Throttle surprise.

[valyidol]

Hi!
Sorry, I do not know how to say it correctly in English. Today my diesel presented a strange and unpleasant surprise. I was driving slowly in traffic. I did not touch the accelerator pedal. Suddenly diesel began to gain speed. I saw in the mirror black smoke. I braked and turned off the engine. After a few minutes I started the engine and drove off. The day is repeated three times. What could it be? I know bad PMD stop the engine. Why diesel accelerates itself?

 

[valyidol]

Sorry I forgot. 94 Chevy Suburban 6.5 diesel.

 

[ak diesel driver]

Change your PMD

 

[WarWagon]

X2! Bad PMD also allow diesel to accelerates itself.

 

[JayTheCPA]

I had an injection pump (IP) do something like this as well but to a much lesser extent (it was brief in duration).

Even if the PMD is bad, consider also doing the 2,000 RPM test to make sure the throttle position and RPM's closely match each other. Naturally, only do the 2,000 RPM test with a known good PMD.

 

[Will L.]

When anything crazy happens with the engine so do #1 is to try different pmd. If that is not it, remove and clean the engine ground wires. Always have a good spare pmd in the glove box.

 

[hookedup50]

The really scary surging(think depends) has always been a PMD in my truck. Slight surging and fishbite tends to be grounds. Thats my truck though. I keep two bumper installed PMD's to keep me going. Call me superstitous, but PMD's behave longer when there is another right next to it waiting to take it's job

 

[GM Guy]

it sounds like your PMD is shot. Leroydiesel.com is where I get my PMD parts.

While you are working on it, clean all the grounds and ensure the lift pump is working correctly. If the OPS is dead and not letting the LP run, Leroy also sells a nice relay kit to take the load off the OPS.

 

[valyidol]

I had an injection pump (IP) do something like this as well but to a much lesser extent (it was brief in duration).

Even if the PMD is bad, consider also doing the 2,000 RPM test to make sure the throttle position and RPM's closely match each other. Naturally, only do the 2,000 RPM test with a known good PMD.

I'm sorry but I do not know what is the test in 2000 RPM. Tell me pls.

 

[valyidol]

I installed another PMD. The problem went away. Thank you!

 

[ak diesel driver]

Slowly increase the throttle until you get to 2000 rpm, Athens slowly decrease back to idle. It should stay smooth and consistent all the way up and down no hiccups or hesitation or surges.

 

[JayTheCPA]

I'm sorry but I do not know what is the test in 2000 RPM.

As AK mentions, it is a comparison of the throttle response / position to the RPM's (per the dash gauge).


Need the motor warm (above 170F) and all accessories (radio, air conditioner, lights, fan . . .) 'Off', and the transmission in Park.

1) Slowly press the throttle to 2,000 RPM and observe the tachometer.

2) Hold the throttle at 2,000 RPM's for ~3 seconds.

3) Slowly release the throttle back to idle.

4) Wait ~ 3 seconds and repeat the cycle (steps 1 - 4) 10 times.


If the IP and grounds are good, the RPM's will closely match the throttle position each time and remain consistent from cycle to cycle.



If the IP and / or grounds are questionable, one (or more) of the following will occur:
> RPM's will want to 'run-away' (keep going higher even though not pressing harder on the throttle).

> The pattern from cycle to cycle is different (example: Cycle 1 has a 'flat spot' at 1,000 RPM while increasing, Cycle 2 has a 'flat spot' at 1,800 RPM while decreasing, Cycle 3 looks normal, Cycle 4 looks normal, Cycle 5 has a 'flat spot' at 1,500 RPM while increasing . . .)

> The RPM's quickly increase, but slowly decrease.

> The RPM's slowly increase, but quickly decrease.

> Any other type of observation that is anything other than consistent each cycle and matches the throttle position.

 

[valyidol]

OK. And what to do if the trottle will not match?

 

[Will L.]

Rebuild or replace injector pump.