GM Multec shorted fuel injectors

So I don't know if this subject has been beat to death or not. I know you've got a few videos already on them but I had one come by the shop and managed to get some great captures. There's a before and after screenshot. I've also included the full captures as Snapon .vsm files inside the captures.zip. Even if you don't have a Snapon product you can download their viewer program, ShopStream Connect , for free.

It was an 89 Pontiac 6000 with the 2.8L V6 distributorless wastespark ignition system and group fire injectors. Complaint was stalling when warm and then hard starting. The stalling was very abrupt. No stumble or hesitation. Just dead. Then it would be a cranking no start. Wait a couple minutes and it'll fire right up.

The problem was shorted fuel injectors and the fix was 6 new injectors. The bad fuel injectors ranged from 4.4 ohms to 10.6 ohms at room temperature. The spec that I found was 12-13 ohms at room temperature. The new ones were all 12.2 or 12.3 ohms.

In the before capture the car is running then stalls after a rev. After which it becomes a cranking no start. I managed to catch the moment where the injectors completely failed. You can watch the current waveforms become squarewaves and the voltage spike dropping right off. The after capture has proper ramps without initial current jumps and has much bigger voltage spikes.

Unfortunately the before and after captures were a few days apart and I goofed on the capture colors... Sorry about that

Before capture:
Yellow - 20Amp clamp on power feed to all ignition coils
Green - 20Amp clamp on power feed to bank2 injectors
Blue - Cylinder #6 injector control voltage




After capture
Yellow - 20Amp clamp on power feed to bank2 injectors
Green - Cylinder #6 injector control voltage
Blue - 20Amp clamp on power feed to all ignition coils




Btw, I LOVE having two low amp probes... Especially when starting out on a job and looking for direction. Then I can just capture a whole bunch of stuff and see if anything jumps out.


Full captures as .vsm files:

Well, those images didn't survive the resizing that well... Here are some fullsize images from imgur.com

shorted fuel injectors - stalling
new fuel injectors

Nice captures, sir! I don't know that we've discussed this phenomenon that much, actually. Watched all the shorted injector videos, but never had one show up at work so I can make money on it! I do seem to get all the Ford EEC-IV cars, though.

Awesome that you linked to ShopStream, too. Noah and I both use it regularly, not for sure about others.

LOVE your avatar, where'd you get it?

I've also got a capture of the ignition bypass timing system before any repair was done. This one will just be a .zip of the .vsm file. The capture is of starting the car, it running for a couple seconds, revving it up, then it stalls. There's a fair amount of hash in it because the battery was a bit low and I just used a chassis ground.

I captured the Ref Pulse input, EST output and the Ign Mod Bypass control wires. I did all of these at the computer as I found that to be easier than the ignition module. The wiring diagram I used from Prodemand.com shows two "IGN MOD BYPASS CTRL" wires. One is.a TAN-BLK from pin A of the ignition module to C7 of ECM. The other is a BLK-RED from pin F of ignition module to D9 of ECM. The TAN-BLK went from 0-5volts when the ECM took over timing and the BLK-RED stayed at 0. I would have to assume the BLK-RED wire is a separate ground just for the bypass control..?

1 - Yellow Trace: TAN-BLK - IGN MOD BYPASS CTRL
2 - Green Trace: BLK-RED - IGN MOD BYPASS CTRL
3 - Blue Trace - WHT - EST OUTPUT
4 - Red Trace - PPL-WHT - REF PULSE INPUT

Haha I normally get grumpy when I have to work on pre OBD2 vehicles. I mean this thing was stalling consistently and not one single code... This time I didn't mind because my Snapon rep just hooked me up with a first gen Verus for a steal of a deal and this was my first time to really test her out. The shop has a Verdict. And although it's newer and you don't have to worry about keys I definitely prefer this Verus over it. The larger screen size makes viewing data and waveforms so much nicer. Not to mention the Verdict is only a two channel scope.

Thanks about the avatar! I actually got it quite a few years ago when I signed up on EricTheCarGuy's forum. I told my girlfriend I needed an avatar and that's what she drew for me. Turned out way better than I had hoped I told her that you liked it and it made her quite happy.

Well thanks for joining up here and tell Eric I said hi next time you're there.
Thanks for the great case study