88 S4 starts, won't hold any rpms
12-16-2008, 09:07 PM
#21
Rennlist Member
Thread Starter
I tightened up the bottom MAF hose clamp snugly, but not wrench-yur-mama-overtight. With the electrical connector unplugged, no loud hissing noise, steady idle at 900 rpms. Couldn't rev up above 2000 rpms this way, though.
Now, *without* lifting up the MAF, I was able to plug in the electrical connector. Loud hissing returns.
I repeated the above 3 times simply because I wouldn't believe it myself were it not consistently and repeatedly happening in front of my own eyes/ears.
MAF connected: loud hissing vacuum/intake noise, rough idle, won't hold rpms when you hit the gas, soon dies if left alone.
MAF disconnected electrically: no loud hissing noise, steady idle, can't rev (even very slowly) above 2000, almost no power for driving (a little reverse and 2nd gear, but it really wanted to die when moving).
Thoughts?!
Also, with the MAF *connected* electrically, the engine runs smooth as silk while spraying in starter fluid through the air intake, and it's easy to rev while so doing.
Further, I disconnected the battery for a minute to reset the engine computers. No change in the above behavior after the computer reset.
Now, *without* lifting up the MAF, I was able to plug in the electrical connector. Loud hissing returns.
I repeated the above 3 times simply because I wouldn't believe it myself were it not consistently and repeatedly happening in front of my own eyes/ears.
MAF connected: loud hissing vacuum/intake noise, rough idle, won't hold rpms when you hit the gas, soon dies if left alone.
MAF disconnected electrically: no loud hissing noise, steady idle, can't rev (even very slowly) above 2000, almost no power for driving (a little reverse and 2nd gear, but it really wanted to die when moving).
Thoughts?!
Also, with the MAF *connected* electrically, the engine runs smooth as silk while spraying in starter fluid through the air intake, and it's easy to rev while so doing.
Further, I disconnected the battery for a minute to reset the engine computers. No change in the above behavior after the computer reset.
12-16-2008, 09:30 PM
#23
Nordschleife Master
MAF connected: loud hissing vacuum/intake noise, rough idle, won't hold rpms when you hit the gas, soon dies if left alone.
MAF disconnected electrically: no loud hissing noise, steady idle, can't rev (even very slowly) above 2000, almost no power for driving (a little reverse and 2nd gear, but it really wanted to die when moving).
MAF disconnected electrically: no loud hissing noise, steady idle, can't rev (even very slowly) above 2000, almost no power for driving (a little reverse and 2nd gear, but it really wanted to die when moving).
The FPR is right next to the MAF connector, on the same side as the plug - is the hissing possibly coming from the FPR? (thinking it could be the change in fuel return flowrate causing the hissing noise)
Check if the FPR holds vacuum on the vacuum connector - its the regulator on the left side of the car by the MAF (the other two are dampers).
edit: Also worth noting that the FPR might have failed even if the Fuel Pressure Regulator holds vacuum - one way to test is see if the return outlet on it (that goes to the fuel cooler on the firewall) leaks or not. There's a page in the workshop manual that details troubleshooting the fuel system including a handy diagnostic flowchart to eliminate possible pressure-drop causes.
Last edited by Hilton; 12-16-2008 at 10:27 PM.
12-16-2008, 10:02 PM
#24
Rennlist Member
Don't get carried away with that starting fluid -- it washes the oil off the cylinder walls. Anyone nearby that can help you out by swapping in a known good MAF? I'm not suggesting you replace it without a confirmed diagnosis(it's $$$) but this would be an easy check.
It's sounding like fuel delivery though. Pull the vac hoses off the regulators. Any fuel in there? Can you attach a pressure gauge to check pressure at the rail? Roger has them for a good price.
It's sounding like fuel delivery though. Pull the vac hoses off the regulators. Any fuel in there? Can you attach a pressure gauge to check pressure at the rail? Roger has them for a good price.
12-16-2008, 10:35 PM
#25
Rennlist Member
If the regulator failed and was returning too much fuel to the tank, there would be a loss of pressure. This could be checked by crimping the return line. A fuel pressure gauge would make diagnosing easier. If the dampers or regulator failed enough to cause a driveability issue from too much fuel, it would be noticable from the rich exhaust.
12-16-2008, 11:14 PM
#27
Nordschleife Master
12-16-2008, 11:23 PM
#28
Rennlist Member
Thread Starter
That makes sense. Now with that in mind, why do I have only the smallest amount of power available with the MAF disconnected (barely enough to pull the car forward at walking speed in 2nd gear and reverse)?
