Megasquirt MAT Correction

[gavinc69]

Hi guys,

Now the weather has changed here I'm getting very lean idle on a hot start, most likely caused by heat soak.

It appears the best way to counter this is to have the MAT correction setup correctly.

Also that should counter the AFR going out since I tuned the motor earlier in the year when it was colder. Now its summer and the auto tune feature is changing and the VE cells again

I've been reading various post's from different forums but am having trouble getting my head round it.
So I was wondering if any of you have had the same issues and what settings you are using for your MAT / MAT correction?

Cheers

[Tippy]

Plug in laptop when hot before restarting. Check green line where MAT is at. In bins, add 5-10% additional numbers. Start it and see how it runs. If it's still lean, give it some more.

[Tippy]

Or, check off "ignore MAT during ASE"

[gavinc69]

Thanks Tippy, will try that. Guess I keep adding until it gets back down to the correct idle AFR.

I did try the check off 'ignore MAT during ASE' but I still get the same issue.

I also tried adjusting the ASE taper which did seem to help, but it then seemed to screw up the idle AFR in general and was too rich, so I reset it all and started looking into the MAT correction.

Hopefully we will have anther hot weekend here next weekend and I'll give it a go.

[Speedy Squirrel]

Why not run closed loop at idle? Set the hold-off for going closed loop to 10 seconds, and make sure the hot idle is around 1000 right after the start. Taper it down after you are in closed loop.

[gavinc69]

I'll looking into that Speedy, not read up on Closed loop idle at all yet. Thanks.

[sjf911]

This was quite a frustrating problem for me. I ended up adding an external temperature sensor behind the front bumper and switched between it and the MAT sensor using a relay driven by one of the programable output channels. I found it took about 5 minutes for the MAT to equilabrate after a hot restart.

[jpnovak]

Where is your IAT mounted? The standard IAT algorithm is designed for outside air temp - like Steve mentioned a bumper mounted sensor on most cars. This works just fine for NA cars where the lack of charge compression is not heating the air.

For boosted cars where the IAT is mounted in the IC or other air tract pathway you have to compensate.

start with values like this and then tune from there.

40.5 -3.0%
42.5 -2.0%
50.0 -1.0%
60.0 0%
100 2.0%
150 5.0%

[gavinc69]

Its just after the IC. I thought that way it would be reading the actual air temp as it goes into the throttle body not just ambient outside temp.

Thanks jpnovak

[Speedy Squirrel]

That is not correct. This is from the Megasquirt site:

"IAT sensor - this should be located in the intake system somewhere after the compressor and intercooler. It needs to sense the temperature of the air going into the cylinders. You can put it before the throttle body or in the intake plenum, either location will be fine. Your intake temperature sensor must be the open-element type. Regular closed sensors are shielded within a brass housing, and will respond too slowly to the rapidly rising intake temperatures that occur when boost comes on. See the Sensors and Wiring section for more details."

http://www.megamanual.com/v22manual/mturbo.htm

Originally Posted by jpnovak

Where is your IAT mounted? The standard IAT algorithm is designed for outside air temp - like Steve mentioned a bumper mounted sensor on most cars. This works just fine for NA cars where the lack of charge compression is not heating the air.

For boosted cars where the IAT is mounted in the IC or other air tract pathway you have to compensate.

start with values like this and then tune from there.

40.5 -3.0%
42.5 -2.0%
50.0 -1.0%
60.0 0%
100 2.0%
150 5.0%

[jpnovak]

What is correct from theory is not what works in application. The standard algorithm will lean the mixture too much as intake temps heat up. Try it on the stim and see how much fuel it pulls when you crank up the IAT.

Just offering a workaround.

[sjf911]

I built an additional IO box for logging and ran my car logging a bunch of temps in parallel including external ambient, engine compartment ambient, pre and post intercooler, intercooler core (discharge side), fuel rail, injector, MAT (multiple locations including a blade MAF), etc. I found that for hot restart, the best fueling (not lean) occurred if I simply used the external ambient sensor until everything had a chance to equilabrate, again, about 5 minutes of normal driving. I even tried switching to a MAF control algorithm to try to eliminate the horrible lean issues with hot restart to little avail.

[Tippy]

Since I'm all silicone after IC, before throttlebody, I'm placing my IAT sensor on the outlet tank of the IC.

I'd think this is close enough due to the high speed rate of airflow.

Again, I'd bump up MAT in those areas it starts lean. This has two benefits:

1. Fixes lean restarts
2. Richens mixture if you have compressor run away temps. Last thing you want to do is pull fuel as the inlet temps rise!

Lean is mean.

[gavinc69]

Nothing is ever simple!

When I came to do it the 'MAF/MAT correction' is ghosted out in the Basic/Load Settings.

I can only get it to 'turn on' or unghost if I go into 'MAF Settings' and change the 'Voltage Input Port' from 'OFF' to either 'MAP' or one of two other settings there as if I have a MAF sensor installed! which I don't think I do, as far as I know its just an air temp sensor.

But if I read right you can't run MAP and MAF together.

Whats the differnce between the 'MAF/MAT Correction Table' and the 'MAT Density Table'?

Do they do the same thing but 'MAF/MAT Correction Table' when you are running a MAF and 'MAT Density Table' used when you are not running MAF!?

[JohnJL]

I m bookmarking this for later use, however one basic thing seems off to me....if you tuned to start fine when it is cold, and now it is warmer, all other things being equal you would be seeing more rich starting as there is less oxygen available in warmer weather. If you are indeed seeing a leaner start than you were then when it was colder then you might have something else going on? Try leaning it out and see if it starts easier.

[Raceboy]

That's why you want MAT/TPS correction table, not just MAT correction. That allows you to adjust hot idle separately from say full load and you always stay on target lambda.

[gavinc69]

After alot of Googling a lean hot start is a common issue particularly with boosted motors and most appear to put it down to heatsoak of the air temp sensor due to its position after the IC which is the MS recommended position.

I've got a simple install just using MAP, no TPS at present.

It looks like you can't use MAF is you are using MAP (so it seems to say in the MS manual). So as I'm only using MAP I guess MAT is right out of the window.

So if I can't enable MAF which in turn then won't let me enable the 'MAF/MAT correction table' it looks like I can't use that table!
If so the only other similar option I can see is the 'MAT air density table'. Now this does look very much like the 'MAF/MAT correction table' (see below)

So the million dollar question is would the 'MAT air density table' allow me to make basically the same adjustments I need to correct my hot start issue?

[Tippy]

Yes. Get engine hot, shut it down, and give it few minutes or 15/30 minutes or so.

Hook up laptop and turn key on. Check where green line is. Add 5 to 10% additional numbers in those bins.

Start it up. Check AFR's. Lower or raise numbers in bins until AFR's are maintained when not heat soaked.

[gavinc69]

Sorry for the delay.....

Tippy, did what you said and it worked out just fine thanks, its not perfect but 90% better than before. Good enough to get on with

[dos531]

Originally Posted by gavinc69

Sorry for the delay.....

Tippy, did what you said and it worked out just fine thanks, its not perfect but 90% better than before. Good enough to get on with

That doesnt look right...