My '96 4.9 Li F150 has a very rough idle (almost stalls) for a few seconds when starting on a hot day and the engine has reached full warm up temperature. This only occurs after the engine has been off for 5 to 20 minutes after full warm up. Then it idles normally. This symptom doesn't happen in temperatures below 70 F. Anyone experience this? What fixes it?

 

My '96 4.9L/5spd starts, runs fine except when starting after engine reaches operating temperature. Violent shaking for a minute or so, then runs great until next warm start. Changed fuel filter, complete tune-up including new IGN coil, cleaned MAF/IAC valve/TB, replaced most vacuum lines. (Mechanic evaluated and suggested complete tune-up, found no vacuum leaks or other issues.) Rough (wild) idle starting hot (warm) continues.

 

Update: I've noticed that on a hot start, it only happened when operating on the front tank. Switching to the rear tank during rough idle smoothed to normal idle immediately. Also, the check engine light came on occasionally when operating on front tank, but never on rear tank. Replaced FDM in front tank. So far, no more problems.Apparently old FDM was not supplying sufficient fuel pressure and was at its worst in hot weather.

 

Thanks Dickie_4.9_F150. My starting problem with '96 4.9L F150 continues. I'm not driving because it is a pain in neck to restart after it is warmed up. Ordered a fuel pump pressure gauge just to make sure that is problem, but from what you say it sounds like a new pump will fix. While I'm waiting for tool, I'll treat the bed bolts with penetrating oil. The plan is to lift bed (SB) for access single fuel tank. (Cleaning the crank position sensor solved a P0385 code issue, but had no effect on "lean" codes.)

 

So far, tested fuel pump (FDM), and it is weak with engine off-key on. Removed bed from truck yesterday. All six bed bolts came out without a problem. Next up: remove the fuel delivery module and start looking for affordable parts. (Also, while waiting for fuel tester to arrive I did an overdue cooling system flush and replaced all hoses.) Repair tip: bed can be removed by lifting it off with a small tractor and backhoe since it is not very heavy. My wife wanted to know what I was going to do with it once it was off? I said we should keep it in the bedroom.

 

Bought Bosch FPL (fuel pump) at local NAPA dealer as recommended by my regular mechanic. They had other brands for less money, but decided on the Bosch. Good choice, I think, as everything changed out perfect and even included tool to separate hangar from pump. Cycled key a few times after installation, and truck started on first try. Next: Replace fuel tank filler hose, body bolts, and tail lights. Images of project coming soon. Oh, truck bed is nearly back in position to do all that. Thanks for all the help on the project. Saved a ton of money.

 

 

Once you get tail lights, fuel filler neck, and body bolts out, two people can just lift the bed off (minus tailgate) and sit on bumper and rear wheels. Get a couple of more guys, and you can pick up and set on grass until fuel pump repair is completed.

 

I completed the FDM replacement repair today, having been delayed by a road trip. Not a lot to it, as I had hoped. Used backhoe on tractor to lift the bed in the air, then pushed the truck underneath. Six bolts all lined up without much hassle. Connected the fuel filler neck and tail lights. F150 started without hesitation, so I guess that was the starting problem I was having. I had previously put the new fuel pump in the fuel tank before leaving on a trip, and set the bed on the bumper and tires. Many thanks to "Dickie 4.9 F150." for getting me going in the right direction on the repair.

 

I realize this is an old forum but the problem is still out there.
Weak fuel pumps exasperate the problem but it is really vapor lock plain and simple. Ford has added an ineffective heat shield between the exhaust header and the fuel line feeding the injectors. I have tried opening my hood after a hot run on a hot summer day and it allows the heat to escape and not concentrate around the fuel line. It works. I am increasing the insulation layer and adding a hood scoop and fan to pull the hot air out.

My truck's previous owners let the tanks run out causing overheating of the fuel pumps which wears them out so they are probably weak. Another issue I noticed is that when I run the rear tank pump it pumps gas into the front tank. This is a problem if I do not watch the level as the front tank can overflow into my scrubber canister and even fill it with liquid gas. It could be pushing my front pump backwards to fill the tank or pushing past the worn vanes. Be nice if there was a one way ball valve in both lines to prevent this.