1996 Ford Thunderbird Idle Problem

Maintenance/Repairs
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1

Hi, I’ve got a problem with my '96 Tbird that’s really stumping me. It’s a long story to begin though.

It started with a little shake when I gave the car gas. I don’t know if it’s related, but I ran over some debris on the highway at around 70mph. [No chance to avoid, unfortunately] That same day it felt like the car was missing, and throughout the week it got worse and worse. It would shake harder and faster with increased RPM’s. By the end of the week, it was so bad I kept the car off the road.

I thought it was a bad misfire, so I changed the plugs, then the wires, then the EGR and EGR solenoid [from JY] and swapped a coilpack from my other car.

It ended up being the harmonic balancer, it spun a 180, not a surprise on an 18 year old car. I’ve never had one go bad, so I didn’t even think of that as a possibility with the way the car felt. [I’ve been working on cars for a long time.] It never threw a code once, so I had nothing to go by to begin with.

So over the weekend I swapped out the balancer, and buttoned everything up. Ran fine! No shake! But then I had gotten a new problem.

The idle started to get bad. Very bad. It would lope, lope then almost try to stall, especially when the fans kicked on or I put it into gear. Since this car is my daily driver, I had it at work while I started to undo the new stuff I put on it. I started with the just unpluging the battery for 3 hours. Didn’t work. So I pulled the new plugs, reverted them back to the old ones. It didn’t help. Leaving work that day I noticed if I pull the IAC connector, the idle smooths out some, but it doesn’t stall. [Suspect, maybe?]

So I figured it was the EGR, or solenoid, so I changed them both out. Nothing worked, still was horrible. Frustrated I changed out the coil pack and wires. It’s still horrible.

I don’t drive the car now, it stalled out a few times in traffic on the way back before I swapped everything back out.

The icing on the cake is that it has not thrown a single code except for when I unplug the IAC. And the only code it throws is barking at me for doing so.

It really feels like a bad vacuum leak. I’ve run down every single line and there’s nothing leaking or rotten. The IAC is in question, but the car was working perfectly fine for at least a year with no problems whatsoever. So how the IAC went bad right after changing everything and a new harmonic balancer is way beyond me.

I’m thinking too maybe I damaged the Crank Position Sensor when I installed the balancer, since it was a one piece unit with the pick-up gear. But, I was careful with it, used a puller installer, and there is appropriate distance between the sensor and gear. Also, I’m not sure, but I doubt the Crank Sensor going bad would effect idle. Any RPM above 1500 is smooth, and doesn’t surge.

Sorry for the long writeup. I’m just stumped with this damn thing, I can’t keep buying things that don’t work, most shops even told me if it doesn’t throw a code their computers won’t be able to tell them whats wrong.

Appreciate the help guys!

2

The IAC may be bad and it’s just a coincidence it went bad around the same time as the balancer. It sure sounds like the IAC. Why not spend the $100 or so for a new one? It could well be the solution for the price of two tanks of gas. Or you could try cleaning the existing IAC.

3

If the idle gets worse when the fans kick in, I’d consider a voltage check and a voltage drop test. If your system voltage is getting way low, it could affect spark efficiency and the IAC. Make sure you check positive and negative drop.

4

I got the alternator checked, It’s perfectly fine. IAC tested fine by an ohm test.
I got it at a shop now, won’t hear back till Monday but last he said was the PCM may be going bad.

5

an IAC doesn’t typically get ohm tested

Replacing the PCM is usually the absolute last thing in any diagnosis, after ruling out everything else

6

Ohm test is not going to tell you if the IAC valve is working properly. It’s an electromechanical device, and the ohm test tells you only if the electrical circuit has the correct resistance. It tells you nothing about its mechanical functioning. It could be stuck open or closed, for example, yet still have the correct resistance.

“PCM may be going bad” translates into “I can’t diagnose the problem and am just guessing.”

If your mechanic is just going to throw parts at it, at least start with the less expensive ones, like the IAC.

7

When I unplug the IAC on my Ford (escort) the engine stalls - I presume because the thing closes up.

Testing the alternator is typically not the same thing as looking for voltage drops. It depends on what was done.