Car Choking/Shuddering while idling

woody

I recently got my Fabia Serviced as I do regularlly..

So it was a Full Service did around 53K, it is now 59K but the car is shuddering at the idling stage, it sounds like it is choking sometimes but not all the time.

Okay so I have a big bore exhaust put on it as the previous exhaust had to be removed as it was to loose and the exhaust garage did not have a standard back box for the fabia until a week later, hence getting the big bore back box.


My idea's are that this big bore exhaust is causing the shuddering and choking at the idling stage, is it a possibilty I am right ? If so I am going to get is switched to a regular fabia back box...

I have got it serviced in a Skoda Dealership and they are pretty good and do a excellent job, so I can only think of that as I drive the car easy and never had done anything stupid...

Idea's Please ??

AMurphy

What amount of the exhaust was replaced, all of it or from the cat rearwards?.

Changing the backpressure in the exhaust manifold can cause running problems at some speeds.

crosstownk

I doubt the big bore exh is a problem - unless it's much, much bigger than the original.

What year is the Fabia - what engine size?

Given its VW-Polo-type engine I'd go with a temperature sender for openers - anyone have any other suggestions?

Owen

The majority of 'big bore' exhausts actually ruin performance unless the engine is re-mapped/jetted to compensate. I'd agree with previous poster that there's a screw up with back pressure, you need the idle speed adjusted if you want to continue with the 'big bore', or put the standard back on and see if the problem disappears. It probably will, it's too conincidental that it happened after the exhaust was changed.

BrianD3

Yeah I agree with the previous poster, if the shuddering started immediately after the new exhaust was fitted then it would be a major coincidence if they weren't related.

If it isn't the exhaust a possibility is ignition coil(s)/pack. A coil went on my car before and the engine was running on 3 cyls and shuddering badly at idle (improved a lot when revved) Initially I thought it might be an engine mount gone but this was quickly ruled out.

AMurphy

BrianD3 wrote:

....
If it isn't the exhaust a possibility is ignition coil(s)/pack. A coil went on my car before and the engine was running on 3 cyls and shuddering badly at idle (improved a lot when revved) .....

a fouled plug(s) would result relatively quickly, which is easy to check.
also, if the car is of recent vintage, this problem should show up as a misfire and detected by the ECU.

V6 Alfa

seriously doubt its the exhaust. Do you have a sports air filter installed, like K&N or BMC?

It could one of many things:

Engine potentiometer
Contant Idle speed actuator
Alternator
Mas Airflow Meter (could be affected by after market air filter)

When did you last change the spark plugs?

Regards
Rob

Ba$tard

Actually, the exhaust can matter dramatically in small engines if the bore has been increased inline. More espically if it is de-cat'ed, most likely not in this case.

It means there is not enough constant positive pressure inline to ensure a constant flow of air from exhaust values to exhaust end, which leads up to turbulance and a jerked irregular air flow as idle, in turn, annoying the vacuum level in the intake.
Most engines get away with it, but I dont know about a 3-cyl engine which has even less constant air pressure coming from it from say a 4/6 cyl.

What a bigger bore exhaust normally means is there is a slight (sometimes)few more HP at the top end and much lower torque at the low end.

The question is again, did this happen directly after an exhaust modification.

woody

Okay it is a Skoda Fabia 1.2 HTP 55 BHP Engine...

Sparks changed 10K ago, Plat long life plugs, Exhaust changed after recent Service, issue started around 2k after the service....

I am going to replace the Backbox to an orginial as a process of elimination and then afterwards check the plugs for fouling etc....

There are no mods to the engine at all.

Thanks for the Help guys

The_Magoo

You could try cleaning the throttle body, regularly fouls up on VAG cars!

woody

I would love to find a course where I could do minor services on my car..

eg.. Changing the Oil,Sparks,Overall Maintenance not requring a mechanic.. Still no Haynes Manual on this car

BrianD3

If the problem only cropped up 2k after the exhaust was changed then I think it's safe to say that it hasn't got anything to do with the exhaust.

AMurphy

woody wrote:

.....

I am going to replace the Backbox to an orginial as a process of elimination and then afterwards check the plugs for fouling etc....


Thanks for the Help guys

I don't think changing the back (rearmost) silencer box is going to make any difference, the pressure is regulated by the resistance of the first few feet, and CAT, not something 5~10 feet from the valves, which offers little if any resistance anyway.

johnnyrotten

The_Magoo wrote:

You could try cleaning the throttle body, regularly fouls up on VAG cars!

I agree. I had a Seat Cordoba with the same probs. Cleaned the throttle body ( which was filthy with a carbon sooty like substance) and it ran sweet .

woody

How do I access the throttle body and how hard would it be to clean

mike65

Assuming its a typical Bosch type design -

Pull the air filter and you will see the throttle body and the flap where the air goes in, push down the flap as far as you can and peer inside with a torch and you will see a 'butterfly' which can be pushed open on its hinge, the soot gathers inside the throttle body and around the butterfly flap, the best way to clean it is with a piece of kitchen towel lightly soaked into carb cleaner similair just wipe round and behold the black grime you fetch out! Be craful not to drop lumps of soot into the fuel/air mix chamber at the bottom (I stuck the vacume cleaner sucker attatchment in when I did my Merc!).

Mike.

AMurphy

is this a cable operated throttle or one of the 'fly by wire' variety.

Not sure you can nor is it healthy to push open one of the "fly by wire" ones, Don't even know if they will open if you switch on teh ignition and press the throttle, I could (should) find out....
A toothbrush also helps, though the carb cleaner may also dissolve it if plastic.
Neat petrol also works, as does any strong solvent, Acetone, etc.

Not sure about the wisdom of sucking flammible fumes through the missus' vac cleaner, where the brush sparks may ignite them and if explosively, may render the vac in need of a vac to clean up the parts.... (one for the urban legend busters)
and or if nothing else, leave the item in a manner that stinks the house....

woody

Solved the Problem,my mate was a mechanic for 2 years before he joined the navy and had a look...His Answer was dirty fuel (petrol)...

So we used this :http://www.stp.com/fuel_complete.html


Then Filled up and gave it a burn for 10 mins on the motorway at 120KMPH and let it idle for a while and problem gone she is puring away...

So I was advised to use the above every 4k miles, not to bad for 12 quid either.

Thanks all for your help

AMurphy

Hmmmm, What fuel do you use normally and is there any recommended clean fuel out there?. What is "dirty" in this fuel?.
It would be interesting to know if the highway part on its own would have done the job, with "clean" fuel.

woody

aparently some sediment can get into tanks and then get filtered into the fuel and hence build up deposits....Some stations have better filtration systems than others I suppose

AMurphy

Given STP additive appears to have solved the condition, I'd venture it was "varnish" build up on the (intake) valves and possibly injector tips.... common enough problem on many engine types and can upset idle. It's in all fuels.

That, or coak inthe cylinder itself, due to short journies.


Next time it happens, see if a quick hot burn will resolve it, without the STP.

woody

Will do cheers dude