Low Idle Issue

[jeffl1107]

Ok, this is the weirdest thing I have seen I have no idea what the problem could be. My truck, 96 Mack RD688S dump truck with E7 350. When the truck warms up, maybe after an hour, the idle slowly starts dropping in RPM's. By the end of the day it is really low, like almost ready to die if I dont keep it up in RPM's. But the odd thing I noticed yesterday is that it seems when I get up off the seat and exit the truck, the idle goes back to normal. It sounds normal while I am out of truck and someone was loading me. But as soon as I get back in and sit down, the idle goes back down again. What the heck could that be. I cant see how it could be something with the seats air system, but was thinking maybe something with the cab. I know it has some sort of air ride system on that and dont know if there is somekind of sensor or something. This has me totally baffled. What do you guys think???

[Mike]

Jeff,

Bare with us we are still thinking.

[Rob]

Ok, this is the weirdest thing I have seen I have no idea what the problem could be. My truck, 96 Mack RD688S dump truck with E7 350. When the truck warms up, maybe after an hour, the idle slowly starts dropping in RPM's. By the end of the day it is really low, like almost ready to die if I dont keep it up in RPM's. But the odd thing I noticed yesterday is that it seems when I get up off the seat and exit the truck, the idle goes back to normal. It sounds normal while I am out of truck and someone was loading me. But as soon as I get back in and sit down, the idle goes back down again. What the heck could that be. I cant see how it could be something with the seats air system, but was thinking maybe something with the cab. I know it has some sort of air ride system on that and dont know if there is somekind of sensor or something. This has me totally baffled. What do you guys think???

This almost sounds like a mechanical governor issue/wear. I can't answer as to the cause of a single human occupant causing the problem, as the truck will not lean under that amount of weight unless it's me in the seat.

Rob

[rhasler]

Jeff, I can't remember for sure if you have a mechanical engine or if it's a VMAC engine, but I've seen this happen on RD chassis with mechanical engines when the cab suspension parts are broken/damaged. When there air bags for the cab will not hold air the cab leans down in the back and limits throttle linkage movement.

[Rob]

Jeff, I can't remember for sure if you have a mechanical engine or if it's a VMAC engine, but I've seen this happen on RD chassis with mechanical engines when the cab suspension parts are broken/damaged. When there air bags for the cab will not hold air the cab leans down in the back and limits throttle linkage movement.

I've had three mechanical engines with worn governors that when the truck is anything but dead level, the idle fluctuates and or hunts for stability. This characteristic also changes with engine temperature as described. Of course I'm talking of the complete truck being off level and not just the cab. I'm certain with cab suspension problems the throttle linkage could be binding, but the heat related symptoms would seem unrelated and I'm assuming the truck idles fine when cold.

Rob

[truckboy]

First, never push the Schrader valve and let fuel squirt out. It's at 30-50psi and fuel flying everywhere around an ignition source is a very dangerous situation. If you want to check your fuel pressure, auto parts stores have free loaner tools that you can check out and return.

Second, it sounds like you could have a vacuum leak. I'd suggest going over each vacuum line and connection and looking for any cracks or areas that look dried out.

[jeffl1107]

Ok thanks for the input. I am going to check the truck over this weekend to see if I can find anything. Yes, it does idle fine for a while. I normally leave the yard with the truck around 5:30am and I would say around 12 or 1 in the afternoon is when I start noticing the idle getting lower. By quitting time around 3:30 it is really low, does not kill the engine but close to it.

My question is with you saying that the cab air bags might not be holding the air and is pinching the linkage to the throttle. If that was the case, why would it not have a low idle all day and just the latter part of the day?

[MACKS]

Just a thought,are you using the ac,compressor might be building up to much pressure.

[theakerstwo]

Its the cab air bag lowering and raising the cab.The pump is idling off of the throdle linkage instead of the idle stop screw. If the throdle linkage was disconnected the idle would be too low. I saw this happen on a superliner years ago and had been to the dealer a few times and they kelp saying it was sucking air.On my super liner in the morning when air is down i can hold the idle to maybe 1000 and as soon as the air bag raises the cab it jumps the idle to about 1500.