1JZ (JZX100) IATs and Fuel Correction. What to use as load axis?

[joeshaw123]

Hi All,

Took my car out on what was probably the first 20°C day of the year last weekend and had some issues with heat soak after a prolonged drive+parked up for half an hour then set off to go home. Idle was very lean, and low load/throttle situations were up at 16/17:1 AFR. As soon as it spluttered into positive pressure, AFRs quickly came down to a sensible number.

Unfortunately didn't have my laptop to get any logs however I plugged it in when I got home and spied IATs close to 50°C despite ambient being around 16 at that point.

I suspect a couple of issues:

1. My IAT sensor is a bit far out of the air flow; I'm inclined to get a longer, plastic bodied sensor so IATs are more realistic at low throttle situations. Thoughts?

2. My IAT Fuel correction table is only set up as 2D, with a linear ±2% fuel per 10°C of temperature difference. Why my mapper didn't add a load axis I don't know.

So, I want to try a load axis so it doesn't pull fuel at low revs/part throttle but torn between TPS, MAP and RPM.

Higher RPM, low throttle sitations aren't too bad, but trying to pootle around off boost results in a spluttery mess.

Any input/advice greatly appreciated.

Cheers.

 

 

[Adamw]

Load is usually not relevant since you are just trying to compensate for the change in air density.  If it is going lean only after a hot restart that would suggest the sensor is reading hotter than the air actually is.  As you say this might be an air flow issue or sometimes the mass of hot metal around the sensor can cause it.  A temp sensor in the intercooler plumbing just before the throttle body often works well as the pipework doesnt have a lot of mass and it is hard to not have the sensor in the air stream.  

The other cause of lean hot restarts I have come across occasionally is with fuel systems with surge tanks with the return going back to the surge tank.  After a hot restart the rail will be 100deg C or so and depending on how the plumbing is oriented etc you can end up with the very hot fuel (low density) just circulating between rail and surge tank and it takes a long time for that to come back down to normal.  

If you wanted to try a 3D IAT, out of the MAP/TPS/RPM options you suggested I would go for TP or MAP as those are more indicative of how much fuel and air are moving through the system which would cover both the possible causes mentioned above. 

Another axis option to consider on the 3D IAT table is engine run time.  So at high IAT's you add some extra fuel for the first couple of minutes or whatever it needs before it fades back to a more typical IAT compensation. 

[joeshaw123]

Thanks for the response Adam.

I use one of the below sensors mounted in the charge pipe just pre throttlebody. Its a cast pipe so does have a bit more mass to it than a normal IC pipe, but shouldn't be too bad. However the mounting boss and pipe wall thickness mean the element/glass bulb bit don't protrude into the pipe very much at all so probably aren't reflecting the IAT very well when suffering from a bit of heat soak. It wasn't just lean on the restart but on a lot of low MAP, low throttle areas too.

My fuel setup is bone stock minus an AEM 340 fuel pump + 730cc injectors so I don't think I'll have too much of an issue with fuel temps.

Current IAT fuel trims look like the below:

I've changed it to the below to pull less fuel out at low throttle. Also put smaller negatives for full throttle high IATs which hopefully I'll never see.

Will hopefully be able to test it out soon but our weather has changed from 20deg ambient last weekend to 1deg and snowing today. Great.

Thanks again.