Part Throttle Stumble.

Hi Everyone. I have a 98 Boxster 2.5 with a manual. 108,000 miles. After we pulled it out of storage this spring the car started a stumble and misfire. I pulled codes P0131, P0137,P0151, P0157, P0300, (01, 02, 06), P1128, P1130, P1318.
After checking all the info, we determined that the coils and plugs needed changed (Coils cracked and split).
So..6 new coils, Bosch +2 plugs, and spark plug tube seals. It runs much better and it still has a cold off throttle stumble. It idles like a dream... But when you start to roll into the throttle, it stumbles. When fully warm, its not as noticeable until you are under load. When you hit WOT the car snaps back to great running like nothing happened, and pulls like a freight train. Sometimes, it barely pulls its own weight at 1/4 throttle.
When the CEL FINALLY came back on, I pulled P0131 and P0151. It took days for this to happen. Even while stumbling and bumbling it never threw a code, stored or pending. I isolated the AOS and the problem stayed the same. (there is no sucking noise from the oil filler cap, just typical engine noises.) BTW, I pulled the TB and T-tube to see how much oil was inside, and it was just a coating. Cleaned and inspected all the rubber and no cracks.
Most times, on any other car... a faulty O2 sensor usually makes the engine run a little richer, but doesn't really affect the performance much. I was told that the Porsche management systems really utilize the O2 sensors for much more and they REALLY affect performance.

Is this true?
Will new O2 sensors fix this?
Thanks in advance. Can't wait to get back on the road reliably.
Tim T.

Quote
TimT
Hi Everyone. I have a 98 Boxster 2.5 with a manual. 108,000 miles. After we pulled it out of storage this spring the car started a stumble and misfire. I pulled codes P0131, P0137,P0151, P0157, P0300, (01, 02, 06), P1128, P1130, P1318.
After checking all the info, we determined that the coils and plugs needed changed (Coils cracked and split).
So..6 new coils, Bosch +2 plugs, and spark plug tube seals. It runs much better and it still has a cold off throttle stumble. It idles like a dream... But when you start to roll into the throttle, it stumbles. When fully warm, its not as noticeable until you are under load. When you hit WOT the car snaps back to great running like nothing happened, and pulls like a freight train. Sometimes, it barely pulls its own weight at 1/4 throttle.
When the CEL FINALLY came back on, I pulled P0131 and P0151. It took days for this to happen. Even while stumbling and bumbling it never threw a code, stored or pending. I isolated the AOS and the problem stayed the same. (there is no sucking noise from the oil filler cap, just typical engine noises.) BTW, I pulled the TB and T-tube to see how much oil was inside, and it was just a coating. Cleaned and inspected all the rubber and no cracks.
Most times, on any other car... a faulty O2 sensor usually makes the engine run a little richer, but doesn't really affect the performance much. I was told that the Porsche management systems really utilize the O2 sensors for much more and they REALLY affect performance.

Is this true?
Will new O2 sensors fix this?


Thanks in advance. Can't wait to get back on the road reliably.
Tim T.

First I can tell you O2 senors can affect performance. Briefly, back in Jan. 2015 my 996 Turbo tripped an O2 sensor error. I forget the error code now. The engine was running fine and I drove it hundreds of miles (from Barstow, CA to Livermore, CA) without incident other the CEL and the same code coming back shortly after engine start after stopping to refuel. I had all 4 sensors replaced and afterwards the engine ran noticably better.

More recently, after just one CEL and error code (I can't recall the code but it pointed to an O2 sensor) I just had the sensors in the Boxster replaced and the engine feels a bit stronger. Oh, the sensors in the Turbo were original and had 132K miles on them. The ones in the Boxster were not original and I don't recall exactly when they were replaced before but I would hazard a guess they had way over 150K miles on them.

Let's see....

P0131 - Bank 1, sensor 1 O2 sensor voltage remained low for longer than 2 minutes.
P0137 - Bank 2, sensor 1 O2 sensor voltage remained low for longer than 2 minutes.

P0151 - Bank 2, sensor 1 O2 sensor voltage remained low for longer than 2 minutes.
P0157 - Bank 1, sensor 1 O2 sensor voltage remained low for longer than 2 minutes.

While 108K miles is not that many miles for O2 sensors my 2002 had to have 2 of its orignal sensors replaced even before that mileage.

However, it is hard to imagine all 4 sensors going bad concurrently. This causes me to dredge up my advice regarding a vehicle that has been unused for any real length of time and that is rodent damage is always a concern.

In the cars I've come across with rodent damage, the symptoms are all about the same: Car runs lousy, seemingly out of no where. CEL is on often with unrelated error codes.

The critters gnaw through rubber, plastic piping, scrape the fuzz from the carpet for nesting material, certainly they'll chew on wiring harness wrapping and continue through any wiring insulation. They can even gnaw through wiring.

The car must be checked over carefully for any signs of rodent infestation/damage. Carefully expose the DME and any wiring as much wiring as you can. Check the various engine compartment wiring harnesses at least those from the sensors back to the DME.

If you don't find any signs of rodent infestation/damage I'm not convinced the sensors are bad.

The codes suggest a failure point that is not at the sensor (or sensors) but removed from the sensor(s). Thus before you replace the sensors you might perform some tests to check the wiring/signal from the DME to the sensors is ok. If you want to pursue this email me and I'll hunt up my Boxster OBD2 error code with tests steps reference and supply this to you.

Even though there is not a lot of vacuum at the oil filler tube with the cap removed with the engine running the engine behavior does suggest to me the early stages of an AOS failure. It was off idle hesitation, a varying idle, but no CEL and no error codes, that were the first signs of what proved to be my 2002 Boxster's first AOS failure. (It has had 2 more since then.)

Still the symptoms are not conclusive. With the O2 sensor error codes I would address those first.

... the issue is with the O2 sensors because you're getting both banks at once.
Sensors don't go bad in pairs.
There has to be something external acting up, such as a vacuum or air leak or even the Mass Airflow Sensor.
Try disconnecting the MAF and see if the stumble is still there. If it's not, replace the MAF.
If it still stumbles start looking for a leak:
Intake tee connectors
Throttle body gasket
Idle valve gasket
Oil fill tube
Etc

Happy Boxstering
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

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Thanks Pedro and Marc.
I've searched and tested pretty much everything to narrow it down to a few possibilities. What I didn't mention, was that when the original misfires occurred, we were traveling on the highway. Luckily we were only 2 miles from home and we limped it in. As I mentioned in my original post, I had the intake tube off and inspected all the rubber bits looking for unmetered air. ( learned about this phenomena with vintage Bosch systems on BMWs LOL). I was also being coached by my friend Brian at Reno Rensport in Reno, Nevada.
I figured that new O2 sensors are in order due to running poorly with the misfires and the P01128, P1130 codes. Not to mention they maybe the factory originals. If anything, I know they will be new.
Still trying to find a smoke tester to borrow for this weekend.
I cleaned the MAF, but don't remember if I tried running with it disconnected though.