Hello all, I have a 1994 Tahoe TBI 350 I am playing with and having one annoying issue. It will stumble from a stop with spitting (backfire with flames) through the TBI if I am in Park and blip the throttle. It will also do this from a rolling stop, complete stop, etc. I have noticed in the datalogger that it will drop to 2 or 4 or 6 or 9* of advance during the stumble and spit. Once it breaks over it will run like a raped ape and pull very hard. It is just this low end issue and can not seem to find why. Any suggestions would be greatly appreciated. -Kenny

I am no expert, but my car had a similar problem. It ended up being a bad cap + rotor, and I replaced the whole dizzy with an autozone unit becauase I wasn't sure if the ign module or p/u coil were usable any more. Also, the points inside were rusty.

I cant say if this is your problem though. I never scanned my car when it was doing it.

My best bet would be that the violent backfires are registering as Knock, and your car is pulling timing to try and fix the situation. Does the SES light come on, because mine never did (however it does work).

Are you datalogging and chip burning?? If so have you changed the timing at all or played with the ve tables?? You may be running lean and causing knock that will induce a timing reduction. Are you running a stock engine?? Or do you have some Mods. What ecm?? Also when did it start doing it?? Sorry for all the Q's but trying to get as much info as possible to diagnose the problem. My current combo did the same thing untill I did some tuning do to the cam/intake/Headers etc.

It is a 7427 ECM and I am using $0D to edit.

The motor is a 350 Vortec crate engine with a TBI intake and Holley 670 CFM throttle body unit. Long tube headers and free flowing exhaust w/cats.

The only tables I have moved were VE up top as it was running rich at high RPM and load and spark. Again up top under heavy load.

It is not backfiring out the tailpipe...it is up through the TBI with flames (at times).

I have noticed on the datalog:

TPS % = 1.00 (not 100%) at WOT and 0 at closed with immediate reaction from TPS as soon as there is deflection. I am checking the TPS which came with the Holley kit for proper outputs. 50% Throttle shows 0.50% 25% = 0.25% etc.
Timing = Set to 10* with service enabled (spark control stabilization off). Shows exact timing advance with light that is stated on datalog for spark reference.
Fueling = 13psi_g BLM is dialed in at 126-128 at idle. AFR is at 14.5 at idle with proper sign wave on 02 sense datalog data.

If you need to know anything else to help...please ask.

Thanks for your input so far guys!!

-Kenny

On the datalogger it is not showing any knock retard when any of this happens.

As far as the timing goes, there is also a transient spark retard based on how fast the throttle is opening. This helps control knock during sudden throttle transitions. Additionally, at low RPM/high MAP, there isn't alot of timing in the spark tables.

Dont forget that it could also be going lean during tip in. That will cause the symptoms as well.

Is there a more "defined" $0D available? I am using the one from TunerPro's site.

Have you played with the Ae enrichment at all?? YOu may need a little more pump shot do to a larger throttle body and minor mods etc. Maybe try increasing the tps Ae or map Ae values where it is popping. I cant remember if the 7427 has this exact set up but its worth a shot.

I played with the AE in all tables one at a time. There are a few. No change.

What I did since my last post was to put the stock TBI setup back on. Backfire through the TBI is all but gone. You can force it if you try hard enough...but not a normal driving scenario will produce it.

Problem...no knock retard showing...but it will drop the timing off to 4-6* on initial roll on. If you ease into the throttle from a start (light feather) it will not lose the timing. If you depress to 1/4 or 1/3 from a dead stop it will drop all timing until it gets above 1600RPM or about 15-17 MPH.

Any suggestions?

Thanks guys!

-Kenny

Disable the Torque Management Spark retard by setting the constant to 0*.

That would make sense.

Is it defined that way in $0D?

I am not seeing it defined that way in the XDF file def...any other ways to find it?

I will look at it with a magnifying glass in the morning.

L414B ;-7.4 deg, max retard during torque management fuel cutoff
;

Thank you sir. I will look tomorrow morning and post back the results.

That was a huge help Fast! Thank you.

I also tweaked Cool Comp Spark v Load as it was pulling timing as well.

Final question I have for you. There is a Base Spark constant. Should that be set to what the actual base spark is (10*)? If left at zero does it become an adder to the spark tables? So base spark set manually to 10* and Base Spark constant at zero...20* on the table is 30* actual?

Let me know an address to send a six pack of your choice!

You might try BRDW, which uses mask $63 used for the Export L05 TBI with L31 heads on it from GM. It has ALOT of subtle changes from the regular TBI 350 bin. That .BIN was my base starting point for my 350 Vortec TBI.

There should be two spark constants, I don't know how they are labeled in your XDF, but one is the Main Spark Table Bias and the other is the Distributer Setting. Either one offsets the table. Stock Bias for the Main Spark Table is -9.84, which allows for negative numbers in the stock spark timing table. Most XDFs have the Bias compensated for in the equation for the Spark Table(s).

The distributer setting is actually subtracted from the main spark table. Meaning if you have the Distributer Initial Timing set at something other than what the physical distributer is at, you will get a net timing change. What that means is that if you put the PCM at 10* and the distributer at 0*, you will get 10* of retard on the whole engine. If you change the PCM to 0* and the distributer to 10* BTDC, you will get an additional 10* advance over the spark table. Finally if they are both set at say 10*, you have a net spark advance change of 0*. I ran ALOT of initial timing at the distributer with my 350, it started easier, ran better with the EST connection unplugged, and changed the firing of the injectors in relation to the longer duration camshaft.

Need to know where can I get BRDW with $63 definition file?