Engine/Transmission (1998.5 - 2002) Engine stumble at idle

[ROADWARRIOR]

Today when I started the truck and engaged the exhaust brake, I noticed the motor was stumbling off and on while warming up. This is the same tank of fuel from the last two weeks and it didn't do it earlier. It drove fine but had a big stumble while idling later and then it ran fine the rest of the day.



I was wondering if I should be worried or if this can happen and go away. The fuel filter is fairly new. My fuel pressure is a constant 12psi. And I only use truck stop fuel with Lubricating Engineers fuel treatment every tankful.



Thanks, Dan

 

[Evan A. Beck]

Well, it could be your intake heaters cycling and putting a load on the engine. This, when combined with the exhaust brake, can cause the lopey idle you speak of. Does your voltmeter move when this happens, and is it only happening right after you start the truck? More info please!

 

[ROADWARRIOR]

This sounded different than the normal grid heater cycle. The engine would make quick stumble sounds instead of the long idle changes when the grid heaters are on. The second time it happened I had been driving for almost an hour. I had my foot on the clutch ready to take off and it sounded almost like I dumped the clutch and the motor was going to die, but then it regained it's composure and the rest of the day it idled fine with or without the exhaust brake. I hope you are right and I am just too paranoid.



Thanks, Dan

 

[lberch]

I too have what I call an "intermittent stumble". It is only at idle and two months ago when it started, it was only when the engine was cold. Now it will do it when the engine is hot or cold, sometimes once, sometimes multiple times over, and sometimes it goes away for weeks. It is definetly a stumble, like a cylinder does not fire for a split second and then it recovers.



I was convinced that it was either a bad load of fuel and or a carboned injector. Went through the suspect fuel. Changed the fuel filter (clean) and got half way through second tank with a generous amount of LUCAS cleaner and the stumble went away for three weeks. It got realatively cool (for where I live) 65F, and whammo! the stumble came back. It was not my heaters cycling.

I thought it very strange that it came back immediately when the temp dropped a little bit.



So far I have checked the lift pump pressure and have 15+ psi @ idle, drops to 10 psi @ WOT (static) and a drop of two psi @ the injector pump.



I measured the APPS voltage out (to see if it fluctuated when the engine stumbled - no fluctuation)



I "re-learned" the APPS to the ECM several times - nothing



No DTC's have been set



I have "wiggled wires and harnesses" on the IAT, Map, Camshaft sensors as well as the ECM & PCM - nothing.



I plan on measuring voltage out of the IAT, Map & Camshaft sensors to see if anything is fluctuating. Short of that I am at loss. If it is a sensor, I almost wish it would just fail so I could fix this. Can't stand intermittent problems.



LB

 

[lberch]

Measured sensor voltage from the MAP, IAT & Camshaft sensors last night. Both the MAP and IAT were steady and rose up and down with the throttle. The Camshaft voltage was strange. It was relatively steady around 2. 8v, but would not move much with throttle. Anyone ever test this sensor like this?



LB

 

[Tomeygun]

per repair manual at my dealership



voltage must read 4. 2 - 5. 5 volts, (correct me if I am wrong)



but i thought that it would set a code if not with-in spec,

please let me know if you fixed the problem...

 

[ROADWARRIOR]

I haven't had any problems since I posted this, it seems to have gone away on its own.



Dan