I have a 1990 Full-Size Ford Bronco with a 5.8L EFI V8 Engine and E4OD Automatic Transmission with about 160,000 miles on it. Tranny was rebuilt in 1999 and trans fluid & filter recently changed (drain & fill). I have recently had the exhaust replaced and the smog pump bypassed. When I picked the Bronco up from the exhaust shop he said the new placement of the Oxygen Sensor was too far back for the OEM pigtail to reach and that I need to buy an extension. The Bronco drove good, but rich while the Oxygen sensor (only 1 and it is before the Catalytic Convertor, remember it's 31 years old) was unplugged. I ordered an O2 extension to match the Ford factory connectors and installed it with no issues. Once installed the Bronco idles terribly surging revs. It will drive, but will not maintain RPM and almost wants to die even while driving. My first thought was that the O2 sensor was bad as my old catalytic convertor was somewhat clogged. I purchased a new Denso 234-4609 Oxygen Sensor off Amazon and installed it. The problem did not go away. The vehicle is almost undrivable with the sensor installed with the pigtail to reach the O2. I even switched out the pigtail with another one and the problem persists. I can drive it okay with the O2 sensor removed, but it burns VERY Rich and I feel like it's going to burn up my brand new catalytic convertor. Please help. This is post carburated engines, but before OBD2 and nobody seems to be knowledgeable about these vehicles. Thank you.
Although not nearly as comprehensive as OBD2, OBD1 does provide some information.
https://www.troublecodes.net/ford/
I would suspect a bad new O2 sensor, plugged up catalyst, a vacuum leak, or other fuel injection problem (sensors) before the computer.
The Catalytic converter is brand new as the entire exhaust system was replaced earlier this year. I will pick up another O2 sensor from the parts store and see if that has any effect. I'm sure the fuel injectors need to be cleaned as well. I drove it some this weekend with the O2 sensor connected and it drove well. The only issue was that it idled much higher than normal, about 1,050 RPM where the typical idle is 800-850.
Be sure to use an OEM sensor if still available, or at least a good brand like Bosch. (I think at least some Fords have used Bosch O2 sensors.)
Thank you for the input. I will grad a Motorcraft Sensor. Also, do you think the MAP (Manifold Absolute Pressure) sensor could be going bad? Would that be worth replacing?
I hate to throw parts at the problem, but I need it to be running right. There seem to be mechanics that know carburated vehicles and mechanics that can work on newer computer-controlled 96 & newer vehicles, but there aren't many that really know the EFI vehicles in late 80s and early 90s.
Unfortunately you're correct, there are fewer and fewer mechanics as time goes by that can work on pre-OBD2 vehicles. There's no detailed data they can scan for, just a few generic blink codes that might or might not give them a lead on what's going on. You have to use other methods, like reading the spark plugs. They'll tell you a lot about what's happening inside your engine like whether the mixture is too rich or too lean or if excess oil is getting inside the combustion chamber.
https://ngksparkplugs.com/en/resources/read-spark-plug
You can test your MAP sensor, there are numerous videos on youtube showing how to do so. However if your engine still has the original part now over 30 years old installed it could not hurt to get another even just to keep as a spare.
https://www.youtube.com/watch?v=fGQaB7HG9Ic
I purchased and replaced the O2 Sensor and MAP sensor both with Motorcraft parts. The Bronco is still searching for idle. I did remove and connect the battery cables while having the lights on to deplete the capacitors and memory. I started and let it idle for several minutes and it was still searching for the idle. I shifted to park, neutral, drive, and others and let is stay for a few minutes. I also ran the AC during the process. I believe the ECM replacement is the next step. Any thoughts or suggestions?
Did you check for any OBD1 codes? Is your AIC (idle control valve) working properly?
Ok, so your O2 sensor was failing, and you replaced it with a new one. But if you had a bad O2 the rich condition may have already caused your catalytic converter to plug up (depending on how long its been bad) then you're looking into a whole lot of other problems that can happen. Check the connections and for vac leaks, Autozone lists both, before CAT and after CAT O2 sensors for a '90. So it seems that you may have two! Yet the Haynes manual's wiring diagrams only shows one. Also 1990 EVTM only lists a single HEGO (Heated Exhaust Gas Oxygen Sensor) located in the exhaust pipe ahead of the catalyst. That's all I know, but it may be a start to figuring out the problem.
Thank you for the response. There is only 1 O2 sensor for sure. I have checked for vac leaks as well as other mechanics and we have not found one. Something is causing the information given by the O2 sensor feeding the computer to change the air/fuel mix improperly.
Could it be that the ECM is bad and I need to replace the ECM? How would I go about testing that?
One way you could test it is by resetting the computer, Here's a video that might give you a better idea of how to do so > https://www.youtube.com/watch?v=BFRMGir6Kgs&t=168s
If you reset it and the problems continue, then you could try replacing the ECU, it's better not to jump to conclusions because I don't know where you'd get a 1990 Ford Bronco ECU.
Thank you, but I don't believe this type of computer reset existed with the OBD1 systems in 1990.
I would be very suspect of the supposed Denso 02 sensor you bought off Amazon. There are tons of counterfeit parts for sale there. I never have and never will buy any important auto parts on Amazon or Ebay. I'll get them from a store locally where I can return them if there's a problem.
