I have a 1980 Bronco Custom with a 351M engine. I have been plagued since I have had this truck with an idle problem. The idle speed wants to slowly creep upward, especially in extremely cold weather. An investigation of the choke shows that it is indeed open when the engine is at operating speed.
I have adjusted the curb idle according to specs, but come cold weather and over time the idle will slowly creep back up to a1000+ RPMs when in gear and at operating temp.
From other posts on this MB, some maintain that such a problem is a vacuum leak, but I cannot find one to save my soul. I have replaced vacuum lines and the only suspicious, last possibility is a small, coffee-can shaped contianer which has several vauccuum lines runninginto and out of it. I am supposing it is a vacuum reservoir of some kind and I had found cracks in the seams when I was looking for a reason why the vent/floor controls would not work. I sealed the cracked seams with JB Weld and that took care of the problem, but now that problem has recurred as well as the idle speed problem getting worse.
Could a leak in that vacuum reservoir (or whatever it is called) cause an idle problem since it would create a leak in the vacuum system? If so, what is that part called so I can replace it?
You are right tombob, that is the vacuum res. You definately don't want leaks here as it would be considered a vacuum leak. I'm not sure exactly if the leak would manifest itself as you describe. Others will probalby chip in to help on that. Replace it and hopefully it cleans up the problem.
I'm surprised your '80 has the coffee can style reservoir. My '83 had the black ball style, the '95 engine I have now has the coffee can, and my '96 parts truck has the bulb type. Anyway, if it had one crack, it can have 2, and cracks don't usually stop - they continue to grow. You can replace the can with the ball, the box, or the bulb types from later vehicles with no problems, and the plastic ones are more reliable. If you go to my sig link and look thru the various truck albums, you'll see photos of each type.
I suspect the idle is creeping up with engine temp because it's leaking at the EGR or Thermactor Air Bypass/Diverter valve, all of which get vacuum from that reservoir. They're controlled (on your engine) by Thermal Vacuum Valves which open as the coolant warms. If you've lost your vacuum map, you can find a copy here and if you need a good document about the vacuum system, PM or e-mail me and I'll send you the manual for the MityVac, which is a WEALTH of information.
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