I'm stuck here. I'm having two problems that are driving me crazy trying to figure out. I started over with a stock bin, then lowered the BPW and IAC counts to achieve a stable 700 rpm idle with reasonable BLM counts. Now here is the irritating part. Something is causing the IAC counts (and idle) to remain high after I put the truck into drive and then back into park; or after I have driven it.....but not every time! I have commanded a max O/L idle speed of 800 rpms and an IAC count of 20 at idle for the temp that is runs at. The engine will idle at 1100 rpms with an IAC count ot 45 or so.

The other problem is a very loud popping in the exhaust (almost like a backfire) after I goose it or accelerate hard then let off.

Anybody want to take a stab at these? I’m thinking that the two problems might be related, but I haven’t figured out how....yet! TIA

 

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Bump

 

Anybody have any ideas?

 

The popping could be an exhaust leak. When letting off the gas, the fuel film in the manifold gets stripped off and richens the mixture. This is what Decel Enlean is supposed to cover. When the throttle is fully closed, and MAP is low enough, DFCO (decel fuel cut-off) will turn off fuel. It could be that cutoff is throwing O2 at some fuel in the exhaust (for whatever reason it's there) as well. Look for small exhaust leaks first, then ignore the problem if you can't find anything. Come back to it after the rest of the tune is working well.

I'm guessing the other idle problem is caused by a lack of a P/N (park neutral) switch.

 

Thanks,

I guess the that getting the O\L stuff and the VE tables dialed in a bit better is more important. It is just that the idle problem is bugging me.

Thnaks again!

 

If its got a 7747, then thats normal. No closed loop idle in park. The ecm basically just stays in open loop idle so long as the RPMs are greater then the stall threshold and lower then 1000 RPM or so.

 

Wow, I didn't know that the 7747 went into open loop at idle, or that it didn't have a P/N switch. I guess I'll have to concentrate on my O/L AFR vs Temp table a little more first. I'm still trying to get it dialed in, it's still quite rich at idle (cold) even after compensating for the Holley's Delphi injectors by backing off of the BPW.

What's really interesting is that ALDL is showing a lean (133) idle in O/L but slightly rich (108-112) while driving. I've backed the O/L AFR vs Temp and the Cold AFR vs Kpa, but I guess I need to back it down some more. I've cut the Cold AFR vs Kpa by half, but am still getting a little black smoke out the pipes and one or two popps while it warms up.

Needless to say this is quite overwhelming untill I learn what I'm doing a bit better.

 

By open loop idle, I mean open loop idle control. IOW, the ECM only moves the IAC when the idle is below the stall threshold or above 1k RPM when in park or neutral.

As far as idle fueling goes, I wouldnt get too hung up on what the BLMs say at idle. Do your tuning while driving, and use the data around the areas that you idle in to fill in the VE at idle, especially if you have a single wire O2. They cool down at idle and arnt as accurate.

 

There should be one if the vehicle is stock. If you disconnect the wire from the switch, then the computer will think its in drive at all times, and control the idle even in park or neutral. The caveat is that there wont be any IAC counts added/subtracted to compensate for shifting into drive and such, which can cause the idle to dip or flare on trans shifts.