Throttle position discrepancy

I don't know about Vauxhalls, but VW/Audi etc cars with drive-by-wire throttles have a dual-track throttle position (or rather accelerator position) sensor. If one track reads much different to the other, then you get this error. Clean or replace the sensor would be my suggestion.

On these, there is a vacuum operated butterfly in the inlet manfold, these often get gummed up / partially seized because of the dreadful way the EGR system works! Dismantle the pipework check and clean this first. Also check the diaphragm is not split. Check all the vacuum pipes to it, and to the control valves.

Also check the oil level- the vacuum pump is the first to knacker from poor lubrication. (also infrequent oil changes- the 20k intervals are a joke)

If it is no better at all, have the codes re-read, and and if they are the same as before, replace the PPS.

Tim..

Pretty much what Tim said. Do you know what codes were stored? Throttle Discrepency sounds like the actual pedal being faulty, but I'm sure that should cause the engine not to rev.

The Vacuum pipe spliting is very common on these, and the usual culprit is the joining peice going to the solenoid control valve that's attached to the bonnet landing panel. Easiest way to check is with a vacuum gauge. You should get at least 0.7bar of vacuum at the supply to all the solenoid valves (should be done with the engine hot, as that's when a weak vacuum pump will show up the most). Any less, and the turbo wastegate doesn't hold shut properly under full boost (it's vacuum controlled on these), leading to Critical System Malfunction codes being stored, and the ECU then goes into limp home mode.

Not sure of the actual wording of the fault, but it is the one where the ECU tells the throttle to go to a position, and the sensor does not return the same position (presumably with some sort of timeout).

I have also been noticing a delay on acceleration, similar to turbo lag. One time the engine light came on just as the engine started to pick up, as though it waited for the throttle to move, then gave up, throwing the error.

As for the pedal sensor, I can rule that one out, because it did it once on cruise control.

If the vacuum pipe was split, I would expect it to happen every trip. It has only done it 3 times in the last week.

Thanks for the replies. I will check for gumminess and split pipes etc.

Bob

I took it back to the garage that read the codes - I asked them to check all the pipes etc while it was in for an oil change, and they found nothing wrong.

I wanted to book it in for them to strip it down to clean out the throttle assembly. He seems to think it is something electronic? He says he will strip it down and clean it if I insist, but does not think it is gummed up. Is gumming up the most common problem? If the EGR comes in the flow before the throttle, then I would go with this (from my ICV / crank gas gumming up / fitting oil trap fixing it experience), but he thinks it is unlikely.

Symptoms other than engine light/limp home include slow acceleration / seems like wrong gear selected all the time, no acceleration until 2000 revs. It feels like turbo lag, and on a couple of occasions the engine light came on just as the engine started picking up (I assume the ECU gave up waiting for the throttle to move just as it started moving). Does this point to any common problem?

Thanks

Bob

Diesels generally don't have throttle assemblies, and certainly not this one anyway. What Tim is talking about, is the swirl control valves, but they generally don't cause the symptoms you're having.

Sounds very much like a vacuum problem, or a MAF (mass air flow) sensor problem. Lack of vacuum will give those exact problems, and so could a faulty MAF sensor.

Would it cause the throttle position discrepancy engine code? I would expect it to be actuated by a stepper motor rather than vacuum.

Bob

The inlet swirl control valves were vacuum controlled. They're position is not critical, as the sole purpose of them is to partially restrict the flow in through one inlet port on each cylinder to create a swirl effect and improve combustion under set conditions. They are held open by spring pressure, and closed by the vacuum. If they get gunked up, they can stick shut or open, which usually causes some loss of power (more so at high revs due to restricted airflow), and smoking.

Personally, I think you've been given the wrong description of the fault codes. I'd suspect it's more likely to be a Boost Pressure Discrepency fault, but the garage is using diagnostic kit that doesn't supply the correct description for the code (and the garage doesn't have a clue about diesels). Can you get the garage to supply the actual fault code numbers? I'd guess it's a P0101, or P1101 code that's stored...

Took it to the VX dealer today. It was a vacuum pipe - a 1 inch long 2p looking bit of pipe was split. I asked them to replace the rest while they had the engine stripped down - the bill shows 3 @ 3.50 each. Strange how the "replace all while in there" strategy had to be suggested - they go on about cam belt addons without prompting when your water pump goes!

Bob