Regardless of RPM, idling doesn't burn much gas. Never spray flammable toxic chemicals onto a running engine - it's expensive, dangerous, & unhealthy. Did you listen around both ends of the throttle shaft? Did you confirm that the throttle ALWAYS returns to its fully-closed position without assistance?

With the engine idling, disconnect each vacuum line & cap/plug/tape each nipple that is NOT necessary for proper engine operation at idle. That includes nearly ALL of the vacuum lines except the MAP (if the engine has one - ALL the truck's details need to be in your signature so we can review them with each reply) and possibly the FPR. The brake booster is a likely suspect since it's among the few large enough to affect idle speed.

 

Details added to the signature. As I remember when I hit the engine with smoke I had a very small amount at both throttle shaft ends. Small amount at the EGR valve and PCV hose connection. I blocked the EGR valve and put a hose clamp on the PCV hose but it made no difference. I'll check the brake booster again but I seem to remember checking that at some point. Yes, the throttle does close without assistance.

The truck is a stock 1995 F-150. 4.9L with a 5 speed manual transmission.

 

That's not a signature - it's part of your post, which will be lost as the thread progresses. We can't remember a month from now (or 9 years as the case may be) if you posted any description of the truck, to go digging back through the thread to look for it. Your signature will appear automatically below each post, and can be edited here: https://www.f150forum.com/profile.php?do=editsignature

And those are not details - they're extremely vague. We assume it's an F150 because you're posting here. And we assume it was built in the 1900s for the same reason. So you can skip those, and use the space in the signature field for all the things we don't know, like whether it has a MAF sensor or a MAP, miles, maintenance history, modifications, damage, repairs, and a complete list of every option. Ford built thousands of unique "stock" trucks, so telling us it's stock tells us nothing. In fact, it can't be stock because many of the factory parts wear out or just go bad with age, and MUST have been changed a few times by now: tires, coolant, wiper blades, light bulbs, belts, hoses, spark plugs, cap, rotor, wires, brakes, fluids... So tell us EVERYTHING - the more we know, the more-relevant our suggestions can be. Is it 2WD or 4? Brown, blue, red - inside & out? Tire size? Alternator output (95A or 130A)? Trim level (Custom, XL, XLT, Eddie Bauer)? Cab & bed sizes? NOT all of its details are relevant to this problem, but you don't know which ones are, so just put them all in now so you don't have to keep adding them each time you want more help. If there are things you don't know, this is an excellent opportunity to learn more about your truck so you know how it's built. Read this page & follow the links on it:

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There's a link that explains common abbreviations, which will help you squeeze more info into your signature. Avoid repeating any detail since it just wastes space. The only details you've posted so far about your truck (going back to the first post) are:
95 4.9L M5ODR2
I know it had 196Kmi 9 years ago, but that's not relevant now. I also know it got a HEGO (oxygen sensor) then, and that its fuel pressure was 45psi then. But not much since. Not even one pic of the truck, the engine bay, any of the vacuum lines, battery terminals (yes, they're very important), wiring, ground points, or the equipment you're using to read codes. This is what you SHOULD be using:

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