Terrible bogging, no codes*
 

*no codes except the airpump which I removed years ago. The car runs great for the first 20 minutes and then looses all power under high load. It's done the same cycle three times now. I did just remove the charcoal canister which could be to blame. Any ideas? Fuel pump or filter? When it's cold it runs perfectly as well unde low load

If you removed the charcoal canister, where is the fuel tank venting to?

It's venting to the outside air. The line and connector is just tucked into the fender well. I tried running with no fuel cap and have the same problem. Of course I'm two days away from the VIR August event.

When it happens, can you shut the car off and restart it right away without trouble?
Do you have a Durametric?

First off thank you so much for your help, not only is this a track weekend it's my first time trial so this is particularly painful. If the car experiences the bogging problem and you shut it off and then right back on and floor it, it bogs right back down. If you pull over and let the car sit (ON or off) for a couple minutes the bogging goes away for a few minutes. It really seems like the tank is unable to vent properly but I think running without a gas cap would fix that. I wonder if the tank can vent through the metal flap under the cap.

I didn't mention that at first when I ran the codes both the pre-cat O2 sensors were bad. I replaced them and didn't see any change. Do I need to reset the ecu?

sound like car is in limp mode - does disconnecting maf have any effect (if not then car is not metering are and just using throttle position - limp mode)? could be low fuel pressure - pump or filter issues. should be a code regardless. did you reset ecu?

No I haven't reset the ecu yet. Can I do that without a Porsche tool? I don't believe it's limp mode because it snaps in and out of bogging rapidly. I think its low fuel pressure, there's a code for that though? I wonder if there are codes I'm not seeing with my generic reader.

there's no code for low fuel pressure, but there is if the ecu can't hit the proper air/fuel mix. now, if it is only happening at high load then ecu isn't monitoring afr, so may be a fuel pressure thing that the ecu isn't detecting - reduced fuel pressure/flow doesn't become a problem until engine is demanding more fuel - crud in filter?

ecu reset is just disconnect battery for a bit.

I'm replacing the filter tonight so we'll see.

When you say bog, is it cutting off quickly or just slowly dying? If the former, I would be looking at electrical. A bad spark plug wire or broken plug issue will only manifest under load in many circumstances.

It cuts off quickly and all cylinders are effected (or seem to be). It cuts in and out rapidly but you don't get that failed coil sound. More of a struggling to run coughing sound. Sounds like a carb'ed car with a really bad mixture. If it were electrical I think I'd get a misfire code.

Welp. Filter didn't fix it.

i just saw your other thread on this issue, as well as your thread on removing the charcoal canister - car was bogging before you removed canister? did you try diconnecting the maf or resetting the ecu - based on the ohter thread you have classic fouled maf?

http://986forum.com/forums/performance-technical-chat/68143-bogging-can-caused-o2-sensors.html

Oops...I posted this and the.n couldn't find it so I thought I did something wrong. I did just pull he charcoal canister which I also included in this thread. I really think some how that's it I just can't figure out why. I just checked the fuel pressure. It's right on the money 3.1 bar

Charcoal canister reinstalled. No luck, same issue. Man this has been a b*tch!

OK BIG CLUE...maybe. It runs great with the MAF disconnected. But I replaced that today. What else could be causing it to run like crap in closed loop if the O2 sensors and new (upstream ones) and the MAF is new?

right part # on the maf (there are several versions, not interchangeable)? reset ecu to get bad fuel trims out of the system? second maf bad (seems to happen a LOT)?

I pulled the battery for 5 minutes. That does it right?

Does anyone know the list of sensors that are ignored when the car is running in open loop (MAF disconnected)?

i think the sensors involved besides o2 sensors and maf are throttle position (there are several - on the throttle, in the pedal) engine speed (rpm - tach) outside air temp and engine temp. not sure which ones stay in play when in limp mode. i have a bosch motronic manual i can email you if you pm me your email address.

Thanks again!

I spent the morning over at a Porsche Specialist and between two techs and a another Boxster to compare values to no one has any ideas. I could have bought a Boxster for what I've spent on resolving this problem. Frustrations starting to come through. For some reason the MAF reading and predictive table values are off from one another, but the MAF appears to be working correctly and showing correct values.

Symptoms
-bogs when hot, the hotter it gets the more it bogs
-runs great with the maf unplugged
-no codes (verified by Porsche shop)
-engine temp correct (verified by Porsche shop)
-maf reading and predicted table reading are off
-tps 82% at full throttle..odd


Parts replaced this week (all oem)
-maf
-O2 sensors (front)
-fuel filter
-air filter


Things I've checked
-fuel pressure 3.1 bar (correct)
-vacuum leaks sprayed everything down with carb cleaner.


Something is making the car adjust its MAF values away from the predictive values. But all systems seems to be working properly.

will affect maf readings:

the new maf is bad?
the new maf is wrong part number?
the new maf got fouled by oil from your filter (same as previous maf)?
vacuum leak (but would be more obvious at idle than wot, and you've already checked that)?

is intake system stock other than oiled filter?

Something that affects the MAF is air temp. Is your air temp probe mounted correctly or is it by chance up against the radiator?

Air temp probe? That's a new one, I did get an air temp sensor fault at one point but was told it was from pulling the maf. It's behind the radiator? I just cut the plastic out behind my radiators:eek:

Edit: can someone confirm the intake air temp is not a part of the MAF? My understanding was they were both integrated.

The air temp probe should be in a grommet in the driver side intake to the radiator. Since you have done so much in such a short time and may not have known it's possible it is now where it shouldn't be?

OOPS Passenger side intake.

Well I just checked in the Bently manual and it says "Hot film mass airflow sensor with intake air temperature sensor" Learn something new, I thought they were independent in the early cars.

Might want to read the last few paragraphs here. Explains a bit about how the MAF works.

Well despite the problems the car made it to the track! I ran the first day in open loop (MAF unplugged) and the second day in closed loop. The bogging is gone, I think it was poor stored values. An overnight battery reset did the trick. However the car is much slower in closed loop with the MAF connected, I ran it four sessions so I think it had enough time to establish expected values. I think the problem may be the throttle position sensor, its currently reading 82% at WOT. Does anyone know if it should be reading 100% on a non-e-gas car?

:cheers: