A curious thing happened. I disconnected the battery to clean off the terminals and when I put it back together, I couldn't get the car to start. My BLM's were never wildly off, so I didn't expect losing power to be an especially big deal. I eventually got it started by holding the gas down after it sparked off, and even at that it was idling very low (650 or so). At the time, the IAC values were like 130-140. I let it go for a while, shut it off, and now it seems to start fairly normally, not great.

So my question is, what do you think happened? It's like the IAC parked wrong or something and got miscalibrated maybe. But then, why wasn't it opening even further to try and get up to fast idle RPM?

Is there anything in the BIN that has any real significant effect on how easily the engine fires up? I swear it used to start up better (I haven't driven much over the winter), but it's fairly marginal in any case. I have to crank it for like 3-4 seconds on a typical start (after the pump stops buzzing). The newer EFI vehicles (not to mention my '86 mechanical diesel) seem to fire up almost instantly.

This is a 7747 by the way. Thanks for any pointers.

 

The '747 saves all 16 BLM values at key off. At key on they are compared against tighter min/max values and will be set within these if found outside of. With a loss of B+ the '747 ECM will set all 16 BLM values to 128 (0% correction).

There isn't anything else in the ECM that will change with the loss of B+. You may want to data log and and if required trim in the fuel tables.

TBI vehicles take more crank time in cold weather then port systems. A port system darn nears squirts the fuel into the chamber. Whereas a TBI being wet flow needs to build up fuel in the manifold first before it reaches the chambers.

In cold weather I'll crank the engine for a second or two, pause, then crank again for a start. One problem I've found is that with the TBI spraying fuel above the throttle plates, is that it will pool on top of them. With the amount of fuel required on a cold start this can be a problem. Hence the pause after the inital crank. To let the pooled fuel to drain into the intake. The cam also has a lot of influenece on this aspect.

RBob.

 

Hmm. Something weird definitely happened in this case, like it really would not start without major effort. I've started it anywhere from 5-95 degrees without much trouble. It seems to be back now, so I guess it's not that important.

One thing I did notice in the course of checking this stuff out is that I have no "crank signal" hooked up to my ECM at all. This is an aftermarket harness and the guy who made it either forgot or didn't think it was important. The starting isn't bad, but I think it could be better. Do you think it's worth hooking up the crank signal to the key switch or starter solenoid? Did every version of 7747 use the crank signal?

 

Many years ago when I did my first retro-fit I went to great pains to hook up the crank signal. Thinking that a crank signal would be used for starting it had to be important. Over the years as I dissected the code I came to discover that it is only used for the fuel pump relay diagnostics. The last retro-fit I did it isn't even hooked up. Maybe some day, but why bother when the majority of calibrations turn off the fuel pump relay diags.

In short, nope, not needed.

RBob.

 

Thanks, that saves me unnecessary work. I thought they modified the pulse width or something for start.

 

As a tech, I have had the joy of changing hundreds of batteries, after several gm cars having idle/starting issues after batt replacement, we started connecting a backup b+ source to retain the ecm memory. In the event we forgot, I found that by "showing" the ecm the iac's hi & low range, the car would usaully start & run normally, this was done by grounding the aldl connector (a+b) for approx 30 sec (drives the iac closed) with KOEO then unground connector (drives the iac open), then turn key off 30 sec. This should set the iac full open & the ecm will expect it to be at 145 counts (full open). Restart engine, idle should be high and slowly return to normal idle. My Vetronics Tech1 & Tech2 have iac relearn options. Sometimes its faster to use my code key then to get the scan tool & navigate through the menus.

 

Thanks for the info. I suspected an IAC issue when I finally got it started and the idle was very low with high IAC numbers. I didn't realize 145 was full open. I think I was just about there and still only idling around 650.