Newly Purchased LRD
 

I just recently purchased a 98 Discovery w/ 99000 miles and no body rust at all. I have some concerns about the vehicle and know my way around a car a little bit. I know it needs brakes, but my main concern is the way it idles. Sitting in park it will sometimes be up at 1200 rpms or so, and then when I throw it in drive it drops down to about 500 (I'm assuming because the engine is putting power to the transmission.) What are the common causes for this in Land Rovers? I already did plugs/plug wires, and I'm going to do a fuel filter soon. Also, the last owner replaced the cats with straight pipes (he left the o2 sensors intact) for more noise (what an idiot). Replacing them seems to be outrageous as i was told by an exhaust mechanic that universal cats would not fit in there and I would have to replace the entire y-pipe assembly. Could it be running weird because of the cats being gone or would that not have anything to do with it?

These trucks run fine without CATs. First thing to do is an OBDII scan for codes. Let us know what the scan returns. If you don't have a scanner or a buddy with one, auto stores will scan for free. They'll try to sell you parts though. Don't buy.

I did have it scanned, the last owner said "yeah it isn't throwing a code" and it wasn't--until about an hour down the road on the way home with it (jack ass). I had it scanned and I believe it was something to do with 02 sensors (at my brother's auto parts store, he said it was the code thrown because of the missing cats). I can't remember the specific code, but I was told it was due to the 02 sensors. Does anyone have any good experience with throwing spark plug defoulers over the rear sensors to get rid of the CEL?

CATs missing does not guarantee you'll get a CEL. I'd definitely have it scanned again and write down the codes. The PO may have cleared the codes, and by you driving it around they may have come back now.

Oh okay I'll do that. He didn't clear it out either, I just assumed it wouldn't go away until I fixed the cats or faked out the rear 02's.

Does anyone know where common vacuum leak locations are on the D1's?

Every line that connects to the plenum is suspect. Check each one for loose connection or cracks. Some are short. Some are long. Carb cleaner can be used to check for cracks. Listen for idle change when spraying.

I'm sorry I'm not very familiar with this motor yet, where is the plenum?

Is it the boxy looking thing on top that says Land Rover 4.0?

Yes you should see five vacuum lines coming off it
Brake booster (medium length hard line)
Fuel regulator (short)
2 x PCV (short)
AEL (long hard line)