Boost reduced when car warms up. Any ideas?

[stroker]

Try to unplug the MFTS and go for a test drive. Dunno if the MFTS overheat signal is wired in AAN harness but if yes this could be it. Faulty overheat signal.

I agree..if you unplug the connector & find that the boost returns..bingo..problem solved.

[s2driveruk]

also check the inlet temp sensor as this can cause problems too if not functioning properly, is the little black plug on top of the IM (at least on a 3b - not sure on AAN but must be similarly mounted just after the throttle body)

Cheers
David

[andrewd]

Right. Unplugged the MFTS and no difference at all .. still gives full boost once or twice, and then reverts to 1.4 bar. Did the same with the inlet manifold temp sensor. No difference again.

Did a bit of web searching and seen a few references to the altitude sensor also causing boost to revert to WG control in this manner. It does this when altitude exceeds 10,000 ft .. or when it fails.

Not had a chance to pull and boost pipework off for closer inspection yet.

The search goes on ... Can't believe I've been driving the car around for nearly 2 months now without the full complement of horses!

[kwlw]

The search goes on ... Can't believe I've been driving the car around for nearly 2 months now without the full complement of horses!

Actually I would suggest to do some investment, and pull fault codes from the ECU when the problem occurs. Maybe it's not so expensive to go to somewhere, where VAGcom is available ? This can save you some time. However, if you are excited knowing your car better - I can see the point digging into details .

I have same engine as you in my coupe, and can tell you, that it's possible to get engine check light . However it is quite extreeme - you need to purchase connector copper part (I've got it from WURTH), put it into your ECU connector, connect wire, led with 1kom resistor (don't use normal light first, just in case ), and tricky wiring to the glove box .

Sau

[porkkis]

Pull the codes. Some problem occurs so that the ECU want's to reduce the boost.

[andrewd]

Agreed. Think I can enable the 'check engine' light by simply putting bulb in, and thus get the codes. Depends how easy it is to get at the back of the instrument cluster on the S4!

[porkkis]

Very easy. You just have to take out the lower cluster trim and perhaps the upper steering column cover. Less than 5min job.

[urquattro-island]

change

change your original dump valve, i had a leaking one on my s4. Think you have one to.

This often comes when the membrane in the dump valve gets hot.

[andrewd]

Haha. What I need is a mate with an S4 so I can swap a few parts back and forth

Off on business for a few days. Will pull the instrument cluster on Friday and see about attaching the check engine bulb. The archived version of this page seems to suggest it's just a matter of plugging a bulb in and attaching the relevant bit of the loom. (The bit about the wiring to the fuse box simply adds a switch to short the connectors on demand.)

[andrewd]

Brief update ...

Removed all the boost/intake hoses from around the turbo area yesterday and gave them a good check over. Nothing seems amiss, all solid, no cracks or holes. Also inspected (as much as I could) the hoses from the intercooler out side to the inlet, and all associated hoses. Again nothing amiss.

Finally I also pulled the MAF out and it's a clean as a whistle.

Attempted to pull the engine codes, but my plan to connect up the 'check engine' light kind of failed!

More later, no doubt ...

[adamw]

any luck findin the problem with the s4?! mines still doin it?!

[andrewd]

Problem solved! For archive purposes here's a link to how ... click.