Boost and idle problems after recent cambelt change
 

I had the cambelt, pulley, tensioner and water pump replaced on my blob recently and around 3 weeks and 60 miles later, having just accelerated hard to join a motorway, my car developed a fault.


I'm not saying it is directly linked to the cambelt but this may be relevant?


Having joined the motorway when I next put my foot down, to move into the fast lane, as soon as the car boosts it started juddering really violently. It would drive fine off boost and I can coax it up high into the revs with no problem if I do it gently and without help from the turbo, but if I push on hard and it boosts then I get the stuttering.


I have removed the dump valve and cleaned the maf just in case but it made no difference. I have driven the car a few times since on very short journeys to see if these little adjustments would help but sadly not, in fact at much lower speeds than on the motorway, the juddering has become what sounds like a metallic sounding misfire and I noticed that sometimes it will boost beyond 1 bar (pre-fault I would see 1.5) and others it wont push past 0.6. but each time it boosts it makes this odd backfiring noise (from the engine rather than the exhaust) - it sounds almost like the dv dumping but machine gun style probably like twice per second.


Initially the idle and off-boost running was fine but yesterday when testing it again I noticed the idle getting a little lumpy and slow driving has become the same, like a minor vibration going through the car.


Could the belt change be linked and maybe the timing is now out or is it likely to be coincidental?


I want to take the car back to the garage for them to look at it but they are an hour's drive away and I don't want to risk that in case of further damage.


Any thoughts?

If everything was fine before the work, ie no leaks etc then id be inclined to suggest its a tooth out wth the timing as its lumpy at idle also if no leaks can be found now

It's an obvious assumption to make if it weren't for the fact that I'd driven it about 60 miles since the belt and it ran fine. It was only when I stuck my boot down to join the motorway that it happened.

Should also mention that no CEL light is shown and no codes are stored

Your boost gauge should read engine vacuum too which can be a useful diagnostic tool.
What is your vacuum reading on idle - ball park figure would be around 15~17 in - hg depending on mileage etc and be a steady reading.
If vacuum drops at idle, could indicate valve problems ( burnt/bent/tight clearances).
Low vacuum but steady at idle, valve timing could be retarded.
High vacuum but steady at idle, valve timing could be advanced.

Hopefully something simple causing your problem.

Mick

Thanks Mick. I don't want to start the car if I can help it but I'll let it idle on the drive and see what readings I get.


Cheers

Yes but the timing being out will affect the car more noticeably when on boost not just pootling about , it could of slipped a tooth whilst boost kicked in that 1st time , if a tensioner or idler pulley slipped etc

True and I had wondered that. Guess I'll have to bite the bullet and pay to get it trailered back there when I can find a free day. Next problem will be if its not timing I'm then stuck an hour from home with a car I don't want to drive...

Hmmm yeah, try the vaccuum tests above and see what they are 1st .

Theres not many tests to diagnose if the cogs slipped a tooth apart from looking, you could remove the cam covers and line them up and see if they are out ? it doesnt involve any mechanical work as such, covers off, socket on crank bolt and turn over by hand etc

I was going to try that but wasn't sure if I could check just by cams or if i'd need to pull the centre cover off too to check any crank markings?

Could they have disconnect the battery and you are now going through the ECU's settling down.

On a side note -- Have you noticed a change in the engine note/pitch..does the engine tone sound "off" at certain rpms?

Battery wasn't disconnected. The engine is now lumpy and wasn't before. Almost sounds like a 4 cylinder car running on 3 cylinders. Its a twinscroll so doesn't sound like a UK impreza, just sounds like any other 4 cylinder motor - well it did until recently.

Any joy on this ?

Not as yet mate. tried a new MAF (working second hand one that someone had) and secured a boost pipe that looked as it the clip may possibly have been letting boost passed but still no different. Was hoping to pull the cam covers off today at lunch but its bucketing snow currently.


Found out the car had an issue about a year ago with the boost solenoid which left it running rough but was sorted by the solenoid being cleaned out so I will also be trying that. Otherwise its trailering the car back to the garage for more investigations.