S4s #5 chip boost problem

[fmxa]

I'm having a problem with a 95 S4s running a Marcus #5 chip. When reaching ~5000 rpm under WOT I get what sounds like a short engine miss & power-loss while observing my boost gauge go from max to zero and then back to max boost in an instant. Anyhow, once this max-zero-max boost condition has occurred, the car keeps on accelerating just fine, so while on WOT I get power-miss-power ....

Any ideas to the cause of the miss/power-loss or tips?

Thanks all!

[Paula&Marcus]

Hi,
Looks like you actually have not a problem ... you just experienced one a the few "They do that all, Sir"-symptoms ;o)
The boost gauges on the newer S4, S4s cars simply are not able to read/show boost more than 1 bar. Instead they show ZERO when boost exceeds 1 bar ... When boost is dropping below 1 bar the gauge is catching up and will display boost like normal.

Ohhhh ... lets reread you posting ... do you get cutouts ???? If so you have overboosting ! This is when boost rises above 1.25 bar the ECU will shut down the fuelling ...
There are many likely causes for overboosting:
- sticky wastegate
- wastegate-capsule faulty
- pressure lines to wastegate faulty or disconnected
- faulty MAP-sensor
- and many more ....

Cheers
Marcus (www.PUKesprit.de)

>> Edited by Paula&Marcus on Monday 7th March 17:57

[fmxa]

Hi Marcus,

I think you're right on the money. I hooked up freescan and MAP is hitting 2.24 and what I feel is exactly like a very brief (~1 sec) fuel starvation or cut off in. Strange thing is, if I keep my foot on the floor, the fuel comes back and she keeps pulling like a train with no further cut outs.

Out of curiosity, how does the ECU determine boost; is it MAP minus local barometric pressure? If so, I noticed my baro sensor is reading low, freescan reports it at 850 while the real pressure was around 1020 millibars. If this is the case, (MAP) 2.24-850 (baro) would tell the ECU my boost was around 1.39 bar and cut the fuel; when in reality it was more like 2.24-1020 or 1.22 bar - sounds plausible?


Thanks!

Paul

[Paula&Marcus]

Hi Paul,
The BARO-sensor 850 mbar figure is absolutely normal ! "They do that all, Sir ! ;o)"

This definitely is not the cause of you overboosting issue.

First I would reset the ECU and let it learn from new.

Cheers
Marcus (www.PUKesprit.de)

[fmxa]

Damn it, I was hoping for an easy fix! Thanks for all help Marcus

[njgsx96]

If you got a freescan log, post it up. It would help us help you diagnose your car better. If you do not have any space to host it, send it to me and I can.

[e-mail]

[Skerd]

How old are your sparkplugs? You would not believe how many symptoms bad plugs can be responsible for. I know of three personally, and a WOT misfire/stutter/stumble was one of them.

[njgsx96]

Here is the freescan log for fmxa, if anyone has a few minutes to take a quick look at it. I converted it to .xls and added a new column for injector duty cycle on the primaries... just another data point...

www.databurstpc.com/data/fmxa-s4s-chip5.xls

[squelch]

According the freescan, you are indeed hitting boost fuel cut, but then your lifting off the throttle and then easing back into it.
This isn't the same as "acclerating normally" I haven't found a pull yet where you didn't lift.

Reset your ECM, if the problem is gone. your home free, if not.. turn the boost down.

Did you have this problem with the stock code ?

[fmxa]

Thanks for the input everyone. I'm going to swap chips this weekend, tried reseting the ECU and no change. Will let everyone know what I find .....

Cheers!

Paul

[fmxa]

Finally had time to start looking into this again.

1) Swapped the #5 chip out and put back the stock S4s chip - same results, still get the over-boost cut out

2) Disconnected the waste gate solenoid - get a steady ~0.65 BAR reading and no over-boost or cut out (vacuum hoses look good)

I believe this rules out the mechanical aspect of the system, (waste gate and capsule) leaving the electronic portion. I'm planning on replacing the waste gate solenoid and MAP sensor next - good idea or not? Any suggestions welcome!

[arium]

I had a very similar thingy occur with mine. Put your foot into the throttle; cut out at about 5K, lift and feather it back down gently and all was fine.

Turned out to be two very high resistance plug wires. Carved in a set of 8.5 mm MSD's and all is now well. Idles much smoother too.

Sure isn't a warm and fuzzy feeling when the fuel cuts out and then slams back in so suddenly. I had visions of renault transaxles dancing around like sugarplums in my head.

Steve

[Cross-eyed-twit]

That looks quite neat mate. Again, I like the reinforced look to the flanges and end bit that holds the turbo.
Curious, how much does it weigh compared to the cast one? I am wonder about long term reduction in stress fom vibration, if it turns out a little lighter.