PO171 Code with random rough idle

[stkelly]

I have an 08 Mercury Mountaineer with 106K miles and the 4.6L v8.

I've been trying to resolve a rough idle for a little while now. The shudder got bad enough where the truck finally threw PO171. Before I got PO171 I noticed that cylinder 1 was reporting misfires in the real time diagnostics screen.

Here's a quick summary of what I've tried to fix the shudder:
1. PO171 code - Check for vacuum leaks, didn't find any. (Any gotcha vacuum tube locations?)
2. Verified O2 functionality (Sprayed carb cleaner in intake and verified O2 sensors response)
3. Checked fuel trims, bank 1 long term 15-16, bank 2 long term 7-11
4. Moved COPs around, cylinder 1 still misfires. (no misfire codes)
5. Checked Ohms on fuel injectors. All injectors have almost the same result.
6. Changed all spark plugs
---- Problem goes away for about 1,000 miles.
7. Shudder returns
8. PO171 again
9. Checked fuel trims - bank 1 and 2 long term 8-11.
10. Checked real time data, cylinder 1 misfire again (no misfire code again!)

I have no problem when accelerating or light towing. It seems I would have more issues under load if was a compression problem with cylinder 1. Would an exhaust manifold leak cause these issues?

Any suggestions would be greatly appreciated!! I feel like I've tried everything the Internet has to offer and still can't shake this issue.

Thanks in advance!

 

[Flag Gibby]

Check your PCV valve vacuum hose. That code is associated with the PCV valve hose elbow on the 5.4L engines being cracked and leaking and for that matter it seems like just about any of the ford V8 motors throw that code when there is an issue with the PCV valve vacuum hose. If that hose and elbow are good then it is possible for a leaky manifold to cause the issue. See this article for a great rundown of the troubleshooting process: http://repairpal.com/OBD-II-Code-P0171-and-P0174

What bothers me is that your fuel trims long term on both sides are too low and make me wonder if you have a plugged fuel filter (initially bank 2 was lean which indicates to me fuel filter because of the split rail design), but a faulty pcv valve hose could let too much air into the intake and cause a lean mixture but the amount of excess intake air makes me think that the air intake box is not sealed or the MAF sensor is where the issue lies at since both sides are running lean based on your fuel trim readings now. Bottom line, go through the process provided in the link since it is clear that you have a good OBD II reader to be able to follow the instructions provided and get to the bottom of it.

 

[stkelly]

Thanks for the feedback. I forgot to mention that I replaced the PCV valve hose before doing the spark plugs. The original had one broken seal (there are two) where it connected to the valve cover. Honestly I could have broken the seal checking it while trying to find the vac leak.

Initially bank 1 was running leaner than bank 2. Does that still point to possible fuel delivery issue?

I'll go through the list and see if I can find anything else. I'll also try a compression test at this point to see if cylinder 1 is off from the others. When I did change the plugs, spark plug 1 seemed to have the most carbon/oil build up vs all the others.

Here's plug 1:

Here's all the plugs:

They are in order. Upper left is plug 1. Lower left plug is 5.

It's hard to tell with these plugs since I had to use the carbon cleaner to help get them out without breaking.

 

[Flag Gibby]

None of your plugs look bad at all, the buildup on that one is from the occasional misfire that you already know that it has. I really think that you have a leak on the intake side of things (upstream of the MAF sensor) that the MAF sensor cannot compensate for that is throwing the code since both sides now read too low on the fuel trims (initially bank 1 was good and bank 2 was low; the banks need to be running around 15 to be considered good). Maybe try spraying some starter fluid around the PCV valve connections to see if the RPMs change or any other areas where vacuum lines connect in on the intake to see if you can find a new leak. What did you remove and put back on when you changed the plugs, because you went from one side running lean to both sides running lean and the plugs had nothing to do with it. Also, you will be wasting your time running a compression test since you don't indicate that you hear any unusual valve noise when the engine is running.

 

[stkelly]

From reading online it sounds like I want long term fuel trims as close to 0 as possible. Anything above 10% in either direction is considered out of spec. That's why I think I got PO171 on bank 1 initially since it was around 15%. I'm not even sure why I got bank 1 again, considering both banks are about the same number, 8-11 now.

