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My Cheesy FAQ Posted by Justin VanAbrahams [Email]  (#32) [Profile/Gallery]
(more from Justin VanAbrahams) on Mon, 15 Jan 2001 19:20:23
In Reply to: turbo (lack of) boost, Ken Westers, Mon, 15 Jan 2001 16:29:50 Members do not see ads below this line. - Help Keep This Site Online - Signup |
This is something I wrote for my website, but haven't posted it yet, 'cause I haven't proofread it... May be useful: (sorry about any formatting errors...)
1. Verify it isn't your boost gauge. Acquire a calibrated boost gauge.
Pull the driver's side front speaker grille, and locate the vacuum line
going to the back of the instrument cluster. Pull that vacuum line, and
connect it to your boost gauge; your stock gauge will no longer be working.
If you're seeing the right numbers (11-12psi on 16v, 7-8psi on 8v cars) then
the problem is your gauge and not the car.
2. If you're not seeing the right levels, it's time to start your
troubleshooting. First, pull the "C" line off the APC solenoid. This will
not only disable the APC system, it will also disable your wastegate. Go
for a brief drive on an open, uncongested road. BE CAREFUL with your
acceleration as boost is completely unregulated. You should get massive
boost, and will probably run into the 15psi fuel cutoff quickly, so be
prepared. DO NOT drive like this for long, as knock will surely set in and
can damage the engine surprisingly quickly. If you get boost, the problem
is 99% vacuum or electrical; if you don't, you likely need a new turbo.
3a. If you did NOT get boost in step 2, pull off the pipe that runs from
the air mass meter to the turbo intake. Try spinning the turbo compressor
wheel gently. It should spin mostly freely; if it does not, your turbo is
seized. If it does, you may have a damaged exhaust impeller, or you may
have a seriously damaged exhaust manifold or engine. Troubleshooting that
is beyond the scope of this document. To get you thinking, the problem
could be anywhere from seriously off timing (ignition or valve) to bad
pistons. That said, at this point you're probably noticing other
drivability problems, too.
3b. If you did get boost in step 2, you have some work to do. First thing
to verify is that you don't have any massive air leaks. Check all vacuum
hoses from beginning to end, and verify that all rubber couplers on the
intake tract are in good shape. I recommend pulling them all off an
inspecting them carefully - small rips or tears (even *pinholes*) can expand
to large size under boost which will bleed off the intake charge quite
readily. Also check grommets in the intake manifold for tightness - they
can blow out under boost but get sucked in under vacuum, thus appearing
okay; be sure to check them with your hands and not your eyes!
4. If you check all hoses and vacuum lines and don't find any leaks or
tears, your problem will lie with either the cruise control or the APC
system and you must check the cruise system first. There are only a few
components - a pressure switch located on the driver's side fenderwall and
two pressure switches located (one each) by the clutch and brake pedals.
Check the fenderwall switch first by attaching a short length of hose to its
vacuum port and sucking and blowing on it. You should hear an audible click
of the switch opening and closing. Test electrically by connecting a
continuity tester to its two leads - you should get continuity at atmosphere
and an open circuit under vacuum. If you do not get continuity, you've
found your problem. You can bypass it by shorting together the two leads on
the car's wiring harness; just remember that while bypassed, the APC system
will be ENABLED while cruise control is engaged, and you'll get full boost.
This can lead to poor cruise control performance and potential engine damage
(unlikely as it is) and should repaired as soon as possible.
5. Next, check the pressure switches at the brake and clutch pedals.
Connect a length of vacuum line to each switch (one at a time) and apply
vacuum and pressure. The switches should hold vacuum/pressure with the
pedal at rest and should not when the pedal is depressed. Additionally, you
must test electrically by continuity. Pull the electrical lines off each
switch and test; there should be continuity with the pedal at rest and an
open circuit with the pedal depressed.
6. If the cruise control checks out, test is the pressure transducer while
you're still under the dash. It's a little metal cylinder above the
driver's feet. Do not get it confused with the overboost switch; it can be
identified by the *lack* of an adjustment screw. Often times when this
transducer fails, you will get extremely low *or* erratic boost levels.
This part is often overlooked, and is responsible for a surprising number of
APC failures. Unplug the two electrical connections and short them
together. Take a test drive, but do note that the APC system will have no
way of knowing how much boost is being produced. The result will be normal
drivability, but boost will not stop at normal levels, meaning you WILL hit
the overboost switch at some point (and probably quickly). Check boost
levels with a calibrated gauge.
7. Next check the APC solenoid by applying +12v to it; under power, the
solenoid should snap. Check that it is fully functional by removing the
vacuum lines from the "W" and "R" ports, and connecting a length of vacuum
line to the "C" port. With no power applied, air passed through the "C"
port will come out through the "W" port; with power applied, air passed
through "C" will come out through "R". Be sure to verify the solenoid does
not leak air through the wrong port at the wrong time.
8. If your APC solenoid checks out, check the knock sensor. Unplug the
wire to the knock sensor and go for a drive, using the calibrated boost
gauge to check charge pressure. Be cautious driving in this fashion, as you
will be operating without the APC's protection. If the car behaves normally
and boost pressure is correct, you've got a faulty knock sensor - replace
it, and do NOT drive for prolonged periods with it unplugged or you WILL
grenade your engine. NOTE: If boost returns to normal levels with the
knock sensor unplugged this does NOT necessarily mean that the knock sensor
is faulty. Trying a known-good knock sensor is a good way to help isolate
the problem. If the problem persists with a known-good component, you may
have more serious problems: Engine problems of one variety or another may
be causing premature onset of knock, such as timing, fuel, or ignition
problems. These may be subtle enough to not affect normal driving but
pronounced enough to set off the knock detector, as it's quite sensitive.
Troubleshooting such problems is beyond the scope of this document.
9. If all else has failed, you may be faced with an unreliable APC box.
Such problems are rare, but they do occur - and will probably occur more
often as these cars get older. The only real troubleshooting you can do is
to verify that the box is getting power and good ground, and that the two
sources are reliable. The next step is to swap APC boxes with a known-good
component.
10. Hopefully, your turbo is working again at this point. If not, email me
and we can see if there is some other stone unturned...
, Mon, 15 Jan 2001 19:20:23 <-- Viewing This Message
, Tue, 16 Jan 2001 09:44:31, Tue, 16 Jan 2001 15:05:25, Mon, 15 Jan 2001 19:18:59, Mon, 15 Jan 2001 17:17:11, Mon, 15 Jan 2001 17:19:48, Mon, 15 Jan 2001 18:00:25
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