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View Full Version : wondering unstable rpm above idle in neutral l



Coal Roller
12-01-2016, 20:25
Last year before I had the IP replaced, if i pushed down on the peddle a bit to increase the rpm it would jump up for a moment then fall right back down to idle, then i would push a little more peddle and it would go to say 1200 rpm but would wonder a few hundred rpm up and down,even though the pedal was not moved. After I replaced the pump it was still difficult for the engine to stay in a stable rpm.

That was a year ago now and it seem to do the same thing but a little less pronounced, it almost sounds and feels like an injector(s) are not delivering a consistent dosage of fuel at a set rpm, giving an "missing" rough kind of sound. This roughness can sometimes be felt when driving, especially in overdrive and with the engine just putting out enough power to maintain speed. It goes away during deceleration and acceleration. BTW the truck starts like a hot dam on cold mornings and idles like a clock, no smoke on start up either.

Recently I moved the remote pmd and cooler off the intake manifold and put it on the wheel well because it was getting hot, even on the heat sink. Now that the weather is cool the pmd is staying almost stone cold in it's new location.

Im thinking that the recently new pmd has been compromised by spending the summer getting cooked mounted on the manifold and that the circuitry is not providing a stable signal to the pump.

I was wondering if anyone else has a problem maintaining a steady rpm, say 1200-1800 rpm in neutral.

Im thinking of buying a JMS PMD at http://www.jmschip.com/jms-pump-mounted-driver-for-1992-2001-gm-6-5l-diesel-engines/
I haven't found too much information about peoples experience with this unit, but it sounds like a whole new design.

More Power
12-02-2016, 08:35
With regard to engine speed, the electronic fuel injection used with the 6.5L diesel doesn't respond like you might expect if the transmission is in neutral or park. In fact, engine speed will vary a lot like you describe - it's normal.

When you drive the truck though, the accelerator pedal should produce a predictable engine speed. That lets you know that the system is working correctly.

Haven't heard a thing about the PMD vendor you mentioned, which is odd. We always cover products that promise to improve reliability or performance for the 6.5. If they do have a "new" PMD design, they should have contacted us.