94 GMC 2500 truck... uncontrollable throttle

[vstech]

I have a link on the other Diesel place forum...

not sure if linking to another forum is allowed, I will link to it if it's allowed.
the link is to the work I've done so far on my truck, and the symptoms as they progressed.

anyway, my truck is impossible to drive. starts fine, but the idle is 2K, and it rapidly jumps to wide open throttle (rack)
if I unplug the Optical Sensor cable, the truck is a little slow to start, but idles fine, and responds to the accelerator normally.

I was hoping someone on here can point me to what to check/fix on the truck. I ordered a new IP, but doubt it's that, since the optical sensor disconnect regains control of the truck.

what does the OS control? I'm guessing that whatever that is, is the problem.

thanks!

 

[ak diesel driver]

have you tried a new PMD?

 

[vstech]

yup. the PMD is a new stanadyne (heh, I mistyped satandyne...) from leroy diesel with 6' extension and sink... I put on a known good one... directly to the IP... no change.

I disconnected the APP, no change.

I disconnected the CPS, no change.

I cleaned all grounds, and battery cables, no change.

I unplugged, and plugged in the computer connections... no change.

[video]http://api.viglink.com/api/click?format=go&jsonp=vglnk_jsonp_14098492241418&key=6b8b55cd222eaa5d60a6e1d117a31de8&libId=53a51049-d598-40d8-a99d-d5e34ec64b43&loc=http%3A%2F%2Fwww.dieselplace.com%2Fforum%2F63-gm-diesel-engines%2F21-6-5l-diesel-engine%2F641762-94-gmc-2500-truck-surging-throttle-4.html%23post6428418&v=1&out=http%3A%2F%2Fyoutu.be%2FkJkWxPZsqRc&ref=http%3A%2F%2Fwww.dieselplace.com%2Fforum%2F63-gm-diesel-engines%2F21-6-5l-diesel-engine%2F&title=94%20GMC%202500%20truck...%20surging%20throt tle...%20-%20Page%204%20-%20Diesel%20Place%20%3A%20Chevrolet%20and%20GMC%20 Diesel%20Truck%20Forums&txt=Cab%20test%20-%20YouTube[/video]

 

[THEFERMANATOR]

The OS gives the ECM feedback for everything the injection pump does. It reads a disc atatched to the injection pump shaft, and the signal it sends back to the ECM tells it the exact timing of the pump, how long it injected fuel, how quickly the fuel solenoid closes, RPM's, and so on. It is the spinal cord of the DS4 injection pump basically, it relays all of the data to the brain(ECM) of what teh inejction pump(body) is doing.

 

[vstech]

yeah... that's what I figured.

I'm guessing I need a new PCM...

 

[THEFERMANATOR]

If unplugging teh OS makes it run, then chances are you have a clouded OS eye. A new pump will fix it if that is the case. Many have replaced just the OS, but finding them can be tough. Also make sure you have the noise filter removed from the OS harness to see if it has failed. Some had problems with the filter degrading over time, and giving false OS readings to the ECM.

 

[vstech]

I swapped in a OS from a used one. no change. should I clean the OS, and reinstall and see if it fixes it? this all started soon after I put in some commercial Biodiesel. and, yeah, the noise filter is gone as well... I tried everything I could think of on this...

 

[Matt C]

IIRC removing the OS filter on OBD1 (94-95) trucks is not recommended. It's been found to not be necessary on 96 and up OBD2 trucks. Someone can correct me if I'm wrong.

 

[THEFERMANATOR]

IIRC removing the OS filter on OBD1 (94-95) trucks is not recommended. It's been found to not be necessary on 96 and up OBD2 trucks. Someone can correct me if I'm wrong.

It was put there to prevent interference with the radio on AM stations, that was it's sole purpose. Removing it will interfere with your radio reception for AM stations, but that's about it.

 

[JiFaire]

That cable is famous for shorts, too. It runs under lots of hot vibraty stuff.

Given that it started after a shot of bio, I am thinking OS, or something in the optical disk. One way or the other, probably crud in something.

Like Ferm says, it's feeding false info to your PCM. Which, in a fly-by-wire setup like yours, is bad news.

so- find the crud and clean it, or swap the IP. Neither are simple, one is cheaper. The easy fixes (other than checking for a short in the OS wiring) are done.

Good job on the troubleshooting, by the way!

 

[vstech]

I changed the OS, and it made zero difference... so I changed the IP. smooth controlled idle again!

now to get all the rest of stuff done that the truck needs...