Runs like crap with MAF connected - 986 Forum - The Community for Porsche Boxster & Cayman Owners

So I recently did the Cayman intake upgrade on my boxster. It's a track only rig with a 3.4 conversion. After I finished the intake and fired it up it ran like crap. Pops at idle and throttle response is terrible. It barks like a dog and hesitates when the accelerator is stabbed. Anyway, when I disconnect the MAF it smooths right out and runs fine. I tried cleaning the old one and then a new MAF with the same result. Any ideas?

Sounds like your MAF isn't metering all the air that's getting into the engine. Must be a leak somewhere between the throttle body and the MAF, right? Or is it possible to undermine the MAF's function by changing where it sits in the airflow by changing the intake path? Seems unlikely to me.

If it was running fine with the stock airbox, my initial thought is that you either need to cut the MAF housing from the old box and use that (cheap, but not optimal HP) or get a reflash for the new MAF diameter ($$$, but you'll get all the ponies).

Is the maf tube the same diameter?

It's the same MAF that was working fine before the airbox change (996.606.124.00) I used the tube section (for the maf) that came with the cayman airbox and did some custom plumbing from there to the throttle body.

Do you have another maf you could switch with? I switched mine and it ran better.

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I tried a new MAF with the same result

what tune are you running?

So are you running the MAF tube from the old or new air box? If using the one from the new air box then try what J.Fro said. If the diameter of the tube is different than the old one, the MAF may not like it.

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It looks like you used the larger diameter maf housing from the 987 airbox. The 987 maf housing is larger than the one on a 986. Use the smaller housing or you will need a custom tune to use the larger maf. Some have tried pushing a 996 3.4 tune with mixed results.

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Not sure how the shop tuned the ecu after the transplant. I purchased the car this way. I used the larger maf tube from the 987 intake and that is probably the cause. Car is going to a shop for a dyno tune today. Why does it run very well with the maf disconnected? Does the ecu default to a baseline? Is the primary purpose of the maf to optimize fuel mix for mileage and emissions?

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Yes . Without the maf it defaults to open loop/limp home mode. Some have said the open loop programming is pretty good ,but I suspect it will run overly rich .

I have the 3.4 tune on mine. I think I heard that the 3.4 tune likes the maf being closer to the throttle body that the 987 airbox gives it.

yessir. as has been said, without the maf the ecu uses the throttle position sensor to estimate how much fuel to provide. called alpha N tuning i recall, and is used by some race companies (gets the maf out of the system for better airflow). in our applications it is a limp mode, however, so, as stated, the ecu adds additional fuel to keep things on the rich side (rich burns fuel and kills power, lean kills engines). the guys who put the 3.4 in your car probably just kept the old tune on it - the ecu is has enough range to adapt to the increased airflow required for the bigger engine. however, note that the ecu calculates airflow by looking at the maf signal and multiplying it by the diameter of the maf holder. increase the maf holder diameter without telling the ecu and it starts doing bad math - there is more air getting to the engine than the ecu knows about and the engine runs lean. your engine will run like crap. your options are to: (a) go back to the original, smaller maf holder. (b) push an oem tune onto the car that knows the larger maf holder. what you do is determined by the year of your car (ecus changed through the years) - 2003 and younger, use an anniversary edition tune. 2002 and older, use a tune for a pre-2002 996. from your pics it looks like you have an e-cable throttle, so no issues with that or with the early 5.x ecus. your tuner will need access to a piwis or pst2 to do this work. (c) aftermarket. there should be more, albeit expensive options here (allthough yours is a track car, so issues like poor idle, etc., may not be a big deal). it can be as simple as a resistor on the maf ground wire that lets you trim the signal going to the ecu. a piggyback ecu (simply put, acts like a programmable version of the resistor noted above). an aftermarket tune where the tuner has modified the tables that convert maf signal to airflow (harder to find than you think - not a lot of tuners can do our 7.2 ecu; may more can do the 2003- 7.8 ecu). or run alpha n - a good tuner should be able to set that up for you.

Radium, thanks for checking in. I should find out today if the tuner I brout the car to can handle the job. (a) seems like it would defeat the purpose of the larger intake using the smaller maf tube (b) this shop does not have the Porsche computer equipment (c) most likely solution. I will check back in with results

There is still a power increase with the bigger throttle body and 987 air filter alone but more airflow would be available with both bigger TB and maf

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Hi John, the throttle body is a 997 part# and I believe is the correct size for the 996 3.4

Yes I know about increased throttle body size on the 3.4 BUT both the throttle body and the Maf housing on the 986 are smaller. I expect you are using the original ecu which was in the body before you swapped engines so it expects to see the smaller maf. You can still benefit from the bigger throttle body and better 987 air filter without a bigger maf and the engine will run better without special tuning.