Engine tries to cut out, then picks up again, what's wrong?

[Police State]

My wife's Renault Scenic’s engine momentarily loses power, and after a second or two, picks up again. This is usually within the first 10 minutes of running the car, but not exclusively so. The effect is that she has to drop down a gear, but even then it hesitates for a split second before picking the revs up. When she stops in heavy traffic or at the lights, the engine shakes at a high frequency, but not wildly or noisily; to cure this she kills the ignition and restarts the engine, after which it settles back down again, until the next time...
Anyone have any thoughts on what may be causing this?

[Munter]

Petrol or Diesel?

[Steve_D]

Could be dropping into limp-home mode but you would expect to get a light on the dash.

Steve

[Police State]

Steve_D said:

Could be dropping into limp-home mode but you would expect to get a light on the dash.

Steve

limp home mode?; it has been doing this for a few days now.

Munter, It is petrol. 2.0.

Edited by Police State on Thursday 9th April 18:39

[Steve_D]

Police State said:

.....limp home mode?

If an ecu is having issues and does not know how much fuel it should be giving or what spark advance to use it has a set of default values that it knows will be safe for the engine and not do any damage. So limp-home allows you to do just that, with no great amounts of power but enough.

In your case after a while it may be saying 'Ah all is well again' or after switching off and back on it resets.

The reasons could be many and varied but will most likely come down to a failed sensor or bad connection/short.

Steve

[Police State]

Steve_D said:

Police State said:

.....limp home mode?

If an ecu is having issues and does not know how much fuel it should be giving or what spark advance to use it has a set of default values that it knows will be safe for the engine and not do any damage. So limp-home allows you to do just that, with no great amounts of power but enough.

In your case after a while it may be saying 'Ah all is well again' or after switching off and back on it resets.

The reasons could be many and varied but will most likely come down to a failed sensor or bad connection/short.

Steve

I see, I never knew about that mode. Is there anywhere I should look in particular, or is it a visit to the dealers?

[Munter]

Police State said:

Steve_D said:

Police State said:

.....limp home mode?

If an ecu is having issues and does not know how much fuel it should be giving or what spark advance to use it has a set of default values that it knows will be safe for the engine and not do any damage. So limp-home allows you to do just that, with no great amounts of power but enough.

In your case after a while it may be saying 'Ah all is well again' or after switching off and back on it resets.

The reasons could be many and varied but will most likely come down to a failed sensor or bad connection/short.

Steve

I see, I never knew about that mode. Is there anywhere I should look in particular, or is it a visit to the dealers?

Like steve says you'd expect it to bring up the "Check Engine" light on the dash if it's in limp home mode.

It's 'probably' something to do with timing the spark plugs. If it's a sensor it might show up when they plug it up to the computer. Might also be duff HT leads or Coil Packs. Could be the fuel pump (but then would it ge better after a time....?).

No expert myself. But when my MX5 was doing similar it was HT leads. Problem is the same symptoms could have been many things which affect the spark.

Edited by Munter on Thursday 9th April 21:45

[Police State]

Munter said:

Police State said:

Steve_D said:

Police State said:

.....limp home mode?

If an ecu is having issues and does not know how much fuel it should be giving or what spark advance to use it has a set of default values that it knows will be safe for the engine and not do any damage. So limp-home allows you to do just that, with no great amounts of power but enough.

In your case after a while it may be saying 'Ah all is well again' or after switching off and back on it resets.

The reasons could be many and varied but will most likely come down to a failed sensor or bad connection/short.

Steve

I see, I never knew about that mode. Is there anywhere I should look in particular, or is it a visit to the dealers?

Like steve says you'd expect it to bring up the "Check Engine" light on the dash if it's in limp home mode.

It's 'probably' something to do with timing the spark plugs. If it's a sensor it might show up when they plug it up to the computer. Might also be duff HT leads or Coil Packs. Could be the fuel pump (but then would it ge better after a time....?).

No expert myself. But when my MX5 was doing similar it was HT leads. Problem is the same symptoms could have been many things which affect the spark.

Edited by Munter on Thursday 9th April 21:45

Ok, The check engine light doesn't come on, nothing at all appears as a fault light. I think I will look at changing the sparks and see if that helps; are changing the HT leads straightforward?

[Munter]

Yes HT leads (in general) are easy to change. But not always cheap.... For me it'd be worth plugging into a computer and seeing if there are any fault codes first.

Last thing you want to do is spend £60 on HT leads and find it's the crank sensor afterwards.

[Police State]

Munter said:

Yes HT leads (in general) are easy to change. But not always cheap.... For me it'd be worth plugging into a computer and seeing if there are any fault codes first.

Last thing you want to do is spend £60 on HT leads and find it's the crank sensor afterwards.

Is the 'computer' you have in mind an OBDII Reader?; I was thinking of getting one of these to run of a laptop, or would it have to go to a dealer to diagnose the codes? if not, is there an OBD that will read all types of manufacturers vechiles?

[Police State]

Police State said:

Munter said:

Yes HT leads (in general) are easy to change. But not always cheap.... For me it'd be worth plugging into a computer and seeing if there are any fault codes first.

Last thing you want to do is spend £60 on HT leads and find it's the crank sensor afterwards.

Hey Munter, sounds like you were on the ball... My neighbour (mechanic) said one of the coil packs has gone. The car is running on 2 cylinders.

[chimtvr]

coil packs are very very common