I don't hear any unusual engine noise at load or idle. I do have the typical knocking cam phaser at hot idle. But you have to be outside the car to hear it.

I only removed the COPs and plugs when I did the spark plug change. I'll keep digging for an intake leak. And try spraying the MAF sensor.

Thanks for the feedback!

 

[fritz24]

What is your initial reading on STFT on both banks when you got 8-11 on LTFT? So adding those fuel trims together will determine which banks is running lean. Have you confirm the lean condition while increasing the rpm? Basically if the fuel trim goes near zero then it is a vacumm leak but if it goes even higher, you are looking at a fuel delivery issue. A clogged fuel injector.

 

[Tech By Trade]

did you replace the coils when you did the plugs? What type of plugs did you use?

 

[Flag Gibby]

From reading online it sounds like I want long term fuel trims as close to 0 as possible. Anything above 10% in either direction is considered out of spec. That's why I think I got PO171 on bank 1 initially since it was around 15%. I'm not even sure why I got bank 1 again, considering both banks are about the same number, 8-11 now.

I don't hear any unusual engine noise at load or idle. I do have the typical knocking cam phaser at hot idle. But you have to be outside the car to hear it.

I only removed the COPs and plugs when I did the spark plug change. I'll keep digging for an intake leak. And try spraying the MAF sensor.

Thanks for the feedback!

Yeah sorry about that, my brain was seeing air/fuel ratios when I was reading long term fuel trims. See the diagnostic tip at the bottom of this link: http://www.easterncatalytic.com/education/tech-tips/fuel-trim-can-be-a-valuable-diagnostic-tool/

 

[stkelly]

So the truck started shaking again at idle pretty badly. I have two pending codes P0171 again and Cylinder 1 misfire.

I checked the fuel trims at idle.
Bank 1 short + long: 20% (too high!)
Bank 2 short + long: 10%

At 2500 RPM:
Bank 1 short + long: 12.4%
Bank 2 short + long: 9.2%

So this points to a possible vacuum leak. Problem is I really can't find one. I've sprayed the motor down. I've double checked every place a hose connects. I've replaced the PCV hose. I may have to cry uncle and have a shop take a look.

I'm still concerned about cylinder 1 still misfiring. I really why it's constantly having issues. I've swapped plugs and coil locations and the misfire stays with that cylinder.

I didn't replace the coils when I did the spark plug changed. The coils were already replaced at 63,700 miles. I replaced them with the Accel coils.

Thanks for the suggestions so far.

 

[Flag Gibby]

Well, I would definitely start looking the intake over for any cracks and put smoke in it to see where it is leaking at. Also, I would take a look at the wire that runs to the #1 COP and trace it all the way back to the controller and make sure that there are no issues with the connectors or the wire at all. Bank 2 definitely is border line on running lean (but the values are consistent) and throwing a code, but Bank 1 has the bigger issue. To me you definitely have a leak that is impacting bank 1 but not bank 2 and part of that may be compounded by the misfire on cylinder 1 since when the misfire goes away you have no code. BTW you never did say when the last time you changed the fuel filter was.

 

[stkelly]

I changed the fuel filter at 64,000 miles. I did a bunch of general tuneup stuff right at that milage. Plugs, coils, oil, gear oil, fuel filter, new radiator and antifreeze. I'll go ahead and replace the fuel filter again. It's pretty cheap and easy to do.

Other than the random shake at idle the truck drives great. It never has any issues under load or hard acceleration. Another interesting item, whenever I clear the ECM the truck idles perfect. After it starts leaning the fuel trims is when things start going down hill. It even seems to stop misfiring after the reset. However, I do know the misfire monitor doesn't work for a bit after an ECM reset.

Last night I sprayed the Mass Air Flow sensor with MAF cleaner. Drove 30 miles one way. The whole way there, the fuel trim numbers were all over the place for bank 1. On the drive back the numbers were much more behaved. Bank 1 started to come under 10 when adding short and long. By the time I was home, bank 1 stayed under 10 at idle.

