As with any old or older car you will have problems. There isn't a question of if, it is a matter of when is something going to go bad or flat our break.
I had my first encounter of this recently...
When I first bought the Mustang and after the new and exciting factor wore off, I noticed that there was a problem. The car would surge when idling. What does this mean? It means if you were to watch the tachometer, you would notice a fluctuation in how many RPM the car would have and you wouldn't be touching the accelerator/gas pedal. In my case, it would rev up to about 1100 RPM and then drop to about 250 RPM and would sometimes stall out.
This being my first Mustang and all, I knew I was going to need some help with this one. I had a friend over, and he took a look at it. Keep in mind, my friend is not a mechanic, but he works with mechanics all day and sees this type of thing happen a lot. As soon as he heard it, he said that it was the Idle Air Control Motor.
I was thinking this is sweet, I will order one direct from ford and install it. I will be all set. Well, about $130 and 30 minutes later, That wasn't the problem. Lesson learned. Do not just go off a hunch about what is wrong with your car.
At this point I turned to the forums at http://www.stangnet.com. This is a wonderful site for any mustang owner. You can talk about your project, get advice on what you should or shouldn't do to mod your car, and also get some technical advice on fixing your car. This turned out to be very very helpful. The first thing I was told was to scan the computer and see if there are any codes coming out of it. Well, now there is a concept. Debug your problem by finding out what the car tells you it is having a problem with. As they stated in the forum, do this first before anything.
I scanned the computer and came up with a couple codes that were going to be helpful in finding the problem. I had an 18 and a 33.
I looked up both of these codes, and the only one that actually pointed to something was the 33. That referred to an EGR valve that didn't report that it was closed properly. I was like, Ok, I will go buy a new one and replace it. Well, an EGR valve costs betwee $80 and $90. This finding the problem is starting to get expensive. I read on in the post of things to check and someone mentioned that you can clean an EGR Valve. That usually fixes the problem, but not always. I was like, ok, save the money, clean the part, stick with an original part for the car especially if it works. Once I cleaned it and put it back on, the 33 went away. Cool, that one only ended up costing the price of a gasket.
So I went back to stangnet and inquired about the code 18 I was receiving. In all of the searching I had done, there was nothing that pointed to anything specific to change. Well, the guys at stangnet came to the rescue. They told me a common reason for this would be an ignition control module going bad. The downside is that usually the go bad and the car no longer runs. Well, my car was running, could this be my problem? How would I know? Well, the easiest way to find out is to remove the part from the car and take it to your local parts store (I took mine to Autozone) and they can test it for you. Testing is free with no obligations. Low and behold they tested it and it was bad. I bought a new one and put it on.
Here is a little background on the Ignition control module. To change this part, you have to loosen the distributor and turn it so that you can get at the bolts that hold it on. There is a special tool that you use to remove and install this part. They cost about $5-7 and can be purchased at any auto parts store.
The reason I told you about the turning of the distributer is because I tried to outsmart myself and be a little lazy at the same time. I marked where the distributer was so I could put it back in the same spot without having to get out a timing light.
Yeah, bad mistake. My recommendation is to not shortcut anything here. No matter how good you think you are, take the time and use the timing light. I thought that I had it in time, the car started, the code 18 was now gone, and it seemed to run good. Well, I took it for a test run and the surging idle was there and it seemed worse than before. Not only that, but now when it stalls it won't start unless I pop the clutch. This is when it stranded me.
I called a friend and she came and we were able to get it jump started. Once I got it home the fun started. The car stalled half way into the garage and wouldn't start. I had to push it the rest of the way in. Keep in mind, my driveway is on an incline to get to the garage. Not fun, but got it in the garage.
Well, my first thought was, the starter finally went bad. The car started hard ever since the day I bought it. So I changed out the starter... This helped with the hard start when it was cold, but did nothing when it was time to deal with the stall from the surging idle.
I am very frustrated at this point. I finally get out the timing gun and check the timing. Whoa, what do I notice, my timing is advanced a lot. That is what was causing the hard/no start when it was warm. And what do you know, I take it for a drive, no more surging idle. Coincidence? No it was not. I did encounter one stall on my test run. This concerned me a bit.
I needed to do an oil change on the car as I had not done that since I bought it. So when doing that I decided to change the fuel filter. The fuel filter was the last piece of the puzzle. The car is running great again.
If you are having surging idle problems, you should check out this post on stangnet... Surging Idle Checklist
My advice is to listen to jrichker as he knows what he is talking about.