Reduction of power

[Dingo]

Ive had this intermittant problem for a while, but it is happening virtually all the time now, it feels like the ECU is retarding the timing and is definately reducing the boost (its home fried and used to boost upto 1.4 - 1.5) where as now its only 1.2 and I can feel it holding back. I only run it on 99 Ron fuel and have boost leak tested it to death!! and I cannot find anything, I have fitted a new set of bosch plugs, measured the resistance of all the plug boots, fitted a new N75 and also a new dump valve, all of which i couldnt find anything wrong with them but just trying to eliminate problems. Its not pulling up any fault codes either.

Someone suggested faulty knock sensors, any thoughts or advise would be appreciated before i start to ramdomly replace any more parts.

PS, its an ABY

[lind4hl]

have you tried tha "mark h mod"?, a silicone hose over intercooler to inletmanifold pipe?

or is the turbo getting bad maybe?

[Dingo]

No it hasnt got the mark H mod but its definately not leaking as I have tested this to death!

[lind4hl]

ok, maybe the turbo is start dying? how far has it been running??

[Dingo]

Its only done 70,000

[lind4hl]

okey ,then i dont really know, better some more expert answer then

edit_ how did the old spark plugs look? black/brown or grey?

[Dingo]

Old plugs looked perfect, i recently had it rolling roaded and it was spot on

[Smiley]

Ok, so no boost leak .. this includes the WG diaphragm, and no fault codes, and the car gets enough fuel and is not running lean
Maybe the WG is somehow stuck open? WG spring?
2cents..

[Knight Rider S4]

I would be looking at the coils again ..I have just done a job on one and had the same problem ..car would run great but when it was under load it was like it was missing a cylinder ...I would check all the wiring for the coils to as the heat under the cover bakes the wiring and can cause it to short againist the cover ....

[Dingo]

Its not missing, I just feel it hold back a bit when it should be on full boost,
I've run VAG and no faults....
Is it possible I have a very small boost leak that I've not found yet and the ECU is just dealing with it

[Smiley]

Small boost leaks - i've had some - don't reduce the max.boost level - at least at the rpm where max boost is reached. But the mixture will be too fat, some loss of power and bad mpg..

EDIT: Yes, ECU does not raise a fault code on very small leaks..

[S2central.net]

That 'holding back' can definitely be the coils (or POS)... Don't expect very obvious misfires to be apparent when coils go bad... Can also be bad earthing of course. Best way to check the coils (or POS units) is to drive up to another S2 owners house and swap them over - the POS units first as they are easy... Then the 5 x coils complete with cover and two connectorised cable (plus earth) - thats pretty quick too.

As to plug boots - I used to have issues a bit like this and solved them with 5 x new plug boots... They all had perfect resistance down the wires but the rubber had perished and broken down over the years such that I must have been leaking HT voltage before it got to the plug(s)... By gently bending the boots you could easily see the cracks - simply caused by umpteen years of heat cycles... I thought it was one or more coils (mine are original at 140k and still going strong) but new boots saved the day for a lot less money !

Also - how fresh is the lambda probe ?

I also suggest you test with another MAF as a crap signal from that will confuse an ECU - only certain faults are detected.

Also - check the air and water temp sensors are in spec... Think the data is on S2central.net - if not - shout back here for help.

VAGCOM rarely detects small boost leaks and generally only detects sensor failures such as open or short circuit. Bit of a PITA I know...

[Paulgo]

Also - check the air and water temp sensors are in spec... Think the data is on S2central.net - if not - shout back here for help.

I had a similar problem recently with my chipped 3B - it was just holding back that little bit of top end boost. Drove me mad trying to work out what was up! Only about 4-5psi but boy what a different when it returned!! Turned out the be the MFTS playing up. Dont think they have to completely fail either to cause problems. Mine started effecting boost without me knowing it and some time before it finally gave the game away with the tell-tale sign of the Temp warning light staying on - the only reason I finally realised it was knackered! Sometimes giving the MTFS a stout tap can fix it temporaily - I did this with mine and it worked long enough to confirm that it was the switch causing the problem as full boost came back briefly!

Paul

[ryster]

i m having simler prob s whats MFTS buy the way

[Tommen]

MFTS = Multi-Function Temperature sensor