Hi all,

I'm new to the forums and am seeking advice to fix my dad's truck. It's a 1991 F150 XLT Lariat, 5.8L with about 130k miles. I have been driving this truck back and fourth to work and over the summer and I noticed that when starting the engine when it's cold, the rpms will go to between 1000-1500..which seems normal seeing as the engine is cold. Then the engine will sputter down to 400-500rpm, then shoot back up to 1000-1500. This pattern occilliates back and fourth for like half a minute....then once the engine is somewhat warm it will idle more or less normally (around 750). This is the first half of the problem. When I was driving it before the engine would reach operating temperature, at certain times (not every time) when I would let off the gas (say to stop at an intersecton) the engine would simply quit....and other times after letting off the gas the rpms would dip down to 500ish..then shoot back up to normal (800ish) just before reaching a complete stop. I have noticed this happening mostly before the engine has reached full operating temperature, but both my dad and I have seen the engine quit after letting off with the engine warm too! If anyone can give me suggestions on the problem...wether it be a fuel problem, sensor problem, etc, that would be great!

Thanks
~Nick

 

Have you run codes on it at all? That's the first place to start. Mechanically, it could be a vac leak.