Weak Idle

[Benzmac16v]

So, the car runs pretty good now, but I have a huge problem with my idle.  Pretty much no matter what I do, the idle will dip to about 700-800rpm when I drop the throttle out of gear.  The real issue with this is driving around in a parking lot, when the additional load from the power steering hits it will stall the car.

I followed the tuning instructions given to AVP in his thread.  I turned all idle controls off, then let the car warm up idling while adjusting the DC reference for my valve until the rpm was 300-400rpm above my target (1100rpm below 71C, 950 above).  Then I enabled PID control and decided to have at that.  Didn't matter what I did, always the same, or worse, behavior.  I got it about as good as I could, then enabled spark advance to +/-5degrees and enabling IAC at 4 degrees.  This didn't do much of anything.

Then I proceeded to get continuously more frustrated with it and figured it was time to ask the experts for some tips/advice.

Here is a log file.  Pretty much no matter what I do, this is the reaction I get.  You can see the first throttle blip results in a low idle that recovers. The second blip I jacked the wheel to show how it stalls.

http://dl.dropbox.com/u/20887362/v3.3_n001612-2011.06.04-12.52.12.vemslog

Thanks,
Jim

[gunni]

Where in the log is this happening.

In any case change your overrun settings your not getting any fuel on the overrun .

[Benzmac16v]

Apparently that was the wrong log file...  But I took the car out for a drive and noticed that 1.1.92 had quite a few issues...  Sometimes my idle speed would just go nuts, bouncing all over.  My tach would flip out when I took my foot off the gas with the car in gear (the rpm output in VEMSTune was still correct).  So I downgraded back to 1.1.88 and have had a little better luck, but its still pretty weak.

So I will make another log shortly, to properly demonstrate the problem.

What is wrong with my overrun settings?  They the cut and resume rpm values are in there and I just set MAP to 0....

Jim

[Benzmac16v]

Ok, here is a good file.  I got the thing to stop stalling (this was only in the driveway though, no real world stuff) when I jack the wheel, but I think you can see this is unacceptable.  It is a very small log, just two blips of the throttle...  The first one has the wheel jacked to the stop to put some load on the engine idle, this is common in parking lots, which is when it likes to stall and make me look like an asshole.

Thanks for the help!

Jim

http://dl.dropbox.com/u/20887362/v3.3_n001612-2011.06.04-18.21.26.vemslog

[boostd audi]

i use pressure based for overrun fuelcut.. make rpm something you will never get to like 10k.

then drive in higher gear and run out to 3 or 4k.  let off the gas, and see where your at.. put the cut just above this.

just make sure that its not higher then where you are on light cruising with low tps, ex: 1~2%

works great for me

[Jorgen]

Ok, here is a good file.  I got the thing to stop stalling (this was only in the driveway though, no real world stuff) when I jack the wheel, but I think you can see this is unacceptable.  It is a very small log, just two blips of the throttle...  The first one has the wheel jacked to the stop to put some load on the engine idle, this is common in parking lots, which is when it likes to stall and make me look like an asshole.

Thanks for the help!

Jim

http://dl.dropbox.com/u/20887362/v3.3_n001612-2011.06.04-18.21.26.vemslog

If you look at the log when this occur you see that you reach a very low rpm before the engine recover. Low enough to go from running mode to starting mode. If you correct your startup settings you will at least not have stalls because of this.

I recommend:

Set max crank rpm to 299rpm, set crank advance to 10-15deg. I have found that most engines like this.

This will not solve the problem but it can save you a few stalls.

I also found that your overrun fuelcut rpm and overrun fuelresume rpm are too close. Try to keep these at least 400 rpm apart. For example 1400 and 1800 rpm. This is found in the Base setup - Rev limit/overrun/safety dialog.

A more important factor is the ignition advance you run at low rpm, increase the whole 500 column to 15-20 deg, all the way to 100kPa. Now the advance is taken out as the rpm drops as you will have less vacuum with lower rpm.

You can also decrease the ignition advance at 1000rpm at the load sites used for normal idle, 0-5deg works fine and will usually decrease the emissions. This will allow you to use more idle air and is more important and effective on cars not running an IAC.

It's also very important to set the idle hardware correctly, your car seems to idle at around 33% IAC opening. Set 32% min duty in the IAC hardware dialog.

I think that these suggestions will sort most of your issues.

Jörgen Karlsson
VEMS installer and developer.

[Benzmac16v]

Ok, this seems to have helped a lot.  The various ways of keeping the idle from crashing (moving the fuel resume a little farther out, and fuel cut even further, or using the map method) and Limiting the duty cycle on the idle valve do a very good job at easing the car into idle.  Then I think the ignition settings helped a lot with keeping it stable once at idle.  I didn't even enable the ignition control for idle and things were significantly better.  I didn't drive around a parking lot, but I have a feeling that it is not going to stall anymore, which is an huge plus.

I had my ignition settings so high because the Mercedes repair manual for the car put it at 18degrees for idle.  So I went a bit lower than that and figured it would be ok, but 5 degrees, with more advance at 500rpm really seems to help it.

It starts about as fast as it did before, with 10degrees rather than 0 (0 was again specified in the Mercedes repair manual) but it doesn't make as much noise when starting now.

Thank you guys very much for the help.  When I get some time Ill stress test it and see if I really have solved the stalling problems.

Jim