This might help out some that are having running issues with their TBI. I am about to try this on the 1991 G20 that I just bought, because it has a rough idle and injector waveforms that look similar the first pattern.

tech tip 139 GM TBI Belling

 

Now that is interesting. No doubt it shouldn't be ringing like a bell. The radiated noise from the ringing is going to affect the sensor signals (O2, TPS, MAP, . . .). Which will cause the driveability problems.

Fast355, when you go to add the cap & check it, first try a different ECM. I wonder if the culprit is really the ECM. There are capacitors in the injector driver circuitry to prevent this issue. It may be that they are either missing (doubt it), have failed, or are the wrong ones.

RBob.

 

I don't think that it is the ECM, because I have had two different ECMs in it and it has idled rough on both. I guess I will be trying the EBL in it just to see what happens.

 

i remember that TSB, it was a field fix for a problem i don't think GM ever got to the bottom of, or if they even really cared about fixing it.

on the vehicles that had that problem, even a brand new Delco ECM (read very expensive) would not fix it, only adding the caps would take care of it.

Fast355, thats a great site , i never heard of it until now.

 

So does this lend new meaning to the term "flux capacitor" ? (assuming that the Delorean had a TBI v6).

A cap in parallel across a inductor (the solenoid inside the injector that opens the injector pintle valve) lowers the input impedance (ZcapZind product over sum, Zcap+Zind) as compared to the inductor alone, and it also can create a resonance condition between the inductor and capacitor but only at the right frequency and that's unlikely to be anywhere close to any part of the spectrum of the injector pulsing.

OTOH if it's role is a shunt for very high frequency noise, thereby acting as a filter for the inductor, then it makes sense. If so, the it would be a band aid because the real culprit would be the true noise source driving the injectors. Of course I suppose it's easier for a shop tech to put a capacitor across the injectors that to pull apart an ECM and troubleshoot it.

I wonder if a small resistor (with the cap) would help, rather than just the cap alone. Fast355 -- do you have a scope + test probes that you can use?

 

The TBI injectors are 1.2 ohm and 2 mH.

The ringing starts when the driver switches from peak to the hold mode. The peak being 4 A and the hold 1 A. The drivers are the NS LM1949, which like to oscillate. The data sheet even shows how to use them in PWM hold mode.

RBob.

 

yeah, don't forget the inductance of up to 6ft of wire too, that will have a role. maybe GM couldn't fix the problem in the ecm due to harness length and connector resistance where a capacitor swamps out the resonance right at the injector, and due to different chassis' routings, couldn't make one fix take care of all cases. We're going to try this on my brothers S10, it displays some symptoms as described, and it kinda has gotten worse since he bought it.

the capacitor really doesn't need that much voltage rating either, something like a 50 or 100 volt cap would also work.

very interesting website, also.....

 

I was thinking the same about the harness. The full size vans have a harness that reaches to the rear of the drivers seat. Also thought about twisting the injector +12 v and driver ground together (twisted pair). Only problem is, the injector +12 v originates elsewhere. So to do that means a lot of work.

I'm also wondering if this is something that comes & goes. I wouldn't be surprised if it did. I've had a scope on the Firebird injectors many times without a hint of this ringing. But that doesn't mean that it may be present at other times.

Should use a minimum of 100 v capacitor. The inductive kickback is clamped at 57-60 volts.

RBob.

 

yeah, you're right. I've never looked at the diodes used in the ecm, I thought they clamped at less than 50v though.

 

Would this even do anything on a stock LO3 ? With a smooth idle ? That may be running rich though, sure looks to me like I am running rich.

 

I have the same problem and all research points towards me doing so also......... Not sure why though...

 

See I could deal with running rich for now, but problem is, gas is pricy here in CA, and I also have a smog check coming up. So if I can lean it all out a little bit, would be nice.

 

check out the adjustable regulator under the ultimate tbi lik, i'd pull a lb of pressure if i weren't so lazy myself lol.

my wb02 readings were from 10.9 to 11.3 last time i tested it which to me, means pig rich. If figure a more power friendly/gas friendly 12-13 to 1 would work better. I plan to back it down a lb or so if i don't get some sort of mods to make it runner a little leaner soon.

Either way i still get quite respectable gas milage for a 15 y/o car with 150k miles and a v8, last tank was 2/3 highway at 80-90mph the rest city and i averaged 265 miles to 15 gallons. haven't done the math but, i'm sure that's over 20mpg so i'm plenty happy.

 

[QUOTE=flaming-ford;3065701]check out the adjustable regulator under the ultimate tbi lik, i'd pull a lb of pressure if i weren't so lazy myself lol.

my wb02 readings were from 10.9 to 11.3 last time i tested it which to me, means pig rich. If figure a more power friendly/gas friendly 12-13 to 1 would work better. I plan to back it down a lb or so if i don't get some sort of mods to make it runner a little leaner soon.

Either way i still get quite respectable gas milage for a 15 y/o car with 150k miles and a v8, last tank was 2/3 highway at 80-90mph the rest city and i averaged 265 miles to 15 gallons. haven't done the math but, i'm sure it's around 20mpg so i'm happy.