Why does it take so long for the engine to unwind when you goose the throttle in park? I could goose my 12V and it would be done as soon as you let off. The 3rd Gen seems to wrap out no matter how hard you hit the throttle and it takes forever to get done. Any ideas?

 

Still a build up of pressure on the rail that needs to bleed/burn off.

 

My guess is the pressurized fuel in the common rail has to go somewhere, so it dumps through the injectors keeping the engine revving.

 

nah.. its software. when you let off the throttle, the TPS goes to 0% and fuel pressure immediately drops.
The delay in the throttle falling immediately to idle is to aid in on/off throttle driveability.

 

It also has a positive charge in the intake system, pushing fuel and ait into the engine, unlike gas that sucks it into the engine.

 

So nobody agrees. Is there a Cummins or scientific reason?

 

Come on, man! You remember... carb vs fuel injected...

Wait till you put a smarty on it... when I'm running SW 7 or 9 on mine, I let off in second gear, it pushes until OD...

 

lol, mine does it too. maybe to prevent turbo bark ?

 

I cant bark the turbo in park anyhow. Thanks for the input. I guess if I wanted it to act like a 12V I should have kept my 12V.

 

I said the exact same thing here a couple years ago, no one believed it.

 

Better check that again, rail pressure does NOT immediately drop when the APPS goes to zero %. It hangs right there at what it was until the ECU ramps it down by dropping the fuel at the FCA and bleeding excess pressure off thru the injectors. There is no other place for the rail pressure to go than thru the injectors and injection events.

You are correct that it is software and ramping the pressure down helps with driveability, but, like everything else on the CR's thats only part of the story. Rail pressure would spike REALLY high if the injection events were not continued because it takes time for the FCA to back the fuel down. Electronic may be almost instantaneous but the FCA is a mechanical device and it doesn't react that fast.

The rail already takes in excess of 30k psi spikes as it is. Dropping injection would drive that much higher and kill the rail PRV that much faster.

Yup.

 

Exactly what I have seen on the rail pressure gauge.

 

Yeah, goose it to hard in traffic to change lanes and things get INTERESTING to say the least.

 

I've been noticing this too since it has been talked about. Definitely software for what ever reason. Rail pressure has nothing to do with it fundementally. The injectors will not inject any fuel unless the ECM commands them regardless of rail pressure.

Just another reason to put a P7100 12 valve in this thing. Best engine ever offered to the public in a passenger vehicle.

Gas engines taper the throttle closed to reduce hydrocarbon emissions but i have no idea why that would hold true for the CRD.

 

Actually,. I may have worded that wrong.. It doesn't matter what rail pressure does when the TPS goes to 0%. It actually duration that is putting the fuel in the cylinders. But it is designed into the software , probably originally put in to aid the manual transmission trucks. It would actually help during gear changes especially for inexperienced drivers.
Winding down in neutral/park wouldn't have much to do with IMAP, since there is only parasitic IMAP in the system with no load.