Hi all, I've been fighting with a vacuum leak and rough idle since I got my 8 around 6 months ago. I've done a new MAF, a new O2 sensor, and done the brake clean trick to no avail. I'm running a 25% short term fuel trim that goes down with more throttle input. I've been totally stumped until today when I sprayed some brake clean down the vacuum line that comes off the accordion hose and goes to the oil injectors. When I did this, the idle smoothed out and my fuel trim dropped way down. Is there a possibility that one or more of my oil injectors is leaking air into a housing?

 

There is a plastic distribution block where that line splits to the oil injectors. It is prone to break

 

Would that cause a lean condition? The vacuum line enters the intake before the throttle body, so I feel like that'd be unlikely. Also I should add that if I plug the line by the throttle body I can feel suction.

 

It is air that is entering the combustion chamber and not going past the MAF for measurement. In the normal scenario the amount is small and not an issue. If the amount is more for whatever reason then it will cause a lean condition at idle. As you noticed an idle leak causes a lean only in the idle zone where airflow is really low, and isn't an issue higher up the load and rpm scale.

Have you checked that the vfad vac line is still connected or plugged on the manifold. That's a really common issue as well.

You could also have a vac leak around an injector or one of the o-ring seals between the different manifold components.

Have seen vac leaks at the purge solenoid connection to the manifold as well

 

The combustion chamber is at or above manifold pressure when it passes the oil injectors, so they're not what's creating suction. If you have a spit hose going to them, that will definitely create a leak.

 

I'm still confused as to why it picks up idle then I spray brake clean down the vacuum line to the oil injectors. do you know what could cause that?

 

Air is being sucked into a crack or gap somewhere along where you're spraying. When you spray, you're either temporarily closing the leak with brakeclean or adding fuel (brakeclean) to the extra air resuling in better mixture. Anywhere in the vacuum system air is only travelling one way: to the intake either before or after the throttle body.

Also SUPER bad idea to spray chlorinated brake cleaner on a hot engine. If it gets on the exhaust components you get some fun gases that put you in bed for weeks.

 

 

UPDATE: I pulled the oil injectors and distribution block. There was a split line to one of the injectors, however, I can also blow and suck air through the vacuum nipples on the injectors. Does this mean they're bad?

 

There is a procedure to test them in the FSM.

I had good luck with a ultrasonic cleaning to get them to work properly again...

Be careful what you use as a solvent as i'm not sure what the diaphram is made out of.... its likely not nitrile.... so you want to use a rubber safe cleaner

 

Even with cleaning, I feel like a failed injector is bad and not worth putting time into. Granted that is simply just my opinion.

Personally, I would just replace all 4 injectors and then annually check and clean them. Replace the vacuum line for them as well with some good quality line and you should be good to go. Don't forget the copper crush washers during the install that go on each side of the oil line banjo.. and make sure the old ones are removed prior to installing the new ones so they aren't stacked up.