Full boost with no throttle, how?

[Skerd]

My boost gauge maxes out correctly for the first second in every gear of full throttle use, then it goez to zero even though the throttle is still fully pressed down. It will stay at zero if I keep it floored, but then if I lift off fully, the boost gauge goes to the max even though my foot is off the gas completely! It does this every time. The spark plugs are new and the turbo was rebuilt two years ago. Any ideas?

[Paula&Marcus]

Hi Michael,
What you are experiencing is normal behaviour for the later (S4, S4s) boost gauges. With boost pressures more than 1 bar the gauge simply is not able to interpret the higher voltage that is coming from the boost-sensor. Thats why it just falls down to zero ... when boost pressure drops below 1 bar it will continue proper operation again ;o) "They all do that, Sir"

BTW, what ECU-code (chip) are you running ? Stock codes are not giving that much boost.

Cheers
Marcus (www.PUKesprit.de)

>> Edited by Paula&Marcus on Saturday 22 January 14:30

[adrianmugridge]

I reckon you are overboasting. The gauge only reads to 1 bar or so, so if you exceed this it goes back to zero.

When I changed the ECU chip in my GT3, it give me about 1.2 bar at overboast and it was too much for the gauge so that's how I found out.

[Skerd]

Thanks for the replies. It just started doing this a few weeks ago. I have had the best Motoconcept chip (1.25 boost) in there for over two years and it has always been perfect until a few weeks ago. It is a 1995 S4s. It was stumbling also under full throttle but the new spark plugs I mentioned fixed that part of it. It definetly use to hold full boost for a long time.

[Paula&Marcus]

Hmmmmmmm .... looks like you are getting more boost now than before ... but why ?
- sticky wastegate ?
- faulty MAP-sensor ? (sensor is giving too low output, could be the sensor itself OR the rubber line that fruns from the intake-manifold to the sensor. Please check this line ! Sometimes it gets squeezed (blocked) or it may have a hole/split)
- Super cool wheather
- Super-Boost-Sparkplugs ?

Who knows .... ?

As long as you don´t get harmfull overboosting (more than 1.25 bar) and engine cutouts (= ECU will shut down the fuel pump for safety) you shouldn´t worry too much about it.

Maybe a Freescan diagnosis couldn´t hurt.

Cheers
Marcus (www.PUKesprit.de)




>> Edited by Paula&Marcus on Saturday 22 January 14:53

[Skerd]

Thanks again. I will have those checked out. Do you know if one thing hopefully unrelated could be the problem. When I had the turbo rebuilt a pack of wires with a plastic clamp in the midle about as thick as two pencils that use to run in back of the turbo was incorrectly routed and has been almost flush with the engine for two years. Well after two years the plastic clamp part is nicely melted and I can only imagine the wires inside are not exactly as should be. Could this be the cause?

[Paula&Marcus]

Michael, could you please specify the wires !?

[Skerd]

Paula&Marcus said:

Michael, could you please specify the wires !?

If I remember correctly from two years ago they said it was the 02 sensor wires. There are about 4 wires total all rolled into one enclosed bundle. There is a plastic part where I guess they join together and you can see about 1" of the actual wires. It is supposed to be routed in back of the turbo by the heat shield, but they incorrectly were wedged into a small opening at the base of the chargecooler. They wind up going into the middle part of the top right side of the engine. The whole thing rolled straight would be a few feet long. I know this description is non-technical but I hope it is enough. Thanks again.

[Paula&Marcus]

Mike,
If your O2-wires are fried/broken you will get different issues like awful idle and way too rich mixture. Your engine will not run properly and will have a quite blackish exhaust note ... this usually happens right after 3-5 minutes after starting the engine from cold when the ECU switches from open-loop to closed-loop-fuelling.

Cheers
Marcus (www.PUKesprit.de)

[Skerd]

Thanks for the info. The idle is perfect, so that is one less thing to deal with. So those are indeed the 02 wires then?





>> Edited by Skerd on Sunday 23 January 14:51