1999 Mustang GT Hesitation Problem

I own a 1999 Mustang GT, 4.6 liter sohc, 5 speed, 120,000 miles. Recently, the car started to hesitate in any gear under load from idle up to about 2200 rpm or so. Plugs and wires were changed at 100,000 miles. No check engine light or codes. My mechanic replaced the fuel filter and added a bottle of fuel injector cleaner. This worked for exactly one tank of gas. Now the problem is back. What do I do?

One thing to try is a decarbonizing of the engine. If there’s enough carbon deposits built up on the backsides of the intake valves it can cause this problem. Ever see that Shell commercial where show the gunky buildup on the backside of the valve? Well it’s true!

Have your mechanic try this:

Get a can of SeaFoam Engine Treatment.

Get the engine up to operating temperature.

Remove the brake booster vacuum hose from the brake booster.

Adapt a smaller hose that will fit inside the end of the brake booster hose an into the can of SeaFoam.

Pinch this hose off with a pair of pliers.

Start the engine and bring the idle speed up to 2,000 RPM’s.

Slowly open the pliers so the Seafoam begins to be drawn into the engine. It’s here where the engine speed must be manipulated and pliers opened and closed to prevent the engine from stalling.

Once all the SeaFoam has been drawn into the engine, shut the engine off for a half hour.

Reattach the brake booster vacuum hose.

After a half hour has past, start the engine and bring the idle speed back up to 2,000 RPM’s until the smoke clears from the exhaust pipe.

Give this a try and see if it eliminates the hesitation problem.


This reminds me of the old days when you could pour a quart of diesel fuel down the carburetor of an air-cooled VW to burn the carbon out of the engine. It worked, too, as long as you didn’t let the engine stall.