F250 Deceleration Stall

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My friend has a 1995 F250 with 115k miles on it. It has the big V8 in it (7.3?).

When decelerating, the engine wants to stall at 15 mph. He replaced the throttle position sensor and the vacuum regulator solenoid.

No luck.

He connected a vacuum gauge from the vacuum regulator solenoid to the EGR valve. The engine is pulling 5 to 15 inches of vacuum at 15 mph, i.e., the vacuum solenoid is opening.

Any ideas so he doesn’t have to “chase more parts?”

Thanks.
 
Long shot, but does the transmission have a lock up torque convertor? Might not be unlocking at low speeds - or sticking - causing a drop in revs.
Again, a long shot.
 
I assume this is the 7.5 (460) gas, and not the 7.3 diesel.
Is there a CEL so you can pull codes? Does it drive normally under other conditions?

Like justnintendo said, an easy thing to check is the IAC. Remove it, scrape any carbon out and clean it with throttle body or carb cleaner. You may also disconnect the battery for a minute to reset the computer.

Otherwise, start with the basics, like an old fashioned tune-up. Check the base timing, idle speed, vacuum at idle, fuel pressure, ignition system, maybe even the air filter.
There is a base idle adjustment screw on the throttle body that you aren't supposed to adjust because it is not going to fix the problem, but sometimes it is useful.

If you have reason to suspect the EGR, you can check the function of the EGR valve with the engine idling. Connect a vacuum pump to the EGR valve and manually apply vacuum. You should be able to see the valve move, and the engine will start running rough or stall.
If you don't have a pump, you can use a vacuum line straight to the intake manifold, but you must keep it plugged until you stick it onto the EGR valve.
 
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Originally Posted By: Danno
Long shot, but does the transmission have a lock up torque convertor? Might not be unlocking at low speeds - or sticking - causing a drop in revs.
Again, a long shot.


My vote goes to check this out. I've seen this a lot with some past automatic transmissions.

An easy way to check this would be to put it in neutral when coming up to a stop.
 
I would check things in this order..


1) Throttle Position Sensor (Replaced)
2) Transmission not unlocking. (Shift to Neutral before 15mph)
3) Idle Control Motor sticking
4) Vehicle Speed Sensor not functioning properly letting the ECU know the vehicle is slowing down.
5) Some vehicle ECU's will turn off the injectors when you are coasting and it might not be turning them back on just before coming to a stop.
you can watch this on a Scan Gauge or Scan Tool.
6) Watch fuel pressure as you are slowing down if possible.
 
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UPDATE:

The engine is a 7.5L gasoline engine.

All “tune-up” parts were replaced, including the gas filter. The spark plugs are not budging, absent drilling them out.

And my friend tried the “shift into neutral” at 20 mph to check the torque converter. Several times. No joy.

Based on advice on this thread, he replaced the idle air control valve.

Apparently, that solved the issue.

He has run the truck the past two days since replacing the IACV and the deceleration issue has not occurred.

Both he and I extend our thanks to those BITOGERs who responded.
 
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