Ok so its back to this again. My truck has been running good but had some low end lag. I went trough every vac line and found a broken yellow line going to the diviter. I fixed that and now it jumps from around 800-1200 rpm when stopped in gear. It shifts perfect and runs great now except for the jump in rpm and when I start it first thing it stalls. After the second start it does fine. My air pump was locking up so I took it appart and removed the fins. Also the line going to the cat has been cut and plugged. Will I have to replace my air pump to make this thing run perfect or can I bypass the air pump for now? Funds are an issue so the simplest and cheapest fix would be best! I am simply out of ideas.

 

The air pump pushes fresh air into the system upstream of the O2 sensor which will cause the system to perpetually compensate for the rich mixture by leaning it out. Some folks will claim this has no effect on the operation of the engine. I don't ascribe to this simply because I know that the system will compensate to the point of leaning the mixture out to the point of overheating plugs and cylinders causing premature spark plug failure and superheating the cylinder walls which will ultimately shorten engine life and reduce performance.

The line to the cat isn't critical IF you replace the cat with a unit that does not need the air injection to reach operating temperature. But it requires getting rid of the OEM cat.

In short, by the time you rework the cat and replace a few sets of early-failure spark plugs, you've laid out enough funds to purchase a new air pump.

The erratic RPMs are the result of the system attempting to control idle while re-learning the fuel mixture settings because of the missing air injection. This usually takes about 30-45 minutes of driving around at both city and highway speeds, cruising and stop-and-go traffic. The restart will "cure" the problem for a while until the system relearns but you will see a change in economy and performance.

 

Thanks grey! That makes perfect sense. I noticed the cat was hotter than it should be when I changed the oil. The line to the cat was plugged before I got the truck so my guess is the cat is no good.Its not stopped up yet but not taking any chances! Question. Did the 91 bronco 5.0 have o2 in the manifold? This one has one in the driver side only and the wires have been cut off. I think the manifold has been replaced but not sure. I cant find a plug that would go to it and I have checked numerous 91 trucks and broncos none had o2 in manifold. I found a few 95 and 96 that had them in both manifolds. This is for the obd 2 set up corect? And my truck has the o2 down by the cat. Its hooked up and looks new. I am going to the junk yard tomorrow to find a good air pump to get me by for now. Next week Im getting a set of proflow headers,high flow cats,flowmaster cat back! Thanks for the info you post on FTE!! I have a feeling you save a lot of people a lot of green backs!!

 

You are correct. EEC-IV used only one O2 sensor in the exhaust pipe at the junction between the two arms of the Y-pipe. (1985-1995 in the Bronco)

OBD-II used three in most instances with one near the base of each manifold AND the one in the Y-pipe. (1996 in the Bronco)

Your catalytic converter SHOULD get hotter than the combustion cycle temperature so that it can burn off the crud that won't burn off in the lower combustion cycle temps. With OEM type catalytic converters, the air injection is needed to help it reach that higher temperature. Basically it adds more oxygen to the exhaust downstream of the O2 sensor to reignite the leftover crap in the exhaust.

 

Well I installed the new air pump today and the truck runs perfect!! I even charged the AC!! Now I can start on the few small dents and one small spot of rust. Shine and show here we go!!! Thanks for all the help!!

 

Good info Greystreak I was thinking about replacing my cat when I redo my exhaust. I was going to do away with the air pump when I put the later model cats on. Glad I read this you saved me some trouble