MOT emissions testing

[JB 3.2]

My car had its annual MOT test on Friday which it passed although for a few moments there were some concerns.

During the emmissions test the engine, as I understand it has to operate at two different engine speeds to monitor emiision levels. Whilst doing this technician was struggling to get the engine to hold at the required level of about 2500 rpm, the car would rev to this or any other point in the rev range but was reluctant to hold the rev's for the time taken to underake the test.

His view was that the engine is mapped to prevent this happening and it is common on modern cars (especially those from Germany) and the wheels need to be rotating to get the engine to hold its speed. They usually get round the problem by jacking the rear of the car up so the wheels are free to rotate while they undertake the emissions test. Not so easy on a C4 where part of the drive is transfered through the front wheels.

They were able to get round the problem in the end, but I'm not sure if other testing stations would have been so persistent.

So my questions are:

Has anybody else had the same problem?
Are 993's mapped to prevent certain rev's being held without drive to the wheels?
and if so what are the resons for this?

My car goes extremely well and returns good MPG (24 mpg overall) so everything appears to be OK on the performance side.

 

[Pickled Piper]

Hi I have a C4 also. I get exactly the same issue during the emissions test. I've always been told it's down to the engine management system but without any specific detail.

It has passed every time.

pp

 

[tudorbroach]

Hi John,

I have noticed that my C4S is also reluctant to hold a constant engine speed and have been meaning to ask the same question.

Let's hope that they all do it.

Regards,

Mark Thompson

 

[Stewart Rix]

My C4 has always had the same issue, but has never been mentioned as a problem at the MOT (that's always been done at the same time as the annual service). When I had the aircon re-gassed I also had to try to keep a fast tickover then, but it was impossible.

 

[clyde]

This is a quirk (if that's the right word) of the C4 and I believe the C2, I'm not sure of the reason though. If you look at the print-out that the test centre should give you, you will note that there is a statement to the effect "engine speed measurement was by-passed". I understand that this is an accepted process for at least the 993 but if it's a garage without experience of Porsches they may be they are not aware of this.

My car also passed it's mot last Friday with it's best ever emissions test results (0.004 CO and 14 HC at fast idle test). This may not seem so amazing, apparantly they can sail through the emissions test with cat by-pass pipes on, however, three years ago the car (with the same cat) failed the mot for emissions after being tested and re-tested 4 times in total. Before taking it back for the re-test I made sure I had got the exhaust very hot to burn any crap off the cat but the second pair of test results were actually worse than the first. It was at this point that I stopped taking my car to the OPC. How can a car fail an emissions test four times and then pass with very good readings a few days later at a different garage? I probably should have taken it up with the OPC but was so relieved to get a new mot I didn't bother.

 

[Guest]

My C2 recently passed its MOT. I had a problem with the emissions but luckily the car was hot enough on the 2nd attempt. After discussion with the MOT tester, he said that driving it until hot on 30mph roads is not enough. He told me to take it for an "Italian Tune-Up", as they say, down the local bypass. Only then would the cats be really nice 'n hot [8D]

Didn't have any problem with holding it at 2500rpm as I sat in the "cockpit". C4 issue?

Cheers
Rod

 

[t11ner]

The reason for this is usually that the engine management is not accurately programmed to run at 2500 rpm with no load. Under normal driving conditions you would not be at a steady 2500 with such a light throttle as the car's weight would cause it to slow down; if you were maintaining the 2500 rpm in gear you would have a wider throttle angle. The reason that getting the wheels turning on jacks makes a difference is most often because you are driving the transmission thus ensuring that you have a wider throttle to maintain the revs.