Disco 2 Rough idle in a V8 D2a — tried MAF, O2, sparks, leads

I hope that someone can help me! Been at this for a few weeks with no luck.

My D2a V8 has a rough idling issue which I can't seem to fix. It also throws up a lot of O2 diagnostic trouble codes. Car has around 120,000 km on it.

It wobbles at traffic lights and when stopped, but runs smooth when it's going.

Confirmed:

• P0130 - O2 bank 1 circuit malfunction
• P0171 - system too lean bank 1 (passenger's side)
• P1171 -
• P1174 -
• P0150 - O2 bank 2 circuit malfunction (driver's side)
• P0174 - system too lean bank 2

(Still getting those after changing both O2 sensors)

Here's what I've tried in order:

1. Run 98 fuel, sometimes E10 and 95, but no difference
2. I changed spark plugs (8x NGK)
3. I tried 3x different MAFs (2x brand new Bosch, 1x from a wreck)
4. I changed air filter, and air filter top lid (clamp was broken)
5. Mechanic changed spark plug leads (aftermarket, grey ones)
6. I changed O2 sensors (2x new Bosch)
7. Cleaned the injectors with fuel additive
8. Cleaned idle position sensors insides with carby spray

What do you suggest that I try next?

Unrelated work done:

• Mechanic: changed rocker cover gaskets (big oil leak before), thermostat, water pump, coolant flush, throttle body base (big coolant leak).
• Mechanic visually inspected at vacuum hoses, doesn't see any damage or cracks.

Stats I'm getting via OBD2:

0km/h (idle, coolant 26-40C)
980-872 RPM
40-20 kg/h MAF
Bank 1: 0.45-0.54V, 7-19% trim
Bank 2: 0.37-0.75V, 2-11% trim

0km/h (idle, coolant 81-87C)
614-703 RPM
10-16 kg/h MAF
Bank 1: 0.03-0.06V, 25% trim
Bank 2: 0.01-0.06V, 0-25% trim

11km/h (87C)
1639 RPM
108 kg/h MAF
Bank 1: 0.89V 17% trim
Bank 2: 0.74V -6% trim

53km/h (87C)
1286 RPM
29 kg/h MAF
Bank 1: 0.04V 15% trim
Bank 2: 0.04V 17% trim

48km/h (87C)
1684 RPM
91 kg/h MAF
Bank 1: 0.39V 2% trim
Bank 2: 0.53V -3% trim

(Trim and MAF numbers have been rounded.)

Have all exhaust connections/manifolds and pipes been checked up as far as the O2 sensors for dodgy joints or little holes?
Also have you tried it on E5 petrol rather than E10 say?

Maf Diver

Idle control valve sticky?
As well as a +1 for checking for air leaks in the system before the O2 sensor and after the MAF

J

I don't know much about V8s but acording to a relaible source the possible causes of those codes are:


P0150 Oxygen sensor (O2S) 1, bank 2 - circuit malfunction = Wiring, O2S, ECM
P0174 System too lean, bank 2 = Intake/exhaust leak, fuel pressure/pump, injector(s), AIR system, hose connection(s)
(* same for bank 1)

seems that you ruled out only the O2 sensors and air filter(the other things done are no factor for these codes) ... for P0174 get the injectors checked cos the rough idle is a symptom for them as IMO the other things like air leaks or fuel presssure issues would affect the engine's behaviour under load too which you said it's not a problem

you should start with a high quality injector cleaner in the fuel tank first, something like this https://www.halfords.com/motoring/e...ditives/wynns-total-action-petrol-212534.html

sierrafery said:

I don't know much about V8s but acording to a relaible source the possible causes of those codes are:


P0150 Oxygen sensor (O2S) 1, bank 2 - circuit malfunction = Wiring, O2S, ECM
P0174 System too lean, bank 2 = Intake/exhaust leak, fuel pressure/pump, injector(s), AIR system, hose connection(s)
(* same for bank 1)

seems that you ruled out only the O2 sensors and air filter(the other things done are no factor for these codes) ... for P0174 get the injectors checked cos the rough idle is a symptom for them as IMO the other things like air leaks or fuel presssure issues would affect the engine's behaviour under load too which you said it's not a problem

you should start with a high quality injector cleaner in the fuel tank first, something like this https://www.halfords.com/motoring/e...ditives/wynns-total-action-petrol-212534.html

Click to expand...

I am now being very daring and disagreeing partially with this, air leaks at idle can often lead to a rough idle but get smoothed out at higher revs.

ooops, smacka ma hand!

