I have a 1998 F150 5.4L 4WD with 192k mi on it. Recently ( suddenly ) the vehicle began shuttering pretty alerting under light load at 35 - 45mph. The engine idle was intermittently smooth then rough.
I quickly drove the truck home and ran a Trouble Code check with my Autoxray EZ-scan 4000 and found the two trouble codes P0302 and P0171. I have checked for vacuum hose cracks and found none.
Replaced the fuel filter, PCV valve, and cleaned the MAF sensor ( dirty ).
Right after cleaning the MAF sensor the idle remained smooth - but stumbling immediately when put in drive. Driving at 35-45mph still exhibits intermittent shuttering and occasional very short bursts of what feel like complete engine shut-down. This all is evident under light to moderate load .
At this point what would you recommend ? Replacing the O2 sensor bank #1 ? Or is there a possibility maybe the EGR valve is faulty - or another vacuum valve somewhere ? Intake manifold leak ?
The severe stumbling and very short complete engine shut downs would indicate that maybe the computer is failing or it's a fuel delivery issue ?
Any suggestions on what rational steps to take next ?

 

Th po171 is likely caused by the po302 as it is a misfire on the same bank.
Get rid of the misfire first and clear the codes
The misfire and the problems you describe are pointing to the coil. Maybe the sparkplug but my guess is the coil
You can swap another coil to see if it migrates or stays with #2cylinder
Good luck and keep us posted

 

your symptoms are indicating a missfire on # 2 cylinder (PO302)...this is in all likely hood this is due to a week COP ( coil on plug)...maybe a bad spark plug.

yes the pO171 is indicating a vacuum leak...prime suspects here are the elbow on the rear, or back side of the intake that leads to the PCV...or the vacuum block and lines to the heating controls, behind the battery. Vacuum leaks can be a bugger to trace down...and any of them lines can do it.

as far as the codes go, did you clear them after doing the repairs that you did??

did you replace the PCV with a Motorcraft unit? aftermarket ones tend to not seal as well and still leak or cause restriction that will trigger issues too.

How are your brakes?? a bit spunggy? possibly

All in all , don't think that changing the O2 sensor is going to help with that code...but at 192k and if they hadn't been changed, it certainly couldn't hurt to change both front O2 sensors while you're at it.

the only other thing is that if you have Not had the whole throttle body housing off...it might be worth your time to do so...there are a couple of internal EGR ports that can get clogged up...give it a thorough cleaning.

can your scanner read "mode 6"?


Yeah what Vapar said^^^

 

Fix #2 missfire first . Then clear codes . Drive and ck fuel trims . They should be + or- 5% . If fuel trim is still High on bank 1 , ck for vac leaks , plugged injectors , ect . Also ck downstream o2 readings as opposed to upstream to make sure you do not have a bad o2 sensor .

 

The OX sensor is the only way the system has to 'detect and report' the trouble.
It's not the cause of the problem.
The 171 code is the result of a fuel table being shifted out of limits.
Because one cylinder is missfiring.
Try to see the cause and effect for reporting the codes.
First, cylinder 2 goes into missfire, the OX sensor for that bank detects it, PCM shifts the fuel table trying to account for it, then PCM diagnostics sets the code for the table shift.
That's the 2 codes. One is the cause, the second is the effect.
They are interelated in a control loop system for both dynamic fuel control and diagnostic operation..
Nice to have a computer but you need to understand what your looking at, and understand the operation if it is to have any value.
Good luck.

 

Thank you all ! A lot of good info to look into .
I'm gonna do this step by step and test drive with the real-time scanner in hand.
I'll get back to ya after I get plug #2 changed first - then the coil.
So far after doing the MAF clean and fuel filter I can't get it to throw a code - but it's still like a "bucking bronco" between 35-45 and pulling hills ..........

 

If under 15% fuel level missfire monitors may be turned off .

