a3 rough idle

[scottisha3]

i have a 1.8 20v agn a3 on tickover it hunts but get revs over 1500rpm its nice and smooth i have replaced the following parts

maf,plugs,leads,coil,all vacume and breather hoses,cam shaft sensor,both knock sensors,crank sensor checked timing about 10 times there are no fault codes or eml on

if anyone has had the same problem or can shed some light i would be thankful as the wife is talking about buying new car and i have to payfor it

[Glen]

To be honest everything I would have thought of you seem to have replaced. Have you checked the cambelts themselves? That would be my last guess as to what the problem is.

Glen.

[scottisha3]

full new timing belt kit and water pump fitted(gates kit)

[Tech]

My guess on that one would be a breather issue but if you have replaced the breather pipes I can't say.  Have you checked to make sure the dipstick tube is not broken at all and the seal on the oil filler cap etc?  If you have access to a fault code reader it would be an idea to check the voltage readings of the oxygen (lambda) sensors and also check the fuel trim.  I would defiantly say this is one that needs diagnosing properly as it sounds like you are wasting money by just replacing parts on it.  

 

Tech

[scottisha3]

hi just a update car has been scanned by 3 different machines no fault codes all the parts i have replaced  are as follows

spark plugs,leads,coilpack,camshaft sensor,crank sensor,both knock sensors,maf,full timing belt kit,throttle body and had it reset,cat, the fault is when car idles it hunts and sounds like missfire get it up to 1500/2000 rpm  runs smooth

[Tech]

scottisha3,

Have you checked the things I mentioned in my above post?  I know you mentioned there are no stored fault codes to go on, but it would be ideal to read the engines measured values to see what the fuel trim and the oxygen sensors are doing.  Also have the fuel pressure checked as close to the fuel rail as possible, both on idle and under throttle.

 

I know you won't want to hear this, but most of the parts you have replaced already (spark plugs, leads, coils etc) were never going to cure the symptoms you have described.  It sounds like your mechanic is clutching at straws and replacing stuff hoping for the best.

 

Check the things I have mentioned before replacing anything else.

 

Tech

[scottisha3]

could you tell me what the reading your are saying should be

[Tech]

Yeah sure, the lambda value should be between 0.95 and 1.05.  The short term fuel trim should not be going out of +/-25% band.  The long term fuel trim should be 0%.  The lambda probe 1 should be fluctuating above and below 450mV constantly and probe 2 should be 450mV providing the cat is doing its job right.  If probe 1 is constantly going above 450mV the engine is running too rich, if it is constantly reading low, the engine is running too lean.

 

Rich running could be caused by a leaking injector, fuel pressure too high, blockage in intake etc.  Lean running could be caused by a blocked injector, misfire, unmetered air entering the intake manifold via a split breather, dip stick tube etc.

 

There is of course a chance it may not be an electrical fault at all, and could be in fact a mechanical fault like worn cam lobes for example (which might sound mad, but I have seen this on a few engines in the past and it always catches people out). 

 

Let me know how you get on.

 

Tech

[scottisha3]

i only have one lambda on my car just before the cat

[scottisha3]

the full story of this car is i bought it with the head destroyed by belt failure i bought a replacement engine but only used the head it was skimmed and pressure tested prior to fitting both engine have agn code but my engine has plastic inlet manifold other had ally manifold this was the only difference i could see i built engine up and started it only to find it was hunting at idle thinking it as something i had done on rebuild i checked and triple checked and replaced all the parts listed only to find out the car was running this way before belt failure 

[Tech]

Right OK.  Sounds like your a pretty competent DIY mechanic then.  Check the values as I stated before at the lambda before cat and ignore what I said about lambda after cat.  Let me know if anything looks unusual.  Have you tried compression testing it?  

 

Failing that, I think thats about as far as I can go on a forum without actually seeing the car in the flesh so to speak.  I hope something shows up for you.

 

Tech

[scottisha3]

just to let you know found the fault and car runs perfect now

[Tech]

Great news!  What was causing the problem?

[Glen]

Fantastic to hear. Back to good old fun motoring.

 

Glen.

[scottisha3]

the fault was with the inlet ports when i fitted the head with same agn code i never noticed the inlet ports were bigger on new head so i had to remove my original plastic inlet manifold and fit the metal one that came with new head i fitted

[Tech]

Ah, well done finding it