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Boost Pressure Surging, Intermittent

March 26, 2014, 10:02:01 pm

I have an issue that rears its (ugly) head after the engine is up to normal operating temperature (~180-185) and the cruise control is on:  I feel "surging" from the engine, yet the RPM's are constant and speed doesn't change.  However, the boost gauge shows the pressure will jump from ~5 psi up to 15, down to 8, up to 17,18, down to 5, up, etc. all within a 15-20 second period.  It will continue to do this until I disengage the cruise control.  Once I am driving with the pedal (manually), the boost gauge and pressures return to normal (no surging).  I figure that the waste gate valve is opening and closing, which would cause the boost fluctuation.  Sometimes turning the cruise back on the surging recurs immediately; other times I may get 10 minutes of normal cruise before the surging returns.

The crazy thing is that it doesn't happen every time.  I recently drove the coach 600 miles over 2 days with no problem.  Then, during a short (150 miles) trip, it started up again.

My U-280 has the MT647 Allison 4 speed, with the original Bendix air throttle and Bendix Cruise control.  I have a feeling that the cruise control is the culprit and they don't make it or support it any longer. 

Has anyone replaced their Bendix cruise and air throttle?  If so, with what?  Can FOT do something like this?  Will this surging harm the waste gate valve?

Any input would be appreciated.

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Re: Boost Pressure Surging, Intermittent

Reply #1 – March 26, 2014, 10:13:58 pm

Don, I do not know but if someone comes up with a credible reason to suspect the Bendix at all this is a simple system to take apart and service. I have done it on the 95 240 due to not going back to idle as normal. The cylinder would be my first line of attack along with cable. Let me know if this is suggested to be culprit and I will explain and send the manual to give info needed to check it out. As I said it is simple stuff to go thru, so do not worry.
JohnH

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Re: Boost Pressure Surging, Intermittent

Reply #2 – March 26, 2014, 10:16:07 pm

Don,

Yes, the Bendix cruise can be replaced with the King Control, leaving the air throttle in place.

Since it doesn't do it when using the foot throttle, you have pretty much identified that it is the cruise control at issue (at least I assume that from your statement).

There are things you can clean on the Bendix.  John just when through this with his U240.  Some good posts on it or PM him.  Also, remove and clean the Bendix exhaust port, as a block in the exhaust port can cause some very unusual throttle issues.

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