Erratic engine operation

[GMC-John]

My 1994 6.5l TD still runs erratically even after warm-up. I would like to know what component is failing if the engine runs away at very light throttle opening. You can run it up to about 1500 RPM but it wants to take off toward 3500 RPM. If it is suspected that this fault is caused by the optic sensor (variable vein position sensor) is there any chance that an fuel additive would help this or is it that the only fix is to change out the optic sensor? I am getting DTC 36. The other question is how do you increase the idle speed on this fuel pump (DS4?)

[DmaxMaverick]

Doesn't sound like the optical sensor. Your truck/fuel system doesn't have a "variable vein position sensor" (that's on 2004+ Duramax turbos). The optical sensor just reads a flash of light through a rotating disc inside the pump. Fuel additive won't help your failing PMD or its connections, which is what your problem sound like. You should not need to adjust the idle speed of the DS4, but you'll need a programmed chip if you do. Anyone who does custom programming for the 94-95 PCMs can do that, but you really need a reason. It will not fix anything. If it's idling too low, something else is wrong, and it should set an appropriate code.

[Robyn]

I agree with Maverick...

The DTC36 is a long "Pulse width error code"

This can be cause by a failing PMD.....But more likely it is a worn out FUEL SOLENOID.

This issue is something that needs to be dealt with by a Stanadyne certified shop.....

The solenoid is controlled by the ECM (Computer) and constantly meters the fuel flow to the engine.

An idle issue would/could also be indicative of a failing solenoid

*****This solenoid is not the one on top of the pump*****


You might try swapping in a fresh PMD and see if this helps.....

Easy thing to do before yanking the pump off.


If you do remove the pump..

Make a note (Map) of all the high pressure fuel lines.

Keep the lines in the pairs they are mounted in.

Scribe a sharp line across the top mounting flange of the pump and the timing cover (Use a straight edge and keep that like nice and crisp)

Then remove the pump......After the rebuild you will be able to replace the pump and get it spot on and the computer will be happy.....

I have done this many times over the years and it works fine..


Keep us in the loop.

[JohnC]

Robyn's advice is spot on. Your RPM is unstable because the solenoid control is unstable. Try the PMD first, and if that doesn't fix it, get the pump overhauled. Have them use your new PMD.

One other very remote possibility: I've seen a pump (one) where the resistor on the PMD was a huge mismatch. This made the throttle twitchy, but it didn't cause a runaway. Your DTC would seem to rule this out, however.

[GMC-John]

Thank you all for your input. Robyn...I am just up here in Seattle so do you have access to a good used or rebuilt pump for this application? We could possibly work out a trade/purchase. I can't see where I can PM you so sorry for the personal reference.
You may be familiar with Stanadyne so are you aware of their Seattle operation so that I can also contact them for help?
Thanks again.

[GMC-John]

Well guys (and girl)......I had the fuel filter housing replaced and this engine now runs like a Swiss watch. Maybe everyone (especially me) didn't understand the amount of fuel that was leaking down past the starter to the inspection cover of the trans. It now idles perfectly at 500 RPM and starts within 3 seconds either cold or hot. The tech that drove it for the evaluation both inbound and outbound said that the fuel leak was so bad that the IP was not even getting enough fuel to idle properly. What really surprised me was that the tech changed the filter housing without taking off the lower intake manifold. I could just barely see it, much less get it off.

[Robyn]

Great news.

Glad it was a simple fix......Getting a DS4 pump overhauled is not a cheap jump $$$$$$$$$$$$$$$$$$$$$$$$:eek::eek: