My '01 Discovery is throwing P0101 and P0505 DTCs and I'm running out of ideas on what it could be. I know my way around gasoline engines fairly well, but this is my first Land Rover and my first European vehicle. I've owned the truck for a bit over a month. It's got about 150K on the engine.

In addition to the CELs, the truck is hard to start when cold (air temp less than 50* F) and almost stalls due to a surging idle. Once warmed up, the idle stabilizes around 800RPM and the surging becomes very intermittent.

So far, I've done the following:

• Replaced hose from LH valve cover to throttle body (PCV hose, I think?) due to crack @ base
• Cleaned PCV w/ CRC Throttle Body Cleaner
• Cleaned MAF w/ CRC MAF Cleaner
• Verified that my air filter is clean (FRAM paper filter)

This didn't solve the issue, so I ran to a junkyard and pillaged a couple of trucks for TPS, MAF, and IACV. I verified all part numbers and of course, these are OEM/BOSCH components.

• Replaced air box lid due to broken clip @ MAF
• Replaced IACV, no change so I...
• Replaced MAF, no change so I...
• Replaced TPS

The truck continues to exhibit the same behavior. If I clear the DTCs/CEL, it will stay gone until I turn the engine off and restart it. Shortly after restarting, the CEL comes back. Additionally, I sometimes see P0327 for a knock sensor. Initially, I was also getting 1174 and 1175, but haven't seen these since replacing the hose from valve cover to throttle body. Tonight, I sprayed carb cleaner all over every thing that looked like it remotely cause a vacuum leak and was not able to reliably get the idle to rise. I got my hopes up more than once because the surge coincided w/ me spraying an area, but as I said... I couldn't make it happen reliably, so I'm thinking they're false positives. I sprayed every joint on the intake tube, all around the air box lid, around IACV, intake manifold, throttle body... Everything I could think that would let air in.

What can I check next? I'm considering the idea that one of the sensors I swapped may be bad, but I'm unsure of how to test them without a high-end scan tool.

Any help is greatly appreciated. I've owned the truck for a bit over a month and haven't been able to register it because I can't pass emissions. Thanks in advance for any help!

 

Did you try doing a smoke test? Check your throttle cable mount on the plenum. Did you check for shorts on any of the wiring to those sensors? sensor plugs/receptacles clean? Did this just start happening out of the blue? Follow up questions for ya:
Do you have an ultragauge or equivalent?
Have your Head Gaskets been replaced yet?
Have you overheated the truck?

 

I have not done a smoke test as I don't have a smoke machine. I've had terrible luck using a cigar in the past on other vehicles, but I recently saw some YouTube videos about making a smoke machine. Might have to give that a go.

I'll check the throttle cables in the morning. I have done a fair amount of "wiggle tests" on the wiring related to the sensors, but most of the wiring for the TPS sensor is buried under the plenum. IACV and MAF wiring seems okay from what I can tell, and the plugs are tight. I cleaned the MAF connector thoroughly w/ contact cleaner, but now that you've mentioned it I did not clean the IACV connector so I'll do that in the AM.

The truck ran pretty good for the first few days that I owned it, and then I got a CEL. The codes were P0505, P0171, P0174, P0327. I cleared the codes, and it was fine for a couple days. I then got P0505, P0327, P0171, P0174, P1175, P1171. Cleared 'em, and it was fine for a couple days before ending up in the shop for an oil pump/timing cover/timing chain and some related odds and ends. The issue is certainly more pronounced after leaving the shop, but I can't attribute it to anything he did as it came in w/ the issue.

I don't have an ultragauge or anything similar, though I may order one if it will help diagnose this issue. I've been wanting to order one, but it's hard to justify since I've been unemployed for a month and won't be working for at least another week.

I have not overheated the truck, though that doesn't mean it hasn't happened. The tech that did the timing cover/oil pump/etc didn't mention anything about missing coolant, and he's very well versed in these trucks so I'm sure he would've thought to check that. I'm starting to wonder if I've got head gasket issues but I pray that's not the case lol

Thanks for the input, bmeier. You've given me some food for thought (and a slight case of heart palpitations re: head gaskets ).

 

Forgot to post what ended up being the solution. Massive exhaust leaks!

 

Where was your exhaust leak(s)? My D2 seems to be exhibiting the exact same problem and the exhaust is definitely now leaking. (I did some serious off roading yesterday and I think something has come loose.)

 

If you've been off-roading, check around the flange area of the y-pipe. I've cracked mine twice, while high-centering the rover. The cats hang down below the frame rails, poor design in my opinion for a "quote" off-road machine, and whenever you high center...the entire weight of the rover bends them upward...cracking the flanges. I've welded mine twice...and they are cracked again. This next build l'm doing is getting a skid plate fabricated to solve this issue.

And also check the passenger side exhaust manifold, l've had three manifolds crack between #2 and #4 cylinder ports.

​​​​​​​ Brian.