I have a 91 EF with a D16Z6 and OBD1 swap, and so far all the issues I've had getting it to run right have been resolved, except for an odd warm up problem. With the engine cold and the ECU just reset, it'll throw a code 1 (O2 sensor), and have a rough idle. It'll start to idle around 2k, then work its way back down to 1k within a few seconds. There's an Autometer lambda gauge hooked up, and it starts out in the rich/stoic area, and then gradually tapers down into the lean area as the engine settles down to about a 1k idle. If I hit the gas, it'll hesitate, but will rev up. After the engine is warmed up, if I shut off the engine and take the keys out and put them back in, the code 1 disappears. If I start it up with the engine warmed, it'll idle and run fine, but the lambda gauge is still reading lean. Any ideas on whats causing the code 1? Is it normal for the gauge to read lean on idle with no load? Thanks in advance.

 

well your engine will run rich on warm up. This being because your ecu is trying to warm up you catalyst. wich is why your engine idles a little high and then you'll notice it go down when it reaches operating temp. Try changing out your post cat o2 sensor (heater circuit) sense thats what your ecu looks at for warm up, it changes resistance when it gets hot and if its not seeing any change it will throw the code

 

I'm running a P28 and only a pre-cat O2 sensor....but I thought the P28 only looked for the one O2 sensor?

 

exactly it looks for 02 sensor fluctuation and thats how it determines its how rich or lean to run. theres a heater circuit built into the sensor and when it gets hot the resistance will go up sending message to ecu to lean out. thats why when its cold the resistance is low and the ecu wants to see that high resistance so it runs rich thats how it knows if the cat is warm enough to start burning combustion gases

 

It would appear that it wasn't the O2 sensor or a code 1.....but it was an IAT sensor and a code 10. I replaced the IAT with another I had laying around and the issue went away. Thanks

 

make sure to read cel lights right before posting lol