Anir

Background:
After getting my 26,000 mile 993TT back after the 11-month accident repair, it has developed a CEL, along with an increasing problem with cold starting, which goes away after running for 15-30 seconds. At the very beginning, the idle fluctuates a lot and the engine wants to turn off.

Also, once warm, the acceleration seems weak, almost as if a fuel filter is clogged, with a sort of subtle staccato effect at full throttle. Still acclerates fairly well, but not as powerfully or as smoothly as before. For at least two years (well before the CEL or current symptoms), I've had an intermittently buggy turbo boost gauge reading (0.2 at rest, and as high as 3.0 under boost), and I have purchased but not installed a new boost pressure sensor, 993.60610300.

Several cans of Lubro Moly Jectron and Ventil Sauber have not solved the problem. The car has received fastidious maintenance, with new air filters, fuel filters, spark plugs & wires, distributor cap/rotor, and idle stabilizer valve within the past few thousand miles.

My preliminary thoughts:
(1) Possible leak from the left intercooler hose, which I plan to replace (the right one is new). I've had the right one blow off at the track once, but this has not recurred to my knowledge.

(2) Faulty O2 sensors?

(3) "Limp home" mode secondary to a faulty turbo boost pressure sensor?

(4) A dirty idle stabilizer valve (replaced 9/02)?

(5) Faulty check (secondary air injector) valve? I found the following from a previous post by Laura: "combination air regular valve 996.113.257.70 cost is about $170". Is that right? Should I replace it? If so, does anyone have a pic of its location in a 993TT?

Here's the results of the OBD-II scan done today with the Auterra tool:

Stored trouble code causing CEL: P0112. If generic (not sure if it is a Porsche-specific code?), it would translate to "intake air temp circuit low input"

Air flow rate MAF = 1.22 lb/min at idle

O2 sensor readings:
- Bank 1, Sensor 1: Max voltage 0.000 V, lean-to-rich threshold 0.440 V, rich-to-lean threshold 0.440 V, time between transitions 0.8 sec, Min voltage 0.00 V

- Bank 1, Sensor 2: R-to-L and L-to-R threshold both 0.440 V (no other readings shown)

- Bank 2, Sensor 1: Same as Bank 1, Sensor 1

- Bank 2, Sensor 2: Same as Bank 1, Sensor 2


I'm not sure what any of this really means, and would be very grateful if someone could point me in the right direction. Unfortunately, I've been unable to find a reliable Porsche mechanic locally, so I'm on my own.

Lorenfb

The DTC P0112 for a 993 is intake air temp sensor. The sensor is located between the MAF and the throttle body.
Your scanner may be able to read the sensor's value and determine its functioning.
Check out the web site ( www.systemsc.com ) on the Codes page for additional info.

Anir

Loren,

Thank you for the help. After looking at the PET, I'm thinking of replacing the following (possibly overkill, but while I'm in there...):

993.606.114.00 "Temperature sensor" (located at leading edge of intercooler housing) - to address fault code P0112

993.606.103.00 "Boost pressure transmitter" - to address intermittently buggy turbo boost gauge readings

993.113.257.50 "Air injection system valve" - for preventative maintenance, and because it could be possibly contributing to my rough idle at start-up (if it's not only the temp sensor)?

I'll also replace the left sided intercooler hose with the updated unit with the reinforced orange ends (and use the Breeze clamps to stop the hoses blowing off at the track).

Does this sound reasonable? Any other preventative maintenance to consider? Part numbers correct?

TIA!

viperbob

Anir - According to the Porsche OBDII manual a P0112 is for a lower bound out of range. This means when cold the voltage is too low. It makes sense that when it heats up things seem fine. I would just replace the temp sensor on the intercooler. That should do it.

Yes the boost sensor can be repalced it you are getting goofy readings on the OBC...

Anir

Thanks, Bob! The West Coast folks are lucky to have you and Steve.

I may replace the air injection system valve (the equivalent of the check valve on a TT?) just for peace of mind, especially after seeing all the posted pics of rusted out examples. Unfortunately, I have driven my car through salt and snow for 3 years.

Anir

One more thought. Could I have a faulty MAF (mass airflow meter)? It would seem really early, since I remember that they are supposed to last about 80,000 miles in a 928. And, supposedly our cars use a more reliable hot-film technology to measure airflow, rather than the platinum hot-wire sensors in the 928 and 968. Plus, a search on the 993 and 993TT boards for MAF turned up nothing.

However, I ran my car hard and hot at the track on several occasions before adding the Cargraphic GT oil cooler. Could that have degraded the MAF early?

The typical symptoms of an aging MAF can include power loss and hesitation during acceleration - two symptoms I am experiencing. Another reason I ask is this quote by Carrera GT in a previous thread on a suspected faulty boost sensor: So, could a bad MAF explain my combination of symptoms - funny intermittent boost gauge readings at the OBC, a CEL with P0112 fault code, rough idle when cold, power loss, and hesitation on acceleration?

In medicine, we talk about it being more likely that a patient has one disease that explains a set of symptoms, rather than a pair of unrelated diseases. Maybe, the same is true for my 993? Has anyone seen a bad MAF in a 993TT?

Lorenfb

The DME has a been programmed with a range value for the IAT sensor, as is the
case for most/all sensors. The DME compares the sensor values against its known
value range. If the sensor's value falls out side the range, it sets a MIL/CEL.

The failure rate on the 993 MAF sensor is not as great as on the 996, but they
do fail. They tend to "drift" in the output voltage which causes a lean/rich mixture.
You'll usually detect a MAF problem when monitoring the DME TRIMs (the DME injection
pulse adaption). The TRIMs should be typically about zero. You can view the
TRIM data via most OBDII scanners. The idle/no-load TRIM as defined
by Porsche is the TRA value (time relative adaption).

Check out this web site ( www.systemsc.com ) on the Glossary page to obtain
further info on TRIMs and other fuel injection components.

viperbob

Cure the first part that you know is bad, check the results and then diagnose from there. A bad sensor on the intake air temp does not allow the computer to properly set the AFR. This can have all kinds of issues in the car. The computer does a "when in doubt add more fuel and pull some timing and boost". I have left the connector off this sensor in the past when quickly changing things on my own car, and you can tell pretty quick that there is an issue...

Anir

Light bulbs goes off...I wonder if the race shop forgot to reconnect the air temp sensor. If it is connected, I'll go with a new temp sensor first, especially since it costs about $35, compared to over $500 for the MAF.

Thanks, Bob and Loren!

Anir

An update: The race shop had simply failed to reconnect the air temp sensor in front of the intercooler. It's running great now.

It only took me four cans of Lubro Moly, several hours of online sleuthing, and a $200 OBD scanner to realize a sensor was disconnected.

On the bright side, it was nothing serious. On the other hand, it's unfortunate that the shop was not more thorough.

Thanks to everyone for the assistance.

viperbob

Glad your car is back to normal Anir.

Look at the bright side. The car is running great, you do not need to spend any money for repairs, you now know the engine operations better than you did before, and you have a new tool... Sounds like success!!! And yes, I would possibly be looking for a new shop.

Anir

Bob,

When are you going to open a shop in Kentucky? Wish you were closer.