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RPM's dropping when coasting, CC on

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Old 04-19-2017, 10:38 AM
tommyl tommyl is offline
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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?
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Old 04-19-2017, 10:59 AM
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Sounds like the torque converter is disengaging. I'm no TC or Trans expert by any means, though, and I'm sure others who know more about this will chime in shortly.
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Old 04-19-2017, 11:03 AM
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I'm a little confused. You say the RPMs drop way off as if the motor shut down.

Are you saying the tachometer drops to zero? Is the engine still running?
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Old 04-19-2017, 11:32 AM
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Don't the injectors shut off when coasting? Would this cause the tach to drop to zero also? It's nothing I have really noticed or paid attention to when driving.......
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Old 04-19-2017, 11:54 AM
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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.
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Old 04-19-2017, 12:44 PM
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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.
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Old 04-19-2017, 12:55 PM
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Quote:
Originally Posted by andym View Post
I'm a little confused. You say the RPMs drop way off as if the motor shut down.

Are you saying the tachometer drops to zero? Is the engine still running?
YES...................
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Old 04-19-2017, 12:55 PM
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Sounds to me like there is a transmission issue. At highway speed, the transmission shouldn't disengage on a downhill.
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Old 04-19-2017, 01:17 PM
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Sounds to me like there is a transmission issue. At highway speed, the transmission shouldn't disengage on a downhill.
OP never said anything about the transmission disengaging, nor anything about going downhill.

I think you have an electrical problem. Have you have codes read? Anything other problems going on with the electrical system? Any recent work?
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Old 04-19-2017, 01:18 PM
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Sounds to me like there is a transmission issue. At highway speed, the transmission shouldn't disengage on a downhill.
Tranny apparently not disengaging if it is maintaining set speed with CC on.
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Old 04-19-2017, 01:18 PM
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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.
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Old 04-19-2017, 03:31 PM
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This isn't a transmission problem if the truck maintains speed and RPMs. It's only the tach that drops to zero - is that correct tommy?
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Old 04-19-2017, 03:59 PM
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Quote:
Originally Posted by Rikster-7700 View Post
Don't the injectors shut off when coasting? Would this cause the tach to drop to zero also? It's nothing I have really noticed or paid attention to when driving.......
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.
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Old 04-19-2017, 04:13 PM
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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.
I hate to be the bearer of bad news, but that's not supposed to happen. It's supposed to stay in gear with the converter locked and the engine spins at whatever speed the converter and driveline spin it at.

Also, if the injectors were shut down, the engine would stall instead of idling at 700 RPMs.
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Old 04-19-2017, 05:04 PM
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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.
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