Thinking of SU carbs there!
Of course modern systems have no moving parts do they? Or do they? Injectors etc?

I mention the exhaust cos the wife's car once failed its MOT on too much oxygen at the O2 hence running too weak. Due to tiny pinholes in the exhaust. The tickover on the other hand was OK. Maybe the engine management made allowance for it!!!!

Stanleysteamer said:

I am now being very daring and disagreeing partially with this, air leaks at idle can often lead to a rough idle but get smoothed out at higher revs.

ooops, smacka ma hand!

Thinking of SU carbs there!
Of course modern systems have no moving parts do they? Or do they? Injectors etc?

I mention the exhaust cos the wife's car once failed its MOT on too much oxygen at the O2 hence running too weak. Due to tiny pinholes in the exhaust. The tickover on the other hand was OK. Maybe the engine management made allowance for it!!!!

Click to expand...

My thinking was that an air leak(or any other leak) where ever it is gets worst under load than at idle though i'd not contradict any other theory when it comes to petrol engines

sierrafery said:

My thinking was that an air leak(or any other leak) where ever it is gets worst under load than at idle though i'd not contradict any other theory when it comes to petrol engines

Click to expand...

On SU carburettors, the spindles for the butterfly valves that let the air into the main venturi would wear in their bushes which thus let in air as there was a biggish vacuum around that area at tickover, once opened the rush of air when the butterflies were flat, or in betweeen that and tickover, meant that any leakage didn't matter.

A compression test will establish a baseline for further diagnosis.

Your error codes *suggest* that it's been trying to richen the mixture, but it hasn't worked...

Exhaust leaks near O2s will cause erroneous readings - air gets drawn in and makes the system look lean, so it tries to richen it up... making things worse.

You need to look at what the long term trims are.

Adding fuel at idle but not at load, tends to be intake vacuum leaks - as more air is drawn in under load.

You say it wobbled at idle though... if it's misfiring, that shows as a LEAN mixture also... because its only the O2 being measured.

Stanleysteamer said:

I am now being very daring and disagreeing partially with this, air leaks at idle can often lead to a rough idle but get smoothed out at higher revs.

ooops, smacka ma hand!

Thinking of SU carbs there!
Of course modern systems have no moving parts do they? Or do they? Injectors etc?

I mention the exhaust cos the wife's car once failed its MOT on too much oxygen at the O2 hence running too weak. Due to tiny pinholes in the exhaust. The tickover on the other hand was OK. Maybe the engine management made allowance for it!!!!

Click to expand...

That is true for EFI too. Both things.

AllanWorms said:

That is true for EFI too. Both things.

Click to expand...

Nice to know that my old skool knowledge isn't completely wrong for more modern cars.

Also wondering if this thing is old enough to have a distributor (? ) then maybe there is wear/play/ looseness in the spindle leading to slightly erratic firing, weak springs etc. But yet again this may be far too oldskool for this engine. and would result in dirty exhaust/unburnt fuel.

Smoke test the engine you have a massive vacum leak

Stanleysteamer said:

Nice to know that my old skool knowledge isn't completely wrong for more modern cars.

Click to expand...

You are right.... it's not COMPLETELY wrong... just mostly ...which happens if you compare a Discovery Series II engine management with a LR Series II ... sorry, i couldn't help myself to not be a bit sarcastic

sierrafery said:

You are right.... it's not COMPLETELY wrong... just mostly ...which happens if you compare a Discovery Series II engine management with a LR Series II ... sorry, i couldn't help myself to not be a bit sarcastic

Click to expand...

You can be as sarcastic as you like.
But I wasn't comparing it with any other LR at all, I was in fact comparing the issues around holes in exhaust etc with a more modern vehicle than my D2 TD5 2000MY, namely my wife's 2004 MY petrol, Citroen, twin cam, 16 valve with more modern engine management. On which I have had to change an 02 sensor and the exhaust due precisely to tiny holes in it which the engine management coped with but led to too "light" a reading at the end of the exhaust which their emissions testing equipment picked up.

As for other stuff, like to do with distributors, I do not know if the engine in the thread even has one although my Disco 1 V8 does.

Anyway the proof of the pudding will be in the eating, and maybe then we will see if my contributions are closer to the truth than either of us know for sure.

Thanks everyone! Here are some replies/next steps:

Stanleysteamer said:

Have all exhaust connections/manifolds and pipes been checked up as far as the O2 sensors for dodgy joints or little holes?
Also have you tried it on E5 petrol rather than E10 say?

