No boost when hot

[jason_m]

Hi all,

have a bit of a problem with my quattro (20v 3b)

It’s fine when cold & will boost, but when it gets warm there is no boost. There’s also a bit of a ‘shudder’ when it tries, and the hotter it gets the worse that shudder can be. Any ideas?

Jason

[K Simmonds]

That's a bit of an odd one, I'm assuming the car runs fine otherwise?

It could be a couple of things, my first thought's are a dodgy N75 or perhaps the wastegate is sticking open once it's gotten hot which is unlikely but possible.

Are you able to pull the fault codes? As that would be a great deal of help.

Checking for boost leaks wouldn't go amiss either as they will often make a car judder.

[K Simmonds]

Assuming you're running a stock spring in the wastegate you could try unplugging the N75, you'll only get 0.4 bar of boost but you'll be able to rule out a duff N75 that way if the car then run's okay with it out of the loop so to speak.

[jason_m]

I’ll see what I can do re the codes. It is a bit of an odd one, yep, absolutely fine otherwise & completely when cold (but I don’t let it boost until warmed up normally)

will try that N75 - not sure re the spring, will assume for the time being it’s standard… it’s a 1986 converted if that makes any difference.

Thanks,

Jason

[K Simmonds]

Fault code's would be extremely useful, have you got a copy of VAGCOM or are you going to have to blink them out?

Though it does sound like a problem with either the N75 or wastegate as they're what controls the boost and are both effected by heat, especially the wastegate.

I've had wastegate's get stuck on other car's at work but never heard of an S2/RS2 one doing it, though that's the first thing that come's to mind..

It doesn't matter what chassis the engine is in as long as it's wired up correctly and you can still get at the two diagnostic plugs.

[newsh]

Try unplugging the MFTS (Multi Function Temperature Sensor), the big sensor under the water rail. One of its functions is to disable boost in excess of spring pressure when the engine overheats. So if it goes wrong it can send the signal at a lower temp.

[K Simmonds]

Try unplugging the MFTS (Multi Function Temperature Sensor), the big sensor under the water rail. One of its functions is to disable boost in excess of spring pressure when the engine overheats. So if it goes wrong it can send the signal at a lower temp.

As always John's knowledge of these car's is second to non!

I didn't realise 3B's had this safety function, it could be either of my suggestions though I feel this more likely than a sticky wastegate, the wastegate's on these engines are very robust and reliable...

Though you should still be getting 0.4 Bar, the same as if the N75 is unplugged, assuming a stock spring is installed which is most likely the case.

Do you have a boost gauge fitted?

[newsh]

Yeah just thinking about the judder, a MAF connector issue or a boost leak can cause a judder but it's usually not subtle.

Could possibly be a really bad coil causing a hot misfire which would kill power meaning a drop off in boost.

We really need to know if your actually measuring boost, and what it's making, when driving ok and when playing up.

Codes would be great.

A boost leak test and static timing check would be a good idea.

[jason_m]

I have VCDS, it hasn’t registered any codes. Sorry for the delay, laptop needed a new disk - hadn’t used it in years! I did learn it has an MTM chip in there though… I wouldn’t say the judder is subtle, it’s defiantly worse the hotter the engine though.

Thanks for the suggestions, looks like I have a few things to try/check now - probably at the weekend if the weather is dry!