vx calais rought idle

[sierrabeachfront]

hi,have got a v6 vx calais that on idle drops a cylinder,are they known for burning exhaust valves as this is what it sounds like or is there another common problem with them,it is high k's 240km

 

[commodore665]

Check the IAC valve on the side of the throttle body , they can get gummed up with carbon deposits , they unscrew easily enough , or even clean throttle body .

 

[sierrabeachfront]

so they dont have an inherent problem with burn valves out then

 

[commodore665]

Sorry mate my bad , might be a dodgy spark plug or lead , not sure on the burnt valves

 

[WHCapriceHBD]

Could well be a burnt valve. At those kms, the seats begin to round away and this gives poor sealing and heat transfer out of the body of the exhaust valve, into the cylinder head seat suffers due to the reduced seating area.
The valve then heats up, with poor cooling and sealing and you have a recipe for a burnt valve.

Best way to diagnose is to swap ignition leads on the affected cylinder. If no change, then its not the spark plug lead.
Next, remove the spark plug, still attached to the spark plug lead, hold it against the cylinder head to ground it, crank the engine and check for spark on that plug.

If it passes both those tests, I would strongly suspect a burnt valve. To test this, compression test all six cylinders and I bet you find very low compression on the suspect cylinder. That would likely indicate a burnt valve.

Source: Cylinder head machinist with my own machine shop. No longer in business, but have kept all my machinery and do small private machining jobs at times. I now work in an office.

 

[WHCapriceHBD]

*Starting around the 250,000km mark, yes any engine that has had a long relatively hard service life (high rpm spurts, traffic, high load, etc) for that many kms/years are good candidates for a burnt valve.

I suffered one in my 280,000km BMW V8.

I just pulled the head off a mates 245,000 km Barina 1.3L Bongo van 3 weeks back and replaced a burnt valve. They had been trying to "fix" the miss at idle by tipping metho in the fuel tank, changing spark plugs, etc. I had a quick listen and said pull the head off. Its obviously a burnt valve. It was.

 

[BT1-355]

easiers way to see if you have a burnt valve, is to put a piece of paper at the end of the exhaust tip and hold it there. Eg if it just blows the paper away from the tip, then its not a burnt value. But if it blows away and then sucks it in you got a burnt valve

 

[sierrabeachfront]

ok,done compression test,all 150 psi exept 2 and 4 at 130,wouldnt have thought this was enought for it to drop a cylinder to dead point,also forgot to mention its supercharged,are things like iac valve in same place?

 

[Nut Kracker]

Here ya go....
It's on the throttle body as normal.

 

[sierrabeachfront]

thanks for that