996 3.4 rough running issue

[Revolution996]

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

-wesley

[Ahsai]

Is the CEL on and have you scanned it with an OBDII or Porsche scanner?

[Loren]

When you change out the MAF the engine must "re-learn" it's idle and throttle response curve. This usually takes 5- 0 minutes of driving and is normal.

Also, you may want to check and see that the you got the right part number for the MAF for your MY car.

[Revolution996]

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.

[Loren]

DId you clean the throttle body and butterfly too?

[Ahsai]

How old are the fuel filter and spark plugs ? Can Autologic show misfire counters in real-time like Durametric? May help if we know which cylinder(s) is misfiring when that happens.

[Revolution996]

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.

[Revolution996]

Oh and the bogging is only at WOT

[Ahsai]

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.

[Revolution996]

I'll try to get those numbers sometime soon. I'll also put a few more drives on it and see what happens. Does anyone else think its weird that a MAF would just fail like that, during driving?

[Revolution996]

Update: the car now has about 40 miles on it since new MAF, and is fine for around town driving, but still has serious flat spots at 3k - 4k and 5.5k - 7k. Also, it threw a CEL but I won't have time to check it this week.

[Loren]

Problems above 3-4000 RPM is almost always MAF.

Are you sure you got the correct part number?

Is it a Porsche MA? or Bosch? Either way what is the part number?

[Ahsai]

Also any mods to the intake like CAI?

[Revolution996]

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 August 29, 2014 by Revolution996

[Loren]

Use MAF cleaner and clean the MAF. See if that improves performance.

If it does, then your K&N filter is over oiled and it is getting on the MAF.

[Revolution996]

Okay I'll try that. I'm going on vacation this weekend, and when I get back ill clean the MAF and run the CEL to see what's going on.

[Revolution996]

UPDATE: Ran the CEL today, and i got P1126 P1133 P1130 and P1128. 

Also, when I cleared the codes, my car started running like crap again and it took 20 minutes of driving to make it not stall at idle anymore. 

Edited September 3, 2014 by Revolution996

[Loren]

Looks like you have an intake air leak.

[Revolution996]

So do you think my best bet would be to take it all off then put it back on again?

[Loren]

Either that or find some one with a smoke machine so you can track that down.

[DBJoe996]

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.

[Revolution996]

Tried that, the engine doesn't like it very much when I take it off. It also audibly sounds like it has a good seal.

[Ahsai]

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 September 3, 2014 by Ahsai

[Revolution996]

Pressure tested the fuel system, and it's only getting 18lbs with ignition on and 28 lbs at idle. Looks like I'll be needing a new fuel pump/filter.

Ideal psi at idle is 47, correct?

[Ahsai]

Forgot about the actual number but 47 PSIG sounds like in the right ball park. Pretty sure 18PSIG is way too low.

 

Could also be a bad fuel pressure regulator though.

Edited September 26, 2014 by Ahsai