Click to expand...

My mechanic said he looked visually but doesn't have a smoke machine. I tried making a homemade smoke machine with a cigar and a siphon, but it didn't work

Yes, tried a few tankfuls of premium 98 and still the same issue. (Running E10 94 most of the time, but also "premium" 95.)

marjon said:

Idle control valve sticky?

Click to expand...

Idle control valve: couldn't find a video or guide that covers how to remove and clean it.

Is it as simple as this? (1) disconnect battery, (2) disconnect hoses from ICV, (3) remove it, (4) clean with brake cleaner and wait 1h for it to dry, (5) refit everything and go?

Or should I just buy a new one?

sierrafery said:

P0150 Oxygen sensor (O2S) 1, bank 2 - circuit malfunction = Wiring, O2S, ECM
P0174 System too lean, bank 2 = Intake/exhaust leak, fuel pressure/pump, injector(s), AIR system, hose connection(s)
(* same for bank 1)

seems that you ruled out only the O2 sensors and air filter(the other things done are no factor for these codes) ... for P0174 get the injectors checked cos the rough idle is a symptom for them as IMO the other things like air leaks or fuel presssure issues would affect the engine's behaviour under load too which you said it's not a problem

Click to expand...

For that first error code, how would I troubleshoot the wiring or ECM? (I've tried 6x different O2 sensors and all end up getting this error.)

Thanks, I'll try a fuel injector cleaner. They've got this in AUS for about £3.

Yeah, under load it seems fine.

Once warmed up and while driving, the OBD2 Car Scanner app reports fuel system status:
- ECU #1: Closed loop, using oxygen sensor feedback to determine fuel mix
- ECU #2: Closed loop, using oxygen sensor feedback to determine fuel mix

While stopped at a traffic light, fuel system status:
- ECU #1: Closed loop, using oxygen sensor feedback to determine fuel mix
- ECU #2: Open loop due to system failure

What is this second ECU?

Graculus said:

A compression test will establish a baseline for further diagnosis.

Click to expand...

Just to confirm, is that the vacuum leak smoke test the others are suggesting?

AllanWorms said:

You need to look at what the long term trims are.

You say it wobbled at idle though... if it's misfiring, that shows as a LEAN mixture also... because its only the O2 being measured.

Click to expand...

After unplugging the battery and driving around for 40 minutes:
Long term fuel % trim - Bank 1: 17.19 %
Long term fuel % trim - Bank 2: 17.97 %

Yeah, it wobbles sideways at idle! Not hugely but noticeably. Funnily, very occasionally it doesn't, especially when the system is reporting both ECU1 and ECU2 running closed loop. But it lasts only a minute and then it starts wobbling again.

My cheap OBD2 scanner app can't seem to report/show misfiring codes even if I had them. I'll borrow a more advanced tool, but that won't be for a week or so. I wonder if there's an iPhone app (can be paid) that can also show this? Shouldn't everything be the same over OBD2 for all devices?

biketeacherdave said:

Smoke test the engine you have a massive vacum leak

Click to expand...

Thanks, will do! Nobody on Facebook/forums near me has a smoke machine and my DIY one didn't work. But I'll manage!

MAF Diver said:

Just to confirm, is that the vacuum leak smoke test the others are suggesting?

Click to expand...

I think it's about copression in the cylinders... it's measured through the spark plug holes

Compression test, (petrol engine) you put a compression tester, i.e. a pressure gauge in the spark plug hole then spin the engine over to see what compression you are getting, cylinder after cylinder.

As for smoke tests. Maybe you can block the end of the exhaust and see if smoke pumped in somewhere leaks out? This one I have never heard of!!!
All good fun.
I am watching with interest!

MAF Diver said:

Idle control valve: couldn't find a video or guide that covers how to remove and clean it.

Is it as simple as this? (1) disconnect battery, (2) disconnect hoses from ICV, (3) remove it, (4) clean with brake cleaner and wait 1h for it to dry, (5) refit everything and go?

Or should I just buy a new one?

Click to expand...

Just chucking new bits is not the best route.
Don’t even know what engine you have, although you have said “disconnect hoses” so I guess you know where it is, and it’s not direct into the intake body?
U tube is a great resource, as is a multimeter and wiring dia to work out what it is supposed to do.

But cleaning stuff is free which ever 1 it is, if it’s working right. So what’s to lose?

J