My FAST tuning issues...

[carbuff]

Hi guys...

I'm trying to get my tuning on my SBC dialed in a little better. I'm having some idle issues that I hope you guys can help me out with. I've been having this discussion at another board also, but I'm looking for a good cross section of advice!

As a quick recap, I have a 377ci SBC, large solid roller cam (252/268, .672/.672, 114LS), converted Edelbrock Victor Jr. manifold, 36# injectors, Holley 1000cfm Throttle Body, and the Bank to Bank Wideband option FAST setup. Phew!

My issues are at idle... My MAP readings are anywhere in the low 50's when it's "trying" to idle (which I think are ok for my starting point)...

I say "trying" because I'm having a horrible time getting it to idle! I went back to my original fuel map, and that seems to be pretty close (I had previously leaned it out too much). The motor is going into Closed Loop now, so I can see the actual and target AFR's (it previously wasn't). The O2 correction at idle is low, so I take it that's a good sign.

The problem is it wants to idle anywhere from 1500 - 1900 rpm! I'm trying to adjust everything, but no luck. I've lowered the requested IAC settings to 10 (low as they will go), and that doesn't help (I see the ECU reporting that's the current value when the engine is running). The ECU is trying to pull out the max amount of timing to lower the RPM, and no luck there (I have it set to request 28 degree, it's showing 22.25, which is the 28 - the 5.75 degree max it can remove for idle speed errors).

When it first starts, it will idle around 1250-1300. As soon as I blip the throttle, it's staying above 1600. The TPS reading is anywhere from 3-5%, and I'm forcing the blades on the throttle body closed as best I can (idle adjustment screw all the way out, secondaries not open, holding the primary closed).

In this condition, the engine is 'hunting'. It stabilizes around 1600-1800, then starts to drop in RPM. Then it seems to 'catch', and speeds back up again. I can't see anything on the ECU which is changing that is causing this 'hunt' (sorry for the poor description, I don't know what other terms to use).

Playing with the VE tables around the point where it is idling is helping to stabilize things, but it stays too high.

When it's "stable", the MAP reading is about 56 or so. When it starts dropping rpm the MAP reading drops quickly, and that's when it catches up.

The only table I can find that controls the IDLE speed is the "Idle Speed vs. Coolant Temp". I have it set to request about 900 rpm once the temps are up.

Help!?

The good news is the fuel pump and fan relays are all finally working! The car will run with the temps right in the range I want (with the dual 12" electric fans cranking away). Also, with the new timing table, I'm not glowing headers anymore!

Thanx to anyone who can provide some suggestions! I'm kinda stumped at the moment...

Bryan

 

[bobc455]

A couple of thoughts.

Why do you have your timing at 28 degrees to idle? That seems extrordinarly high to me. One possible cause of "hunting" is too much timing at idle. Try changing this to 15 degrees or so, see what happens.

"Playing with the VE tables around the point where it is idling is helping to stabilize things, but it stays too high. "

The only reason to play with the VE numbers is to make the actual A/F ratio match the target A/F ratio. If these are close (minimal O2 correction, like you suggest), then you should be playing with the target A/F ratio instead.

What is your target A/F ratio at idle? Another reason for "hunting" is too lean of an idle. In theory, you should be at 14.7:1, and FYI mine is set around 14.5 for the best idle.

If your idle is just plain-ol' too high, then you have air getting into that motor somehow! If the IAC is at 10 and the throttle blades are closed all the way, then start looking for vacuum leaks. Are you using the right base gasket under the throttle body? Are all of the vacuum lines sealed up (including the two small ports on the back of the throttle body)? If it gets to the point where you're going crazy, try putting a carb hat over the throttle body- then if there is a vacuum leak, your perpetrator should start hissing very loudly. If you have an automatic transmission, did the vacuum line fall off the governor? Is there a power-brake connection that isn't sealed up well? Your mission (should you choose to accept it) is to find out where the air is getting into that engine.

If you've checked all of this stuff, let us know and we'll try to keep trying to help!

-Bob Cunningham
bobc@gnttype.org

 

[carbuff]

Bob,

Thanx for the ideas... Since my first email, here is what I found...

Adjusting the timing at idle does make a difference. I'm shooing for about 22 degrees at idle, based on several suggestions I've received from others with similar setups. I only set it up to 28 because the ECU was pulling it down to 22.25 with the IDLE timing correction.

I have also changed my idle AFR targets. Instead of 14.6, I'm not sitting at about 13.5. It seems to like these better. I should have stated that I was adjusting the VE tables to try and get them to match the Target AFR. Again, others have told me that a slightly more rich idle target on a motor like mine may help smooth the idle.

But the big discovery...

It appears that my throttle body wasn't sealing against the Mr. Gasket carb spacer that I am using. I need the spacer to allow my Holley throttle linkage to clear the fuel rails. Looking at the impressions of the TB and spacer on my gasket, the opening in the TB for the IAC circuit appears to be able to draw in air from the outside... UGH! I am not in search of a spacer that has a wider body on the rear that will allow my TB to have a surface to seal against!

I probably won't run the motor again until I do this, but I will keep everyone posted on what happens (and probably ask more questions too!

Thanx again!
Bryan

 

[carbuff]

I took some pics of the 'problem' and posted them at NastyZ28... If you want to take a peek, go to:

http://www.nastyz28.com/ubb/Forum8/HTML/002327.html

They are pretty self-explanatory...

Bryan

 

[bobc455]

Interesting.

I'm still willing to bet that you can get it to idle nicely at 14.5 or so, I doubt you'll have to drop down to 13.7 (and I think it will smell better too!).

Let us know how it runs after you fix the leak!

-Bob Cunningham
bobc@gnttype.org