1994 Nissan Sentra SE-R going VEMS, then turbo.

[rob@vems.co.uk]

I think thats as bad as it gets - the answer is to look at the grounding and sheilding.

I think theres something in the LCD code that updates it, you might want to play with the values in Extras->LCD Settings

[BenFenner]

I was waiting until I was absolutely sure I'd fixed some problems before posting updates.

The cold start problem is fixed. Long story short: the injectors weren't firing on closed valves, they were firing all willy-nilly. Now they are firing at the same time as ignition, and it has made a HUGE difference. I have a little tweaking to do, but it starts up in the morning now without 10 minutes of cranking.

Changing the injectors to fire on the closed valves has messed with the left half of my VE table, so that's gone through a little change. Also, the warm-up enrichment needed work since the change. This was all expected. The idle is much more stable now too... Also a bonus.

Still working on the car, it's just been slow since I've been busy with the end of the school semester.

Rob, I still have to check into those LCD settings, but from my memory, I don't recall anything to control the updating of the LCD. =/

[rob@vems.co.uk]

Good to hear that you're running, its one of those things that you'd see if you had access to a six gas analyser, but reading HCs is not something thats easy to do.

As for the LCD, its dnb that asked for the reset feature, I think he'll be the one to ask.

[BenFenner]

Cold start below 23C was a problem until I changing the injector firing timing. I thought all was well... Now cold start below 16C is a problem. Not nearly as bad as before, but it still takes a minute or two of cranking/resting to get the car to start in the morning. I'm still exploring fuel delivery and idle opening values, but I've seemed to hit a wall. Hearing stock cars fire right up every day in the morning is giving me much respect for the folks who design factory starting systems, and making me realize there's got to be something I'm missing.

I came to a revelation the other week when looking at the starting diagram that Rob made.



I realized I had my cranking/afterstart threshold value set too low. I had the value set at 300 rpms so that once the car fired the values would change drastically because the rpms had passed the threshold. I thought that the control was designed to "catch" the firing condition, change values drastically to help the situation, and keep those values for a bit until...

The diagram doesn't show that at all. So, I put the value at 400, and now at 500 rpms. The car will crank then fire and still not get past 400 rpms. It will only get past the threshold now if it cranks, fires and catches. This is the way it's supposed to work I believe, and is why it only takes a minute or two to start in the morning instead of the 10-15 minutes it used to take.

Still, I'm musing about how to get this fixed. I know I'm close. It feels like maybe I'm firing fuel on two closed valves, and two open ones. Except... That can't be right...



On the LCD front, dnd has given me good info to follow to try to prevent the problem and I'll do that shortly, but it might be that the screen refresh option was only put into the x.0.x firmwares, and not the x.1.x that I'm running. =/

[rob@vems.co.uk]

The diagram doesn't show that at all. So, I put the value at 400, and now at 500 rpms. The car will crank then fire and still not get past 400 rpms. It will only get past the threshold now if it cranks, fires and catches. This is the way it's supposed to work I believe, and is why it only takes a minute or two to start in the morning instead of the 10-15 minutes it used to take.

So you've got your cranking PW sorted, now you need to look at what happens with the PW reading when the system transitions from cranking to running, thats your afterstart enrichment and then warm-up enrichments.  I'm betting that its plenty rich for cranking, then drops too lean.  Datalogs can help here

[BenFenner]

Yah, I've been using the realtime data display for a while now to make sure I know exactly what's going on with air and fuel. I don't remember exactly what's going on at the transition between cranking and afterstart, but does it matter? From what I can see, the car should have cranked (200 rpms), fired (400 rpms), and caught (800+ rpm spike) by the time afterstart comes into play. I'm having trouble getting over the 400 rpm point.

[rob@vems.co.uk]

I've never had cranking threshold above 300 for anything other than a test - when you're below the cranking threshold you're also stuck at the fixed cranking advance.

I use the real-time data too - just carefully watching the PW needle drop from its cranking value down to its idle value effected by enrichment, you can normally see where the engine starts to lean misfire - if its right after the point of cranking, then the afterstart enrichment needs to be higher, if it tails off as afterstart enrichment tails off then its your warm-up enrichments...

[BenFenner]

So the threshold should be 300 rpms or so? I had it right the first time?

When the car starts, it starts like a champ and holds idle. Otherwise, it cranks and fires (rpms jump from 200 to 395 or so when it fires) but won't really "start". At first i thought this firing needed to be "caught" and helped out by either adding fuel, removing fuel, adding air, removing air, or a combination of fuel and air changes. This didn't seem to work for me, and raising the threshold to 500 rpms has helped cold start, so I thought I was on the right track. You're saying my first assumption was correct?

