Idle Question

[DOHC Vobra]

Here's a wierd one that I haven't been able to figure out... I've setup my idle so that it idles perfectly when the car is warming up and when it's completed warmup. Now, when the cooling fan turns on, I get an annoying "rolling" idle... Anyone have any ideas why that might happen? Could it be the batt voltage is going lower due to the cooling fan being on???

Justin
'97 Cobra Coupe
http://dohcvobra.corral.net
http://www.guieectuner.com

 

[Craig Smith]

Yes, it could. Watch the voltage on the laptop when this happens, that will tell you if it's a problem.

 

[bobc455]

Happens on my car, drives me bonkers. I've tried and tried but haven't been able to squash the problem. Passengers think that I don't know how to tune a car (and in this area they may be right)!

And because I have a low stall convertor, when the engine surges it really pushes the car forward- very, very annoying (especially when in a traffic jam on a hot day and you're already irritated because you're late to be somewhere).

The responsible gremlin is a low-voltage correction parameter that is non-user-adjustable (you can see it when you press F9 on the DOS version of Calcom, or you can add it to your dashboard of any other screen). I forget the name of the parameter offhand, but it will be obvious.

This is not a problem on certain vehicles with certain types of charging systems, but it is a major pain in the butt for those of us who have a less-than-ideal (but still plenty sufficient for all other purposes) charging system.

-Bob Cunningham
bobc@gnttype.org

 

[bobc455]

I wonder if de-tuning the response of the IAC would help.

Can anyone give a decent explanation of how to tune the IAC P and D gain? I would guess that I want to decrease the D gain, but I'm not sure.

To me, it would be worth a sluggish IAC reaction to get rid of the surging.

-Bob Cunningham
bobc@gnttype.org

 

[Craig Smith]

Bob,

I think lowering the IAC gains would be a good idea for you. Especially given that your ECU is up to date and you still have trouble getting the idle you want under pretty much any condition, there is a decent chance that the IAC is simply trying too hard to keep up when the electrical system is loaded down.

Here's what I wrote in the manual on this subject:

"An excellent way to better understand these settings is to liken them to a sensitivity control on a cruise control circuit. If the circuit was set for maximum sensitivity (gain values both set to 255) the accelerator pedal would go to the floor as soon as you dropped below the target speed, and the pedal would drop to an idle as soon as you were going too fast. If the circuit was set for minimum sensitivity (gain values set to 0) the accelerator pedal would hardly move if you were going too fast or too slow, and you would almost never be traveling at the desired speed. This same concept applies to how your IAC motor will regulate idle speed."

In other words, somewhere between 0 and 255 for each of these settings lies your sweet spot. The P gain setting will affect how far the IAC motor will move in response to idle speed fluctuations, and the D gain will affect how fast the motor will move in response to those same fluctuations.

Before I start talking way too much again (which I do) I guess I would say cut your gain settings down to almost nothing, and see if your idle problems persist when the fans kick on. If they do, go get a cold one, call it a day and we'll tackle it again later. If the problem seems to go away, I would raise the settings as high as you can get them WITHOUT the problem coming back.

I hope this post is more helpful than it is time-consuming! lol

 

[TwinTurboIroc]

wasn't there a thread on this a while back? i've been readings these forums for a while to try and gain a lot of knowledge with the FAST unit to minimize my problems in the future....

if memory serves, i'm almost 99% sure that this issue was solved with just a bump in the VE table around the idling cells....

let me do some digging

 

[TwinTurboIroc]

just do a search for "cooling fan"

a few topics

might help

 

[bobc455]

The Demo software comes set with P=20, D=10.

I've already changed mine to P=16, D=3 (have been this way for quite a while- a year or more?).

I will try to lower even further and let you know what happens. It shouldn't cause any problems when the engine is warm because my throttle blades are open far enough to catch the engine when I put it in drive, the IAC is barely on the verge of opening when I go into drive (IAC target = about 11 or 12 usually).

DOHC Vobra, have you ever touched your P or D values? Just curious.

-Bob Cunningham
bobc@gnttype.org

 

[DOHC Vobra]

Twin Turbo - Thanks for the idea...I'll go search on "Cooling Fan" and see what I find.

Bob - Hmmmm...you know, I've never messed with the idle motor parameters... Let me see what they are set at...

Okay, they are currently set at P=32 D=4...not sure if that's normal or not, just what they were since I recieved this FAST box... I'm thinking that maybe I'll try what Craig mentioned above and see if there's any difference.


Thanks Guys!
Justin

 

[bobc455]

Well last night I played with the P and D just to see what would happen.

