engine hesitation when maintaining a speed

If you tuned it up with new plugs and plug wires then it sounds like it coud be some bad gas and/or dirty fuel injectors. You'll usually notice a bad injector at a higher rpm when there's more of a demand for fuel. There's one for each cylinder on that 4.6 motor. Try a couple cans of Gumout fuel injector cleaner in the gas tank to see if that helps clear it up. If it seems a little better , do that a couple different times, otherwise you might be looking at some fuel injector replacements and they aren't cheap. I don't remember if that car has separate coil packs or not, I believe it does. if so, you might have a bad coil pack too which would make it cutout at high rpm. Take it to an Autozone parts store and see if they can put a scanner on it, that should tell you what's bad and they test underhood stuff for free. Good Luck. Kenny.

first thing you need to do, is a scan for trouble codes. sounds like a cylinder miss fire. could be that one of the Coil Over Plug (cop) is bad. a quick scan can verify an issue.

Could also be a torque converter solenoid starting to go bad. At speed it's supposed to lock it in, but if it doesn't it'll almost feel like the transmission is slipping a little.

I'm having the same problem. All coils and plugs have been replaced. Seems to be most noticeable around 50 mph, in high gear, under light to moderate acceleration.

Dont have an answer but my 05 mercury doing it too I get the same codes when they check it I changed the number 1& 2 cylinder plugs and it says sumn bout either a throttle problem or intake problem. I put a can of sea foam in my tank and filled it up and a half of can in my crank case to clean my injectors and engine block out it cleared up for a few minutes on the highway thr check engine light went off but whenI got off highway 95 came to the stop sign and gave it a little gas it was still skipping

This engine is known to have this problem. Mercury/Ford put out a notice but no recall. I have an 01 marquis that we already replaced one spark plug with an insert but #7 also needs replaced. From what I've read, the aluminum wears over time so it needs tapped out an insert put in and then put spark plug in.

My 2002 with 4.6 also has this issue, but I notice mine during light acceleration but only when weather get cooler (30 to 40 degrees) and misty out it will hesitate so bad I swear it's gonna drop the drive shaft. I've had into my local shop twice and naturally it wouldn't do more were there any codes.

Bob, if it happens when the humidity is high you can try using a windex type spray bottle to spray around the coils and it may act up. the boots on the coils act like plug wires and can degrade over time allowing the high voltage to "trace" away from the plugs. Your vehicle also supports mode 6 data that can store data that doesn't set the mil off. Mode 6 requires a more advanced diagnostic computer than most shops have. The Ford diag computer should make quick work of it.

I have a 2004 Mercury Marquis with 75000 miles. I am getting the same hesitation.Took it to Autozone.They told me put some heet in it to see if it was water in gas. Then had them check for a code but check engine light was not on, so nothing came up. Bought some Lucas injector cleaner and put that in. I need a check engine light to come on to see what coil is bad to change it out. Hate to change all coils and plugs out if dont need them.

Hey Thomas did the Lucas injector work did it solve the problem

Run the codes if u get cyl 4 7 misfire they are usually the worst they get most stress ir anyother cylindet misfire swap the coil packs with a cyl that never hesitates other than 4 or 7 if it follows change the packs.. I have a diff issue lately a real stumper with the ac on only every 20 seconds at higway speed trys to shift rather violently then back normal then 20 sec again for as long as the ac is on so far im guess somthin in ac but just thought id put out some bait and see if anyone can help me out damn these cars are awesome tho right love mine

Put a dab of di-electric grease on threads of plug, spring connector in boot, and two wire connector at top wipe of excess on two wire connector it will short if not use a artist paint brush to apply there. On the threads it will stop it from seizing, and tearing out the threads, saves big bucks.

Redman, that's funny bc when I got mine the ac blew super hot air no matter what. Watched YouTube video on how to bypass temp control module bc they are crap I these cars and known to cause this issue. It worked and my ac has blown super cold air for about 2 months. In this time starting about 4 days or so, my car started just dieing out of the blue while driving. Always if I've stopped at a light or am just starting to pull off from making a stop. Very random! It gradually has gotten worse! Now at low speeds accelerating usually between 30-50 it shutters or kicks hard! Doesn't stall out as much now but it really struggles and kicks when accelerating at low speeds. If I gun it as soon as I take off I can usually get to my desired speed without car kicking and feeling like it's going to die on me. It's got new MAF, TPS, fuel filter, fuel pump less than a year old, valve seals, oil, tranny fluid with Lucas in it for stop slip (good for old tranny's) i cleaned the ICU already and the injectors, new valve cover gaskets...the car has 201k miles but the engine is a replacement before I got it and only has maybe 56k miles on it. WTF y'all?! And just 3 days ago all of a sudden my account kicked back to blows extremely hot air right about the time that it was really kicking bad on my and I stopped. To restart car. Idk how this could have happened bc I set the module to cold and then disconnected the power from it so it wouldn't switch back to hot!? I've been running circles trying to figure this issue out but I'm just stumped. Also when it started kicking and dying randomly I got a running rich code but that was right after I put an injector cleaner in my tank so I assumed it had something to do with that and didn't pay any mind.. if anyone can point me in the right direction I would so very much appreciate it! Tia!

