M1009 Slight hesitation then surging at highway speed.

[mktopside]

I'm not even convinced this is a problem, but wanted to know if anyone else experiences the same thing.

When driving at 55 or slightly above, flat grade, constant throttle, there is a very faint (almost imperceptible) momentary "lag", then subsequent (equally as faint) surge. It's faint enough to where it doesn't register on the speedometer and is really only noticeable after a few minutes at the same speed. There is a very faint audible change in the sound of the engine, like you let off on the throttle 2mm, then it surges and returns to normal for 15-20 seconds before it does it again. The frequency of the lag/surge increases on slight grades, but isn't noticeable on grades where you would have to depress the gas to maintain your speed. You can "power out" of this happening by depressing the gas, but it will return after a little while after you stabilize at your new speed.

If I had to make a standard automotive comparison I would say it feels like older cruise control hunting to maintain speed. If it happened at 30mph, I would say it feels like very slight slippage in the trans....but at 55+ I'm not sure that is the case.

All other aspects of drivability are normal, and other than this possible non issue the truck runs and sounds great. If the truck didn't run so well this wouldn't even be on the radar.

I'm an ASE certified automotive master tech, so I'm not a total moron; but my experience over the years is primarily with foreign cars and not diesels.

For all intents and purposes everything is as it should be. Fluids clean and at the proper level, nothing obviously wrong or out of the ordinary. Fuel return check valve was clean and clear (light visable through the glass ball) when checked.

I don't want to start throwing parts at it to make it go away just yet, but due to the barely noticeable nature and speed required to produce the problem I'm not sure how much I'll be able to figure out at idle in the garage. A hunch made me pick up a Racor spin on setup, and an electric fuel pump at West Marine while out at lunch.

So, anyone experienced this before? I'd like to head it off if it is something that shouldn't be going on.

Mike

 

[OL AG '89]

Have you tried replicating the problem in a lower gear. Same engine rpm's, slower speed... see if its a transmission problem or engine problem. I would think if you replicate in 2nd with same rpm's it would be an engine issue. If not it could possibly be a trans problem....
These are not "electronic" so as you know, pretty basic for troubleshooting. May also check the TM's troubleshooting section, may have something in there.

 

[southdave]

shot in the dark maybe somthing in the Vacume system for the tranny or your vac pump it self?

 

[mktopside]

Have you tried replicating the problem in a lower gear. Same engine rpm's, slower speed... see if its a transmission problem or engine problem. I would think if you replicate in 2nd with same rpm's it would be an engine issue. If not it could possibly be a trans problem....
These are not "electronic" so as you know, pretty basic for troubleshooting. May also check the TM's troubleshooting section, may have something in there.

Thanks for reading my post!!


I haven't tried it with the trans in 2nd, but Ive been paying attention at slower speeds and can't detect it until I've been on the highway for approx 20 miles (this may only be because that is the only straight and smooth part of my commute. DC metro traffic)

I acknowledge it could be trans related, but trans slippage would account for the slight deceleration, but I can't envision how it could make the surge in acceleration afterwards.

I used to run a simple water meth injection system on a VW VR6 with a Vortec V1 blower. It ran on a Hobbs pressure switch set to kick on at 6psi. When accelerating at full throttle when the boost hit 6psi there was a momentary lag as the meth turned on, then a massive burst in acceleration. It feels the same, but much less massive and more cyclic in nature.

Do diesel injectors ever "float" when going bad? Like hover in a momentary netherworld of half way open/closed until engine/acceleration needs cause a change in pressure that momentarily allows them to close? Is Injector spring pressure affected by engine temp?

I just haven't worked on enough diesels to compare anything to, I have my M1009 and my M35A3. Not knowing stuff is kinda cool again

 

[mktopside]

UPDATE: I'm 95% sure it's fuel related. There are two large long hills right next to each other on my commute to work. I coasted down the first one to make sure that the trans was in third. Normally I wouldn't accelerate that hard to get up the second hill, because the inertia on rolling down the first carries you up and over the second. This morning there was little traffic so I let it roll down the first, then gave it just enough gas to keep it in 3rd, but not enough to engage the kick down. "Lugging" it as much as you can in an auto. Truck ran like crap with the load of the hill and in 3rd gear. Not slippage, just crap.

I'm going to change out to the spin on filter tonight and report back. I've never experienced a clogged fuel filter, or a filter with water in it before on a diesel, so I think it is probably as simple as that.

Mike

 

[68impalass]

there is a small "bag" filter at the end of the pickup tube in the fuel tank. Sometimes they get clogged up with "stuff" and it gives the indication the fuel filter is clogged.

 

[kipman]

Mine has the same problem, but only after putting on a rebuilt IP.

