ECU quirk

[3D914]

In trying to diagnose my current FI woes, I've been spending a lot of time with the MPS and the ECU (see post:http://forums.pelicanparts.com/showthread.php?s=&threadid=98920&perpage=20&pagenumber=2.

I noticed that prior to some of my testing the knob on the ECU when turned CCW (leaner) would cause the idle to increase. While turning CW (richer) would cause the idle to decrease.

Somewhere in my testing of the last several weeks it has switched. CCW on the ECU now decreases idle, and CW increases. Since my FI problem is related to fuel mixture, this doesn't completely surprise me, but I'm wondering how it came about.

Any ideas?

[jkeyzer]

Well, if you go way too lean, your idle will drop too. The idle speed goes down if you go too far with the mixture either way. So maybe you are just on the other extreme now?

[pbanders]

I'll say it again - you should go to a shop that has a quality emissions analyzer and get your idle CO measured. Adjust the ECU knob and verify to yourself that CW does make it richer and CCW is leaner. See if you're WAY off of where you should be - for example, "really lean" is 1% or less, and "really rich" is 5% or greater.

Don't expect D-Jet to ever have the idle stability that a modern EFI system has. Most modern engine management systems have an idle stabilizer device that dynamically adjusts the air bypass to maintain a rock-steady idle. D-Jet is for the most part an open-loop system without feedback (if you neglect the speed control signal from the trigger contact points, which isn't used in a feedback manner, AFAIK).

Common idle stability problems are that when the car gets hot, idle tends to bog, and when you switch on heavy electrical loads (e.g. lights + heater blower), it bogs. The goal here is to set the idle to the best compromise setting (assuming you've first assured yourself you have everything set to spec: mechanical, ignition, timing - FIRST!). I go out, run the car hard, then come stop, switch the lights on, and set the idle to the lowest RPM I can tolerate (about 800 rpm or so). You'll find that if you do this, your lightly loaded (warmed-up) idle may be as high as 1200 rpm. That's about as good as I've been able to get it.

[3D914]

Brad,

Thanks for the "seat of your pants" perspective. I can relate completely to what you're saying. I was just bugged by the fact that it (ECU knob) was behaving completely backwards fromn usual.

What's weird is that just this afternoon I was down at the Porsche garage to get some feedback from Don. I took the freeway so the engine temp was nice and warm (3/4 on my dufus temp gauge). When I pulled into the parking lot to let it idle it wanted to die again. So I held my foot on the pedal at about 1200rpms for a bit and took the foot off. It dropped to around 700, but still idled so that I could get back and adjust the ECU knob. I turned it CW (richer) expecting the idle to pick up, but it dropped. So I turned it CCW (leaner) and idle picked up. I needed to turn it all the way CCW to get it to idle at ~950rpms without dropping off at all.

SO what ever occured that appeared to flip the ECU knob results has dissappeared. I've adopted your approach to setting the idle Brad. I guess I'm expecting too much from this 30 year old system.

Thanks again all,

[hardflex]

Brad- is the ECU knob a completely different circuit, or more like a fine tuning of the MPS setting. Does changing the MPS adjustment screws affect what happens at the idle level?

[Bleyseng]

It's a different circuit and has little to do with the MPS. Yes, changing the MPS has alittle affect on the idle but not much. IF the MPS is set lean or rich it will affect the idle some, but then the engine will run crappy anyway.
Geoff

[pbanders]

There is a circuit in the ECU that adjusts the mixture as a function of engine speed (speed control). When the idle switch in the TPS turns on, the idle circuit in the ECU "takes over" the speed control circuit's job in setting the mixture. Most ECU's have a small mark in the plastic surround that sheaths the idle mixture knob, that mark signifies where the effect of the knob is "zero" - in other words, the point where the mixture setting is exactly the same as if the speed control circuit was still active. D-Jet was originally designed without any idle mixture adjustment - but apparently, mechanics didn't like that so Bosch added the mixture knob to later ECU's. When things are all stock and in good operating condition, the idle mixture knob should be very close to the factory mark. See my web page for all of the details on these circuits.

The MPS affects mixture under all conditions by the setting of its primary mixture adjustment. Like Geoff said, if it's affecting your idle, then it's affecting your mixture at all load settings.