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996 3.4 rough running issue

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Hello, I just joined Renntech today because I had looked through all the other forums and have read countless threads, and none seemed to help. So I decided I would ask you directly.

I have a 1999 996 C4, and out of the blue, I was driving one day, and I lost power, and the engine bogged down, so I put in the clutch, and the engine died. I tried to restart it and it would backfire if I gave it throttle, then it would die. So I didn't see any obvious vacuum leaks so I unplugged the cord from the MAF sensor, and that got me home. It wasn't running perfectly, but it ran. So I just got a new MAF today, and it for the first 5 minutes of driving, it would drive terribly and misfire and backfire, and if the revs got too low, it would die. After a few restarts, it seemed to be working better, with the occasional backfire and bog at low RPMS or if I was in the wrong gear. So now I can drive it, but under full throttle, it lacks a lot of power and misfires quite a bit at 5500+ RPMS. Is there anything I'm not taking into consideration? Any help would be appreciated, thank you


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Ran an Autologic on it when it first started acting up and only got ABS and PSM codes, no CEL. And yes I bought the part from Bosch, but it has the same Bosch number and same 996. number

And when I installed new MAF, I disconnected the battery & touched the ends together to reset the computer, and I don't have any CEL or ABS or PSM lights so far.

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When I first got the issue I pulled up the misfire counter and it showed zero, even when running like it has been. And I haven't changed the fuel filter since I've had the car (10k miles) or the spark plugs, but the engine only has 35k on it.

And Loren I didn't clean the throttle body or butterfly, mostly because when I took off the intake it looked shiny and new, so I didn't think cleaning it would really have an effect. Do you think I still should? I just took the car on another 20 minute drive, and its fine below 4k rpm, but 2nd bogs from 3k to 4k, and especially from 5.5-7. Not hearing any backfires anymore though.

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If it still persists after some drive cycles and no CEL, it would help if we can get fuel trims FRAU, FRAO, etc. and such, could be a fuel pressure issue. Will also help if you can verify your MAF reading is ~3.6g/s @ 700 and ~19g/s at 3k rpm with engine fully warmed up and a/c off.


Also you can check how much the MAF reading is at 5-6k, on my 3.6, I've seen 60-120g/s.

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Yes, it's the right part number, Bosch part number: 0280218009 I ordered it from Bosch and it has the same exact number from Bosch and Porsche on the actual MAF as the factory one I took out.. No mods to intake except K&N drop in filter

It does have a crate replacement engine from Porsche in it, but the MAF I pulled out has the same part number as the one I put in. So I don't think that's an issue.

Edited by Revolution996
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What happens if you remove the oil filler tube cap while the engine is running?  I am suggesting trying this because I had a number of the same symptoms (bogging, backfiring, slow throttle response, rough running) and it turned out to be that the oil filler tube had two major splits in it.  It is plastic and it splits open in the corrugated section.  Replaced it, no more problems and the engine sings like it is brand new.  I go along with the other suggestions, an intake leak somewhere or something like the oil filler tube.

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Those codes are saying the engine is running lean (too much air or too little fuel) during both idle and under load. If you're confident about the MAF, I think it makes sense to:


- use smoke machine for air leak

- check the purge valve that's connected to the left of the throttle body to see if it's stuck open

- pressure and flow test the fuel system

Edited by Ahsai
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