advice on problem please

Hi to everyone. I wanted some advice before I go off to a main dealer and pay out big money. I have an early 996 tiptronic with the following problem. Under normal driving it is perfect. However under heavy acceleration it hesitates, almost similar to a mis fire before it sorts itself out and takes off towards the horizon. Sometimes it is hardly noticeable but other times you wonder if it will ever go. I have had my local mechanic run diagnostics on it and nothing shows up. There are no dash lights indicating any issues either. He has checked all the obvious but no joy. Has anybody experienced this before or have any idea what may be causing it so I can make a decision on my best course of action? Any help would be greatly apreciated. Many thanks

Try changing the MAF sensor first. It's a common problem and it should cure the problem.

+1

oh, and :welcome:

Many thanks I apreciate the advice.

+1 usually a quick cure of this problem

whats a MAF sensor?

MAF = MASS AIR FLOW Sensor. Calculates the mass of air entering the engine to allow the engine to inject the correct amount of fuel and apply the correct ignition timing. They are known to become a problem causing poor acceleration and drivability. Normally it won't be as bad at cruise speeds as the engine will switch to closed-loop mode where it relies on the o2 sensor and at lower airflows the maf inaccuracy won't be as apparent. cheers!

Am I right in thinking that the MAF and the Oxygen sensors are different?I have just had an Oxygen sensor replaced after the engine check light came on. I had noticed a couple of days before that the car was a little hesitant a few times when pulling away. This was covered under the extended warranty but if you don't have one expect a bill of £300 - £400 from the OPC

Yes, oxygen sensor or lambda sensor sits in the exhaust stream reading the air in the exhaust. Basically a narrow band sensor is only able to accurately read a high or low reading, with a very fast switch point. The idea being, the engine enriches the fuel mixture one way to make the sensor read less air, when it sees that it leans it out in the opposite direction until it reads lean and repeats. This happens fast enough that the average air/fuel ratio reaches the stoichiometric target of 14.7:1 Air/fuel ratio. This is the best balance of nasty chemicals within the mixture and provides avg fuel economy. Some cars have "lean burn" engines ie Honda where they run to 17:1 and on.... This is hard to tune however and the engines are designed with lean burn in mind not performance.... Newer vehicles have wide-band lambda sensors made by NTK, Bosch etc that can accurately measure the full range of mixtures allowing the engine to trim more accurately. They are still not normally used at full throttle (WOT) though. The engine relies on the MAF or MAP if equipped, throttle position sensor, engine temp (both water and air temp) etc to provide the right fuel. The sensors also normally have a heater within them to get the sensor working ASAP. Anything else you want to know? I used to own a UEGO wide band meter which was permanently installed in the car for tuning and engine safety..... Oh, lastly, Porsche and many new cars to meet EURO 4 + have two o2 sensors per bank, one pre and one post the catalytic converter. This is to detect when the cat fails..... the second sensor is checking to ensure the post cat fumes are what it should expect to see...... Edited to add a Wiki link fyi: http://en.wikipedia.org/wiki/Oxygen_sensor Next lesson will be on Octane rating of fuel and ignition timing.... why it's not worth putting in higher octane than the engine is tuned for.....

Tell us more Jeffrey :typing:

Well.................. Obviously this is ECU dependant within reason. I cannot for the life of me get anything specific on the 7.8 version of the Motronic ECU to get the finer info from it but I am searching. 7.8 is the 3.6 specific Porsche model. It's part of the 7 Series of Motronics.... Ok, so there are two ways of working out what fuel and ignition to use. If you have a MAF, generally the MAF will provide a load for the engine. A Maf is a heated wire strung across the air stream. The cooler it becomes the more air entering in simple terms. Ok, so you have load, you know RPM, then we get the fuel or ingition from a 3D map.... see image at the end. The end image is an ignition table from a Nissan ECU we retuned. Right, so simply, when the car leaves the factory the beekers have preprogrammed a base map that will run the car, provide best power, mostly best fuel economy and an ignition map that will suit all conditions. Now, the trimming on top of these base maps occurs as part of the calcs that are run to determine the final values the engine uses. So it says. I have x-load provided by the MAF, I have r-rpm, I have a table value there of Y. Now, I take into account air temp, water temp, ambient temp, knock sensors etc and produce a final trimmed figure. This is where it gets fun..... Now this does vary between each ECU hence my I want the 7.8 specs.... but generally, an ECU won't go above the ignition figures it has mapped. So, if your car has a sticker recommending 98, it's tuned for 98 in the maps. Now if you use 92, no problems. The Ecu as part of doing it's calcs will be trying to advance the ignition timing for a 98 ron fuel, will pickup the pinging on the knock sensor and retard the timing to suit the fuel. It will then maintain that knock adjusted value for a while and then gradually wind the timing back in, get knock, wind back again. When you go back to 98, it will wind back again. It does take time though.... So, will it keep winding until it gets knock? Not normally. Some systems I've seen have had the ability for a small, few degree adjustment above but generally not. So if you put 100 in a 98 car that won't increase the timing for the better fuel, what will happen? Well simply, your power goes out the exhaust (assuming your car wasn't pinging on 98 previously). Basically as you increase the RON you are slowing the fuel burn down - the time combustion takes - fuel becomes less volatile. But, it then takes longer to burn, hence why you increase the timing, advancing it. You need to start the burn process earlier in the cycle so the peak cylinder pressure occurs at the right point, which it not the top of the stroke, but slightly before.... If you use high octane fuel, peak cylinder pressure will come too late losing you power......and out the exhaust goes the horsies.... Now that all relates to Wide Open Throttle (WOT). Cruise can be tackled a couple of ways but normally they use the same process to get base values, but now they take the o2 sensor into account. So it wants to take the base value, lean it and watch the o2 sensor for the swing, enrich and watch for swing etc. Smarter sensors - wide band can just trim to lock a particular a/f ratio and hold it..... It's a tricky balancing act. I gained 10hp throught the rev range on the dyno with 3 degrees of advance during one of my tuning sessions with the knock headphones on....basically highly sensitive headphones plugged into knock sensors, circuitry etc to allow you to hear when the knock was starting.... And then there was air fuel ratios......and exhaust gas temps.... and on it goes.... Map below is RPM on the Left(Y) Load across (X) and values are the timing in Hexadecimal. These values are absolute. You need to take into account the already 10 degrees of manual advance applied on the CAS (Crank Angle Sensor - sensor that works out where in the cycle the engine is). So all the hex figure need 10 degrees added to them for actual timing..... Again, the above is a simplified version of the process. To meet some of the new euro emission requirements they are getting more tricky with ECU's and let's not even talk about what traction and stability add to the mix.....

Jeff, that must have taken ages to type up..... :clap: But my car is still faster / more responsive when using shell 100 rather than 98 :grey:

Ah, but this is a seat of the pants test..... I wonder if we had the opportunity to play placebo's with you whether you'd guess it Now while yes, you may feel it is better, and it probably is, but have you done a pre and post dyno effort to prove the "feeling"? We put a few octane boosters on the dyno once. Nothing much to see from those lovely things apart from holes in your pocket. It's convenient when you friend owns a dyno dynamic dyno you can use whenever you want.... What I never told you was that most cars have a base timing adjustment, the CAS (Crank angle sensor) that you set the base timing with. If you know you are going to use a higher octane fuel than normal, you can advance the ignition across the board using this. It's quite possible the Porsche Motronic is able to advance beyond the basic octane rating, if only I could get the data on the system. Need a Bosch friend....

Calling Sid Spanner !