Rough Idle and Fuel Trim issues

Sam Mechanic Missouri Posted   Latest   Edited  
Resolved
Driveability
2004 Ford F-150 Heritage XL 4.6L (W) 4-spd (4R70W)
P0171 — System Too Lean Bank 1
Rough Idle

So we got this F150 in and customer had said it had a rough idle and CEL. First time reading codes I had a P0175(Rich B2). Watching LT and ST fuel trims B2 was -25 and B1 was +25. I checked purge and clamped off, no change. Watched B2S1 O2 and it seemed to stick sometimes so we substituted a known good and no change. My boss got to looking at it and found a weak spark on cyl 6. So we put one coil and spark plugs on it. The B2 trim evened out to within 5% of 0 LT and ST. Still having a climbing lean trim on B1. Smoke test and found a vacuum line broken and a small leak at the bottom of intake. Fixed that and inspected intake runner and valves for carbon buildup while I had it off. Also went ahead and moved all of B1 injectors over to B2. Still having lean trim. Did a cylinder compression test and every one had 170-180 except had cyl. 1 shoot up to 250 psi three different times. Thinking an exhaust valve isn't opening… Removed valve cover and I did a compression reading and cyl 1 was back down to 180 psi, worried it was a lifter problem we replaced the exhaust valve lifter and inspected rocker-which felt smooth and no flat or hard spots. Put it back together and ran fine (no rough idle but fuel trim was up)until engine reached about 170F. Also checked backpressure at B1 and has less than 1 psi at idle and about 2.5 at 3K. Probed B1S1 at signal wire and datastream matches meter at probe. Ended up substituting B1 O2 with known good and no change in symptoms. No exhaust leak between manifold and O2. Everything plugged(PCV, purge, booster, egr, FPR and no change in trim. Trying to pursue the next path in diagnosis. Not hearing any valvetrain noise but we could have another lifter collapsing maybe I can try hot compression test. Not hearing any timing chain slap but may need to make sure it is in time. Engine runs smooth and has power the only issue is the p0171 and rough idle more pronounced in gear.

Also B1 upstream range is .2-.8 and B2 range is .2-.6 maybe a compensation computer is making for trim but that would mean it is richer than b2 at a +25 enrichment- engine protection logic or overcompensating

0
Enrique Diagnostician
California
Enrique
 

When at first you said extremely positive on one bank and very negative on the other the first thing that came to mind was a few cases where the O2 sensors where swapped. might be worth taking a look. although you seemed to take care to the negative trim on bank 1. just a thought.

0
Ð Awarded
Sergio Instructor
Florida
Sergio
 

Unplug upstream HO2s either B1 or B2, with the scanner read which bank in inactive.

0
Ð Awarded
Sam Mechanic
Missouri
Sam
 

Yep B1 O2 is B1 on data confirmed

0
Ð Awarded
Sergio Instructor
Florida
Sergio
 

cat Converted clogged !!! be careful if you use a back pressure gage, it will pass away if the engine runs with an almost totally clogged exhaust System. Just keep the engine from starting while checking the exh back pressure.

0
Ð Awarded
Sam Mechanic
Missouri
Sam
 

Checked with back pressure gauge at idle already , you think .5 psi or less would be enough to need digital pressure transducer to check with scope? Not sure if we are gonna hit the head on the nail without advanced scope, we have a 4 channel but no special sensors

0
Ð Awarded
Sergio Instructor
Florida
Sergio
 

Both banks? Or just one ? 1 psi is clogged. You can practice the back pressure gage test on a known good car. It should be “0” cero psi. Did you rev the engine up during the test ?

0
Ð Awarded
Jason Technician
Illinois
Jason
 

1 psi of back pressure on a converter is normal. Sure zero is ideal but one psi will cause no issues. when you start getting to 3 and up … then it’s got issues

0
Ð Awarded
Darren Technician
Wisconsin
Darren
 

Just thinking out loud… what’s the best way to check injector flow for all injectors , in the car. ?

0
Ð Awarded
Cody Diagnostician
Illinois
Cody
 

I went to a class with john thornton and he had a case study that acted similar. Ended up being one of the cats was restricted. automotiveseminars​.​com/free-training…

+1
Ð Awarded
Agree
Sam Mechanic
Missouri
Sam
 

Did have 0-.5 psi on B1 at idle

0
Ð Awarded
Cody Diagnostician
Illinois
Cody
 

That case study is on his free training video page, its' the lower one. I'd try an in-cylinder pressure transducer if you had you to replicate his test. Too bad you couldn't swap the cats to see if it moved. Other than camshaft timing being out I'd look into exhaust backpressure further.

