Autonerdz forums

Hi. New to the forum and to using a scope to diagnose problems. I only have an OTC/CJMAX Palm Scope (with Pico envy) so far and grabbed these scans with it. The vehicle is a 1993 (OBDI) GMC Van with Central Port injection(CPI). It starts and runs, but stumbles and often dies as you try to accelerate. I have checked and/or replaced the usual cast of characters to no avail. New fuel pump, ignition module, MAAP sensor, TPS, no vacuum leaks, new cap and rotor, new fuel filter, good fuel pressure. After it stalls, it can only be started again by squirting some gas into the intake. Then, it will run until the gas burns out of the intake. If I let it cool down for 30 minutes, it will start normally again.

I hooked up the scope to the fuel injection port on the computer and got the following pictures. The first shows it idling and running well, with the characteristic central port waveform.



After holding the throttle open to about 2000 rpms I get the following set of pictures, as it first stumbles, then eventually dies.









I am brand new to this scope diagnosis, so be gentle. I suspect that either the wiring harness to the injector body is bad, or the injector unit is failing. I suppose it could also be the computer failing, but I sort of doubt that. I am going to remove the upper plenum to gain access to the CPI, but wondered if anyone sees a clue in the scope pictures?

Thanks. I introduced myself in the Introductions section, but I am not a mechanic or technician. I am an engineer by training. I look forward to learning from you guys.

Thanks Mick. I will do it and post the results. This whole scope thing is new to me, so I am trying to learn the techniques. I think I also need to expand the trace in the area where the injector actually opens (where it drops to zero and the pintle opening bump occurs) so we can see what is going on there.

I have seen a few of those injector(tbi) fail and look good on the scope, but I`m like yourself..well worse--scope illiterate. I just finished up a 95 transport with an intermittent stall..looked like an injector fault but it turned out to be the dist signal itself. Could not see it on the scope though.

Regards,
Dave

I just wanted to close the loop on this one. I finally got a chance to remove my upper plenum, and take a look at the connector and wires. The connector was coated with oil and the connector prongs were too. I also noticed that that the connector had some plastic broken off. I cleaned it all up and put it back together, and ordered a new connector. It seems to run just fine now.