I am having a engine surging issue on my 1992 f150 with the 4.9l and 5 speed tranny. I can't figure out what can be causing it... it only happens when I let of the throttle going downhill and try to give it gas when I get to the bottom. It ONLY happen going down hill not if I do it going up hill or on semi flat ground. I've already tried replacing the MAP sensor, TPS, and IAC. I'm drawing blanks as to what else it could be. The engine only has 5k on it since rebuild it has a new clutch and it just barley started doing this in the past 500 miles. Any ideas would be appreciated!

 

Maybe the fuel pressure regulator? The engine makes the most vacuum coasting down hill, maybe fuel is leaking into the vacuum line at the fuel pressure regulator?

 

Where is the fuel pressure regulator located? I was unaware that the 4.9l had one I thought it was regulated by the fuel pump? Sorry I'm fairly new to working on cars.

 

Pull the vacuum line off the fuel pressure regulator and see if it smells like fuel. There should be no fuel in that vacuum line.
Your symptoms are that after coasting down hill, when you put your foot back on the accelerator, the engine over revs? Edit: Describe the surging in more detail. Do the RPMs stay high or go up and down?

 

I guess surging is not the right word it's more like a bucking feeling. Almost like fuel/air has been completely cut off. It only lasts for like 2-3 seconds then goes back to normal... a loss of Fuel pressure due to high/inconsistent vacuum would make sense. I'll give the regulator a gander. Doesn't look to hard to change out.

 

If there isn't fuel in the vacuum line, or fuel odor, I wouldn't change it out. I've heard too many stories of bad "new" parts. Plus, it would be extra work that's not needed.

 

I've smelled fuel coming from under the hood before. Never been bad enough to worry about. But I will definitely check it out before I replace the part.

 

Yes, and yes! Unless you're willing to bet real money that the part is bad (which is exactly what you're doing with a replacement), look for and test for proof first. Brake booster and EGR system are other things coming to mind towards your issue...

 

Update: I pulled the Vacuum hose and there was a little gas in it. So I went ahead and replaced the pressure regulator. It smoothed out my idle quite a bit and made my problem much much less then it was. It still bucks a bit going down hills but not nearly as bad or as frequent... but after looking it looks like a lot of the vacuum hoses running into the throttle body are original and there may be a good chance they are weather/age cracked. Is it possible this is being caused by a rogue vaccum line? If so are there any good ways to check them?

 

I would check fuel pressure with a gauge.You might be able to situate it so that you can monitor fuel pressure as you drive. The pressure for your 92 should be 45-60 psi. The regulator's job is to keep the fuel pressure from going too high. I would also expect a rich code being thrown if that were the case.
I would try disconnecting and plugging the EGR vacuum line and see if that makes a difference.