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1993 Bronco XLT 5.8l
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My 1993 5.8l seems to be dying out when given a little throttle upon startup. If I let it sit and idle for 15 minutes before hand, it will drive fine. When I start it up it idles at 2,000 rpms and then sometimes drops down to 1,500. When you put it in reverse or drive it drops down to 1,200. When you give it a little bit of gas it drops down and stalls and won’t start back up for a few minutes. I can’t seem to find any noticeable vacuum leaks and I have cleaned the entire upper intake, throttle body, and IAC valve. I also replaced the TPS. I’m kinda at a loss because I can’t think of what else it could be. The previous owner has said that they replaced the fuel filter last year. I know it has an exhaust leak but I can’t imagine that would be the issue?

Also to add, I hear the fuel filter whining as it should when I turn the key so I don’t think that’s the issue. And I’m not showing any check engine lights
 

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yo Nick,
Try a Self Test for Diagnostic Trouble Codes by my pal, BroncoJoe19 @ Code Reader.....

Or purchase a coder reader such as Equus 3145 Innova OBD I Code Reader for Ford EEC IV Engines at Walmart & most parts stores.

It would rev up and then drop the idle so low it would die unless I would catch the idle with the accelerator. ... ...
I decided that more improvement could be had if I had a thicker gasket, because the original IAC gasket was very thin (0.018"). I had some 0.030" gasket material so I made my own with the same 1/2" cut out. This gave enough bypass to noticeably raise the idle speed and almost eliminate the dying. After maybe 50 starts in the last few days, it only died twice. After so many years of catching it with the accelerator it is darned hard to leave my foot off it. It is clear to me that by trial and error one can tune the air bypass without the need for the expensive Ford kit. I may combine the two gaskets for more bypass, but the idle is about as fast as I would want now, especially for driving in snow."

Idle Air Control (IAC) Sludge; Poor Idle TSB 91-25-07 for 85-92 Bronco & F Series & many others; "...Hard cold starts, hesitation and stalls on initial start-up or during idle or decel may be caused by sludge in the throttle body and/or idle by-pass valve. Sludge deposits or oil film on the throttle body bore and plate or the idle air by-pass valve may cause one or more of the following conditions. Hard Cold Start, Stall On Initial Start-Up, Stall During Idle, Stall During Decel, Rough Idle, Rolling Idle, Hesitation During Acceleration. A new idle air by-pass service kit (F2PZ-9F939-A) is now available for service use to correct sludge contamination concerns of the throttle bore and plate only. It eliminates the need to clean the majority of past model throttle body applications. Cleaning is not required on sludge tolerant throttle body designs released for 1991 and newer model years..."
 

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If it is not a Bronco, it's just not worth driving.....
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1,900 Posts
Just a guess from what you stated in your post, but I think it might still be a vacuum leak even though you have looked for one already. Since it will run after 15 mins ( engine heated up ) I think it is a vacuum leak that is somewhat sealing itself by expansion of the surrounding metal of the engine. My first thought would be something like the intake to plenum gasket. There could be other areas that would seal themselves up from metals expanding.

What I would do is to use the propane torch method of looking for the vacuum leak. Crank the truck and let it idle, then open the valve on the torch ( do not light it :p ) and move it around the gaskets, hoses and such to see if the engine rpm changes. I suggest you do this when it is cold since the problem either goes away or is significantly lessened as the engine heats up.
 
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