forum.audiv8.com Foren English Audi V8 Forum 2 MOT problems,help please-not steady mid rangeRPM

I need some help from this forum

I know there is a huge knowledgement here in your collective mind, based in previous experiences ,so please help me to pass the MOT.

1st trouble
car with ignition on and engine not running: I can turn on the parking, low and high range ligth beams.
just start the engine and headligths turn off , only parking ligths remain.
all fuses are ok, at least in the left wheel weel fuse box.
strange, not?

2nd trouble
car has correct idle speed and o2 ,CO, and lambda measurings. but , and rises and fall about 100 or 200 rpm every second, gas meter register instant reading oscilation between, 8L/100 and 12L/100km it shows 8 for one second, 12 for next second, 8 another time and 12 again....and 8 again... changing every second. you can feel also a very sligth difference in pulling power at your seat in highway crusing with this rpm oscillation.
this causes MOT fail here in my country as CO levels are high at 3000 rpm test ( they test all cars at idle and at mid range , 3000 rpm)
any ideas about what will be??
I repeat, at idle works perfect.

Hi Jose

1st trouble

not as strange as you think...

Turn the ignitionkey a LITTLE bit left and right when the engine runs... what's happen?

If nothing happen, so it can be the "Lenkstockschalter" (steering column switch?) whitch has kopper/brass shifters they often carbonize (verkohlen) or sometimes bad connection, so dismount the whole part and watch what's going on inside.

2nd troulbe

take the plug from the lambda out, the small single wire... watch attachment

1st problem solved, thanks to Roy, it was the ignition switch!!

2nd problem still there , I took me some time to find the lambda connectors I compared your picture and i was sure they should be near the distributor, but they are hard to see, but when find I check with my digital voltimeter and it's signal voltage was jumping all the time between 0,2V and 0,80V tipical jumos were between 0,3 and 0,6v at steady mid range rpm, and it you press accelerator full down for half second, some jumps reached 0,8 volts, but always jumping, as the rpm.

I found that idle rpm was ok at 500 rpm steady , but the readings of lamda sensor alsa were jumping all the time.

also found that when cold started, lambda was 0,40V steady the first 30 seconds ( i suposse it was until lambda sensor get hot)

all this measurings was made with sensor plug connected to car and with a third wire taking the signal to my voltimeter. is this done ok? or should I started the engine with sensor unplugged and see voltages with it unplugged ?
can a engine be started with sensor unplugged doing no damage?

that's all for now,
this was first time i tested a lambda sensor , so no idea if all this is normal.

Any comments or ideas of what to test next are welcomed!!

thank you for your help

In Antwort auf:

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can a engine be started with sensor unplugged doing no damage?

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I've done this for the last 50k Kilometers or so... the motor run a bit fat, not more... this is the reason why you shouldn't measure -> your measurements can be wrong, but the unplugging never fail (and no damage).

Hi Roy,

thank you for your help and sorry for the delay in answer, I was out this weekend.

Monday I disconnected the lambda signal connector and tested the car, everything ok, smooth idle and smooth and steady mid range rpm.

does this mean my lambda sensor is not ok?

I waited until today to tell you because I wanted to try to pass MOT to give you the good news.

Car went to MOT today and IT FAILED!!!

you have reason , it goes fatter, richer mixture, but I do not suppose so much richer.

now I can tell numbers with lambda sensor NOT connected

lambda factor 0,98 OK ( MOT friendly 0,97 to 1,03 )
CO at idle 1,10% (MOT max 0,3)
CO at 3000rpm 1,30% (MOT max 0,5)

now I'm going to buy a new set of spark plugs to try maximise burning efficiency, and maybe a new or used good condition lambda sensor, and see what happens ,not?

car mileage is 150.000 km

any comment is welcomed !!

The most problem of lambda sensors are the grounding....

original it's with 3 wires, 2 for heating, 1 for signal and the grounding by exhaust pipe.

you should buy a 4 poled sensor (with extra ground wire) and put the 4th wire to a really clean ground point.

Or you try by use a clamp (or so) to mount a separate ground wire to the hull of the sensor.

The CO values implicate that the catalytic converters were not hot.

The car, respectively the exhaust system, really has to be HOT when you make MOT, otherwise the converters don't work properly. Having the car on a sharp ride for a couple of minutes before the test, ideally on the motorway, is of huge help!

Hi Roy

thank you for your help, this week I got holidays and repaired and mot'd the car,

you were rigth, I bougth a new replacement Lambda sensor , I attached a earth wire with a clamp to the body of the new sensor to assure a good electric ground and car changed totally, the mid range RPM is now perfect, with no weavy up and down rpm, just constant rpm

I added a new set of spark plugs for better burning of the fuel and WOW, exhaust gases gone, passed MOT perfectly.

now all is OK, both problem repaired thanks to Roy and all the other members for his help in this trouble.

best wishes
Jose

You're welcome