I have a 98 f-150 with the 4.6L . In the past few weeks I have been dealing with a issue that I cant solve! When its idling it shakes ,not alot but enough to notice.Almost like a misfire but throws no misfire codes. I replaced the plugs and wires with motorcraft plugs and 9mm motercraft wires and 1 coil pack. I replaced just one because weeks ago I got a cylinder 6 misfire code.Also replaced both downstream O2 sensors and cleaned out the IAC valve and throttle body.Also replaced map sensor with OE replacement. The damn thing still does it.The cylinder 6 misfire came up once after about 500 miles or so again.I ohmed the injectors,all 8 at .013. # 6 came at .014 but went to .013 after about 1/2 a second,so I think the injectors are good. What the hell can it be? The only code it ever had was cylinder 6 misfire,it came up 3 times in the past.HELP!!! I like my trucks to run perfect,so this is driving me crazy

 

Do you use fuel injector cleaner in the fuel tank ? If you have not used any cleaner regularily,you might want to clean the injectors with a cleaning kit(a can of fuel with a cleaner,a hose with adapter for the schrader fitting on the fuel rail). Have you checked compression on all cylinders ? AS a wag,you might want to reset the pcm(disconnect neg battery cable for approximately 30 minutes).

 

I have used cleaners,I cleaned it out with seafoam thru the vac lines and tank.Alos I have a mac scanner,the big dog! Cleared codes and also unpluged the battery for about 2 hrs when I replaced the battery last week.Compression is all ok on both banks,all within 5psi of each other.No coolant or oil loss.And gas milage is ok,about 250 to the tank,it has been getting less but it has been quite cold lately and that affects the milage alone.

 

If if only occurs at idle, I would start with replacing the IAC, they can be cleaned but generally replacing has been the concensus on the best approach. When was the last time the fuel filter was replaced? I have the same issues as you, so those are going to be my first fixes, stopping at the auto parts store on the way home to get them!

 

I replaced the fuel filter 2 months ago,it was pretty nasty. Its just driving me insane,the rpm's dont even budge.just you can feel the lope thru the floor,just like a misfire.

 

Have you checked the vacuum lines for cracks/leaks?

 

No, thats next.Its -32 deg now here in this cold *** tundra we call Minnesota and my garage is hostage to my bike and current car project .Does anyone know what the vacum should read out to on a vacum guage? I heard 20-25 psi on vacum is good unless it dives up or down.But there are no leaks or cracks on the lines on the left side(driver).I checked those,but next is behind the throttle body vac. lines,but too cold to dig in now. But there seems to be no vac leak sounds anywhere.I have been wrenching for years and know what too look and listen for,and this has me pulling my hair.Although,Ford rocks cause she still fired right up in the sub zero tems after sitting for 2 days.Unlike my Grand *****,and had to jump the neighbors Chev truck.Haha buy a ford!

 

OK ,got the code again. Cylinder 6 misfire.Is it possible to have a bad plug? Someone sugested a fuel system cleaning. It still has the rough idle,like a shudder when idling only,in gear or in park. I have replaced all plugs,wires,fuel filter,air filter,MAF sensor,Both rear O2 sensors,pcv valve, one coil(cyl2,4,6,8),tried every pour in injector cleaner,seafoam thru vac lines,cleaned throttle body until spotless,cleaned IAC valve,reset computer,reset with the mother of scanners,used preimum,mid,and regular gas.Still no change.What the hell is it? I would be most gratefull if someone can help diagnose this. I try so hard to avoid the stealership,they want 200.00 to diag. Unless I drive it 65 miles to the cities then its 95.00. Im not chasing a ghost,cause I can feel the stutter in the idle.Other than the shudder at idle,it runs like its brand new,and i seem to have lost 5-8 mpg since this issue has come around.

 

Not very familiar with the 98's but if you think you have a bad plug (which is possible) just get another one and see what happens. Are you sure when you swapped the plug wires you didnt get any crossed up??? Can be easy to do if you get distracted.

Finally are you sure it's just not running rough cause of the x-treme temps your dealing with this time of year?

Good Luck!

 

I did think that the temps could be a factor,but it started when we were at 20-40 deg,Ill try swaping the plug next and post results

 

Well,replaced that plug,still there. Ohmed the injectors,all the same ...I bout a professional fuel system kit (pro cleen) and flushed the injectors.750.00 later still feel the miss,in fact its warming up(40-50's) and getting a bit worse.But no check engine light. WTF I really need help! Bonus,I have a awesome new tool..The injector cleaning kit works NICE! But what the hell can it be.

 

Im having pretty much the same problem with my 97' that sat for 3 years. Hopefully somone can give us some insight.

 

Ok so last night I used the wideband O2 on this pain in the a$$ and my air fuel is premo! 14.7:1. So that rules out fuel issues and valve/cylinder issues. Now what? Also check engine light has not gone on since I did my fuel system cleaning. Someone has to have some insight! Im too stubbron to go to the dealer. Besides it seems anyone that did go to the dealer still has the problem. I bet half of the shake/misfire issues on this forum are related to this same issue.

 

I haven't had my issues since it has been relatively warm around here, but I would still lean towards a vacuum leak, even a small one. It only happens to you at idle, so I'd try to exhaust this fix next. I think Chester recommended in another thread somewhere, getting a can of starting fluid and spraying it at connections for the vacuum lines one at a time. If you hear the sound of the engine smooth out/go up....you found a leak. It's entirely possible that the hoses are in need of replacement.

 

bumping this thread up, hoping someone would know the answer. I too have been having small rough idles in park and gears. I'll check for vac. leak too....as one of the line to the DPFS sensor was cracked, though I've replaced it since.