That's the thing, every time I change or do something the fuel trim numbers seem to get better for a short while. Within 100 miles or so they're back to all over the place for bank 1. Same thing happened when I changed the plugs. I reset the ECM, fuel trims and misfire data were good for almost 1000 miles.

I'll try tracing the COP cable like you suggest. I'll also get a smoke tester. Seems like a good investment at this point.

Thanks again for the feedback!

 

[stkelly]

Wow, just found out how expensive smoke machines are. So I'll be home growing one instead!

 

[Flag Gibby]

Your fuel filter should be changed every 30k miles so it is definitely past due if you are at 106k miles now. Yeah, a home grown smoke generator is the way to go. Also, when you change something don't reset the ECM because you want to see if the system self corrects on its own; that way you can tell what was causing the issue. Also, if you cannot find any leaks after running smoke through it pull the valve cover over cylinder #1-4 with the pcv valve seated/mounted in it and verify that it is working properly and not sticking since that one originally had a bad seal on it.

 

[stkelly]

Finally replaced the fuel filter and checked the compression on cylinder 1. I got 160. I was going to check all the cylinders but stopped when the next cylinder I checked was 160 as well. So the cylinder 1 misfire has nothing to do with lean condition. No change on fuel trims with the fuel filter.

Once I've made a smoke machine I'll see if I can find this elusive vacuum leak.

 

[stkelly]

I still haven't had a chance to home grow a smoke machine. But I looked at my vacuum system in more depth. I am pulling 23 Hg at idle with no A/C and 21 Hg with A/C. The vacuum responds exactly like it should when I rev the engine and drive around (I taped the meter to my windshield!).

I'm wondering about my catalytic converter. I charted my fuel trims and O2 sensors. The second O2 sensor on bank 1 seems way out. Not sure if it's something else causing the reading to be so bad. Here's the charted data over the same time period:

O2 Sensors:

Here's my fuel trims:

Engine RPM:

Thanks again for the input!

 

[fritz24]

Your not looking for a vacuum leak. I can suggest base on the fuel trim that it is a fuel delivery issue. Could just be a loose connector of one of the injector in bank 1 or a clogged injector an try mixing a bottle of injector cleaner in your fuel tank like stp multipurpose or techtron as per your choice.

 

[stkelly]

Thanks for the feedback! I was really pulling my hair out on the vacuum system. It really didn't seem like a vacuum leak. It's amazing how much more you can glean from a chart vs looking at numbers.

Cylinder 1 has had a constant random misfire issue. The spark plug is new, the COPs have been moved around and compression is good in that cylinder.

At this point I'm going to assume the fuel injector for that cylinder is flaking out. I ohmed the injectors a while back and they all read the same number so I thought I ruled out the injectors. But it's such an intermittent problem that maybe it's reading fine most of the time. I've already run BG44K through the system. I'll try another can, if that doesn't change anything I'll replace that injector.

Thanks again for the input!

 

[stkelly]

Finally Found the Leak!

So I finally found the vacuum leak! I would have never thought to look where I ended up finding them. I've sprayed down the intake manifold a few times but never on the very back by the two adjustment arms.

Question is now, how did both sides fail at the same time? I don't see a replaceable seal for those arms so it looks like I may have to replace the whole intake manifold. Is there maybe another seal internally that failed? The O2 sensors peg rich as soon as I spray those arms. The engine starts to stutter as well.

Here's a stock intake manifold picture where I highlighted where the leak is happening:

Here's a picture of the two sides in my truck pointing out where it's leaking:

Bank 1 Side:

Bank 2 Side:

Thanks again for all of the help and feedback so far!

 

[Flag Gibby]

Glad you finally tracked the leak down. I would just silicone those areas and see if that takes care of the issue and maybe check all the mounting bolts on it to make sure they are tight before using silicone on the problem areas.
Also, each intake does have it's own o-ring/gasket and most automotive stores carry the manifold gasket sets.

 

[BootyDo]

If you don't mind sharing, what did you use to plot those charts?