Why are my idle VE cells much lower than surrounding cells

[Tompo]

This has been bugging me for a while now...my idle VE cells are set to 39, but the cells adjacent to my idle region are 50-60.

I don't understand, the target AFR is the same, the ignition timing is the same, why should there be such a difference - something doesn't feel right?

I've attached my msq - don't laugh at the file name, I've been dicking about like an asbo trying to get some pops and bangs on over-run.

I don't know if it's related but my cold idle is a lot leaner than my hot idle - I've ramped up the WUE to combat this but then it over-richens the rest of my map under non-idle conditions - I haven't got a log of this unfortunately - I will try and get one tomorrow.

Car is a 1990 1.6 mx5 turbo, MS2, 440cc RX8 injectors - I'm not sure what info is relevant, I will provide more if necessary.

Any help much appreciated!

[Matt Cramer]

Can you also include a data log of running it through these cells?

[Tompo]

I've uploaded a log of the car idling from cold to 80+ degrees. There's a few blips of the throttle in there where I've tweaked the WUE% slightly.

I've also uploaded a snapshot of my VE table so you can quickly see the difference in VE values at idle.

The mx5 uses a PWM idle speed control valve along with a bleed screw on the throttle plate... Could I not just adjust my fueling to ~55 at idle and use the idle screw until I achieve my desired AFR? I initially thought this would increase the idle speed but MS and the idle speed control valve should sort the speed out, right?

Thanks for taking the time to look - feels like there's something glaringly simple that I've overlooked!

[ashford]

injector dead time is probably too long, definately too much accell enrichment too

[Tompo]

Hi ashford,

injector dead time - I've read about this in the past but never found anything concrete on it - or at least for my 440cc rx8 injectors so I've just left it alone. I know aftermarket injectors come supplied with a deadtime, but is there a way to effectively measure it?

How does dead time effect things?

Accel enrichment, agreed - definitely needs tuning but shouldn't explain my idle VE being so much lower than surrounding cells?

[billr]

There is a good video on how to check DT, somebody will post a link. There is really no good substitute for measuring DT with your drivers and fuel pressure. Any vendor data you may find is only a guide, not terribly exact. Dead-time is most critical when pulse-widths are lowest, like with over-size injectors or at idle.

[billr]

What type are the injectors, high or low Z? If low, are they being driven with P&H, or dropping resistors?

[Tompo]

High impedance injectors, no resistors in line.

will try googling more about how to measure deadtime - could this really contribute to the effect I'm seeing? 'a little insight into the symptoms of an incorrect dead time would be great

[Tompo]

Found this video on dead time and voltage compensation

https://www.youtube.com/watch?v=88VC1X4A-84

Seems reasonably straight-forward, just need to buy some graduated cylinders. I think that's all I need, I just unplug the injector from the head and place it over a graduated cylinder right?

I don't have any fancy voltage supply so I'd just have to use my car battery

[billr]

I think this is the one I'm referring to:

https://www.youtube.com/watch?v=lJgi8l-2XG4

One symptom of incorrect DT is that the VE values at low MAP (think "idle") don't seem to make sense. Sound familiar? Another symptom is the VE values have to be changed if number of squirts per cycle is changed.

[Tompo]

OK yup, sounds like I need to tune my dead time then, cheers!

I read a post on the mx5 forums where people had tried to tune dead time by flicking from 2 to 4 squirts per cycle with the engine running and changing dead time until no change in AFR is noticed. Sound any good?

[billr]

Yes, that is an often-used way to check if DT is about correct.

[slow_hemi6]

You should also keep in mind that if you have been using veal and your AE is not contributing enough fuel then veal will compensate by increasing the cell value.

[Tompo]

I thought VEAL ignored AE? I'm happy that the region around idle isn't being effected by AE as I do drive with very gradual throttle application whenever I use VEAL to get the best results.

I'm going to attempt to tune my dead time and see where that gets me and I'll report back.

Thanks for the help guys

[Tompo]

Took some measurements to calculate my deadtime by idling the car and logging AFR and MAP and plotting injector PW against MAP/AFR.

This gave me a deadtime of 0.9ms as oppose to 1.0ms and has raised my idle VE from 40 to 47 so that's a big step closer to the surrounding cells.

I might back the deadtime off a little further until the idle cells match the surrounding cells a little closer and see how I get on.

Thanks for the help!