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Old 12-19-2009, 12:22 AM
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My engine stalls when braking!

I have a 98 F150 and am having a problem with the engine stalling when braking. Also, when it is in park it idles low as well. This happened to my wife today while she was out running errands. I haven't been able to drive it myself but I am thinking that it may be either a vacuum issue or a brake booster issue or both together. I am going to perform a starting brake test on it when I get home on Monday but wanted to know if anybody else has had this problem before and if anyone has any ideas. Sorry, I don't have more details at this time but i am just trying to gather iformation so I can formulate a plan and have some idea where to start my troubleshooting. Thanks for any and all advice anyone can provide.
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Old 12-19-2009, 01:07 AM
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I forgot to mention is that their is no engine light coming on either. One question I do have is since the engine is idling low when in drive and in park, is this indicative of a bad IAC? I would think so but then again I am not a mechanic, just a lowly DIY'er. Is there a way to tell if the IAC is bad other than the engine light coming on which it is not?
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Old 12-19-2009, 02:37 AM
Bluegrass 7 Bluegrass 7 is offline
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The IAC would be the first suspect for being gummed up.
Remove it and clean with intake cleaner made for the purpose.
Also clean the inside of the throttle body around the throttle plate area.
A little background on why the motor may stall upon braking is as follows;
The speed sensor tells the PCM what is happening to the road speed by driver action (braking).
This does several things. It unlocks the convertor, shuts down the fuel and keeps the IAC open just enough so the motor won't stall.
As you see, the IAC cannot respond so the motor stalls.
The gum around the throttle plate has limited air flow vs opening angle as well.
Also the PCM has tried to account for the problem by shifting it's tables to some degree. After some drive time, the PCM will return to it's more normal operating table values as you drive.
The IAC is not looked at for much more than an open electrical circuit or malfunction bad enough to cause a code for some other reason like to low an idle speed etc.
The TPS is part of the throttle body operation so is not able to tell much about anyting but it's own travel or electrical circuit.
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Old 12-19-2009, 03:49 AM
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Cool, so I was on the right path, just wasn't sure what direction to take. I appreciate your help with this. I will clean the IAC/throttle body next week when I get home and hopefully thats all thats wrong. Probably wouldn't hurt to look over the listen for vacuum leaks with the engine running and inspect all vacuum lines as well. I'll repost when finished to let you know how things turned out. Thank you again.
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Old 12-24-2009, 05:03 PM
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Well I popped the hood the other day and discovered that the throttle bypass tube was split half way through. I replaced it and cleaned the IAC as well and it no longer stalls or drops idle. However, today while I was out and about, my brake light and ABS light both came on and found that the left rear drum brake was sticking. I got down and checked the brake cable while my buddy set the parking brake and noticed that the cable leading to the drum in question really didn't move at all. How much movement should I see when the park brake is set? Obviuosly the rears are drum brakes and don't have ABS. Has anyone else seen this happen? I am going to break out the troubleshooting manual this afternoon and see what I can find. If anyone has any tips or pointers they would be greatly appreciated. Thanks.
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Old 12-25-2009, 10:01 AM
eallanboggs eallanboggs is offline
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These cables have a protective sheath. It can fail and the cable inside binds. Time for a replacement. Might find one on eBay.
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Old 12-25-2009, 11:29 AM
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Sounds like time for a replacement for the cable. Remove your drums and have your buddy operate the parking brake and see if they move at all. I had to replace mine on my 2001 F150. Also the lever that the cable attaches to was rusted solid. Had to soak my lever in PB blaster to break it free, cleaned it up real good and put a little antiseeze on it. My rear cable had swollen at each frame clip, I think the cable had worn through the inner lining at each clip point and allowed moisture to enter and rust.
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Old 12-26-2009, 01:05 AM
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The cable did look a little worn but when I had my sister push on the brake pedal, operation seemed fine, until she released the brake pedal anyway. I noticed that the wheel cylinder didn't retract on one side(shoe facing front of the truck stayed put). Might need to replace the cylinder as well. I'm about due for brakes anyhow so I guess now would be as good a time as any to replace any worn parts. Just out of curiosity, does anyone know the average lifespan on these cables? I have replaced the cables assembly once already about 5 years ago, does this sound normal? Thank you for all your help.
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Old 12-26-2009, 01:19 AM
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Have you replaced your rear brake pads? If you have, did you replace the brake return springs and hardware? Every time I do back brake shoes I always put on all new hardware(brake drum all in one kit - auto zone) these springs and hardware do get a little warm and lose their strength.
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Old 12-26-2009, 04:52 AM
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Yes, I try to do the same thing. If I am doing the brakes I will replace all the hardware as well. You are taking it apart anyway so you might as well replace it right. Thats the way I look at it. It has been quite some time since my last brake job though so they are about due anyhow. The shoes didn't have but maybe a 1/4 inch left on 'em.
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Old 12-30-2009, 02:42 PM
stan49 stan49 is offline
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im having a simular problem, i have a 1997 f-150 supercab, 4.6l 4x4 130k, auto.
when the engine is cold it starts and runs fine but when i go to stop of slow down the engine seems like its going to stall the rpm's drop to about 300 then bounce buck up to about 800. then as soon as the engine gets up to operating temp its fine.
replaced the IAC, fixed vacume leak at the pvc, changed the speed sensor in the rear dif(abs light on no dealer can seam to read it) the only other thing that i can think of it the egr valve or possable mas air flow dirty, it does have a k&n on it. please help me out i am lost.
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Old 12-31-2009, 09:40 AM
robin1t2002 robin1t2002 is offline
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I don't know if the trucks have an IAT sensor (Inlet Air Temp) but my crown vic was doing the same thing. I removed the IAT sensor and noticed it was very dirty. I cleaned it and no more problem. If it has an IAT it should be located in the air inlet system.
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Old 12-31-2009, 02:50 PM
stan49 stan49 is offline
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i have checked the IAT its tests good cleaned just in case.
mass air flow cleaned and checked its good. the EGR Vacuum regulator tests good. and the DPFE sensor tests good. im changing the egr right now we will see if it helps. now i did notice if i stop or slow down in nutral it doesnt do it at all.

Last edited by stan49; 12-31-2009 at 02:55 PM. Reason: adding more
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Old 12-31-2009, 06:01 PM
eallanboggs eallanboggs is offline
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If you tested all those sensors why didn't you test the EGR as well rather than just changing it. The EGR is not that hard to test.
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Old 12-31-2009, 08:54 PM
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it wasnt the egr, the ports in the throttle body were complty closed from carbon, but it didnt fix it, but i did somthing different if i manully shift it down to first its fine, so its getting stuck in second gear i dont know what would cause it to do that. any ideas
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Old 12-31-2009, 08:54 PM
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150, 2004, 97, 98, braking, dies, drops, engine, f150, idle, idles, low, rpm, stall, stalls

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