Volvo Maintenance Hints for 7xx/9xx - Bill Garland's Nuclear ...

attery voltage when the gas pedal is depressed a bit. You say you cleaned the
throttle body BEFORE the symptoms started appearing, so perhaps there's a chance
you accidentally didn't adjust the TPS position properly?
The other thing to check is the engine coolant temp sensor (ECT). Look at pin 13 of
the FI unit. When the engine is warm you should see 350mV. See Diagnosing ECT
Failures for more information. In addition, since you've disconnected the mass airflow
sensor (MAF), the computer may have gone into limp-home mode permanently which
will cloud the diagnostics, so you should really check and reset any error codes stored
in the system before you do anything else. However, unless the Check Engine light
has come on the MAF disconnect didn't trigger any code setting.
High Idle at Startup: TPS Failure. Symptom: the idle surges immediately to 2000-
2500 rpm after a cold start, then gradually declines to 950-1000 rpm as the engine
warms up. The idle rate seems dependent on temperature: lower temperatures cause
a higher idle. Diagnosis: TPS failure The TPS internal microswitch does not sense the
idle condition, even though a "click" might be heard at the switch. As a result, the
ECU does not control idle through the IAC valve and idle is too high. See Throttle
Position Switch for information on diagnosis, adjustment or replacement.
Idle Surge, High Idle, Poor Idle: Vacuum Leak [Inquiry:] Idle surges at times
between 1200-1600 RPM when car not under load. Idle Control Motor? Other?
[Response: WBain] You have a vacuum leak. Check all lines especially at the throttle
body. Also if it's a turbo, look for a bad intake manifold gasket.
Intake Manifold Gasket Leak. Mine has had a very rough idle. I checked the FAQ and
cleaned the throttle body and checked for leaks in the hoses. I was finally able to
isolate a leak in the intake manifold gasket. The propane test didn't find the leak. The
only way I was able to isolate the leak was with the mechanic's stethoscope with the
probe removed and only using the tubing to get very close to the gasket and follow the
contour of the manifold. Replaced the gasket (approx. 2 hours) and the car runs fine.
[Tip from Gary Goms] Vacuum leaks can be easily diagnosed by several methods. I
believe the safest and easiest method is to use a modified mechanical stethoscope to
listen for vacuum leaks throughout the system. To modify the stethoscope for locating
vacuum leaks, replace the probe with a length of plastic hose or fuel line. Alternate
methods include listening for a change in engine speed while spraying propane or
aerosol carb cleaner around manifold and vacuum connections. The various brands of
"smoke" machines are also very handy diagnostic aids to use when attempting to
locate vacuum leaks at the manifold-to-cylinder head gasket, throttle body shaft,
throttle body-to-manifold gasket and vacuum hoses.