Random misfire and lean under boost

[zachatttack]

I did the timing belt and water pump on my car the other day because the belt had a tear in it. After they were done everything was good. then after driving 50 miles I noticed I was running a little lean. And at idle i was only pulling 10-15 pounds of vacuum. Does this mean vacuum leak? Then I took it for a drive and it was running fine. There was a little stumble into boost but I thought that might have been the awd kicking in since the tires aren't the greatest. Then after a little while under normal driving I just started getting a random misfire and my car didn't want to accelerate. I pulled over to the side of the road and shut my car off. Then started it back up and the misfire was going. So i went to see what happened under boost. And it ran really lean once boost hit like it was stumbling for fuel. But normal driving it was fine. My fuel pressure was fine. Does this mean coil pack? Bad injector or what? The code I got when this happended was a p0300 random miss fire and that's it. Also I have dsmlink v2.

 

[GRAVEDIGGER]

If it's running lean that's a fueling problem. You are not going to be reading lean when you have an ignition misfire, instead it would read rich. I would start with a fuel filter replacement and see what happens. Good luck!

 

[hocka09]

This sounds almost certainly like a case of a large vacuum leak. Take for instance if the PCV hose somehow came off of the intake manifold; this would allow the engine to pull in large amounts of un-metered air into the cylinder, making for a lean combustion event at idle since the ecu is expecting X kg of air, when there is actually (X + leak) kg of air making it into the cylinder. A/F is a ratio of air mass to fuel mass, meaning a higher mass of air will cause it to go above 14.7:1.

As for your misfiring under boost, there would be metered air escaping to atmosphere at the manifold since the air charge is at a higher pressure than ambient, modifying the A/F by (X - leak) kg of air mass. This much lower than optimal (<8.5:1) A/F can result in partial burns of the air-fuel mixture, which present themselves as random combustion events or misfire.

Definitely give the vacuum lines a good look over and if you have never replaced the lines this could be a really good time to buy some and throw a fresh set on there. It'll cost you all of $20 worth of silicone tubing. If you're not in a green state, I would recommend deleting the emissions equipment as on a vehicle this old it usually causes far more problems than it's worth and with the waning population of older cars on the road they really have a minuscule effect on overall emissions if you ask me. If you need help with what you need/how to replace the lines or delete the emissions equipment feel free to PM me or search around on here.