RPM's dropping when coasting, CC on
Thread Starter
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More Turbo
Joined: Feb 2004
Posts: 509
Likes: 3
From: Findlay, Il
RPM's dropping when coasting, CC on
RPM's are dropping way off when coasting downhill while towing (as if the motor shut down), CC is on and maintaining desired speed. When at the bottom of the hill, all of a sudden, with a big bang(in the motor) the rpm's have came back up and you drive on down the road as normal. This seems to happen only when towing. "99 F250 7.3 I haven't pulled any codes as of yet. Whats happening?
Hotshot
Joined: Nov 2006
Posts: 11,287
Likes: 263
From: North of Greenville
The only time I see the TC disengaging and allowing a corresponding decrease in RPM is when I'm coasting downhill in my neighborhood. When the RPM slowly decreases to about 1100, and at the point, if I'm not pressing the accelerator, the TC drops out and the RPM will drop off to the engine idle speed of about 700-800 RPM. I see this every day on two particular hills in my neighborhood. I don't think I've ever seen it happen, though, at anything near highway speeds.
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
Yes the injectors shut off when you're coasting, so long as the converted remains locked. As soon as your speed drops below X, the converted unlocks and the injectors start firing again, just as Pete says.
I've noticed this on long downhill runs. EGTs will drop down to about 175 until I touch the throttle.
The tach measures how fast the engine is turning. It has nothing to do with the injectors.
I've noticed this on long downhill runs. EGTs will drop down to about 175 until I touch the throttle.
The tach measures how fast the engine is turning. It has nothing to do with the injectors.
Thread Starter
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More Turbo
Joined: Feb 2004
Posts: 509
Likes: 3
From: Findlay, Il
YES...................
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Cargo Master
Joined: Apr 2010
Posts: 2,405
Likes: 4
Sounds to me like there is a transmission issue. At highway speed, the transmission shouldn't disengage on a downhill.
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Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
I think you have an electrical problem. Have you have codes read? Anything other problems going on with the electrical system? Any recent work?
Thread Starter
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More Turbo
Joined: Feb 2004
Posts: 509
Likes: 3
From: Findlay, Il
Tranny apparently not disengaging if it is maintaining set speed with CC on.
Hotshot
Joined: Jan 2011
Posts: 18,849
Likes: 179
From: Puget Sound
Yeah... mark me under the column "TC unlocking when it shouldn't".
Torque Pro, AE, FORScan, Car Gauge Pro, or whatever OBDII tool you have - log your speed, RPMs, Gear, and Torque Converter Slip. That will paint a picture of what's going on in there.
Torque Pro, AE, FORScan, Car Gauge Pro, or whatever OBDII tool you have - log your speed, RPMs, Gear, and Torque Converter Slip. That will paint a picture of what's going on in there.
Lead Driver
Joined: Dec 2006
Posts: 7,092
Likes: 145
From: San Diego, CA
No it does not. One of the "features" of a CA truck is that fuel is shut off to injectors when you let off the go pedal. It is different for each gear but if I am going more than 45mph and let off the pedal completely the injectors shut down and the RPMs go to ~700RPM (idle). When I get back on it the injectors start back up and off I go. No bang though.
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
No it does not. One of the "features" of a CA truck is that fuel is shut off to injectors when you let off the go pedal. It is different for each gear but if I am going more than 45mph and let off the pedal completely the injectors shut down and the RPMs go to ~700RPM (idle). When I get back on it the injectors start back up and off I go. No bang though.
Also, if the injectors were shut down, the engine would stall instead of idling at 700 RPMs.
Lead Driver
Joined: Dec 2006
Posts: 7,092
Likes: 145
From: San Diego, CA
I stand corrected. Many years ago when I bought my truck I read that this was one of the differences with the CA model (along with a GPCM). My interpretation was that it did not fuel at all. My engine is definitely quiet when I'm coasting above 45mph. Once I step on the pedal it cackles to life and starts to pull. I guess that it just cuts back to idle. I learn something new every day.