FIX MY FOX
by Matt Highley
Q
WHO YOU GONNA CALL?
I think I have a ghost in my ’89 GT. When I’m driving,
the A/C system will just blow in the defrost vent by the
windshield. If I’m sitting at a light or driving steady down
the highway, it works normally. It’s still cold, but it just drives
me nuts.
Andre Thomas
Austin, TX
A
No need to call an exorcist on this one. There’s
a one-way check valve in the vacuum supply system
under the hood that supplies the duct system under
your dash. It’s a little black plastic valve located on the
passenger side of the upper firewall that allows vacuum
into the duct system and closes off the system when
vacuum is temporarily lost. This can be found at any
Mustang parts retailer for around $12.
Q
NO TACH
No matter what I try, I can’t get the tachometer to work
with my MSD box. I even bought the MSD tach adaptor
(MSD-8920), and it does not work. I am almost ready to
mount an aftermarket tachometer, but I don’t want that huge
thing bolted to my dashpad. Do you have any suggestions?
Daniel Parsons
Norman, OK
A
I’ve run into this issue in the past with the Ford
Duraspark ignition system. The instructions tell
you to hook the white wire to the tach output on
the MSD box. This does not work on the Duraspark
systems. You need to wire the white wire from the
tach adaptor to the signal wire from the distributor.
If you’re using the stock Duraspark system, it will be
the wire with the orange strip. Just splice the white
tach adaptor wire in to this circuit.
90 FOXMustangMagazine.com
Q
SENSELESS MASS AIR PROBLEMS
My mechanic has had to warranty my mass air sensor three times. The problem
is it will start to run rough and the idle will surge. It takes about three months for
the sensor to go bad. What could be causing this?
Ted Leof
Wilson, NC
A
There are a couple of possibilities. First would be if you ran through a
deep water puddle and was actually able to get any water past the air filter
into the sensing wire on the sensor. But since your problem is recurring,
that’s probably not the problem.
Next, make sure there are no shorts in the wiring harness going to
the sensor. The likely culprit for this complaint is a K&N air filter. K&N
recommends maintaining your filter by spraying an oil that they supply
onto the filter. The problem is, over time this oil will coat the sensing wire
on the sensor. So the sensor is not bad; it’s just dirty. Auto parts stores do
sell a mass air sensor cleaner in an aerosol can. This sensor can be cleaned as
many times as you need to, but it’s also very fragile, so be careful.