Misfire under acceleration. vw polo

[industryspark]

VW Polo 1999. It has a slight misfire under acceleration (more noticeable under heavy acceleration). I removed the plugs and cleaned them up about 2 months ago and it seemed to noticably improve for a few days before going back to as it is. I changed the plugs today and again it improved for a few mile before reverting back. I will check the plugs to see what the plugs look like for clues in the morning.

Few questions i would appreciatte answering if anyone can spare the time.......

1, I was told by a mechanic a few years back that there was a slight leak at the throttle body. Would this cause the slight misfire and would it be more noticeable under heavy load?

2, could it be a HT lead slowly breaking down or would a HT lead not fail in this way (i.e work or not work).

3, i put a new dizzy cap and rotar arm on last year-they looked ok when i checked them but could it be something the eye cant see with them?

basically i would appreciatte any pointers as to where to start and what i can and cant rule out.

regards.

 

[karlosthejackel]

on polos of that age leads are always a cause of misfire under load. i generally change the whole HT side(leads,cap,arm plugs) but seen as you have done most just replace the leads and youll be sorted!

 

[industryspark]

thanks for the input carlos. I replaced the whole lead set in January 2009(sorry forgot to mention) as the lead that controls the 1st cylinder failed. They are a bosch set so not cheapy's. Would they fail this quick?

any ideas why it would seem ok with fresh plugs in but then revert to slightly missing?

 

[karlosthejackel]

ah....hmmmm well the misfire is in that area somewhere unless its on the injector side which is more unlikely. Bosch are usually very good. Check inside the leads(the bit you put on the plug and bit you fit to the dizzy cap) for corrosion and water ingress. Check in the cap for cracks and rotor arms can fail internally with heat etc as can the plugs. I guess without being in front of the car and having a dig around myself i can only say look for evidence. Misfires under load are harder to detect as you cannot generally make it happen while you are under the bonnet testing!

 

[industryspark]

ok thanks karlos. i will have a dig round in the morning.

 

[karlosthejackel]

just looking back at the start of your thread. Are the plugs black when removed?

 

[industryspark]

Yes and No i guess.

They are a lightish tan brown on the 'tip' but the three 'prongs' have a little blackness on them. The old ones i replaced today have been in 18 month though. No one plug looked much different in appearance to any other.

What were you thinking Karlos?

 

[jetex]

If your plugs are Bosch, try and change them to NGK if they do an equivalent. Bosch electrics tend to be first class -- except the plugs in my experience. Another tip is to run the engine somewhere where it's pitch black. You will then see the plug leads glowing with a blue haze if they're leaking the H.T electricity. You may also see the coil shorting out ?? Good luck.

 

[industryspark]

im going to check the leads out tomorrow but i think thats a red herring-surelly one wont have failed after 18 month. The plugs are ngk so no worries on that front. i changed the dizzy cap and arm at te same time as the leads so im thinking they are unlikelly to have gone.

How likelly is it that the slight misfire/power loss could be due to the throttle body leak?

how can i tell if the coil pack was faulty? anyway of testing it, ect?

regards

 

[jetex]

Don't trust ANYTHING. I've taken new parts out of a box and they've been faulty. The air leak may be the cause of your misfire, but this would normally put the engine management light on. The only way to check the air leak would be with a diagnostic code reader, or exhaust gas emission check.