If so, what is the general idea behind the changes made after the threshold. So you want to add a bit of fuel, a lot of fuel? Take fuel away a little, or a lot? What should the idle control do? Add air, remove air?

I feel I just need a little schooling the the theory, and I should be able to get mine working.

[rob@vems.co.uk]

The threshold is there to set the point below which we consider the engine to be turning on the starter motor.  Once the engine fires the after start stuff comes into play - this is designed to "catch" the engine as extra fuel is added (AS Enrichment) and of course the ignition advance goes to that of the low load and RPM on the tables.  You may find that the engine is not drawing much of a vacuum as it starts and that it reads mid-way up the kpa scale on the map and so you can perhaps add some ignition timing to accelerate the engine a little.

So mess with the AS enrichment and bear in mind the temperature that you're doing the start at AS is scaled in the usual -40degC to 77degC manner - are those days that it starts warmer than the days that it doesnt?  Is it harder to start when warm? (The old school rules still apply - too rich = difficult to start when warm, too lean = difficult to start when cold).

[BenFenner]

That helps a lot. I was going about it most recently in the wrong way. So, once the engine fires, to catch it you typically want to add more fuel. Anything going on with the air? Should it get more or less?

Right now it starts extremely well at warm temps (50C or above) and takes more and more work to get it to start as it gets cold. I'll give it some more time and tell you how I'm doing soon.
Thanks.

[rob@vems.co.uk]

The system adds more fuel, its all about the way it transitions from the super rich starting condition to the correct mix running conditions - its tricky to get your head round (how they did this with carbs I'll never know...)

And things need to get significantly richer as they get colder.

[BenFenner]

I understand that things need to be richer when it's colder... I just need to know, over-all, what happens during the transition between cranking and afterstart? Or, what should happen? As, I've gotten the system to do anything I've wanted it to. I've gotten it to take away fuel, add fuel, etc.  I'm familiar with every value that is taken into account, and modified them to do either job.

What should be happening after that threshold is reached? Less fuel, or more fuel to "catch" the firing condition? Less air or more air to "catch" it?

[BenFenner]

I did a bunch of messing around with cold start this morning. I changed the cranking/afterstart threshold to 300 rpm and did a whole bunch of things. I concentrated on removing a bit of fuel after the threshold. Nothing seemed to work. Although, I found the values in the spark table that are used for starting, and they were stupidly low, so they went up with a positive effect on firing strength and frequency (but no help in "catching"). At the end of 30 minutes before work, I was forced to put the threshold back to 400 rpms so the car would start.

I even made it so after the threshold was reached nothing changed (simulating a higher threshold) and the car wouldn't start... That's very odd.

Anyway... Still musing and waiting on Rob's info as to what should really be happening.

[rob@vems.co.uk]

I started replying to this last night and my machine decided that it was time to sleep midway through typing.

By putting your threshold at 400 it means that the PW and advance are held at those set for the cranking values.

The part about making the PWs produce the same values as at cranking is interesting, are you alternating injectors when cranking or batch firing?

Also take a close look at the idle control settings, theres some idle afterstart settings in there.

What should be happening is that the fuel amount should be reduced from the high levels required for the start to the lower levels required as the engine warms up and the charge sparks off easier.

[BenFenner]

I'm not alternating injectors, I'm firing "all banks". I changed this to "alternate" once and it seemed to only slow the rate at which the engine would fire.

Right now I have a -44C pulse width opening of 14 and a 71C opening of 2.5. With these values, and a warmup enrichment of 118 around the 0C areas, I get almost no change in fuel when the threshold is overcome. This is with an afterstart enrichment percentage of 65% at -44C and 5.9% at 71C. The afterstart enrichment value of 118 I know is close to what it should be, if not spot on. The 65% value seemed to be a default. Raising that or lowering it will change the fuel delivered during afterstart by a small margin. Bringing it down to 0% or up to 100% seems to be good enough to change fuel delivery enough to make a difference. In the realtime display the injector PW drops or raises after the threshold is breached accordingly depending on which value I've set. I've also gone and changed warmup enrichment to lower values to remove even more fuel if I felt like it...  No amount of fuel removal seems to do any good. How much should I remove? Right now it cranks with 12.2 pw or something. Should it go down to 11 pw? Or a more drastic change to 8 pw or so?

I'm not using PID for idle control yet, so I'm using the cranking idle settings, and then it moves to the regular idle settings. I've messed with this as well, but what should it do? Stay the same opening the whole time? More air or less air after threshold?