I'm hoping some experienced wrenchers can pinpoint my starting problem with my 1996 F150 4.9 I6 based on the evidence i have gathered. I just don't know enough about how the truck's sensors work with regard to startup to diagnose this. The facts:

First, the truck stumbles and shudders like crazy if you attempt to start it WARM. IE, after its sat anywhere from 5min to 2 hours roughly. After maybe 20 seconds of shuddering it smooths right out and runs perfect. It will also sometimes stall a time or two during this period, but again, once about 20 seconds passes it smooths out and runs fine.

Second, the truck ALWAYS starts perfectly when the truck is cold. So if the truck has sat 2 hours or more it starts perfect with no issues at all.

Third, when the truck has been fully warmed up, if i shut it off and IMMEDIATELY restart it it starts up fine.

Last, the truck is throwing codes P0171 and P0174. I'm not sure if these codes are related to this problem or if i have 2 problems.

Now I understand the truck follows a preset fuel map for cold starts. Whatever this cold start process is, it works. I have no problems. I'm hoping this is the clue for those in the know as to what my problem is. Also, the fact that it starts fine cold makes me think the fuel pump is fine. I have cleaned the MAF sensor, which wasn't dirty. I'm at a loss as to where to go with this. Any ideas are appreciated!

 

My 1996 F-150 4.9 4X4 shortbed with 195,000 miles on it is doing the EXACT same thing !

To give you the history here, I bought the truck about a month ago, the previous owner must of had the problem because he changed the fuel filter just before I bought it.

On the way home with the truck I stopped at a store to get a drink and when I went out to start the truck it did it, I was like what the heck and figured I was screwed. THe truck straightened out and I drove it home. THe trip was about 200 miles and the truck ran fine. When I got home, I cut it off and went back out and it did it again. So my search begin as to what was going on.

I tuned it up with new plugs, wires, cap, rotor, air filter and it still did it. Next I got on line to try to figure it out. I read about the idle air control valve, took it off and cleaned it good, still did it. Next I got a new PCV valve and hose, put that on, still did it. Put a new coolant temp sensor on, still did it. Cleaned the Mass Air Flow, yep, still did it.

Finally gave in and took it to a shop after reading it might be the fuel pump getting hot and not working right (fuel pump is an electric motor) the shop tested the fuel pump pressure and said it was good, and held pressure good. BUT I found out they did not drive the truck any distance so the fuel pump was not on long to get hot. Not satisfied, I borrowed a fuel pressure guage and hooked it up and drove the truck 25 miles turned it off and let it sit 10 minutes and it did it again, while it was studdering and shaking the fuel pressure guage dropped to 20 lbs (not enough pressure) then when it started the idle right the fuel pressure jumped up to 38lbs and held.

So I really think it is the fuel pump, but before dropping the tank and putting a new fuel pump in, I bought a new fuel pressure regualtor and put that on just in case, it still did it.

So now after all of that, I have a new fuel pump on the way and will drop the tank and put the new fuel pump in. I really think it is the fuel pump that is worn out and when driving any distance the fuel pump motor is getting hot and not working right.

Those codes 171 and 174 are lean codes which means the fuel system is lean which means probably a weak fuel pump motor.
Hope this helps you out.

 

Update: Truck is also throwing the p0385 code. One other thing i've noticed. When i reset the check engine light, the warm start problem goes away until the check engine light comes back on. Why would this be the case? That to me again points away from the fuel pump.

 

The CEL comes on when the parameters have been met to throw a code. Your crankshaft position sensor could be to blame, but I would check it out with a scan tool for a good square waveform before you put a new one in.

 

This warm start problem has been solved. I replaced the fuel pump with a new Bosch unit and the problem is gone. The check engine light has stayed off.