86 surges while coasting downhill
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86 surges while coasting downhill
So i have an 86 F150 4X4 with a 302 and AOD auto. As stated while coasting it surges. To go a little more in depth it started about 2 yrs ago and just recently got WAY WORSE!!!!!! If i am say doing 55 and go down a hill and let off the throttle it would jerk and start rapidly slowing down. Im, a experianced diesel mechanic and it felt just like i was using the engine brake. As soon as i touched the accelerator it would jerk again and coast normal. But recently the jerking has gotten MUCH MORE VIOLENT and now it does it all the time when I let off. It doesnt stall when i Stop and and getting like 9mpg now opposed to the 13mpg i use to get.The tranny has a shift kit and upshifts and downshifts fine through all gears. The fluid is full, not burnt, the trans is not slipping under acceleration. But its doing this so violently now i can no longer safely drive it. This old girl is my baby and any help would be greatly appreciated.
1. could it be the torque converter locking up under coast doing this ?
shawn_gumby is online now Report Post
1. could it be the torque converter locking up under coast doing this ?
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From: Lyman, SC
Do you see any difference in the situation between having the shifter in drive as opposed to overdrive?
Also check the transmission throttle valve cable at the throttle body butterflies....with engine off pull it out all the way and see if it returns smoothly.
Also check the transmission throttle valve cable at the throttle body butterflies....with engine off pull it out all the way and see if it returns smoothly.
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From: Virginia
Sounds like your throttle stop screw is off, or there may be something wrong with the tps. I would pull the codes from the computer before I did anything else.
If you do not have any codes, I would get it idling and then pull the plug off the idle air control valve. The engine should run really slow, barely turning over. If it quits right away turn the idle stop screw in some. If it's idling normally, then back the stop screw out till the engine is barely running.
If the computer sees the rpms are high, and the tps is in the idle position, it assumes you are coasting and turns the fuel off to the injectors till the rpms get to a certain value, and then you will feel a "bump" as the computer turns the fuel injectors back on. As you push on the gas pedal you are taking the throttle out of the idle position, and the computer will turn the injectors back on.
If you do not have any codes, I would get it idling and then pull the plug off the idle air control valve. The engine should run really slow, barely turning over. If it quits right away turn the idle stop screw in some. If it's idling normally, then back the stop screw out till the engine is barely running.
If the computer sees the rpms are high, and the tps is in the idle position, it assumes you are coasting and turns the fuel off to the injectors till the rpms get to a certain value, and then you will feel a "bump" as the computer turns the fuel injectors back on. As you push on the gas pedal you are taking the throttle out of the idle position, and the computer will turn the injectors back on.
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Joined: Dec 2007
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From: Lost
Also, you could have a rear axle in need of attention. The 8.8" isn't as durable as the old 9" was. What you are feeling could be due to worn pinion bearings/spider gears/axle splines. Any one, or a combo of more than one. They're all prone to wearing out.
I'd hate to see you throw money at the trans, and the problem be elsewhere......
I'd hate to see you throw money at the trans, and the problem be elsewhere......
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OK. Thanks for the replies. To start off my odb hookup has never worked. I've tried multiple computers and always have gotten "computer will not respond". I've even tried a new computer and got the same thing so I can't pull codes. It doesn't matter what gear the trans is in or what gear I have the shifter in. It does it every time I coast. I pulled the tcc cable and it staps back with no drag. I will try to pull the IAC off when I get home. Thank you guys so much for helping
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From: Denver Metro Area, CO
If it's happening even with the transmission in neutral (effectively disconnected from the engine) this is another good reason to investigate your rear differential & drivetrain.
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