0
Ð Awarded
Cody Diagnostician
Illinois
Cody
 

I see you tested back pressure on bank 1, have you checked it on bank 2?

0
Ð Awarded
Eric Owner/Technician
Wisconsin
Eric
   

Idle is not the best time to check exhaust back pressure, an idling engine does not have enough air flow out the exhaust to show a restriction. If you did find a restriction at idle it is likely the engine wouldn't even start. You need to connect your back pressure gauge, start the engine and perform a quick snap throttle test, at least to 3,000 rpm. A snap throttle will increase air flow…

0
Ð Awarded
Sam Mechanic
Missouri
Sam
 

I did hold steady at 2500 to 3k and about 2 psi which according to my tool procedures is in range for v8, I have read good running vehicles with same back pressure, maybe Monday I’ll try snapping the throttle to see and check under load, I have verified o2 location B1 component responds to B1 data

0
Ð Awarded
Karl Diagnostician
Massachusetts
Karl
 

I would verify cylinder pressure when exhaust valve opens if you have tooling to do so for in cylinder diag. Or even a relative compression test. It is possible for timing to be off on bank 1. The pcm is only monitoring bank 2. Your fuel trims are screaming a fuel imbalance . All or most of the air the engine is taking in is going through bank 2 causing it to be lean. Bank 1 is rich becuase it…

0
Ð Awarded
Stephen Owner/Technician
California
Stephen
   

Your fuel trims are complete opposite. Check that the O2 sensor harness wasnt swapped from either side. The harness connectors will reach, and its an easy mistake to do. Wiring diagram will tell you which is left and right. Sorry, I just caught your post that you checked that

0
Ð Awarded
Richard Technician
New York
Richard
 

I agree with Karl, either the timing is off or you have an exhaust restriction. A relative compression test would be a good starting point. If you can road test it go WOT and see if you hear the cats being clogged or feel a large restriction.

+1
Ð Awarded
Agree
Thanks
Bentley Manager
California
Bentley
 

Did you do a VE test. I think that would point toward timing, exhaust restriction or take it off the table.

0
Ð Awarded
Enrique Diagnostician
California
Enrique
 

Don't just check the front sensors, the rear ones will affect fuel trim too.

0
Ð Awarded
Sam Mechanic
Missouri
Sam
 

According to shop manual on this specifically ford says downstream is only catalyst monitor, however just for fun I unplugged and had no change to trim

0
Ð Awarded
Jesse Owner/Technician
Pennsylvania
Jesse
 

Hi, does anyone use the ats escan on this type of problem?

0
Ð Awarded
Darren Technician
Wisconsin
Darren
 

Yes, quite often

0
Ð Awarded
Mike Mobile Technician
Illinois
Mike
   

B2 cat is clogged, try a relative compression test also

0
Ð Awarded
Sam Mechanic
Missouri
Sam
 

Checked today B2 backpressure matches B1, idle(.25Psi), 2500(1psi), load @WOT(1psi), snap WOT(2psi) all at or under 2psi Ordered Maxiscope yesterday(we only had old low resolution scope with few functions), but by “ear” relative compression does sound a little off. Customer wanted vehicle back in the mean time I'll have to bench this until he lends it to us again, fuel trims are back to +20 B1…

0
Ð Awarded
Sam Mechanic
Missouri
Sam
 

Truck came back in today, primary and secondary waveforms looked good on scope, relative compression is even across but when doing cam crank correlation I've got 7 teeth between cam/crank and the only known good i could find on 4.6 was a lincoln town car which had 6 teeth between. Can someone confirm this has jumped if you have known good?

0
Ð Awarded
Eric Owner/Technician
Wisconsin
Eric
 

Here is a known good for a 2006 F-150 4.6L from my files. diag​.​net/file/f6fy1bzef… ​

0
Ð Awarded
Darren Technician
Wisconsin
Darren
 

If you can’t go in cylinder…. try cranking vacuum

0
Ð Awarded
Sam Mechanic
Missouri
Sam Resolution
 

After pulling cam crank correlation waveform I suspected a slipped chain, and confirmed with a known good waveform. Pulled valve covers and bank 1 is in time but bank 2 is about 1.5 teeth off. This engine only reads can position on bank 2. Noticed the outer chain guide on bank 2 is broken away from the bolt causing the chain to be loose on that side if you can see in picture (one chain per bank…

+1
Resolution Ð Awarded
Stephen Owner/Technician
California
Stephen
 

Nice job! Thanks for posting resolution

0
Ð Awarded
Bentley Manager
California
Bentley
 

Thanks for posting the resolution!!

0
Ð Awarded