WHEN YOUR CAR IS UP TO SPEED AND YOU FEEL A LITTLE JERKING....THE CAUSE IS YOUR OVERDRIVE IS CHANGING IN AND OUT AT THAT SPEED.....TURN OFF OVERDRIVE AND IT WILL BE OK.........JIM

2003 Mercury Grand Marquis. 238,000 miles. The car jerks when I gently speed-up from about 50 mph. I have used gasoline additive Lucas 3 different times. No luck. I have purchased full tanks of gasoline from 3 different gas stations. No luck. I have tried not using overdrive. No luck. Replaced fuel filter. No luck. I have opened the hood at night in the dark looking for an arching spark. No luck. I'm trying to determine if it is a coil/spark problem, fuel problem, or transmission problem. Next, fuel pump pressure test. I don't think a fuel pump would give me a 'grace period'. They either work, or are junk.

Dude, spend the 80 bucks, and order all new coils from amazon and buy new spark plugs, then take the 45 minutes and put them in....it'll be like a new car...trust me....I've done that with my girlfriends THREE Merc grand Marquis's....

Will my car keep running with this issue of hesitation. It's a 2005 with 93,000 miles.

Y66GB is right. Most likely, worn out spark plugs have damaged your ignition coils. It might last without correction, but it will only get worse. Replacing them fixed the problem and makes a huge difference.

My 96 Mercury is doing the same crap! I've replaced plugs and plug wires, fuel filter, air filter, and both coil packs. It's starting to get all over my last dang nerve too. I have to let it warm up no less than 10minutes and if I try to floor it she just boggs right down. When I'm on the highway she goes to stuttering between 70-75mph and she will not go past 82mph. So now what do I do? I feel like setting her Ford butt on fire.

The 4.6 engines are prone the the heads cracking around spark plug holes. antifreeze will leak out and short plug and coil out causing the miss fire code,

So I just had my 2004 Grande Marquis checked out by an experienced mechanic and we discovered there was a serious hole that had formed in the vacuum hose just under the plastic cover going to the block. We replaced it and the problem was resolved.

I got a 05 marquis when I stop or go round a curve it hesitates and dnt go pass 20 mph an when I put it in drive it take a few seconds to go

So for an answer to the original question on this thread, I would have to say it has something to do with the idle bypass control valve, an electronic throttle body issue, plugs, wires, individual coil pack problem, bad or dirty injector, or manifold gasket leak which that model is particularly prone to. Any of those or a combination of those is my opinion. I would try to catch it when the light is on and stop at oreillys or auto zone and have them scan it for codes. Missing under a load is a #1 indicator of weak or no fire, i.e. fouled or bad plug. Supposing it has individual coils, it would be cheaper to to replace the plugs and wires first probably, and could only help the situation. If the threads are bad like someone mentioned you'll know when you pull the plugs. Check the condition of the plugs and see if they're fouled or burnt and in what way. Water will leave certain characteristics, as will oil. You could even clean and re-gap the old ones, as long as, they're not completely melted or destroyed. After that I think it's just a guessing game without detailed code analysis and some resistance testing on individual electronic parts. Well that's my two cents anyway. Little Texas, OUT.

Just to to add to repairs, I read that sometimes, dirty fluid, filters in transmission could cause this issue, Made sense because if feels like it’s in middle to rear of car, had it serviced at a transmission shop and made no difference. Transmission shop put the high mike additive in also. The original post is a perfect description, it’s not a mis or a back fire it’s like a shudder, somewhat smooth, but give it a little gas like, your gonna pass someone and goes away or when I take off never a problem, only happens when feathering the gas maintaining speed. Someone mentioned a toque converter sensor, that sounds possible just because it feels like it’s right under you when it does it. I tried Turning off the overdrive also. 100k on it

My 2000 Grand Marquis kept feeling like it was going into overdrive often, even at low speeds when it's not supposed to. A transmission place change the fluid, filter and even replaced engine control module. That did fix that issue, but there have been several other issues that were kind of similar. Had to replace ignition coil on number 5 and then there was a lifter issue on number eight cylinder and runs like a top, except when I come to a stop light, it runs like glass smooth but intermittently has a Miss I guess, kind of even bounces like it's had a plug wire pulled loose or something. "Ford experts" where I live have had it back and forth and back and forth trying to figure it out! That's where I'm at...ideas?

I have a 2004 Marauder that does the same bucking/hesitation at various speeds. I've changed coils, fuel filter and had diagnosis done. Nothing could be found to be the, or a, problem. I have found that if I turn off the traction control, via the traction control switch by the steering column on the Marauders, the hesitation doesn't happen. Leads me to think the traction control programming in the ECU might be involved with the hesitation/bucking that randomly occurs.