Partock

I've tried about everything to fix this issue and I'm about stumped. My 2006 f150 4.6 with 181,000 miles mildly bucks when maintaining between 45 and 50 on a level surface. Feels like it's trying to slip/buck when coasting down from 50 MPH. It didn't start until I had an engine mount replaced and had never done it prior to that and I've spent a lot of money attempting to fix it. I know this is a long story but here's what happened.

Took it into the shop for a coolant flush, belt replacement and a complaint about the font end rattling. Found a worn out engine mount and had that replaced with the other work. after that, the truck would buck really bad when maintaining 45-50. Took it back and found all the COPs were worn out. (the COPs had about 95,000 miles on them and were due anyway) I had all the COPs replaced and it ran better, but still bucked. took it back and they tested all of the Coil packs as good but low compression on three cylinders. Note, NOT burning oil or using oil nor blowing smoke. They ran BG engine restoration on it and claimed to get more compression. Truck runs great but still bucks some. They suggested a new engine and I gave them the big NOPE. Took it to another place and they suggested the transmission was going out. Trans had a bad shift in passing gear for a while and it had about 85,000 miles on it. Rebuild the trans and still bucked, but it's mild. Decided to replace the throttle body since it was original equipment and did have some low idle issues. Truck idles so smooth that it feels like the engine died. RUNS GREAT, feels great, still bucks and I'm tired of spending money on it.
I would be eternally grateful for anybody that might have a suggestion and sorry for the long first post.
I've been reading suggestions on here for a while so I know this group has some great input.

JCR 56

Have you changed the plugs, I didn't see that listed. Always use Ford /Motorcraft ignition parts.

fordguy2100

Well if you changed the cops with motorcraft and you're sure they aren't getting wet or have a pinched/cracked wire then I'd make sure you swap all the plugs out too. Go with motorcraft. Other than that failing a compression test means your engine is on borrowed time and anything you do to fix this bucking could be for nothing if the dying engine is the cause.

powerranger262

Change your plugs and get a new mechanic.

Partock

Plugs have been done and I make sure to use motorcraft products.
As for low compression, would that cause some of the bucking issues?
I'm planning on buying something new in a few months and handing that truck over to my teenager. Just about fixed everything else.

Troyl321

Vacuum hose to intake might have come off. Trace the vacuum hose that connects to the brake booster to the rear drivers side of the engine. You can access it from under the truck but it's not easy. The hose is known wear and leak or disconnect and causes alot of issues . In my opinion that's the first place I would look if my truck starts to run like crap.

Partock

Thanks. I'll do that next.
Since my last post, I've replace the fuel filter, the fuel pump driver module and the last motor mount. The truck feels GREAT, except for that shudder that doesn't happen under acceleration.

redfishtd

That is a typical misfire you just haven't found it yet . Did you change boots ,that's where most people fail . Changing cops is not a maint function OEMs rarely go bad they just get corroded clean them . Oems cost like $50 each aftermarkets are like $6 so does that make sense . I never had any luck with them . I have 7 oem and one guaranteed duralast after 201k . I probably could have cleaned that one but I was out of town broke down .All connectors need cleaned and must lock on .No junk oil etc in hole .
Magic speed is where rpms low,od on , engine lugging under load misfires show up . A hill will help show them up . All plugs are to be torqued 25 to 28 foot pounds not by feel if not you can get blowby which will eat plug and may launch plug and cop taking threads with it . Loose plugs are common .

kAptnSp0ck

Did you have a full tranny rebuild?

ShirBlackspots

Since the 4.6 is similar to the 5.4L, the videos I watched regarding bucking under light acceleration at 45-50 is because the spark plugs need replacing. I had the same, but very minor problem, after I changed my plugs around 87,000 miles, it went away.