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Mail From: (email redacted) (Robert Whitley)

Several have recently requested help with unstable idle problems. To that
end, here's what happened with my idle surge problem in my 89 Mustang LX
5.0L. My problem only showed up after warmup and when the car was fully
stopped and resulted in a rolling idle which cycled from nearly stalling to
about 800 rpm at about 1 Hz. I think the lesson is to buy to factory manual
or Probst's book and to systematically attack the problem. Also, the EEC-IV
is your friend.

I checked the TPS, the idle air bypass, the EGR valve, the PCV, and ran the
KOEO and KOER tests. All the sensors/valves checked out OK. I changed the
fuel filter with no effect, and I reset the idle with no effect. Initially
the computer said that I was running lean on both banks and I was distressed
that the answer didn't appear to lie among the easy candidates. I figured
that I had an air leak or a fuel pressure problem. Generally, I was
following Probst's book and using his rolling idle diagnostic table, but I
was doing the easier stuff first.

Things got busy at work, so I lived with the problem for a couple of weeks.
Then, abruptly, the surge went away. I ran the EOEO and EOER tests and got
a Thermactor Air problem (the two small solenoid valves which control the
air injection are called the Thermactor Air Bypass, TAB, and Thermactor Air
Diverter, TAD). My theory is that either the bad component finally failed
in a benign condition as far as the idle goes or the EEC-IV eventually
figured out that the TAB or TAD was bad and shut the offender off. I think
the credit for solving this problem goes to the EEC-IV.

So, after reading about the TAB/TAD in Probst's book, I bought a Sunpro
vacuum tester. The vacuum lines in question are close to the firewall near
the throttle body. I used the tester to check all the pieces in various
stages of disassembly. I discovered that the TAB and TAD valves look
virtually identical (more on that later). The outlet of one would hold a
vacuum while the other would not (they both held vacuum on the inlet). I
figured one was bad since they were behaving differently. The harness hot
lines checked out OK (KOEO) and both solenoids had the proper resistance.
All the connecting lines held vacuum.

At the time, I thought both the TAB and TAD were identical since I didn't
notice the difference between the connectors and since I could only see one
part number (I removed the TAB/TAD mounting bracket with both valves
attached and could only see the PN side of one of the valves. Actually, one
part number is appended AA and the other A. The dealer ordered a
replacement ($27) for the one whose part number we could read which was the
one which held vacuum on the outlet. The part came in the next day and did
not hold a vacuum on the outlet (so the replacement behaved differently than
the original). I was lucky, the one whose part number we could read turned
out to be the one which was bad.

I installed the part. Problem solved. No codes on KOEO or KOER. Stable
ide. Haven't bothered to figure out if I replaced the TAB or the TAD.

By the way, the TAB/TAD is listed in Probst's book as potential cause #13
out of 13 for an EEC-IV idle surge problem.

Rob W.
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(email redacted) (Robert Whitley)
89 LX 5.0L Hatchback 5 Speed