map leans out the stock system. meaning if you assume you need more fuel across teh board for more power it's not entirely true.

[dave_in_michigan]

In theory it would certainly allow some adjustment to the overall fuel delivery. My only word of caution are that in the power commander map for airbox cover removed there are still some areas that the map leans out the stock system. meaning if you assume you need more fuel across teh board for more power it's not entirely true.

Understood. And thanks. That's good to know.

Yeah, this is certainly no magic bullet. But I'm not planning on buying a real controller, and I have some concerns about engine health running too lean with the airbox mods + exhaust. I'd rather run a bit rich if anything to be on the safe side.

No question that a real controller will be far superior for optimizing power and correct tuning.

[dave_in_michigan]

Well I opted to wire up a 5k potentiometer, because I'm still not certain which value is right for my current setup, plus I haven't upgraded the exhaust yet, so I might want to change it after I do that.

Stuck home watching the kids right now, so I haven't had a chance to test it yet... Seems to run ok sitting in the driveway. ?

Splice is easy. You can see the sensor here, too. It simply pulls out of the left side of the airbox, just to the rear of the battery.

I went ahead and mounted the potentiometer, since it'll be a while before I get an exhaust upgrade, and I won't fix on any final resistance values until then. I positioned the knob's set screw so that it faces me between 2k and 3k, so I can tell about where I am in resistance value for that range.

[LiveSideways]

Sweet, Dave. Curious to see a ride report. Are you running the stock air filter along with stock exhaust? Any airbox mods?

[a16tony]

Since I haven't had a good reason to take things apart yet, does any one know if the TPS is adjustable, and would it allow for a slightly richer fuel cure if advanced?