Mild Thumping At Idle

[bigdog66]

OK, I have a 98 3500 12v with an 47RE auto. Everything is stock except I have removed the silencer ring and added a BHAF. When the transmission is in drive there seems to be what sounds like a very mild thumping along with a simultanrous rough vibration. I was thinking if it was possibly a rod knock. The truck only has 55,000 on it and it was purchased used. The truck did not do this when I bought it. I changed the oil about 1,000 miles after I bought it and again 5,000 miles later. I am using Valvoline premium blue

15w-40 and Fleetguard filter. This is my frist diesel so this maybe normal, but I don't think so. Never has run hot since I have had it and there are no leaks at all. There is no noise or vibration when engine is operating at speed and at idel oil pressure as per guage on truck(stock) is approx 42 psi and when driving it stays around just below the high line for normal maybe 70 psi? Thanks for any help



Chip

 

[Extreme1]

At idle, in gear, foot on the brake, what RPM is it at?

 

[illflem]

A/C should also be on

 

[bigdog66]

Yes to all the above and the rpm is approx 800rpm. it happens with or without the a/c on

 

[bighammer]

BHAF on the underside of the hood? (or on something else) Just a guess

 

[drees1]

Sounds similar to my problem, also posted. I'm thinking fuel knock and maybe a bad injector or something in the fuel system. Mine has done it for the 12k of 136k I owned it, and I think a rod problem would get worse fast. Any responses to my post might also apply to yours.



Doug Rees

 

[dieselburner]

Check for wetness around any of the injectors or high pressure fittings. All it takes is a slight leak at idle to cause a combustion knock on a diesel. I had this same problem and found #3 injector had a little discoloration down by the threads. After tightening, the knock went away. By the way, use a crows foot wrench to tighten the injector if it needs it. That way you don't need to remove the fuel lines. Also be sure to tighten fittings/injectors with the engine OFF. High pressure fuel can can blow right under your skin.



Sometimes people mistake a loose lifter as a knock because it "rings" under the valve covers and can be audibly transmitted to other areas under the hood of the truck. You can usually use a long screwdriver or a short broom handle with one end held up to your ear and the other on different parts of the engine to find what is knocking. Good Luck



Steve

 

[bigdog66]

That might be it

Now that I think about it, the truck has had a rough idle for a little while. When I changed the fuel filter it was very dark brown and brittle. No change in the idle but seemed to spin up RPM quicker. I will check the injecters and lines and re-prime the the pump also. When went to the Cummins place to see about a valve adjustment the mechanic told the best place to bleed the fuel system the large double nut on the front engine side of the injector pump v/s the banjo fitting. Sorry about the tangent but dieselburner's post got me thinking. Oh, I recieved my first oil test kit today. To bad I changed my oil last week. Thanks for the replys and keep it coming.



Chip

 

[bigdog66]

I forgot

I does seem to get worse with the a/c on