1993 F150 4.9L Five speed
My truck fires right up with no hesitation, but idle RPM runs from about 500 to 1500 back and forth till the engine warms up. It never dies, just an uneven idle. Once engine is warm it settles down to steady idle speed. I do not have a tac and am just estimating the numbers. But when warmed up it does not sound too fast. The engine runs strong through the gears, no problem or hesitation.

Being a '93 it is fuel injected.
Any ideas. I have searched thru the forum and not found this identical problem with a Ford inline 6.
Tanks

 

I have owned my 1990 F-150 (4.9L, 5-spd) for two years and have been wrestling with engine issues ever since. It turns out that my engine issues were caused by several things: several vacuum leaks, EEC-IV w/ leaky capacitor, faulty fuel pressure regulator, kinked fuel return line, leaky Mass Air Pressure sensor, and faulty EVAP canister purge valve. It's taken me awhile, but my engine performance has incrementally improved as I have corrected these issues. But, while it runs much better than it used to, it still isn't perfect. In fact, my last remaining issue sounds just like the one you described. On a cold start up, my engine RPMs "hunt" as you described. If I rev the engine for a few seconds, the idle will then hold stable. I have no ECU codes, and I have checked/tested the IAC and air temp sensor.
I paused any effort to work on this issue as I am currently replacing my pushrod cover gasket, and to repair a hole in my exhaust. But, once complete, I'll be back to this issue again, so I'll be watching this thread for ideas.

(One of the first changes I made to my truck was swap out the dash instrument cluster with a junkyard one that had a tach)

 

Need to pull codes.

You have a surging idle. That's common with a vacuum leak. You need to inspect all vacuum lines. That includes the brittle nylon lines. They're notorious for having hairline breaks and you don't even know it.

I briefly read through your trucks description. You may have 2 vacuum reservoirs, depending on transmission and if it has cruise. You need to inspect each one. Even the slightest pinhole leak in those canisters can affect idle quality and control.

I just fought this off and on again with another truck and when i pullled vacuum on the canister, it would hold but only briefly. Swapped it out and my idle is back to normal.

 

Vacuum leaks can be tricky to locate. I used a hand vacuum pump to test individual components, and later bought one of these to ascertain that I had found all the vacuum leaks (turns out, I had). Pulling codes may provide insight, but I found more problems with a hand pump than by pulling EEC-IV codes.

 

I had the same issue last year with my 302. I had replaced all of the vacuum lines in the engine bay as well as both reservoirs so it made me a little crazy, but in the end it was a stuck pcv valve

 

I have owned the truck only a few months and had a mechanical inspection. Oxygen sensor, fuel filter, and air filter were replaced. There was other work done but these are all that would affect fuel/air mixture. Vacuum has been suggested by several here who I expect have more experience than an old 1970’s shade tree mechanic like me. So, I will be looking into that.

I really like this old truck and want it running like a swiss watch and am able to take my time doing it and hopefully learn a few things along the way.

Is there a good manual specific to the 4.9L without paying Ford a large sum? Have not found anything in the public library.

Thanks for everyone’s feedback. Will report back if I have something significant to say on this fix.

 

I bought used copies of the 2-volume 1990 Ford F series Service Manual (via eBay). They're useful but not as useful as I had hoped. They do not include information on emissions equipment.

 

Thanks for getting back. I will keep looking. Good luck with your truck.

 

Check for codes as previously suggested, but you will most likely not find any related to your issue. A rolling idle is typically caused by a vacuum leak, worn throttlebody, bad/worn TPS or sticking throttle cable. It's a process of elimination.

 

I hadn't considered the throttle cable, but it could be my problem. With my truck, it's not so much a matter of the engine not being warmed up, because even if it's ten seconds after I started a cold engine, after I hit the accelerator, the RPMs return to idle and the "hunting" seems to go away. That seems to indicate that it could be an issue with the cable (rather than the engine or sensors not being up to temperature yet). I have cleaned/tested/replaced several culprits so far, but I haven't given any consideration to the throttle cable.