WIth my old settings (P=16, D=3) it would maintain idle nicely in neutral, and oscillate (surge) just a bit (about 25-50 RPM) when in drive. Not perfect, but tolerable and acceptable.

Then I switched my fans on, and left it in gear. Engine started to surge as usual. I played with both terms all over the map (raised one while lowered the other, lowered both, raised both, etc.) and the surging got worst for most changes and never got better than original. Even with both settings set to 1, the car still surged as usual. Voltage was about 12.9 (idle RPM is 700, fans draw about 30 amps, so the voltage is bound to dip there).

The problem with dampening the response further is that when the car is cold and I drop it from park into drive, the engine speed dips and the car won't recover fast enough and it stalls. So I have to keep some response there, just for when it's cold (wanna change the P and D settings to a 2D graph vs. temperature!).

So DOHC Vobra unfortunately I have nothing to offer for help.

I wonder if I set my "minimum TPS for idle" below the TPS minimum (when the throttle is resting on the throttle stop screws) if that might help somehow or another, but I doubt it.

The only thing I can think of is to run a rich idle (13.0 A/F?) and back the timing way off. But I hate to have to do that just for the occasional time when the fans come on.

-Bob Cunningham

 

[TurboJim]

Bob, I did 2 things thatr might have fixed it, cuz mine doesnt hiccup or stall anymore putting in gear or fan turn on. If you run closed loop idle, try idling in open loop. (Enable atr 1500rpm) And the second thing was raising the idle 50 or so RPM. I notice my voltage is much more stable at even that little higher idle. Also, might try to minimize the "idle error timing corrections" because I think the ECU starts simply overcompensating by trying to screw with everything, and well, mine used to stall.

 

[bobc455]

Thanks, Jim-

My closed loop is set to come on about 250 RPM above idle speed, and I've already zeroed out the timing idle correction.

Something I've thought about doing is opening up the throttle so far that my IAC plays no role at idle. So I might idle at 900 in neutral, 750 in drive (or wherever fate takes me), and have the idle speed set for like 600 in the ECU.

The only thing the IAC would be used for is the warmup when the fans aren't on.

It's a bit of a band-aid method, but I'll try it and post how that goes. (And after trying to work for about 7 years with a Pro-Jection, I am the king of band-aid methods!)

-Bob Cunningham
bobc@gnttype.org

 

[bobc455]

Problem Solved! (I think)

Alright, if you don't ever hear from me again on this subject it means I either drove into a light pole or have finally solved this thing for good.

First thing I did was to increase my throttle opening with the setscrew so that the IAC was stuck shut under all conditions. Now the P and D terms could go to hell and I could care less. Solved? NO! The engine still jumped around. I had the throttle blades open far enough that IAC target stayed at 10. Idled around 925 in neutral/park, and about 700-725 in gear, idle speed set to 600 RPM. Turned fans on, and the G** d***ed engine still went up and down even though the IAC targed stayed at 10.

(Note: If you do make this change, remember that you may have to change the IAC target for startup. Also remember that you will get slightly less engine braking because now the engine will always have a bit more air going into it during deceleration.)

So back to the basics. Running too lean at idle can cause suring. I watched the VE number, it needed to be nudged a bit so I played with that (first chance to play with this since my cam change). Got my actual AF right near my target AF- numbers were better but the problem was still there. FWIW, batt corr % was about 1.6. Not much at all.

Timing. Ah yes, timing. Well here was my answer. At idle I had about 25 degrees. When I first tuned up the car a long time ago, at idle the car loved more timing- the more timing I gave it, the lower the IAC target was, so more timing must be better. So with my knowledge that too much timing can cause a surging motor, I tried backing off the timing just for kicks. And problem solved! I took 5-10 degrees of timing out (I might go back and take more out later) and the surging stopped when the fans were on.

What is the relationship between the fans, timing and surging? How many licks does it take to get to the Tootsie Roll center of a Tootsie Pop? The world may never know. Why did the engine not surge in drive with the fans off, but it did surge in drive with the fans on? A total mystery to me. But whatever the reason, it worked.

So DOHC Vobra, try backing off your timing and see if the problem goes away.

Sorry for babbling- signing off now.

-Bob Cunningham
bobc@gnttype.org
(Who believes that if you include all the details, someone may learn from your mistakes but unfortuantely that leads to posts that are too long)

 

[TwinTurboIroc]

Now the 64,000 question is....

if you put everything back to how it was before you started fiddling with your bandaid method and just backed off the timing.....would the problem STILL be solved from that one adjustment?

 

[bobc455]

I think so, yes. I think the timing was the key. I would even bet on it.

Although I'm still completley befuddled why the electric fans affected it.

-Bob Cunningham
bobc@gnttype.org