3.0L Duratec engine misses badly

2002 Taurus SEL 3.0L Duratec engine, ( 93,000 KMS)
Have annoying engine miss at idle, most noticeable when in drive, stopped at traffic lights.
Miss is that noticeable that it is felt throught car by all occupants
Service Engine Light did not illuminate.
.
Ford delaer ran diagnostics and road test, no faults recorded. They recommended replacing plugs / fuel and air filters, done by Ford dealer, problem still exists.
Ford dealer suggested wait till Servicre engine light comes on, and then return to garage. Several thousand KMS since, and still no light illuminating.

Acceleration to highway speeds do not feel smooth.
In recent days I have noticed that when heater/AC is turned off, miss is
gone, turn on heater or AC and engine begins to miss.

When the heater is switched on, you can hear a heater or ac vent door closing or opening, and within a minute engine begins to frequently miss.

Seems very strange that turning the heater or ac system on, could cause this miss.

I have already spent $600.00 for tune up / diagnostics and no closer to finding problem, do not want to continue troubleshooting at these costs.

Could the heater / ac systems be causing problem through the engine
vacuum system

Any suggestions or ideas.

Thanks

Let the repair facility know what you just said. If it misses with the a/c on and stops when it is off, that will give them an area to check. Does it idle lower also with the a/c on? Could be carbon build up somewhere.

I'd check for a vacuum leak.The heater door uses vacuum to open and close it.If you know someone with a vacuum gauge you could verify the loss of vacuum.For a vacuum leak to cause that kind of problem I suspect a rather large leak from a large hose.Try going over all the hoses that are easily visble firstly then try to visually check the more hidden hoses.It could be a hose has come in contact with the exhaust manifold and melted.A vacuum leak that large should be heard as a whistling sound as the throttle is opened.

my car was having the same problem. i checked all the hoses, when i changed the spark plugs i bought new upper plenum gaskets torqued it down too spec. i also replaced the pcv valve which was junked up. after that my car runs great.

another thing is that, (idle air control) IAC sensor could be worn out and full of carbon. the pcm controls the idle, of how much air is entering in front and behind the throttle plate. this sensor is expenisve too replace (around $45.00). but occasionally it would pass during a DTC scan test if its worn out.

try this tip:

have your car warmed up at operating temp. open the hood and listen too the IAC sensor, if you hear a pop when you step on the gas its probably bad.

Just an update to let everyone know my FORD dealer has resolved the problem.
They replaced the coil packs ( found several cracks in it), the miss got worse, then they replaced the plug wires and the problem has been resolved.

Thanks for the suggestions.

Just an update to let everyone know my FORD dealer has resolved the problem.
They replaced the coil packs ( found several cracks in it), the miss got worse, then they replaced the plug wires and the problem has been resolved.

Thanks for the suggestions.

Thanks for the info. I have been trying for a month to fix mine. As soon as I installed the new coil. All the problems when away.