[HQRP] Diesel impulse noise

[Ron L. Sparks]

Gang,

Thanks for all the input on the diesel QRM.  I am the "culprit" that asked
Ed the question.  What I am hearing is about S1 at an idle and S3 at highway
speed.  The vehicle in question is an F-350 Ford with the 7.3L Powerstroke
diesel.

The noise is very clearly some form of impulse noise and directly varies
with engine RPM.

Here is some info I have been able to gather/clarify:

1. Since the late 1980's or early 1990's many diesel manufacturers have
adopted computer control to reduce emissions and improve economy.
2. Older and non-computer diesels are clearly all mechanical and would not
have this problem.
3. The method of computer control varies greatly depending on manufacturer.
I have experience only on BIG diesels in this regard (2000 Hp and up).  In
these big engines there is a computer driven servo that shifts the lobes on
both the injector timing cam and the valve cams.  This would be very tough
to implement in smaller vehicle size engines.
4. The Ford Powerstroke uses high pressure engine oil to work as a hydraulic
pump of sorts to cause the high pressure trip required for the direct
injection system.  The technician tells me it works at about 2000 psi.  I
have not been back since I discovered the RFI so I cannot tell what trips
the hydraulic system.
5. There can be as many as four or five separate computer systems on this
vehicle.  It is "sensored to death".  I have been told by another diesel
tech that if the oxygen sensor ages or dies the engine will not start/run.
That means that the computer is definitely controlling something in the
injection timing and/or duration.  Ditto the available chip modifications to
add Hp and sacrifice economy (and vice versa).
6. Other Ford and GM owners have bitterly complained of this RFI to both
Ford and others.  Seems like mine is one of the quieter ones at S3.  A local
ham dealer told me that one of his customers was told that it was the
electronic fuel pump.  Mine is so directly and immediately RPM dependent, I
doubt that is what I am hearing.
7. Ford answered my e-mail promptly with a "go see your local dealer"
response.  I sure don't hold much hope there (except that I'll bet it would
be expensive).
8. The injection system is plumbed from the fuel header to some sort of big
blob that is integral with the front of the engine (like it drives off of
the timing chain).  The lines then go from there into the valve covers.
Lots of wires connect to various plugs on the intake manifold and valve
covers.  It is impossible to visually tell how the thing adjusts itself.
9. Probably the best hope will be to build a small coax noise sniffer probe
for my receiver and see if I can localize the component(s) causing the
noise.  If I can find it, I might can shield/filter it.
10. The noise is definitely coming in via the antenna, which is mounted
about 2 ft behind the cab at the bed top level.
11. I might look at the spectrum of the noise and see what the optimum
filtering scheme might be.
12. On the plus side, what I gain from this QRM is great fuel economy (17
MPG in a 21 ft long vehicle), good acceleration, and very good torque.  Oh
well, you can't have everything (easily) <grin>.

Thanks for the space.  I will keep you posted as the saga unfolds.  Further
comments are appreciated.

Regards,

Ron
KC5ODM

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