Lurching\Loss of power - MAP sensor?

[Orange7]

Good evening, I hope someone may be able to help me with a re-occuring problem. Sorry for the long post.

Car: '97 1.3MPi

A few months ago my car was off the road - a rough idle which became worse until the whole car would lurch violently when driving. This problem came and went until the car would struggle to start or stall immediately upon starting.

I was told the MAP sensor was to blame and that this was a very expensive part, the mechanic offered to source a second hand part which I agreed to. I left the garage happy. The next day the problem was back, I cleaned the throttle body as described on this forum and had 2-3 months trouble free motoring from the car. The only difference was that the car had a tendancy to hold the revs just above idle for a second or so after releasing the throttle. I had not noticed this before.

Today the problem re-occured, I was accelerating to 60 when the car began to lurch violently for several seconds, then run OK for 30 seconds and repeat. The car stalled at a junction and I got out to investiagte....

Visual check - OK
Attempt to restart - struggles to start then stalls
Blast of carb cleaner into the throttle body - struggles to starts then stalls
Unplug MAP sensor - car started and got me home 5 or so miles without a single lurch.

Can some one suggest what may be the cause of the problem - is the MAP sensor the problem or would unplugging any sensor cause the engine to run on default ECU settings ?

Is it likely to cause any problems driving without the sensor plugged in ?

Finally could someone take a look at the picture (couldn't attatch it but it some of the crimps are pushed back into the shell of the connector, I'm not sure if they would still make contact with the sensor. there appears to be some blue/green corrosion on the crimps also)- Can anyone suggest how the damage to this connector was caused ? The connector is keyed and will only go in one way. Is it likely that the mechanic did this on purpose to ensure that the sensor did not make contact with the plug and therefore stopped the lurching ?

Thanks for the advice !

[KenONeill]

MAP sensor? Do you mean MAF (Mass Air Flow) sensor?

[Orange7]

Could well be - The mechanic told me that this was the manifold absolute pressure sensor - Its mounted below and behind the throttle body on the inlet manifold.

The photo of the plug that I couldnt post last night.

[KenONeill]

Not sure if it's the same thing or something different then. But that connector needs cleaning or replacing. Note the verdigris on the pin 3rd from top.

[ap0gee]

If the 'holding revs up a bit' thing is like on my wife's Fabia (1.4 MPi), I was under the impression it was to do with emissions control. Something along the lines of the fact that the ECU holds the engine right at the point of cutting out when idling, even though it's "happier" idling at 100rpm or so higher than that. Hence, the revs drop to the "happy" point, before the ECU drags them down lower. Could be wrong with that though. In fact, based on the accuracy of posts I've put up here lately, it could in fact be complete fiction. Maybe it was all a dream...

Seriously, though, it might just be that - especially since you've cleaned the throttle body. It might have been sticking far enough open that the revs didn't drop that extra bit when idling...

[KenONeill]

No guarantee that this is reliable for a car I've no direct experience of, but ap0gee's given me an idea (well 2 actually).
1) Check that the throttle cable isn't so tight that it stops the throttle closing fully.
2) Based on my ZX, there should be an adjustable throttle stop, which you could use to tickle the idle speed up a hair (careful, and don't take it up more than 100rpm in any case).

[TeflonTom]

holding the revs is because the throttle body needs to be reset wit hdiagnostic equipment..

it is indeed a map sensor... the felicia doesn't have a MAF. by the looks of that plug it is badly corroded and that could be the cause of the problem... you may have to replace that plug or the entire loom.. they are cheap as chips from a breakers yard and they are dead easy to fit. iirc you only need a 13mm spanner to undo one earth clamp... can be done in under 15 miniutes. the damage to that plug was probably caused by sombody trying to remove the terminals without the right terminal extraction tool

the map sensor fault could also be caused by a blocked breather allowing oil to collect in the air box and the condensation from this trickles down into the throttle body and the intake manifold... some of which ends of on the map sensor causing it to give false readings or fail completely

[TeflonTom]

Originally Posted by Orange7

or would unplugging any sensor cause the engine to run on default ECU settings ?

you can unplug most of the sensors(not all at the same time) and the engine will still run in LOS mode. except the crank(hall) sensor. without which you wont get any sparks

[Orange7]

Originally Posted by TeflonTom

holding the revs is because the throttle body needs to be reset wit hdiagnostic equipment..

Thanks for the replies. Any idea how much this might cost Tom ?

Originally Posted by TeflonTom

the map sensor fault could also be caused by a blocked breather allowing oil to collect in the air box and the condensation from this trickles down into the throttle body and the intake manifold... some of which ends of on the map sensor causing it to give false readings or fail completely

This rings a bell - The mechanic mentioned that the sensor was oiled up when he took it out. There was also a small amount in on the plug when I took it out. Not sure how it got in there but it looks like I may have to check the breathers this weekend. Would it be obvious if oil was collecting in the airbox ?

[KenONeill]

Originally Posted by Orange7

Thanks for the replies. Any idea how much this might cost Tom ?



This rings a bell - The mechanic mentioned that the sensor was oiled up when he took it out. There was also a small amount in on the plug when I took it out. Not sure how it got in there but it looks like I may have to check the breathers this weekend. Would it be obvious if oil was collecting in the airbox ?

Not very; had issues with oil in the carb on an Ascona (original Solex), and you could wipe out the air cleaner on it with a couple of sheets of paper towel, so it's visible, but not a swimming pool.

[TeflonTom]

normally a puddle forms in the lower section. it's really common on the petrol polo's and the 1.6 engine.. and where the engine sits on a slight slope leaning down to the left the oil runs down into number 1 cylinder and fouls up the spark plug

maybe a kindly member in your area may be able to hook up your car to vag-com to reset your throttle body alignment

[Orange7]

Had a check that the breathers were not blocked today - checked two rubber tubes from the filler neck on the rocker cover to the throttle body. No sign of blockage and the air box was free from oil.

Are there any other areas I should check - Didn't want to start pulling the tubes to the charcoal canister about if I dont need to.

Would I be able to tell anything from taking the sensor out and looking at it ? The car is running well now but I know it has the ability to let me down which can be scary if it happens at the wrong time !

Tom - Now that the engine has been run with the MAP unplugged will the ECU be stuck in default mode even though I've plugged the sensor back in ?

Thanks for the suggestions so far.

[TeflonTom]

no it should relearn the settings within a few miles of being reconnected.

next thing i would try looking at is the distributor/coil