I own a '94 Ford Thunderbird 3.8 V6 Super Coupe with approximately 148,000 miles on it. I have been experiencing rough idling, particulary with the brake on in drive. It is not quite as noticeable with the transmission in park or neutral. Sometimes the engine will completely stall, but always (at least until now) restarts easily. I was getting a fault code 6-9 months ago which turned out to be the cam position sensor. I have recently replaced the Idle Air Control Motor after attempting to clean it first as recommended by a Ford dealership mechanic (when cleaning it I believe the diaphragm cracked and I was idling at 2000+ RPM). I changed the spark plugs about 16 months ago, but have not changed the spark plug wires. I also have a slow coolant loss which I cannot track to an external source. I took the vehilce into the Ford dealership prior to 120,000 miles when they were checking 3.8s for potential head gasket leaks and they claimed there was none at that time. However, I have had a slow loss for years. I replaced the radiator, hoses, and thermostat approximately 2.5 years ago.
Let me address the most common problems that I know about on Ford 3.8L engines.
The rough idle MAY be dirty injectors or carbon deposits in the combustion chamber. The problem could be exacerbated by broken or weak motor mounts. This engine has "oil filled" motor mounts that sometimes just "fall apart".
To see if you have bad motor mounts "power brake" the engine in drive and see if the engine moves more than a couple of inches. Then do the same test in reverse.
If you have weak or broken mounts you will see the engine move a lot and make a clunking sound when you let off the gas. (the clunking sound is the engine falling back down on top of the broken mounts)
If the motor mounts are good then I would recommend you find a shop that can do more than just clean the injectors. They need to do a upper plenum cleaning procedure as well as cleaning the injectors. Ask them if they have a machine called Motor Vac. This machine cleans the injector filter screens before the car is started. These injector screens can't be cleaned by pushing cleaner through them. This is why this Motor Vac machine is so useful to clean them properly.
Now to the coolant loss problem!
Find a shop that can look for a external leak with a blacklight or a UV light!
Antifreeze has green dye in it and when it dries it still leaves a trail that a UV light will spot. Look real close around the water pump and timing cover. If no leak is found then the intake or cylinder head gasket MAY be faulty!
If they are faulty this might explain rough idling as well as rough running when first started.
The best way to find this problem is find the best shop in your town! Just ask your friends who has a good shop then make an appointment!
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