Hesitation at 35-45 mph

I have a 99 Merc. Gnd. Marquis,in a fuel conserving accelaration stumbles between 35-45 mph, ( stumbling not noticed under heavy accelaration in that range). W/ no ac on at a stop light car runs fine but w/ ac on at the stop light engine then runs ruff and when accelerating. I changed fuel filter and added Lucas fuel injector cleanor to the gas tank. Water misted the electric connections at the Ign. coils and fuel injectors. Any ideas.

forgot to mention that I saw no sparking or decreased idle speed when I misted the elec wiring.

EGR valve staying open during acceleration can cause this. Also, you might try cleaning the air-flow sensor as it could be sending a false signal to the ECM…

Misting the low=voltage connections does little or nothing on these coil-over engines. Any problems are hidden down in the spark plug wells…The 100K mile spark plug change interval Ford claims is REALLY stretching it…If it was an ignition fault, it would be profound at WOT and trigger the CEL…

Engine has 79K miles and belonged to my Mother in law. Last December had to change out the Intake manifold (the old all in one comosite intake manifold), changed all the spark plugs, 4 Ignation coils, fuel filter and various hoses along w/ the O2 sensors. Worked great untill last week.

This could be one of the coil on plug coils starting to go bad. Sometimes it is the “boot” between the coil and plug letting electricity arc to ground. Perhaps a code is stored in the computer to tell you which cylinder is acting up.

You may have two problems. I am not following the W/ no ac… very well, but here goes. If you have a problem at idle and a little pressure on the accel pedal fixes it, you have an IAC (idle air control) valve problem. Replacing it will fix it. Cleaning it might fix it.

Does the stumbling occur continuously between 35 and 40 or does it occur transiently (for a second or so) somewhere in that range? If the latter and it feels like you are driving over rumble strips, you might have transmission shudder. Has the trans been serviced every 30K like it is supposed to be? You need a complete fluid change with the correct fluid, Mercon V. It takes about 15 quarts. The torque converter has a drain plug which should be used and the pan dropped and filter replaced.

It can be difficult to tell the difference between transmission shudder and miss fire.

Did you ever figure out what was causing the stumble? My 09 Nissan Pathfinder is doing the exact same thing. At exactly 40 mph. The A/C works intermittently also. It comes on as I get below 40. It’s driving me crazy to figure out what it could be. Lol

The only thing your 09 Nissan has in common with a 99 Mercury is that they both have 4 wheels. Seriously, you would probably have better results starting your own thread.