This has been driving me nuts for a whlie now. The offending vehicle is an 88 B2, 2.9, 4WD, 5 speed. It sat for almost 2 years straight and ran fine before.

Symptoms are: Low RPM backfire through the intake. Consistant surging in all RPM ranges and throttle positions. Cold engine will constantly backfire and doesn't have enough power to get the vehicle moving.

When we first hooked it up to the nifty Snap On computer it gave enough codes to write a book. Reset all the codes and ran it for a couple days and checked it again. Ended up replacing the O2 sensor, Air intake temp, MAP, and coolant temp sensor. Did the tank fuel pump as well because it was howling pretty bad. Also "cleared" out the cats.

After replacing all those items the computer shows no error codes but it still saying the system is lean. I shrugged it off and figured it would end up going away on it's own. I thought something just needed to work itself loose after sitting so long. So, I head out on a 2500 mile road trip with it and deal with the surging, which isn't that bad at highway speeds but it's still there. The mileage is around 16 to 18 which is pretty good for a stock geared truck with 31s on it.

So now I'm 2500 miles from home, 3500 miles or so from when I first started working on this problem, and still have no ideas on what the problem could be. I didn't personally check the fuel pressure but the friend that supplied the shop for me to do my work in said it checked fine but I am starting to wonder about that since it's acting like it runs out of fuel and then picks up right away. The part about it that gets me is that it's consistant. I can time it in my head and know when it's going to cut out at a certain speed. I'm completely lost for ideas. So let's hear what you all think here.

 

I would confirm the fuel pressure. Have you changed the fuel ilter, air filter, plugs, wires, cap and rotor?

 

You know the 88 Bronco II has 2 fuel pumps, Right? There is one on the inside frame which is the high pressure fuel pump and there is one in the gas tank which is the low pressure pump. Which one did you change? Also I would have to agree with Kenn. Did you change your fuel filter?

 

I guess I did space that we did a basic tune up, plugs, wires, cap, rotor. Air filter is relatively new as well. Only thing I didn't change that's suggested here is the fuel filter.

As for the pumps, I changed the one in the tank. The frame mounted one was changed less than a year before it went to sitting. Probably less than 10k miles on that one.

I guess tomorrow will be full of rolling around under it and seeing what I can see. Thanks for the ideas. Will update if I can't find anything with these suggestions.

 

Ok I have this narrowed down to a relay. Not sure if it's eec or the pump though so I'm just going to replace both. My problem now is, I have 3 relays on the fender and all are black. I don't know which is which. I'm pretty sure the EEC is the middle one from what I have read but I don't know which is for the pump. 2 are mounted on top under the cover and one is down a bit and to the rear. Anyone help me with which is which?