My 2001 V8 has (and always has had) an uneven idle and uneven throttle response in the low/mid range but no apparent problem at big throttle openings. The problem feels like a misfire and my first thought was air leak (vacuum leak) but I'm not so sure - I think it might be the electronic throttle actuator/sensor on the pedal mechanism.

The reason why I'm questioning the pedal is that the problem seems to disappear when I engage cruise control. It is my understanding that the pedal is effectively bypassed when cruise is engaged, with the signals to the throttle body coming directly from the ECU/ECM/PCM or whatever it's called on these cars. If the problem was an air leak, coil, spark plugs, the idle air valve, throttle body or throttle actuator motor than I would expect the problem to still be there witht he cruise engaged - the actual pedal electronics are the only bits that I can see that are not shared.

My bluetooth code reader doesn't throw up any codes. So, I have 3 questions:

Does this sound likely or am I barking up the wrong tree?
Is there any definitive test that I can carry out to determine if this is correct?
If I replace the throttle pedal assembly, will a 3L one or RHD one fit/work?

Thanks

PS: I tried a search but couldn't find any reference to a similar issue.

 

i'd put money on vacuum leak

 

Or an actual intermittent misfire...

 

If it's the pedal you can watch its reading using OBD live data and will see it fluctuating. Chances are you won't, but it's an easy thing to do.

 

If it is a vacuum leak (or more correctly air leak - vacuum doesn't leak!) or misfire, why does it NOT occur when using cruise control? Surely such problems would be evident at all times?

 

At least read LTFTs, hot engine, parked, at idle & 2500rpm.

You've hardly any facts, you need more. Guessing can be annoying & expensive.

 

Are you saying that there's a noticeable difference at (say) a constant highway speed with cruise engaged vs. using the gas pedal?

 

I have seen PPS (pedal position sensor) problems with the X200.
I have replaced several in years past but there was usually a DTC logged in the module.

The pedal has no cable so it has two (I think) sensor tracks for position sensing.

I would get a diagnostic scanner and watch the position sensors for 'glitches' in the linear track(s).

There was a TSB or Service Action for the PPS on the early X200 but I think it was for a pedal 'squeak'.

If you have a spare PPS you could plug it in and see if the throttle responds smoothly.
A salvage yard might sell one cheap.

bob gauff

 

Yes, it has two and they're deliberately different so that codes flag and you won't get uncontrolled acceleration like with some cars. (Post #4: read data.)

 

Yes - there is no sign of the problem when using cruise control
That is what makes me suspect a throttle pedal problem rather than air leak or coil/plug problem (I've done my best to eliminate those). My code reader shows no codes though.

 

So just use OBD live data. Job done.

 

Unfortunately the car is now out on loan to a friend for a couple of weeks - so I'll have to wait a while now.

 

Do you have a recommendation for a programme/application to do that?

 

Thread in General Tech.

 

Correct me if I'm wrong, I understand things the gas pedal has two variable potentiometers which read from 0-100 as the pedal is moved. They are wired opposite to each other so at idle, one reads 100 while the other reads 0. At full throttle the opposite occurs with the first at 0 and the other at 100. Logically there is a mid point where each reads 50. The system constantly reads and adds each expecting a total of 100 at all time. If it sees a different number a code is set and possibly the engine goes into fail safe/reduced performance.

If the gas pedal is at fault here, I would expect a CEL long before any engine roughness becomes apparent. The engine not responding correctly to the gas pedal would seem to require a dual failure (both potentiometers) that read incorrectly but somehow still give a combined total reading of 100. Seems impossible.

Monitoring the raw data is the best way to prove/disprove.

Is my understanding of the system correct?

 

I'm not sure how they read but the overall conclusion is right so there ought to be a code. In honesty I see no reason to monitor the data as there's no code, but currently it seems there is some doubt that the fault may lie with the pedal system so looking at the data will show the truth. I fully expect it to show that the fault is not the pedal.

The same live data can show the fuel trims, which may well reveal something.

 

Windows and android obd software available from the link in my signature

 

Thank you

 

Coming back to this problem now that the car is back from loan to my friend. During that time it overheated due to a seized cooling fan (now fixed) but thankfully doesn't seem to have suffered any further injury. It did give me the chance to try different software for the scanner.

Now, back to the throttle issue.

Using the Android software mentioned by Police666, I have been looking at the live data and the results are that when stationary (not in gear), with or without the engine shutdown I only get approximately 21% throttle movement and the engine will not rev past 3000rpm (is that normal?).

I unplugged the connectors from the throttle body and the car imediately flashed up the "emergency mode" message on the dash and settled down to a high idle (I believe that this is normal for limp mode).

On the road the vehicle seems to drive reasonably well, revs through the range OK, accelerates but is rather uneven in its throttle response (as it always has been in the time I've owned it).

Throughout all of this it is showing no codes through my bluetooth code reader (although that shows codes OK with my 2003 Discovery). So I've come to the conclusion that the ECU is not talking to the code reader properly (even though the live data works) and I'm going to bit the bullet and take it to the dealer for a diagnostic scan. After all, with all the issues that it has had (overheating, battery disconnected, fan seized, stepper motor and TPS disconnected) it should have displayed a few fault codes! Once the dealer has told me what's wrong I can set about fixing at at my labour rates ($0 per hour) rather than theirs.

Thanks for the help and support, guys

 

That's normal.