Hi all

My 98 1.3mpi is really rough on idle and progressively through the rev range although it seems to smooth out towards 3,000rpm.

it feels like it is missing slightly and. on 3 cylinders until revs are increased

My thinking is that it is one of the following:

- Coil pack ignition bar on its way out

- inlet manifold gasket blowing slightly

- fuel filter clogged

Anyone got any other suggestions as i am gonna change the fuel filter and inlet manifold gasket together first due to the cost factor to see if it is them?

The previous owner put a new throttle body on it and there is no cutting out or fluctuation in idle speed and i keep it clean with carb spray. I put a new timing chain on it, spark plugs are good, air filter recent and all connections seem clean and ok.

cheers for any help

First thing I'd do is check and set the valve clearances and do a compression test.

You could have a duff spark plug.

Are the symptoms the same when stationary as under load?

Broadly speaking, if the symptoms are similar then it's ignition side, if it only occurs under load, it's fuel side.

If the inlet manifold gasket is possible, then perhaps the seal around the throttle body is also a possible cause (since it's been disturbed).

You could have a duff spark plug.

Are the symptoms the same when stationary as under load?

Broadly speaking, if the symptoms are similar then it's ignition side, if it only occurs under load, it's fuel side.

the idle is lumpy although under load it seems to run fine above 3000rpm

I guess you've checked that the plugs all have a healthy pale grey complexion & that the gaps are all correct? (It could also be a coil-pack - if you go this route, buy one & swap it for both of them alternately).

I'd also go along with the possibility of it being the throttle body gasket, especially if it backfires at all - double especially if it's been like this since you bought the car.

It could also be a loose or split vacuum hose such as brake servo, engine breather or vacuum hose to rocker cover.

Failing this, yes, tappets & compression test.

thanks for the replies

the car has never been what i would call smooth since i got it over 20,000 miles ago but i just thought that was how they were. I recently changed the plugs as it was due a service and noticed that one plug near the timing chain looked slightly different from the other 3 in that although it had a light grey colour around the tip there was a bit more soot around the base.

Changing the plugs made no difference. The tappets were checked about 2,000 miles ago and adjusted but i think i will have another check just in case. The car doesn't backfire either and is completely oil and water tight with no consumption whatsoever between services and virtually no traces of soot in the exhaust pipe.

I think your points about the throttle body gasket could also be an explanation but i can't hear any whistling noises or anything

damn, this is so much easier when you can drive the car yourself

Rich mixture on No. 1 pot, could be a leaky injector. Incomplete burn could be coil-pack, but as it affects only the one pot & it's a "wasted spark" system it would affect both, so it's not that (save your money!).

And you say it runs rough up to 3k even with your head under the bonnet? Swap No. 1 plug for No. 4 plug & check them again after a quick run, that would eliminate or confirm it being a duff plug - and injector fault.

It does still point to the throttle body gasket, even th... Was the throttle position sensor changed with the throttle body? There are two sensors on it, one built in & one changeable - this is the changeable one. It has a 3 position switch (I think) and it could be that it's trying to tell the ECU that it's still ticking over... Get the throttle opened up & it returns to normal.

At least your driving style will be fun having to keep it over 3k all the time ;)

It does still point to the throttle body gasket, even th... Was the throttle position sensor changed with the throttle body? There are two sensors on it, one built in & one changeable - this is the changeable one. It has a 3 position switch (I think) and it could be that it's trying to tell the ECU that it's still ticking over... Get the throttle opened up & it returns to normal.

thanks. i have no idea what was changed with the throttle body although i have cleaned the MAP sensor to the back from which there was a slight bit of green corrosion. This cured the flat spots i was experiencing over 3,000rpm. The throttle sensor i don't know but it doesn't have a problem holding an idle it is just lumpy.

I was thinking of the SPi with a separate throttle position sensor. It would seem that the previous owner had the same problem & came to the same conclusion that it was this sensor causing the problem...

Back to square one it would seem.

(I've not actually seen a Simos 2P engine, so I guessed the coilpacks were separate. Now I come to look at the Haynes closer, I can tell it's all one unit ) I'm also pretty new to EFi engines, being honest, though I have got mechanic training & a logical turn of mind.

There's also the "infamous" thermostat/temp sender problem...

I also notice that you've posted in this thread: http://www.briskoda.net/forums/felicia-fun-forman/felicia-mpi-hesitation-please-help/53316/ - there was more after you left: Fuel filter change helped, as did cleaning the Hall Sender.

Also try spraying some carb cleaner near the manifold, if the revs rise you have a gasket gone.

Cool tip!!! :P