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Don Knight
Don Knight, ASE Certified Technician
Category: Mercury
Satisfied Customers: 1736
Experience:  40 Plus years on all makes and models
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Mercury Grand Marquis: I am working on a 94 Mercury Marquis,

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I am working on a 94 Mercury Marquis, with 50,000 original miles(sweet ride). My friend brought it to me with the following problem. Engine surging while driving. They had already had someone look at it, and they replaced the MAF. When that didn't cure the problem, they removed the new MAF and reinstalled the old one.
When I drove the car I found that at about 30-40 mph at just under neutral throttle, the engine would start to surge. The idle would "Almost pull the car like Cruise Control. At this point a service light would come on. Returning to the garage, upon putting the car in park, the engine speed would be in the 1000 rpm range (no tach, just by ear guess) Checked the throttle cables, no bind. Started to pull the snorkel from the throttle body, and when I pulled the hose off (3/4 inch) going down to the back of the manifold, the engine raced even more until I plugged it with my thumb. at this point the engine came down to a normal idle. replacing the hose on the snorkel brought the engine speed back up to the high idle. Upon doing some research I found references to the IAC. I located it, and gave it a light "Tap" as soon as I did this, the engine speed dropped to normal. I repeated this (revved the engine several times till idle stayed high) several times, and when I tapped the IAC the speed dropped to normal. SO.... I purchased a New IAC valve, and installed it. The surging, while diminished some, is still there, and when put in park, will still run high. Now, tapping the IAC does Not reduce the idle. Goosing the throttle will make it drop until it is throttled up again. There is a noticeable "Howl" coming from the air snorkel when driving, and the IAC is in the open position. one last thing, When parked, and at high idle, I unplugged the MAF, and engine speed dropped dramatically, almost to the point of stalling. At this point I pulled the MAF. I inspected it, it looked ok, but I cleaned it non the less. No improvement... Help!!

Donald Knight :

Good morning.There are a couple of things I would do.First check the throttle bore and make sure that it is clean.If it is carboned up(black stuff) it can cause the type of problem you are having.If it is dirty, cleanm it with some throttle body cleaner(available at any parts house).When you clean the throttle bore wet a clean cloth with the cleaner and wipe the surface especially where the throttle plate rest.Next it would try another IAC motor.I know you just got one but I had a problem with those in the past.I went through three motors before I got a good one Talk about drive you nuts.I would also recheck for any vacuum leaks using the throttle body cleaner.Just spray around the engine along the intake manifold and the vacuum hoses.If you notice the idle change at any point look there for a vacuum leak.Ford had a problem with some of the hoses where they bend(90 degrees) but the "hole" would be on the underside and you could not "see" it.Please let me know if you need more assistance,Don P.S. Have a Happy 4th..


I will try the Throtle body cleaning, I remember having issues with my 88 Town Car 5.0, and That taking care of it. With the IAC, It doesn't appear to be "Sticking", because when I disconect the MAP, it drops down, which leads me to think, the IAC is being held open electricaly???

Did you disconnect the MAP or the MAF?Either one will affect the idle.I would check the throttle bore and any vacuum leaks first.
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Customer: replied 4 years ago.

Hello Donald, Well, Tried the throttle cleaning, and check for vacuum leaks, all with no success. Pulled the IAC back off, and cleaned and reinstalled the original... GRRRRRRRR the car is running smoothly. I have had luck with such parts from the Advanced store, but will have to see how they feel about taking this one back, or replacing "IF" they have one... I think if they don't have one. I might steer my friend to either a NAPA, or Ford Dealer.

Thanks again Lonnie