This occurs only when idling in nuteral. The BLM's go up into the 150 range, with the IAC counts staying steady. But as soon as I touch the throttle or drop it into drive, the BLM's drop into the upper 120's, low 130's. If I let it idle for a few minutes in nuteral, the CEL comes on. I'm not sure is this is related to my problem (very rare) of the engine dieing when I let off the throttle after driving a hundred miles or so on a hot day (hotter that 90 degrees), but something is telling that they could be related.

I was wondering if anybody could explain how the idle logic works on a '7747 running a AMUR bin (ie: what the ECU is looking at, and how the data is evaluated). I've searched, and searched but I can't find exactly what I am looking for. I've found several threads that are related, BUT the poster has changed thier cam, or have made other modifications; so the information is skewed in that direction. I've tried some of the suggestions, but with unspectaluar results.

As far as the rest of time, datalogging is showing my BLM's between 120 and 135 while driving. I haven't had to much time to play with the VE tables over the last few months except for a tweak here and there.

Additionally, I am also experiencing a big flat spot (hesitation, as in open the throttle and nothing happens) when accelerating from idle to about 1200 rpm. My thinking is that this is related somehow to the lean idle issue. As always TIA!

Anybody?

I need more details. I am going to assume that this is a 350 in a Van or from a Van being that AMUR is typically a 350 Van .Bin file.

What has been done to the engine? Exhaust, Headers, Intake, TBI mods, Fuel pressure, ETC.

What is the IAC count at idle? Too much IAC opening at idle is like a vacuum leak.

What is the MAP reading at idle in Park/Neutral and also Drive.

To me it sounds like a vacuum leak or an EGR valve that is not seating at idle.

At this point in time the EGR valve seems like the most probable cause, due to the fact it clears up above 1,300 rpm or so at part throttle. Most TBI calibrations open the EGR valve around 1,100-1,400 rpm with the TPS around 2-4%, which will activate the EGR valve and the EGR compensations in the ECM.

Sorry to take so long to get back to ya (toldja I'm busy! ). The engine is in the '88 Blazer that I've been playing with for the past few years. The engine is totally stock with the exception of a Holley 670 TBI (which stated all of this in the first place) and a free flowing exhaust. I'm running 33" tires, but I've swaped in a set of 4:10 gears, so I'm very close to the stock RPM's going down the road. My BLM's are a little lean (130-136) while driving, and I've actually managed to get a few cells at 128 lately and I am still working on the rest.

I haven't thought of the EGR since it's been replaced within the last year, but I'll plug the vac line tomorrow and see if it does anything.

Lastly, I've been asking how the C/L idle logic works (ie: what the ECU looks at and how it makes adjustments) , but nobody seems to either know exactly or isn't saying. Got any ideas?

TIA!

Does the 670 TBI unit have round plastic caps over the injectors?

For fueling, Tuning Guide sticky up top, section on '8063/'7747/'9746, 747fuel link (or similar).

RBob.

RBob,

The only connectors are the locking wiring connectors.

I've searched till my eyes have crossed, but have yet to find what I'm looking for. I'm sure that the very lean idle is part of the hestiation that I have off idle unless I baby the throttle open.

Try as I might, I have yet to unlock the secret of getting the idle right.


Edited to add:

After re-reading a LOT of the stickies, am correct in thinking that in order to richen up the C/L idle, I need to significantly raise my VE 2 table since it adds fuel accross the board at idle RPMs?

TIA