2004 Discovery 2. Replaced up and down O2s. Replaced MAF with Bosch MAF. Replaced Cat Y pipe. Replaced Throttle Body gasket.



I am getting a P1174 and P1171 after a drive on the highway. I saw that on deceleration my Upstreams my short term fuel trim goes to +20%. This only happens on deceleration from high speeds. I have not done a smoke test for vacuum leaks. I suspect that it is upper intake or fuel injector O-ring. Pics of readings below.

Decel and drops to 0%


Decel and Short term fuel trim goes to 20+

 

Going to zero on decel is normal. It is called deceleration fuel cutoff, it basically saves fuel. Your issue is elsewhere. There is another forum member chasing the same issues, read that thread and jump in there.

 

I read a bit of that thread. . . Lots of info.

 

Replaced the injectors and upper plenum gasket. . . Codes are gone.


Pretty flat

More like a square ring instead of an O ring

 

I had the same codes that I chased (MAF, manifold gasket, PCV, etc...) for a long time. Even after a smoke test that showed no leaks I finally found the source of the leak. With the engine running I sprayed small burst of starter fluid under the intake manifold. Right in the gap behind the alternator and immediately heard the rise in engine speed. Culprit was not loose manifold bolts or gasket. It turned out to be the fuel injector o-rings. I ended up buying a rebuilt set on eBay and have been driving without any 1174 or 1171 issues since.

Food for thought. After replacing the o-rings I started to get random cylinder misfires P0300, P0303, etc... which ended up being a bad coil. Ended up taking the whole manifold off again for the 4th time. Just something to note for maintainence while you are in there.

So far this has been my fix. Best of luck!