Im having some issues with spark scatter at idle when there should be none. Its not anything mechanical. With the est bypass connector disconnected, the base timing is rock solid at all rpms. I also have a new module and the problem persists. The problems come when the ecm takes control. It looks ok at higher rpms, but as soon as the motor goes to idle the timing is all over the place. The thing is that Im using a closed throttle table thats flat as a pancake below 1k. The stored timing also looks to be in order. Whats the deal? Its especially bad at lower rpms. The timing goes all over the place and the motor wants to stall out. Is there something in the code to look at, or is it a possible hardware problem on the ecms side?

 

Some ECMs/masks will use what's called "Spark Stabilizer" during idle which may be causing your problem. Spark timing is adjusted based on the idle RPM error. Speed too high = remove spark. Speed too low = add spark. Try zeroing out these tables to see if the problem is resolved.

 

The mask I have doesnt have the idle spark control. IIRC, the tbi ecms that got that where the later 8746's and others similar to it. Its really weird, I dont see anything that should be causing the spark to float around like that at idle, unless Im missing something. Is it possible for the commanded dwell or anything like that to cause the timing to vary?

 

how noisy is the map signal ?? a noisy map signal will cuase issues espcially with a surging idle. chain slack running up and down and what not

 

The timing set is only a year old or so cam and dist. gear are almost new as well and there isnt much slack in the system. With the ecm out of the loop, the base time is rock solid at all rpms and under revving.

The seperate timing curve for idle is based on rpms alone so the map is out of the way. With the ecm hooked up, the lower the rpms are, the more scatter there is. I know there is code in the loop that adds and subtracts timing based on the rate of rpm change but that should be out of the picture at steady throttle, but maybe its not.

This is just driving me nuts

 

I disabled that transient throttle/rpm routinen that the stock tbi code has. The throttle response is much nicer and its not bogging real bad anymore. Ill have to keep that routine in mind. Looks like it would make for real nice traction control.

As I thought, though, no effect on the idle. Ill have to keep digging.

 

Are the RPMs going so low that it's going into stall saver mode and reverting back to module control?

Is the timing moving in rythm with the RPM moving, or is it fluctuating even when the RPMs stay steady? I could imagine it being the ref wire, possibly, or the pickup coil maybe, if you're sure there's no idle spark stabilizer.

What about the open throttle table?

How much timing are you giving it?

 

It may get low enough to revert back to the module, but the fluctuating rpms seem to lead to the stall in the first place. Its a vortec headed 350 with around 9:1 CR. Im giving it around 24 degrees at idle from 400-1000 rpms. If the rpms where flakey but the timing stayed steady then it would be a simple problem with the ammount of timing. Instead the timing floats around when it should be steady.

The easiest thing to do was go though the code to look for some sort of crazy stock routine or something that I put in that could be doing it. Found nothing that would do it. I did find, in the tbi masks, the transient rpms spark routine that was messing up my throttle response and a routine that controls how fast the timing can come in from the tables after the engine starts. Its amazing how you only can find the problems when you arnt looking for them.

Im going to try a new pickup coil tommorow. Maybe its getting weak. I hope its that, because if its something deep in the harness thats screwing with the signal to the module or causing lost drps, Im screwed.

 

Come to think of it, though, it would be unlikely that it would be the p.u. coil since it seems to trigger just fine without the ecm.

 

thats the logical path. got and oscope ?if so grab a few shots of it misbehaving. i one time saw a 5volt output fial on a gm ecm and it was no longer able to keep the module overide voltage up to control dweel it would come up and fall down every few millisceonds. set a scope for 5 volts roughly 50msec and log the ground est enable est input and ref out. itll show itself rather quickly.

 

Found the problems. First, there was a mistake made by me that caused part of the dwell routine to be skipped. Fixed that and made it run a little better.

Biggest issue is that the MAF table wasnt tuned at very low flows. The engine would be fine but would then cross over to the lowest part of the table and go super lean and get stuck there in a funk. Didnt notice but the blms where momentarily shooting up to 180 in C/L and it was still lean (or misfiring). Didnt see it was more then just a momentary fluke untill I switched over from winALDL to the high speed datalogger. The timing chain and dist. gear where tight so the chunky start/stop idle was probably causing lots of twisting in the cam. So, it looks like I was trying to fix the effect of the cause, and not the cause of the effect.