P1124 & P1126 - Rough Idle - Low Fuel Pressue

[beachdrifter]

Hi 996 Friends,

 

So recently I embarked on fixing some things with my 996.  I replaced the AOS (now no smoking and oil filler cap pulls off with no vacuum), replaced vacuum lines, canister, and some switch over air valves, replaced coils and plugs, and replaced alternator.

Car ran pretty good for the weekend.  Then out of nowhere, I get the codes above, when I get home I turn the car off, and after a few hours turn it on and car starts to sputter, and idle REALLY rough.  I know I don't have a vacuum leak, or at least I don't think so, because AOS is new and car ran good for the weekend.  When I connected the fuel tester to check fuel pressure, both standing and running fuel pressures were reading 45-48psig.  When I pressed the gas with the car idle fuel pressure didn't move much at all.  Now, I think this is low?   Does anyone know what should be the standing fuel pressure reading and with the car on?  For this test I took the airbox out and had the MAF disconnected. 

 

What could be some possible causes that are likely culprits, fuel pump?  could it be a faulty fuel pressure regulator?

 

Thanks for your insight,

-Randy

BTW, Car is a '99 with 80K miles.  I assume it has the original fuel pump, filter, and anything else fuel related. 

Edited April 27, 2017 by beachdrifter

[JFP in PA]

8 hours ago, beachdrifter said:

Hi 996 Friends,

 

So recently I embarked on fixing some things with my 996.  I replaced the AOS (now no smoking and oil filler cap pulls off with no vacuum), replaced vacuum lines, canister, and some switch over air valves, replaced coils and plugs, and replaced alternator.

Car ran pretty good for the weekend.  Then out of nowhere, I get the codes above, when I get home I turn the car off, and after a few hours turn it on and car starts to sputter, and idle REALLY rough.  I know I don't have a vacuum leak, or at least I don't think so, because AOS is new and car ran good for the weekend.  When I connected the fuel tester to check fuel pressure, both standing and running fuel pressures were reading 45-48psig.  When I pressed the gas with the car idle fuel pressure didn't move much at all.  Now, I think this is low?   Does anyone know what should be the standing fuel pressure reading and with the car on?  For this test I took the airbox out and had the MAF disconnected.

 

What could be some possible causes that are likely culprits, fuel pump?  could it be a faulty fuel pressure regulator?

 

Thanks for your insight,

-Randy

BTW, Car is a '99 with 80K miles.  I assume it has the original fuel pump, filter, and anything else fuel related.

 

Fuel pressure spec is 3.8 bar+/- 0.2 bar, or 55 PSIG +/- 3 PSIG.  When was the last time the fuel filter was changed?

 

From Porsche OBD II Manual:

P1124 Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Rich Threshold

P1126 Oxygen Sensing Adaptation Area 1 (Cylinders 4 - 6) - Rich Threshold

Possible causes:

- Intake air system leaking.

- Fuel pressure too low.

- Volume supply of fuel pump too low.

- Fuel injectors fouled

What the DME computer is saying is that it thinks the mixture is too lean (too much air) and that it can not compensate by making the mixture richer (adding fuel).

9 times out of 10 this is an air leak.

An air leak can be a loose or damaged intake hose, a cracked oil filler pipe, or a bad AOS.

[beachdrifter]

2 hours ago, JFP in PA said:

 

Fuel pressure spec is 3.8 bar+/- 0.2 bar, or 55 PSIG +/- 3 PSIG.  When was the last time the fuel filter was changed?

 

From Porsche OBD II Manual:

P1124 Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Rich Threshold

P1126 Oxygen Sensing Adaptation Area 1 (Cylinders 4 - 6) - Rich Threshold

Possible causes:

- Intake air system leaking.

- Fuel pressure too low.

- Volume supply of fuel pump too low.

- Fuel injectors fouled

What the DME computer is saying is that it thinks the mixture is too lean (too much air) and that it can not compensate by making the mixture richer (adding fuel).

9 times out of 10 this is an air leak.

An air leak can be a loose or damaged intake hose, a cracked oil filler pipe, or a bad AOS.

Thanks JFP, 

 

I have reason to believe the fuel filter has not been changed ever.  I bought the car used a few years ago and never replaced it.  Should I start with that based on the fuel pressure being in the 40s?  

 

Also forgot to mention I had replaced the oil filler tube also,  so I know that doesn't have cracks.  

 

Thanks. 

[DBJoe996]

With the amount of work it takes to replace the AOS and the engine components removed and reinstalled, and all the other stuff you did, I would start by backtracking over everything you did very carefully.  Like said, 9 times out of 10 this is an air leak.  Did you center and tighten all the plenum clamps? Are you sure you got the intake manifold down tight? Is the throttle body torqued down right and did you use a new seal on it? Are you sure you got the AOS lines snapped in right with new O-rings?  I know by experience this is a very difficult job that requires many steps.  I would start by double checking everything you did.

[JFP in PA]

4 hours ago, beachdrifter said:

Thanks JFP, 

 

I have reason to believe the fuel filter has not been changed ever.  I bought the car used a few years ago and never replaced it.  Should I start with that based on the fuel pressure being in the 40s?  

 

Also forgot to mention I had replaced the oil filler tube also,  so I know that doesn't have cracks.  

 

Thanks. 

 

I would definitely change the fuel filter.  Along with checking all of the connections again, I would also use a digital manometer to check the vacuum level of the AOS.  We have had little luck with aftermarket replacement AOS units due to the high rate of "right out of the box" failures, and no longer use them.

[beachdrifter]

Thanks to everyone who responded to this thread.

 

So it seems after I was able to change the AOS, Alternator, vacuum lines, canister, air valves, idle control valve, plugs, coils, tubes, and 02 sensors, when I put everything back together, it seems at the intake plenum boot one of the main vacuum lines on the left side going to the check valve was pinched in between the boot and the intake plenum.  This basically caused the vacuum to be completely blocked or choked, and i'm sure was causing a bad air leak as well.  

 

It's amazing how important these little tubes are that plug into the intake boots.  Any obstruction to these will cause the car to run like total JUNK.  I mean I had white smoke coming out, car was sounding like chug, chug, and idle was HORRIBLE.  OMG.. after I recentered that little tube and put the throttle body back together, the car came back alive again, and now is purring like a kitten!!!

 

Thanks EVERYONE!!!

 

-Randy