My son has a 94 crown vic 91000 miles.Car runs very nice and runs strong.Occassionally at around town speeds and at cruise speed on highway car stumbles a bit from time to time.Car kind of bucks for lack of a better word.At around town speeds car will occasionally idle a bit to high with your foot off the gas and as car de-celerates it will buck a bit again.No service engine light comes on.Recent complete ignition tune up and fuel filter change.Car idles nice and smooth.Havent had a chance to look under hood yet.Any tips on what to look at first would be appreciated.

 

Hmm. Sometimes a mechanic forgets to re-attach a vacuum hose. Listen for a tell-tale hiss around the intake manifold.

 

Crawled all over the motor and underneath,pinched off what lines i could to no avail.if there is a vacuum leak i cant find it.Not sure what to look at next and im not gonna start changing parts.Seems when you first start the car the idle goes up and comes down just a bit after a couple seconds but could stand to drop a little more.

 

Ignitionis tuned up and there is anew fule filter. Let's assume that was done right.


Consider this post:
this is for natedogs crown vic
been there did that many times
there is a vaccum tee to the right side of the t/body that crosses into the pcv system and intake, over time the rubber tee fitting rotts out causing a vac leak and is very common the part is available from your locak ford store, the repair is so common they will know what you want by description
ppauley/mustang cobra freek
ppauley
to clarify - on the pre-1996 crown vics, this tee is located on the intake on the passenger side rear of the throttlebody - find the PCV valve and follow the hose toward the intake - you will find the T

I agree with the poster - it is one of the more common issues with a Ford 4.6L V8

Watch Youtubes on how to find vacuum leaks.


As far as changing parts goes we are on the same page. You could do the following things:
Seafoam treatment. It removes deposits on the intake valves and in the combustion chamber.Techron fuel system cleaner is also an option.
Clean the MAF sensor with CRC maf sensor cleaner.
Clean the throttle body with CRC throttle body cleaner. That includes the idle air control valve.
I believe this engine has intake manifold runner controls. Check if the little white bushings are still there ($3.00 for a set).
Clean the electrical ground connections. Also make sure the brake lights work correctly. The PCM will know when you step on the brake and adjusts the idle accordingly.
Check fuel pressure.


I think when you diligently remove parts, clean them and reattach them you are bound to stumble across the cause of the rough idle.

 

All good advice thankyou very much.Did what i could today and will tackle all your suggestions next weekend.Not enough time today as my son works nights.Thanks again i really do appreciate your help and will update as i go along so hopefully it will help someone else in the future.

 

just for giggles, toss in a couple doses of injection cleaner, and fill-up gas from a different brand than you normally use. If possible fill-up with non-ethanol and see if there is a change after a few day's.

 

It could be sucking air at the filter it could also have something in the fuel it doesn't like. Has the car been setting up?

 

I did have him put a couple cans of seafoam in it and he said it seemed a little better.Im going to clean throttle body and sensors and change a couple vacuum fittings sunday.I will look at air filter connection also.Car is a daily driver and only does this from time to time so a cure is kinda hit or miss.

 

With the "Seafoam" did you follow the instructions on the can or did you just put it into the fuel tank?
It takes about 20 minutes to do the proper "Seafoam" treatment. I take the extra time to get into an industrial area so I don't have trouble with my neighbors.
You use a vacuum connection to drip 1/3rd of the "seafoam" (naphthalene +) in the engine while it idles. Then you turn the engine off and wait 10 minutes while the solvent works on the deposits. Another third goes into the tank and the last third can go into the oil. Then you drive leaving a huge white cloud behind you. That cloud is the solvent going through the exhaust...

Also: disconnect and reconnect all electrical harnesses. Maybe this improves a questionable contact somewhere. That's a good way to use the 10 minutes of soaking time.

 

(Not the air filter) fuel filter connections. Also check fuel cap and tank vent. If your car has a charcoal set up I would bet that it is clogged. These kind of problems are usually in the fuel system. Although a intermittent ignition problem can have the same sort of symptoms. As stated just pulling the electrical connections apart and putting some dielectric silicone paste in them cures a lot of problems that you can't quite put your finger on.