Runs Smooth... And Runs Ruff... Why???

[icanfixall]

Sometimes my engine runs so smooth you can't feel it running but you can hear it running. I have the rotational parts very well balanced for a 3000 rpm engine. Then it runs like I'm loosing part of a cylinder. Its nothing like a valve leaking or a stuck injector. It just runs ans shakes like a ruff running engine. When this ruff idle is felt the power seems down too. But when the smooth idle is felt adding the skinny peddle shoots you off down the road lickty split. It revs up really easy and smooth but when its ruff idleing the rpm seems to slowly climb with stoborn resistance. Fuel mileage has dropped 1 or 2 mpg but thats not a big thing. I know if it was a bad valve it would be happening all the time and same with a broken ring. So my thoughts are a sticking injector or something in the pump. I run a Carrier lift pump. A moose pump and a 7.3 Motorcraft fuel filter. Any ideas or thoughts are readily accepted here. Thanks all.

 

[Wyreth]

Just a WAG, but could you be getting a little air in the fuel? Just every now and again?

Have you tried the atf in the filter flush yet?

 

[freebird01]

thats kind of what i was thinking. random air intrusion....maybe a cracked hose or a loose clamp somewhere?

 

[PwrSmoke]

My truck has done that since the first day I drove it 25 years ago. Sometimes it will idle glass-smooth and others it's a bit rough. I never feel any difference in power but what I have noticed is that it's smoothest when it's the hottest. Like a long freeway run. It's not so rough that you feel like it's dropped a cylinder, just a little rough. I also used to think it ran smoother on summer diesel vs winter. In '90s Colorado, they had Total stations and their summer fuel had a very high viscosity vs others and the truck seemed to run the smoothest on that. Also, I ran a tank full of B40-B50 in my truck about 5 years ago... really smooth on that too. Not sure if any of this helps but I always had it in the back of my mind that it was fuel related in my case.

 

[mobilemech]

I have had this very condition happen before, what I found was the rubber line from the hard line on the frame to the pump was starting to break down. would not leak fuel but would allow air ( randomly ) into the system. I see you run a carrier pump but you still have to supply the pump, could be an area to loook.

 

[mariner45]

I would look into a sticking injector - I have had that on newly rebuilt injectors. If you have a pop tester you should be able to narrow it down to likely culprits and then test to be sure. Even a new injector can sick and it is not a bad idea anyway, to take them all out and test them.

There are other things that could be the reason, but the sticky injector would be the first thing I would check for.

Cheers

 

[mobilemech]

a stuck injector dosent usally clean it self and then have the problem return, but it is a good idea to check. how many miles on the fuel system? did it just start this or has this problem surfaced after changing parts?

 

[vegas39]

I'm going with bad injector. I pulled my injectors out after we got back from California because mine had been running a little rough for the past couple years and lately I devoloped a knock in one of my cylinders. The shop that tested them found 1 bad one and after correcting the problem and reinstalling them, my truck is running much better now, aside of my hot air intake problem.

The white truck we brought back from Calvins is one of the smoothest running diesels I've ever seen and it as far as I know, is still on its original injectors. I really think these Delphi injectors are crap and would be curious to see what the difference is internally between the Delphi and original Stanadyne unit.

 

[Agnem]

Air or bad injector. Air will make an engine stumble, and miss in non-consistent ways. An injector can do that too, but more often than not an injector will cause a steady miss. Unfortunately, an injector's performance does and can change with temperature, or the amount of fuel being injected, so sometimes pop-testing a cold injector doesn't reveal very much. In most cases, you just have to swap out the whole set to be sure.