P1128 & P1130 and running fine turned into barely running engine

[Lowski]

1999 Boxster 2.5L
71,xxx miles

I am looking for some feedback on the source of my woes. Currently the engine is running very rough at idle when cold and is not drivable. When blipping the throttle the engine will bog down. Letting the engine idle for 10 minutes or so it runs better and engine responds when blipping the throttle. I have not tried to drive it yet after warming up.

The Story……….

The other day on the freeway I got a check engine light. The codes were P1128 & P1130. I didn’t record the freeze frame data, but it very similiar to the numbers listed below for the second occurrence. The car was running fine. I cleaned the MAF sensor and cleared the codes.

The next day I drove it to work and on some errands with no issues. Approx 80 miles with a mix of highway and city.

The second day I drove it. On the way home from work on the freeway I got the check engine light again. The same P1128 & P1130 codes were present. Car was running fine. The following freeze frame data was on my basic code reader.

Load 8.6%
FuelSYS1: CL
FuelSYS2: CL
Short fuel trim 1: 2.3%
Short fuel trim 2: 4.7%
Long fuel trim 1: 29.7%
Long fuel trim 2: 28.1%
VSS: 70MPH
RPM: 3022

The car was still running well. Figuring it wasn’t a big deal, I parked it for a few hours and cleared the codes. After clearing the codes I move the car out of the driveway I thought it was running a bit rough, but I thought maybe I am just being over sensitive.

The next morning, I fire up the car to go to work and it doesn’t run worth a crap. Since one of the systems of the P1128 & P1130 codes is an air leak I go and check the oil filler cap. When I took it off the car quit running.


Troubleshooting……

Searching the various Boxster forums the common causes for the P1128 & P1130 codes are as follows.

Air intake leaking.
Fuel Pressure to low.
Fuel injectors contaminated.
Fuel supply volume to low.

MAF sensor clean = no change. I feel the rough running engine may be symptom of something a bit bigger. Original MAF.

I put a bottle of fuel injector cleaner in, but I probably only burned 1 to 2 gallons before things got worse.

I am leaning towards an air leak issue based on how the engine is running. I sprayed all the intake connections and vacuum lines with carb cleaner. No change to engine RPM.

I took the hose that runs from the AOS to the throttle body off. I found about a teaspoon of fresh oil in the line. I haven’t had any of the standard AOS failing conditions. No clouds of white smoke at start up. The oil cap comes off easy while the engine is running. There is a slight vacuum at the filler. Original AOS

I have ordered a smoke generator so I better trouble shoot any potential air leaks.

Summary:

I am leaning towards the AOS as being bad. The fresh oil in the line, age of the AOS, poor engine running when cold. The fact that it runs better when hot I think is indicative of air leak sealing as it warms up.

What I am struggling with is, can a failing AOS generate the air leak type symptoms I am seeing?

[jdiba]

Lowski, am experiencing the same 2 codes, although the engine seems fine in terms of how it runs.
I am taking it in to my Indy next week. He suspects a vacuum leak. But it could be any of the things you mentioned.

We'll see.

[Anker]

A failing AOS is a vacuum leak.

[kk2002s]

When I got those error codes, it was the tube from TB to AOS.
I had cleaned TB and I put hose on incorrectly relative to the bend. was not making a tight seat at the TB side. Car ran fine but I could hear a slight hiss once I started investigating. Once I turned hose to bend other way no more issues as the ends seated correctly.
I have replaced that hose and the AOS some time after that issue
This was self imposed

[Lapister]

I despise those squeeze type end connectors on hoses. I've secured both ends with a stainless hose clamps.

[Lowski]

It looks like I have solved my P1128 & P1130 problems. Below is what I have learned and figured out along the way. Sorry for the long post……

I noted originally along with the P1128 & P1130 codes the car wouldn’t run very well. In retrospect I believe this was self-induced due to a clearing of the codes and then immediately trying to drive the car. I have now learned when clearing faults with a scan tool you need to let the car idle and learn the new values before trying to drive it. I know some of the e-gas models have a key on and off procedure to help facilitate this.

During my troubleshooting I assumed I had a vacuum leak. Besides reading all the various posts from others that found vacuum leaks I didn’t have anything concrete telling me I had a vacuum leak. But off I went…….

I bought a smoke generator to find the leaks. I ended up buying one of the homemade examples made out of a paint can on eBay. For $70 bucks shipped it was quicker and easier then sourcing all the parts and building it. It works great for how simple it is. My only criticism was the wick choice used by the builder. The builder used a fabric wick out of an oil lamp. The wick burnt up completely after about 10-15 minutes of use. A tiki torch works just as well and holds up a lot better.

