Idle steadily increases for no reason. (Vid)

I did a cold start on my M923-A1 (15 degrees) and she started up fine but the idle keeps steadily increasing like it’s a runaway. I’ve been told it’s from air getting into the fuel system but I wanted a second opinion.

Were you using the hand throttle?

Dang hoses can look good but they will fool ya.

doghead said:

Were you using the hand throttle?

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Nope! She would start off idling normal then slowly climb to 1500 Rpms before I shut her down.

My 819 with the 250 does the same thing until the ip warms up, the rpm creeps up, I set my hand throttle to about 750 then as the rpms go up, I reset the throttle a bit, I have had other 855s do the same thing over the years, once warm they stay at a set rpm dead on. I think it is more to do with COLD clearances then air, the warmer it gets the better the fuel meters.

Remove the bolt from the throttle shaft. It is a 7/16 socket and wrench. Remove it and make sure the splines are turned clockwise to close. Start the truck and see if it idles high. This eliminates any linkage issue first.

That’s a good question! We had the same thing with a M936A1 wrecker steady climbing RPMS while working the crane. I also had the same thing happen with a civilian Peterbilt dump that had an older Cummins in it. Must be a Cummins thing because never had a Caterpillar do it with any model.

Ok finally figured out how to post videos, now you guys can have an idea what’s going on here.

I think your throttle linkage is not adjusted correctly and or your return spring is not functioning.

Throttle problem.

Also make sure up in the engine side of the doghouse that the hand throttle cable isn't wrapped funky around its throttle arm down there where it meets the pedal.

Yeah but if that was the case wouldn’t It immediately rev up like that after start up rather than creep up?

No. Vibrations can cause it to creep up. Especially on the splined section where the shaft and the throttle arm meet. If that bolt is broke, loose or the splines are stripped it will slowly creep up.

Does the throttle return to idle once you pop the gas pedal to full and back or does it stick WOT?>

The idle will creep up a little past 1500 rpms then Go back down to normal idle and repeat the whole process again. If I pop the pedal the RPMS jump but instead of falling back down it just continues to creep up.

It’s just weird because when I parked her she was fine and I didn’t have any problems. The only thing that changed was the temperature. Then again that doesn’t mean something came loose or broke in the linkage cable like you mentioned.

Could be an internal pump problem as well . Something sticking as well. I would go drive it and see what it does.

STILL think it is nothing more then cold clearances compared to operating temp clearances, the cold clearances are wide and sloppy allowing more air to be drawn in there by not getting the correct air/fuel mix, once at op.temp the clearances tighten up for a metal to metal contact which means the fuel is now metered correctly WITHOUT the unwanted air from sloppy clearances., As I said before, I have run into this more then a few times with older 855s (these were not mvs), I am not sure of a answer, on a few the ip was replaced, with these some the issue went away while on others there was no change. I even had a 855 powered crane that had this issue, just a extra 1/4 hr warm up starting off then great .

19Stroker95 said:

Yeah but if that was the case wouldn’t It immediately rev up like that after start up rather than creep up?

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I believe so. If I remember a little air in the system it would idle but if you blipped the throttle it would climb but slow to return.
Guys it's been a long time since I have worked on a PT system and most on the 809 series. Did they change the return line system on the 939 series. I thought the 939 had a check valve on the return. If the check return valve stuck would that not bring the rail pressure up bringing up the rpm?

Your fuel return valve is most likely sticking, or not holding the correct pressure. I've had this same thing happen on a few of these engines.

Floridianson, Two Stroke Madness,
Some M939 trucks have a fuel return valve but not all. Mine never had one but Simp's truck did. Mine never went through any type of rebuild but his did. I'm not sure what determines which trucks have them versus which ones didn't.

19Stroker95,
Look at the driver's side of the engine to the right of the front intake runner. Is there a round can with 2 metal lines plumbed to it? That's the fuel return valve which if sticking can raise injector rail pressure and cause your idle to speed up without throttle input.