Generally a lean mix would indicate a leak in the intake side of the engine, or in some instances the exhaust side. You need to check all joints and gaskets on the intake side, including the injector seals, to ensure there's no unmetered air getting in. in the same way ensure there are no leaks in the exhaust side as once again this would dilute the exhaust gases before they reach the O2 sensors which would read the extra air as lean burn and signal the ECU to adapt the fuelling.

If you are sure there are no leaks, but the problem persists check all the injector electrical connections. Finally you may have to check the injectors are all working properly, as if any of them is not injecting the correct amount of fuel it would be read as a lean burn.

 

A friend of mine fought exactly those same codes for two years before he sold his truck. We tried smoke tests and everything else we could think of at the time. In the time since he sold his truck there have been several threads here about leaky injector O-rings being the source of this problem. My theory is that problems with injector O-rings are not revealed by typical smoke tests because smoke tests do not create as much pressure in the intake system as the vacuum generated by a running engine. If you've eliminated all other vac leak risks go ahead and replace the injector O-rings. I understand it's a more invasive task than some of the other fixes but it's not all that difficult. Just order the parts, including the upper intake manifold gasket, and spend a Saturday making your Rover all better.

 

I had my D2 for 7 years and fought a constant battle trying to sort out a lean mix problem. No matter what I did it simply seamed to evade me, until it dawned on me that while I had made doubly sure all the obvious seals and gaskets were sound I hadn't, nor had any mechanic, thought to look at the injector seals. So I set to and changed them. With the new ones in and everything back together, took it for a run and miracle of miracles it ran better than it ever had and a check with Nanocom Evo showed no more lean P codes.

It is important, when doing the injector seal, to clean the sockets in the manifold and the corresponding ones in the fuel rail and use an little engine oil to lubricate them so them slide in without being distorted. It only takes a small bit of dirt to undo all your hard work.

 

I'm following this thread too and that is real useful information. Thanks! When I have pressed my fuel rail down I've always been pretty sure I was screwing up the brand new seals because it was so difficult. Now I know why... no lubricant.

 

I want to ad that I just found this 11 page thread on fuel injectors. Really informative on cleaning and fixing them. Fuel Injector Thread

 

update: Just got home from my safety and emissions and it passed!! After the advice from the forum, I went and got an "official" smoke test from a mechanic. There was smoke coming out all over! There was quite a bit of smoke coming from under the upper and lower intake manifold (not sure what the upper one is called). With everything still attached to the car, it was very hard to tell exactly where it was coming from so I decided to replace all of them. Took the car apart all the way down to removing the valley gasket. Cleaned the surfaces up real good and started re-assembly. I noticed a few small scratches (not from me) on some of the sealing surfaces so I actually put permatex 1 minute gasket maker on both sides of all my gaskets (except the valve cover gaskets) to make sure the voids were filled in. Yes, I know the rave manual said to put the gaskets on dry, but I am so tired of these codes, I wanted to make sure it was sealed up. As I was putting it back together I did replace the injector o-rings. We have been driving the car around for maybe 75-100 miles to get it ready for the emissions test. Well I am happy to say that there are no pending or no codes and that I am at 7 ready and 1 not ready for the emissions test (you can pass emissions with 1 not ready). I know that is not much driving but I can tell you that before the work it would put p1171 and p1174 to pending within about 20 miles and the full blown codes would turn up not too long after that. The only thing that worries me, is that it still goes to +25 on the short term fuel trim on deceleration. But the difference is that is just bounces to +25 the immediately goes down to a normal range just around 0. The mechanic that looked at it the first time said that when it hits +25 two times it will turn on the lean code. It does not go to +25 every time but it has done it many times but no codes. Is it because it does not stay at +25 that is it not turning on the lean codes? Are my lean codes going to come back? Is it normal for a disco to hit +25 on deceleration? Am I safe (at least I passed emissions)? I did not re-do the smoke test to see how it was after the work. Should I do another smoke test?

 

Good to hear you fixed your problem. If I had to guess you had some bad injector o-rings.

What gaskets did the RAVE tell you to fit dry that you used RTV on? Please don't say the Upper intake gasket. I guess if there was a deep cut then you may of had no other choice. Although you will probably be okay if you used very little RTV, the upper intake gasket is a metal crush gasket and is designed to be sandwiched/crushed down into place between the two surfaces. I would imagine that if you had a lot of RTV on there it could get squeezed out into the air intake and possible work it's way to an intake valve. You are probably okay though if you used a small thin layer and left some room around the edge for it to expand.

Also, I used RTV on my valve cover gaskets and many others do also. It won't hurt anything if you use a nice thin layer and don't get gunk in your valves or anything. My valve covers haven't leaked a drop in over 50k miles