2005 Turbo S misfire under high load / boost conditions

[danny tattersall]

Hi Guys,

My 2005 Turbo S is suffering from a severe misfire under heavy load / high boost conditions. I have seen an indicated 1.4 bar, although I am unsure whether or not that is just a spike, and am unable to push the accelerator fully to the floor in high gears for long because the misfire will occur. However, if I push the accelerator 80 - 90% of the way down and control the boost pressure at around 1.0 bar there is no misfire apparent. There are no check engine lights or fault codes stored (although I only have a generic OBDII scanner) and I have recently replaced the MAF in order to try and resolve this problem. The question is can anybody suggest what the likely fault is? Could the diverter valves be sticking in the open position causing overboost and subsequent overboost cut-off? Are the diverter valves purely mechanical or do they have an electrical input to open / close them as i think that they are purely mechanical and perhaps the pressure in the pipeline that the diverter valve 'sees' is controlled via a separate electrical device.

Thanks in advance for your assistance,

Danny.

Edited February 2, 2010 by danny tattersall

[Danyol]

Is there a modified program in the ECU ("flash"); what do you normally see under boost on the gauge? take a look under the car to see the actuator rods for the waste gates are still in place. They are mechanical and pneumatic actuated via a tube to a piston. If the gate isn't moving the ECU will see excess boost then start dumping boost/retarding timing (the engine will protect itself).

Makes me question if the ECU is flashed since normally under "limp mode" the boost will be lower (no check engine?)

[Boston Duce]

Bad plug or coil pack? If you are changing plugs, go with Bosch FR6 instead of Beru.

Did somebody try to adjust the turbo waste gates? If you put too much boost the ecu will try to protect itself.

Bad diverter valves (mechanical) usually honk like a goose (no joke) when they go bad.

BD

[wross996TT]

My initial guess is plugs & coil packs.

[Danyol]

WROSS, Agreed the ignition could be the issue but 1.4 bar doesn't make sense?

[danny tattersall]

Thanks for all the replies. I can confirm that there is no check engine light and that I don't think there is a problem with the wastegates because I recently had a restricted 0.4 to 0.5 bar boost after messing around with the electrical connector on the MAF sensor and the car was effectively running on the wastegate spring default pressure. This has obviously been resolved but I have too much rather than too little boost. My first thoughts were that the spark plugs and / or coil packs might be faulty but the fact that it is boosting to 1.4 made me think that an overboost situation might be the cause. I'm not sure whether or not the ECU has been flashed. Is there any easy way to check it out? I have a Galleto 1260 that can interrogate the ECU to see what ECU file is installed if somebody knows what I need to look for.

I was under the impression that the ECU can electronically allow the boost pressure to be higher than the wastegate sprng pressure by bleeding off some of the pressure from the wastegate actuator hoses. As the Diverter Valves look like they are purely mechanical it would stand to reason that there must be another device in the system that taps off some of the pressure.

Thanks again. Any more suggestions or responses to the additional questions above would be greatly appreciated.

Danny.

[Mikelly]

Keep in mind that with the colder dense air, he could easily spike higher, and the OEM boost indicator might be slightly off. That coupled with the crap OEM wastegate springs on a flash/tune, and I could see a possible 1.4.

The coilpacks/plugs would be a safe bet, especially with the "winter" blend of fuel in most locations this time of year...

Mike

[Danyol]

In case you missed it there's a Coil snd Plug DIY in the TT DIY; I used it a couple of years back.

One more question; your replacement MAF; where did you get it? How long have you had the car, if you're unsure whether it's flashed; has it ever run well? my concern is running lean at the top end, this would be very harmful to the engine.

Edited February 3, 2010 by Danyol

[danny tattersall]

In case you missed it there's a Coil snd Plug DIY in the TT DIY; I used it a couple of years back.

One more question; your replacement MAF; where did you get it? How long have you had the car, if you're unsure whether it's flashed; has it ever run well? my concern is running lean at the top end, this would be very harmful to the engine.

The MAF is an original Bosch unit that I sourced from a reputable supplier. I have had the car for nearly 12 months and I think that this problem has probably been there from since I bought it but I didn't notice it due to the high speeds etc that you need to be doing in order for it to manifest. As soon as the misfire occurs I back off the throttle immediately so as not to cause any damage to the engine. The car is booked into a Porsche specialist next Friday so hopefully they will be able to diagnose the cause.

Thanks again,

Danny.

[ari1856]

Does it happen at right around 4500 RPM? If so you need new plugs. A few over boosts and you will foul the plugs.

That said, 1.4 BAR seem very high. You are probably running into the factory TB shutdown which is the factory ECU protecting your engine.