I just got my mother-in-laws 1986 Bronco II Eddie Bauer. It has 141K on it. So far I have replaced spark plugs, wires, cap rotor, radiator, muffler, fuel filter, and fuel pump in the gas tank, oil and oil filter. I took it to a local shop and had the throttle body cleaned. The air filter has been checked and cleaned. Here is my problem:

Before I got the truck it would occasionally start to run rough for a minute or two then straighten up and run correctly. It would do this once in a blue moon. My father-in-law took it into the local ford mechanic but the vehicle would never exhibit the problem when it was in the shop (has than ever happened to you?!). Anyway, after I did all the service mentioned above, the truck started to exhibit the problem more frequently. Sometimes it does it when it is first started, sometimes it does it while driving down the flat. It runs rough and small puffs of black smoke comes out the tailpipe. Sometimes the engine will even stall if at a standstill. If you give it a revving up it may clear up. This problem is progressively getting worse, however, the truck has never left me stranded. If it stalls, I can always restart the engine and continue on. When the truck is running correctly it's a dream to drive. But when it starts to run rough it's very frustrating! Any advice?

 

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Welcome to FTE!!!!

I always like to start with pulling the codes and testing the fuel pressure.

 

Yeah I am having the exact same problem only mine is an 88 and I have gone a little more extensive in my replacement list. I have yet to change out the oxygen sensor and that could be the culprit. I do know what you mean about the frustration thing though manno! Anyways if I figure it out I will let ya know. I'm gonna pull the codes tommorow. Darn 2.9's are one hardcore engine though aint they! LOL.

 

My Bronco II seems to be getting worse. I thought maybe the hot weather may have something to do with it (a sensor getting sideways) but this morning the outside temperature was around 65 degrees and the engine started to act up. I noticed that the exhaust was puffing very faint black and smelled a little like rotten eggs (catalytic converter?). I stopped on the side of the road and shut off the engine for about 15 seconds. Started back up and everything was fine. Very strange! Have you switched out your O2 sensor yet? This Bronco II is a dream to drive but this problem is very irritating!

 

I took my Bronco II into the local mechanic and lo and behold the truck ran without a glitch! He had it for 4 days and everything ran great. However, the next morning when I was driving it to work it started up again. Go figure! Anyway, the mechanic told me to try a few things:
1. When it acts up, hit the EEC with a stick and see if it straightens up. I'm not too keen on beating up my computer so I decided to go to action 2.
2. Disconnect the EGR vacuum line and run the truck for a while. If it doesn't act up it may be the EGR valve. So far, the truck seems to be running OK. I am however, not convinced that it is only the EGR valve. I am currently test other components (the EGR vacuum solenoid, vacuum lines, and looking into the signal the EEC delivers to the controlling solenoid). I'll post my results when I get them.

 

As Ken suggested earlier, if I were to inspect the EGR system, the first thing I would do is pull codes from the computer. See Ken's tech info post for a good set of instructions.

 

After a lot of troubleshooting, brainstorming, and swearing I finally got my '86 Bronco II to straighten up. Mine would start fine in the morning and run OK for about 1 to 2 minutes. Then it would run very rough with black smoke coming out of the tailpipe. If you jumped on the gas the engine would rev up and start to run OK again only to return to rough running and black smoke. Finally, the engine would run smooth after a mile or two of this rough running, black smoke nonsense. This would happen very sporatically, in all types of weather. I traced the problem down to a bad O2 sensor. Although its output was OK when I tested it, it would act up whenever it wanted to. This sensor operates with very low voltage. My sensor's leads had a small nick in the insulation causing a "leak" in output. To put it in a nutshell, the computer thought the engine was running lean, so it would tell the injectors to deliver more gas to the cylinders. Because of this, the engine would run extremely rich and start to miss and puff black smoke out the tailpipe.

My Bronco II now run great for a vehicle with 148K on it. I get between 17 and 19 miles per gallon (I live and commute in the foothills down the the valley floor). Hope this tidbit helps all who read it.