rough idle, knock upon acceleration - Diesel Forum

radeon1314 · 11-17-2009, 02:55 PM

as title states, the idi in my sig has a rough idle. but upon acceleration of engine, while in gear, there is a knock, only 6 or 7 times real fast, then the engine runs smoothly, with no knocking. but then when coasting right after i let go of the pedal, the idle becomes rough again. Same situation whether the truck is not in gear. rough idle, acceleration with subsequent knock and then smooth. and the knocking and rough idle is intermittent, some days its fine and smooth, others rough and knocking.

any imput?

Thanks,
Kevin

osufireman · 11-17-2009, 04:40 PM

Has the IP ever been rebuilt? how old are the injectors? are the return fuel lines leaking?

I have a knock in the mornings upon start-up, but it goes away in a few seconds. I found it's attributed to a leaking fuel line, air intrusion in Cylinder #5 injector when it sits over night.

A bad injector could be popping too soon if it's worn out, causing the fuel to be injected before Top Dead Center. I'm not too familiar with bad IP's but I'm sure a worn out one will cause problems with inadequate fuel supply, or the wearing out will affect the timing of it. Fuel injectors can be tested by diesel shops or if there's an injector shop around (we have one here in Oregon) that would work too. It might cost a bit for the labor.

Can you tell where the knock is coming from with the hood up and listening to it?

Also, do you use any ULSD lubricity additive??

cdnsarguy · 11-17-2009, 06:23 PM

250K miles then I would suggest valves/guides/rockers may need a good going over. Consider doing a compression test....

As OSU stated could be injector induced how old are they......

radeon1314 · 11-19-2009, 02:44 AM

injectors and pump were replaced somewhere around 140k. dont quite remember exactly.

and after driving today, its kind of a tapping more than a hard knock.

Will be doing a compression test this weekend. what numbers should i be looking for?

and there is one of the returns that is slightly damp with fuel, the cylinder closest to the fuel filter.

also, is there a procedure to check the valves, guides, and rockers?

Thanks, Kevin