Ok people, need some ssistance with an issue that's doing my head in.
First some history, after failing an MOT on some rot, and then discovering a whole more I managed to find a decent shell with a standard motor which had spun a bearing, which was perfect for an engine change as mine was forged.
In the end, as the standard engine had only run 20,000 miles since I rebuild, I used that head on my forged block, and all went well with the swap.
The problem now however, is that I can't get it to hold at 14.7afr at idle or fast idle, with the best I can do around 13.3.
The puzzle is that as the engine warms up the afr performs as expected starting rich but gradually leaning out and during this process, the afrs will actually hit around 14, but as soon as you touch and release the throttle they drop to 13.3 on idle and don't change.
The car is running standard with the exception of a turbo back exhaust, induction kit and FMIC.
I have checked the TPS calibration, changed the O2 and ECU coolant sensors and checked the wiring. I've also run through all vac hoses to confirm no leaks.
The car does have a Walbro GM250 fuel pump fitted but having researched this it flows the same as a standard fuel pump.
The slightly odder things is that, having it in diagnostic mode and altering the timing slighlty brought the afr back to the correct 14.7, however, once you come out of diagnostic mode it runs slightly richer then back to the 13.3 as soon as you blip the throttle again. This might suggest that there is a duff sensor somewhere that is not used during diagnostic mode but is during normal operation but A I'm not sure if this is how it works, and B, there are no fault codes present.
I do have a couple more things that I might try but before I give up on the car does anyone have any ideas what this could be or what to try next?
Also, am I right in thinking that the engine operates in two modes, the first when under vacuum using the O2 sensor to control mixture based on water and air temperature, and the second when under boost with the mixture controlled by the map sensor for a given air and water temperature, and if this is the case, it is the map sensor signal that defines this?
To me there only seems like a small number of inputs that control mixture but having checked them all out, I'm at a loss as to what is causing this.