SeanB23

Ever since I replaced my HG, put in a 3200 RPM spring, and cranked up the fuel screw, the motor in my truck has vibrated somewhat at idle. I mean, you can see the motor shake a little bit.

It was much more pronounced at first, but I think I've pretty much bled all the air out by now...it still shakes a tad, though. Quite smooth once rolling, this shaking is only at a stop. Tried cracking each injector line, this results in a similar rough running for each cylinder until the line is re-tightened.

Replaced all the seals on the lines from the filter through the IP. Three sets of two seals on each bolt, the new ones from Cummins are much nicer than the old ones that were on there before.

Tried adjusting the idle screw up and down, but it's the same no matter what.

Could there be a leak somewhere that I'm unaware of, or is this just normal on 6BTs that have been "turned up" ?

Or maybe a little crud got lodged in an injector ? I am running B20, this does have a tendency to clean out the entire fuel system on a truck that's never had much Bio...I'm of the opinion that rotary pumps require lube with this ULSD, and top-shelf B20 is the best option. Perhaps I should change the filter yet again.

Opinions ?

brainfade

They aren't the smoothest things at idle (when I start mine in the winter it's like sitting in a paint shaker) but it sounds like you may have more vibration than normal.

Might try a heavy dose of cleaner, alot of guys have good things to say about Sea Foam. And yes, B20 will scrub the tank and lines pretty well.

Another possibility is a damaged injector tip. If one got dinked up a bit during handling it could cause a rough idle but run ok at speed.

Doesn't sound like an air leak if it runs good and makes good power at full throttle.

I'd just run it for a while, as long as you're not doing any heavy towing or running high EGTs for long periods it won't hurt anything and it may clear itself.

I'm sure some others will have more ideas.

wannadiesel

Any idea on the idle RPM? They will idle as slow as 450 RPM but it gets to be hard to see out the mirrors...

SeanB23

Is there any way to gauge the RPM, or is it just a "by ear" kind of guesstimate ?

wannadiesel

If you have any friends who are into R/C planes they will have an optical tach. A piece of reflective tape on the crank damper and you are all set.

I use a scan tool I bought at a yard sale to check RPM on '92 and '93 trucks.

Alwaysworking

thats what i use,

running bio fuel will kill the diafram lift pump.

IRoss

I turn the fuel screw in until the mirrors stop shaking

TractorMike

Mine shakes pretty good at idle sometimes.. It all depends on how hard i've run it and how hot it is

SeanB23

So I was looking at the lift pump. One of the fittings that goes into the top has a plastic part to it that is broken. It's hard to describe, it's an L shaped fitting that comes out of the top of the lift pump and connects to a line going, I'm guessing, to the tank.

The fitting itself on the top of the lift pump looks to be brass, the plastic L-shaped part that connects this to the tank line is black plastic and gray plastic. The gray plastic part on the inside is cracked and missing a big chunk.

Could this be letting air in ?

What is this part called, part number, etc ? Is this a commonly available part or Cummins only ?

Thanks...

wannadiesel

It's the quick disconnect fitting. It's possible you are getting air there. It's a Dodge part if you can get it.

SeanB23

Autozone doesn't have it, Schucks doesn't have it, Cummins is closed until Tuesday. Do people ever just yank this out and replace it with something common, like a length of fuel hose with clamps ?

Or if it is a must have, where can I find part name/ number ?

wannadiesel

Cummins won't have it. It's a Dodge part, a quick disconnect fitting. It does not look broken from the fuzzy pic provided.

SeanB23

It is broken, sorry about the fuzzy pic. The grey plastic part that sits inside the black plastic part of the fitting is totally cracked and a big ol' chunk is missing from it. You can just turn the whole thing back and forth freely, but *that* could be normal for all I know. It's definitely missing a chunk, maybe I'll try to get a better pic.

wannadiesel

OK, as long as it can't just pull off that's not necessarily a problem. Sealing is accomplished by the black piece and the o-rings inside it. If it does not leak when the fuel tank is lightly pressurized, don't worry about it.

SeanB23

Here's something weird I just noticed while repairing other things on the ol' clunkier.

When I first start the motor up after it's been sitting for like 12 hours or more, it runs very smoothly for about 3 or 4 seconds. Thereafter, and upon starting unless it's sat for a good long while, the vibration and shaking at idle returns. Doesn't shake much or at all at speed.

This has to be evidence of a tiny air leak somewhere, right ?