I smoked the engine on three separate occasions. The first time was a bit of a learning curve of where to hook it up and I didn’t end up finding anything. The next two times I hooked up to the rubber hose that attaches to the right side of the throttle body. I believe this hose is part of the secondary air system. On the next smoke session I found a leaking oil cap. Changing it with a new one made no difference. I smoked again and found a small leak in the accordion section of the oil fill tube. Changing it with a new one made no difference.

Additionally during smoking I always removed the MAF as not to foul it. Before reinstalling it I cleaned it with MAF cleaner. It ended up making no difference any of the times.

During the above troubleshooting I ended buying a Durametric off eBay. I started looking at my mass air flow sensor values. At idle my output voltage was around 1.3 - 1.4 volts and my air flow around 12.5 - 13.5 kg/h. Very close or within DME 5.2.2 setpoints of 14 +/- 0.1 volts and 15 +/- 1.25 kg/h. At face value it looked like my MAF was ok.

At this point I am started to get frustrated. No obvious vacuum leaks and the MAF looks ok. The next things on the list are fuel related. I figured this would be a good time figure out what these fuel trim things are all about. I started with the following YouTube videos. I found them very informative and really helpful to explain what the values mean.

Part 1: https://www.youtube.com/watch?v=5WnM_NsOtd8

Part 2: https://www.youtube.com/watch?v=cARQ0jZZ4Qc

With my new found knowledge I excitedly hooked up my shiny new Durametric and ran promptly into a wall. Durametric for the DME 5.2.2 does not report short term fuel trims (STFT) and long term fuel trims (LTFT) like a generic OBD-II tool. It does report adaptations for the “idle” range (RPM 1700 and below) and the “cruise” range (RPM 1700 and above). These values are listed as TRA/TRA2 and FRA/FRA2 respectively for the DME5.2.2. Other versions may have different labels for the same thing.

The DME 5.2.2 setpoints I found in workshop manual online are:

FRA Bank 1 = .96 +/- 0.03
TRA Bank 1 = 0 +/- 0.03
FRA Bank 2 = .96 +/- 0.03
TRA Bank 2 = 0 +/- 0.03

I have also seen some online references to TRA being a value of 1. I don’t know how current my manual is, but it’s all in the ballpark.

My Durametric gave me this values in this general range:

FRA Bank 1 = 1.29
TRA Bank 1 = -.29
FRA Bank 2 = 1.29
TRA Bank 2 = -.22

First thing I noticed was the negative TRA values. The car is running rich and taking fuel away at idle. If I had a vacuum leak the TRA should be positive. To validate my theory I took off the oil cap at idle and over several minutes I watched the TRA values change from negative to approx. 0.2 – 0.3. The workshop manual provides oil cap off TRA values and mine were in the ball park. Putting the cap back on made the TRA values swing back to negative.

Knowing I didn’t have a vacuum leak I wasn’t sure how to explain the negative TRA values and the positive FRA values. The car is adding fuel at cruise. Maybe something in the fuel regulator or fuel delivery, but that does that jive with the fuel removal at idle.

I did some more internet digging and found a few forum posts of Boxster owners with the same codes and similar FRA &TRA values. A new mass air flow sensor fixed their issues. After some hand wringing I decided to put in the new $190 MAF sensor and see what happens.

I cleared the codes with the Durametric and watched the FRA & TRA reset to 0.98 and 0.0 respectively. As the car idled the FRA values held steady at 0.98 as expected and the TRA values dropped negative to previous negative values (unexpected) and then after about 3-5 minutes they increased and steadied at -0.03 which is right in the acceptable range. I took the car for a drive at this point and the FRA values hovered right around the 0.99 – 1.0 range.

I have driven the car a few times now for about 50 miles (highway and around town) and the values have steadied at:

FRA Bank 1 = 1.0
TRA Bank 1 = -0.1
FRA Bank 2 = 0.99
TRA Bank 2 = -0.03

No check engine lights yet and I passed my emissions exam today. So I am cautiously optimistic the new MAF sensor has done the trick. I am a little worried about the TRA bank 1 being a bit higher than TRA bank 2. With the old MAF sensor it was higher too. I wonder if I have an injector on that cylinder bank going bad.

[Gelbster]

Thanks for all the details. Very comprehensive and with useful explanations.

[Cunningr]

Now these are the type post that will benefit someone in the future, great write up.