I did all that to mine, eventually I pulled off the Idle air control or IAC took it apart and cleaned it with carb cleaner and it worked, idled good, it has worked for longer than a year. cost-$5.00, 1 hour.
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1. Vacuum leakage Check throttle body and intake manifold for tightness. check all vacuum hoses.
2. Faulty fuel or pump or electrical.
3. Plugged PVC valve.
4. Air filter clogged
5. Leaking head gasket.
6. Camshaft lobs worn.
All of these porblems can cause lopes or erratic idle.
You could have a clogged fuel filter, if you haven't changed it in awhile. You could have a leak in the intake system somwhere also. When you slow down, you get less power to the engine. It may need spark plugs and wires as well, if in need for a tune up.
4.9 litre ?? you having a laugh its a 10ton truck engine not a pick up -global warming machine,remove the pump/sender unit and check the gauze filter also look inside tank for a something floating around ,i think the idles is not a related problem to the vehicle losing power but their is a possibility you could have a problem with the little vac operated thing on the fuel injector line that allows the fuel through only when the engine is running
I think your 88 mustang contols the fuel air mix with a manifold absolute pressure sensor. Ive seen were the vacume line going into one has worked loose or broken cousing rough idle and throttle. llso check that the idle air control valve isn't plugged up with soot. This would also result in rough or loping idle but would be fine once the engine is hot. Let me know if this was helpful, good luck.
Top candidates for this problem are: Clogged Fuel filter Faulty Fuel pump Clogged fuel injectors Dirty / worn spark plugs
Of these, only the clogged fuel injectors are likely to respond to a gas tank additive, and even then you need to get the right kind of additive. If your fuel filter has not been changed in a while (couple years), start with this. It is fairly easy to change. The fuel filter is under the car near the the driver's side rear door. If this problem started "suddenly," then the fuel pump might be the culprit. Fuel injector / spark plug issues are not likely to stop the engine right away, but would tend to cause it to miss cylinders, idle rough, or start rough.