 

[mktopside]

there is a small "bag" filter at the end of the pickup tube in the fuel tank. Sometimes they get clogged up with "stuff" and it gives the indication the fuel filter is clogged.

That's good to know. If it still does it after I replace the filter I'll look at that next.

 

[edpdx]

I have a suburban that does the same thing. It started about the time I noticed my dash lights and amperage lamp surging ever-so-slightly. Have your alternator(s) tested.

 

[mktopside]

I have a suburban that does the same thing. It started about the time I noticed my dash lights and amperage lamp surging ever-so-slightly. Have your alternator(s) tested.

I fought with a dim Gen 1 light a few months ago and ended up replacing Gen 1. No dash lights are on, and everything appears to be charging properly at this time. I assume your Suburban is a diesel? I'm not making the connection as to how problems with the alternators could effect the drivability of a mechanical diesel.

 

[pistonium]

I've experienced plugged fuel filters several times, and once the symptoms were similar to what you describe; so maybe your filter change out will fix your problem. I think usually the filters clog so much so fast that you sense a loss of power, but if the 'plugging' is slow and not severe, I think you can sense it in accelerator issues as you describe. I had this happen to me the second time I had to buy B5 and my tank got cleaned up by the biodiesel. (The first time was a 'loss of power' filter change).

One other time I had un-called for accellerator action, the IP (apparently) was the culprit. Look here: http://www.steelsoldiers.com/cucv/56488-injection-pump-rebuild-some-pics.html
for the cause of old IP's causing this kind of issue. I replaced my IP with a recent rebuild and the problem went away.

Good luck -

 

[mktopside]

I've experienced plugged fuel filters several times, and once the symptoms were similar to what you describe; so maybe your filter change out will fix your problem. I think usually the filters clog so much so fast that you sense a loss of power, but if the 'plugging' is slow and not severe, I think you can sense it in accelerator issues as you describe. I had this happen to me the second time I had to buy B5 and my tank got cleaned up by the biodiesel. (The first time was a 'loss of power' filter change).

One other time I had un-called for accellerator action, the IP (apparently) was the culprit. Look here: http://www.steelsoldiers.com/cucv/56488-injection-pump-rebuild-some-pics.html
for the cause of old IP's causing this kind of issue. I replaced my IP with a recent rebuild and the problem went away.

Good luck -

I've been driving it maybe once a week for the past few months. Only over the past week have I been driving it every day. I noticed the problem for the first time Friday when my tank was down to about 1/8th. The only thing I had done different on that tank was running a quart of two cycle oil I found while cleaning out my garage. Maybe it cleaned out some gunk in the tank and stopped up the fuel filter. We'll see, if the fuel filter doesn't fix it, I'll throw the electric pump on in case the lift pump is starting to poop out. I'll do it in stages and report back.

 

[Capt.Marion]

Is it just me, or does a whole quart of 2-cycle oil sound like a lot to add in a fill-up?

 

[Woodsman]

Is it just me, or does a whole quart of 2-cycle oil sound like a lot to add in a fill-up?

In one thread someone inserted a link to a diesel forum that discussed lubricity additives examined in a test. One of the additives mentioned was 2-stroke oil mixed in a 200:1 ratio. Unless my math fails me that should be about 16 ounces for a 25 gallon fill-up. I'm new at all this diesel stuff so I'm trying to learn, but a friend told me he usually uses a quart also, which was backed up by some military motor pool guys.

For what it's worth...

 

[mktopside]

Is it just me, or does a whole quart of 2-cycle oil sound like a lot to add in a fill-up?

Nah, all the cool kids are doing it

Problem solved:

Fuel filter. I installed the Raycor filter/water separator, added a bleeder valve to one of the ports, bled the system, fired it up and drove home. It's like a new truck with power I didn't know was there. Not a hint of hesitation in any way.

Thanks for working through it with me guys.

 

[OL AG '89]

mk,

glad to hear it was something "simple"
That's what I love about these trucks!!!!
If you are experiencing a problem, SOMEONE has the answer.....

 

[Capt.Marion]

Awesome, glad to hear you fixed it! General rule of thumb: if your diesel ain't running right, check the fuel filters!

 

[lavarok]

Mine has the same problem, but only after putting on a rebuilt IP.

Kipman - I see the OPs fix was the fuel filter, did you ever fix yours?

 

[kipman]

No, mine is in the rebuild of the pump, they sent me a second pump, it was better, but not perfect.

 

[BIG_RED]

Is it just me, or does a whole quart of 2-cycle oil sound like a lot to add in a fill-up?

I throw a whole quart in with every fill up on the '09. To make up for ULSD fuel. Seems to help the ip. Accelerates just slightly better with the 2-stroke oil.