 

changed the #2 sparkplug - no difference in performance - still "bucking" badly under light-moderate load especially 35 - 45mph. Idle alright.
OHM Tested cyl # 1-3 COPs, and Injectors - all look good. Inspected #1&2 COP for boot tear or spark break-through .. they look good.
Called Ford Dealer today to ask about their Injector Cleaning process - it's $120 and uses some strong chemicals that dissolve carbon in the injectors. But the service manager said to try other things first - he said they seldom replace bad injectors. He suggested the most common problem is COP boot breakdown or sparkplugs. He said to swap the COP's around to see if the trouble code follows ( as others here suggested )
This service advisor also suggested using 2 cans of TerraClean in the tank for injector cleaning......... sounds kinda scary !
I have a new COP coming I'm gonna try in the next few days when arrives.
I'm also gonna do a scanner "capture" during the bucking and bring it back here for you guys to speculate upon .....

 

Just curious, but what did your 3 COPs read on the primary side and on the secondary side?

Ohm testing will only tell if your COP is not bad. It won't verify if the COP is good. To do that you need to load it with current and watch the waveform with an oscilloscope. Then you will be able to see if it is breaking down under a load.

What did you find when you did what he (and others here) have suggested?

 

As pdq says,that test will not verify that the coil is good

 

PROBLEM SOLVED !
Last nite I pulled the # 1&2 COP's out to test them for Ohms - they read 8k ohms across the two connectors.
While I had them out - I applied generous amounts of dielectric connector grease to the insides of the COP boots with a Q-tip and made sure the spring loaded contact was clean.
Today first test drive - "VOALA" the truck idled perfectly and ran like a Champ in light, moderate and heavy load driving. I finished off an hour later driving a steep mountain pass under constant load with no problems !

So, Here's my take on this one - the COP boot on #2 ( and maybe #1 ) were leaking ignition spark down the side of the plug insulator or through the boot itself ( even though no evidence of electrical penetration was evident ) Contributing factor may be supressor sparkplugs installed 4 years ago have enough resistance now to enhance this spark leakage.
I assume the PCM sees the lean mixture due to the misfire and overcompensates with additional fuel swing - which causes the severe "bucking" at light to moderate load.

I want to thank all of you who gave me the incredibly great input ! This website 'Rocks" !!
PS - If anything changes and the problem manifests again - I'll be right back here for more fun ......... in the mean time - I plan on pulling all the COP's and coating the insides with the dielectric grease - and maybe a sparkplug change as well.
Thank you All !

 

Just coment:
If the boot tips are buldged oversize or there is a spark caused pin hole you can't see, the issue may come back.
At least you pinpointed the issue.
The grease loses much of it's effectiveness in a short time after the first heat cycle and tends to harden somewhat.
If you can look at mode 6, test 53, you can track when any rise in missfire counts occurr, as an early warning the grease is not holding off a leakage path from the boot.
Good luck.

 

Thank you Bluegrass . Yea, I realize the COP needs to be replaced. You're right - it won't take long for the effect of the new grease coating to wear off and be experiencing the same issue. I'm gonna replace the COP in a few days when the new one arrives. Thanks again for the help !

 

I'll get back to ya after I get plug #2 changed first - then the coil.

 

Ok, got the #2 Plug and Coil replaced, the MAF sensor was cleaned, new PCV valve, fuel filter. And the vehicle runs smooth and normal until it's warmed up then has a slight stumble under load at 40-50mph overdrive. It also has lost power coming up a steep mountain grade - indicating there's misfiring or fuel delivery problems. Most of the time sounds very smooth at idle. Around town engine often sounds perfect.....
I did a "Capture" on my EZ-Scan - engine well warmed - very slight hill :
Load Value 59.6% , Coolant Temp 197 degrees, Short Term Fuel 1 (- 5.5%)
Long Term Fuel 1 (+ 1.5%), Short Term Fuel 2 (-6.3%), Long Term Fuel 2 ( 0.0%)
Vehicle Speed 47MPH, Short Term Fuel Trim 1 ( -7%), OS 2 Bank 1 ( 0.705 V ), OS 1 Bank 2 (0.650 V )

My thought was the next step is wait till it stumbles at idle and pull COP connectors off one by one ...... or could the bank 1 oxygen sensor be getting faulty and intermittent ? Should I just have a beer and sit on the tailgate and think about it ?
SO, anyone care to take a stab at those capture readings ?