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90+ had the base time at 0 deg while the previous years called for 6 deg BDTC. Have you done any ignition work? The popping and fluctuating rpms sound like possible misfiring.

 

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Sounds like maybe the shop didn't disconnect the timing control on the firewall wire harness. If they didn't your initial timing is probably at 28* retarded even though it's reading 6*. Not only does this kill power, the gas mileage will be cut in half. The instruction's are also on the under hood sticker wich your missing!
In 1989 the initial timing went from 0* to 6* on the 305 TBI and plug gap is .35 plug's are AC-R45TS.
It is possible the shop screwed up because my son took my bird
to 2 differnt shops to get it tuned up and both of them set the timing without disconnecting the control link. Even though it ran, it ran like crap and the mileage sucked. So he sold it to me cheap.
On the first day I had it I figured out what they did wrong and had it running like a watch! I offerd to sell it back but he already
had bought a truck.

 

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Do you know where the connector is? I would check it! Maybe they forgot to hook it back up!

 

I think SQZBOX nailed it dead on. My buddy recently picked up a tbi car with the same exact problems. That popping your hearing is probably unburned fuel igniting in the cat, at least that's what was happening in my buddy's car.

Once we disconnected the timing control wire and put the timing light we found it was significantly retarded. Once we set the timing where it needed to be the idle smoothed out and the power nearly doubled.

Before we fixed the timing we were also getting the ESC code. But for the most part that went alway also. We figured the ESC code was from a cylinder loading up, misfiring and being read as knock.

 

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Here is the wire that needs to be unplugged for the timing.

 

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Well I geuss I should have added that even if it is connected it may not be set right. It needs to be dissconnected, have the base timing set, and then reconnected.

 

Have you checked the base time? With the connector disconnected, anywhere from 6-8 degrees BTDC is fine. If that looks ok then check the rest of the ignition system for loose or burnt wires or any other sort of problem.

 

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Dont let it go on. There is obviously something wrong. Does the motor lack pickup when you hit the gas or is it just uneaven and rough? Both misfires and retarded timing will cause some popping through an exaust. The retarded timing makes teh motor sound real sick, though, and it doesnt have any pickup. The missing will cause some power loss but most importantly the arcing can damage things and will cause the motor to run rough and idle erratically if its only intermittently firing the cylinder(s) in question.

 

A good shop should have set your timing with that wire disconnected, but that doesn't mean they did. I went to a highly recommended shop(a few years ago) to handle a distributor problem on my car and when I got it back it was running really bad. I took it back, they got out the timing light and showed me it was set correctly. I pointed out that they didn't disconnect the wire and when they did the timing light showed the timing was retarded about 20 degrees.

Take it back, it only takes them a few minutes to adjust the timing, if they don't disconnect that wire, do it for them.

Again, your problems mirror those on my friend's car. Someone had obviously tried to set the timing on his car without disconnecting the wire in question. Once we set it correctly the problems disappeared.

 

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Just for future reference here are the underhood sticker shots.

 

And vacuum.

 

Exacly how the car sounds will tell alot. Through the stock exaust too little timing will sort of sound like a continuous popping sound with a motor that jsut generally runs like crap since the mixture is still burning when the exaust valve opens rather then be fully burned on the downstroke of the piston. This will also cause the exaust to get so hot that it glows, which in turn will trash all the stuff near the exaust thats not made of metal.

The intermittent miss will cause a spitting noise out the exaust with fluctuating rpms. It could be a plug boot thats not on all the way or a damaged spark plug. Id take it to a reputable shop or a dealer thats known for good service if the present shop can't fix it right for free on your next trip in.