Stidara

I have recently replaced the head gaskets on my impreza and I had an issue with valve clearance which is all sorted now but I think there is a small misfire on the vehicle the management light isn't coming on since I sorted the valve clearance . It might all be in my head but is there any way to check to make sure

chrisn8601

iTrader: (1)

What makes you think your getting misfire? Also are the new head gaskets the same size as the oem ones to keep compression ratio the same?

Darrell@Scoobyworx

iTrader: (5)

Are you 100% on the timing been right ?

Stidara

I'm sure it is , when it was done it was all on the Marks but when it was turned over by hand it looked half a tooth out to me but they said it was fine

Darrell@Scoobyworx

iTrader: (5)

When timing these up i put the belt on the crank pulley and refit the guide. Then fit the belt around the left pulleys and feeds round to the right bottom pulley. Now i pull the belt up to roughly the same position it would be with the bottom idler on before fitting the the right pulleys as if you don't do that they are slightly out when you release the tensioner and refit the bottom idler

Stidara

I'm tempted to send it somewhere and get it done again just to make sure it's right

propofol

Is that right and left as you look at engine from front or right and left as you sit in the driver's seat?

propofol

Really hard to determine an intermittent fault like this!
Spark, fuel, compression.
If worse when cold and wet or under load, I would go towards ignition system. Might be worth checking fuel pressure although this wouldn't be abnormal if you have a dodgy injector. Could be compression problem, esp if your timing is off or an issue with new head gasket.
IMHO these kind of problems are always the worst to track down!

Stidara

It just feels ever so slightly miss firing on tick over and in a high gear at low rpm , might just be me but want it checking , where's the best in Nottingham to take it ?

Stidara

Any more input people as it's sending me mad now

propofol

Have you looked really closely for vacuum leak? Won't cause misfire but will cause rough idle in a MAF engine (unmetered air resulting in positive fuel trim). This will improve as you increase the revs as well as less positive fuel trim numbers.
Plenty of youtube help on finding them.
Fuel injectors:
low tech - listen for injector opening/closing with screwdriver
high tech - injector balance test with timing tool and fuel pressure gauge. Look at secondary ignition waveforms for a lean misfire (too little fuel in cylinder or air inlet gasket leak)
Ignition:
low tech - unplug each coil/spark plug depending on system and listen for dip in revs. if no dip then that cylinder not contributing (only useful for a dead miss really)
high tech - primary and secondary waveforms with scope checking for ecu control of coil and spark occurring in correct place (i.e inside cylinder).
compression:
compression testing +/- leakdown test. won't pick up subtle problems with valve opening/closing however.
Could also look at O2 sensor - best done with scope +/- scantool.
Could check cam and crank signals to check for any glitches/dropouts - scope needed.

Hope this gives you some direction.
I probably haven't covered all variables either!

Stidara

Thanks for that mate

I was told yesterday it could be something todo with the two valves I had no clearance on , as I brought the car with the head gasket gone they could now be seated further into the head and be slightly damaged ??

propofol

Not sure I completely understand what has happened with head gasket change.
are they saying the valves could have been damaged? As the head is domed shaped the only way they could get damaged is if they stuck open and got hit by the piston or the timing was out so they were open at TDC.
If the valves are damaged then you would lose compression and that would certainly cause a misfire. Someone may be able to feed an endoscope through the plug hole and look at the valves? Otherwise it's a heads off job
I was incorrect in saying a vacuum leak would cause neg fuel trim numbers - it will of course produce a lean condition resulting in positive fuel trim!

Stidara

When the head gasket went and the engine over heated could it out damaged some of the coating on the valve ?? Causing them so seat further in the head possibly not seating fully now and causing them misfire

propofol

I see!
I guess that's possible. The valves and seats should have been checked when the gasket was done.
More likely the head or block got warped by the overheat however.
Did they check that the flatness of head and block were within tolerances?
If not they should have had a trip to the machine shop