Vacuum Leak?
Thread Starter
Race Director
Joined: Jun 1999
Posts: 14,924
Likes: 9
From: Merritt Ils Fl
Vacuum Leak?
Yesterday I finished the reinstal of a stock cam in the 85. Install was normal as was initial start up, timing set (6BTDC) and base idle adjust. Idle was smooth at 500 with the A4 in gear. First test drive was a bit rocky at first but got progressivly better,reasonable power and quick starts. Running errands.
On start up at my last stop the idle jumped to 11-13 hundred and stayed there. During the short drive home,2 miles, SES appeared. In the garage a visual check turned up nothing. SES code was 44, Lean Exhaust. Rechecked all fastners with a torque wrench and cleared codes. Drove it last night, same high idle and SES Code. Drive was short,7 miles one way, and Avg MPG climed the entire trip. I checked Instant MPG and got 17-18. This is at 50 mph with TCC engaged ( with the big lump stick I was getting 26-27).
My guess is a big azz vacuum leak but am at a loss as to why it would wait 20 or so miles to show up. Why would a vacuum leak knock the MPG down that far? I could understand a drop but that seems too far.
Any suggestions or should I re-do the top end.
On start up at my last stop the idle jumped to 11-13 hundred and stayed there. During the short drive home,2 miles, SES appeared. In the garage a visual check turned up nothing. SES code was 44, Lean Exhaust. Rechecked all fastners with a torque wrench and cleared codes. Drove it last night, same high idle and SES Code. Drive was short,7 miles one way, and Avg MPG climed the entire trip. I checked Instant MPG and got 17-18. This is at 50 mph with TCC engaged ( with the big lump stick I was getting 26-27).
My guess is a big azz vacuum leak but am at a loss as to why it would wait 20 or so miles to show up. Why would a vacuum leak knock the MPG down that far? I could understand a drop but that seems too far.
Any suggestions or should I re-do the top end.
Melting Slicks
Joined: Jan 2001
Posts: 2,940
Likes: 5
From: Chesapeake VA
If you can't hear any leaks (and I can see where that would be difficult with a 1K+ RPM idle), you could do a base idle check to make sure the IAC is closinging/responding correctly. If the base idle holds at a lower RPM, you will have a better chance of listening for vacuum leaks and if it dosen't hold at a lower RPM, then you can start by having the IAC checked out.
Le Mans Master
Joined: May 2005
Posts: 7,354
Likes: 2,279
From: Central Maryland
Pinpoint the leak, and work from there. When you find it, the cause of it, and the fix for it, may both become obvious.
Chasing down a vacuum leak is usually pretty easy if you have a propane torch handy. Crack open the valve a little bit on the torch to allow some propane to escape, but do not light it. Start the engine, and let it idle. Play the nozzle around in any area where manifold vacuum is present, and if any propane is sucked in through a vacuum leak, you'll get a noticeable change in the idle as the propane is burned off in the engine.
Be well,
SJW
Chasing down a vacuum leak is usually pretty easy if you have a propane torch handy. Crack open the valve a little bit on the torch to allow some propane to escape, but do not light it. Start the engine, and let it idle. Play the nozzle around in any area where manifold vacuum is present, and if any propane is sucked in through a vacuum leak, you'll get a noticeable change in the idle as the propane is burned off in the engine.
Be well,
SJW
Le Mans Master
Joined: May 2005
Posts: 7,354
Likes: 2,279
From: Central Maryland
Originally Posted by Muffin
Do you tech gurus agree that a vac leak is most likely my problem? I can't think of any other reason for an engine to act this way.
Lean AFR means either too much air or not enough fuel. If it were not enough fuel, the idle shouldn't have climbed so high, so it looks like too much air.
I'd also verify that the EGR isn't misbehaving.
Be well,
SJW
