my car will miss under low RPM high load (35mph in 4th gear) It never threw a code, but the car will shake\miss. However that is the only time it does that, idle is smooth, WOT no problems. The engine has to misfire a certain amount of times in a given time to set the Check Engine Light.

 

I know this thread is about 8 years old, but man I have to ask, did you ever figure out the cause of this problem? this is the EXACT same thing i’ve been struggling with for months. I’ve replaced a couple emissions components, fuel filter, spark plugs, cleaned MAF and nothing has fixed it. No trouble codes either

 

Some of you might remember my thread where I went on the dyno after a cam install and made **** power. My tuner said it felt like it had a misfire, but wasn't throwing any codes. I changed the spark plugs, and felt a definite increase in power. Before I go back, I want to make sure I don't have any problems.

My car shakes lightly at idle but it only started doing it after the cam install. I figured it's just because of the lope. It doesn't sound or feel like I have a miss. It revs smooth.

However, when I unplug the the harness on any of the coils it doesn't change the idle speed or quality that I can tell. I'm still not throwing any codes, even after removing the wires to a coil pack and letting it idle like that. That makes me think that I still could have a miss and not know it. How can I make sure I'm getting spark to each cylinder?

If you dont feel a drop in the idle when you unplug the coils means its not the coils with the problem. You might want to look for a vacuum leak. I had one on my car. it was real bad at idle but smooth in high rpm. Try cleaning out your MAF, check the gaskets on your intake manifold and plentum, double check all your vacuum lines are not loose. Try these few things and let us know how it sounds. Would be nice to have a video :naughty:

 

If it were a vacuum leak, wouldn't it idle high?

 

SPARK PLUGS

 

I literally just replaced them a few days ago

 

My car blew cop's twice recently and never threw a code. It missed and shook like a bastard.

 

What did you replace them with?

 

I'm not going to start a spark plug debate. The spark plugs are brand new, they're not the problem.

 

My car had a miss for about a week before it finally threw a code. When it did, I replaced the COP and all was good.

 

You replaced all of them, or just the bad one?

 

I replaced just the one that was bad. After about 2 weeks of misfiring, it finally set a code, so I was able to pinpoint which cylinder it was.

 

It kind of shakes at idle, it's hard to tell if there's a problem because the cams are choppy. Maybe it's just me.

The SCT is definitely saying there's random misfires on all cylinders at a rate of maybe one every five seconds, which isn't enough to trip the check engine light

At least the short term fuel trims are equal, which was the problem when I went to the dyno. He said they were 15% off from bank to bank.

The front O2's are good too, oscillating between 0 and .9 volts

 

Yes, COPs can fail causing misfires that don't trigger a CEL. A cylinder contribution test, which is typically only availalbe at a shop.... I use a Snap On Modis. This function allows you to view the output of the coils as the engine runs and greatly aids in identifying COPs. To give you a bit more information on this topic....... One thing that I have found is that identifying one bad COP doesn't always fix the problem. Replace the failing unit, then perform the contribution test again. Quite often I find at least one more COP that has diminished output.

 

What he said. My car ran fine, but under a load had a definite miss. No codes and the unplugging a coil trick didn't work. Fuel pressure was good, everything was fine. It drove me nuts trying to figure it out. finally i took it to a shop and 1hr and $60 later i was told the secondary side of my #5 coil was weak.

 

Yeah I was hoping it didn't have to come to that, but I might need to bring it to a shop so they can hook up their fancy equipment and tell me why I'm an idiot

8 new coils are like 300 bucks too... damnit

 

First I'm gonna do a leakdown test to completely rule out mechanical problems

This is turning into way more of a pain in the ass than I had anticipated

 

Not a bad idea, but my money is still on a COP. In lieu of dropping lots of coin on a full set of Ford/Motorcraft COPs why not try a full set of boots/spring wires? They're only about $50 per set.

 

If it's any consolation, my 262's have a little shake at idle..

 

shake does not equal misfire



misfire does not equal shake

 

Could one bad coil cause the rest of the cylinders to misfire intermittently?

 

I don't have an answer to that question.... but using the cylinder contribution test you'll be able to single out each and every COP individually and visually see what they are doing. In my experience, when running the contribution test and you have only one misfiring COP...... replacing that COP and running the test again will identify another misfiring COP (or more than one). I don't know that one causes the other, but one can mask the other.

 

Interesting. Thanks for your input.

 

Hope the info helps you...........

 

Yup! I had a misfire due to a bad COP. It missed above 3000rpm and WOT. Replaced all 8 cops, ran perfect.

Only code I was throwing was from my offroad X.

 

I don't have much experience with Mustang OBDII systems, since I just got mine.. But I do have experience with older BMW systems. Pertaining to "random" misfires, the computer has a threshold of number of misfires over time before it would actually illuminate the CEL. Before it would, you could scan the computer and pull out shadow codes or pending codes if you had the right scanner. If it were a single cylinder, say a failed COP, you would get a fairly immediate CEL with fuel cutoff for that cylinder. So, under that type of system you could theoretically have random misfires without a CEL. I would take a look at your fuel trims, or maybe try a smoke test or spray around a can of carb cleaner to try to rule out a vacuum leak. The cam might exacerbate a rough idle due to a vacuum leak, but I'm not really sure of that. Start with the simple things and work your way up.

 

I've got an SCT tuner that I can datalog with and pull real time data from the car's computer, I'm not just pulling codes. You're right, there is a misfire threshold before the CEL lights up. It's showing random misfires on all cylinders, at a rate of 1 every couple seconds which could be robbing me of power but not enough to trip a CEL.

 

If anyone knows their ****, I can do some test runs and send them the live link file

 

Pulled the plugs after less than 100 miles, they all pretty much look like this

They're all dry, with an off-white on the grounding strap that changes color right at the 90* bend, and sort of a half-ring of dark brown around the lip where the threads start. The porcelain is spotless.

 

Am I correct in thinking that they look pretty good?

 

That plug is fine.

Ive had a random misfire at idle on my 5.4 3v since i bought it. Id bring it to work to test somethings and it wouldnt misfire, next time it would. 100% PITA

Sent from my droid using my fingers.

 

Just fixed my misfire. Clean your MAF and check your vacuum lines. I pulled all of mine out and inspected them. Try unplugging all of your cops one by one. see if there is a change is idle. I really think you have a vacuum leak... if anything replace all your vac lines its cheap and will ease you mind. Lmk if anything of this worked for you.

 

Roger. I'll check vacuum and clean the MAF next, then I'm just gonna bite the bullet and go get the COPs tested the right way. I'm pulling the coil plugs off one by one, and they all do something, but some change the idle more than others.

 

Cold, dry compression check results:

Drivers side:

150, 150, 150, 140

Passenger side:

150, 140, 150, 140

If I did a test after running the engine for a while to expand everything, I'm sure they would jump up a good 10psi


Looks pretty good. At least I know the valves didn't touch.

 

I took the boots off all of the coils and cleaned them, then put them back together with some grease around the bottom of the boot. It misfired like a bitch all the way to work, CEL came on, then the misfire went away as the engine warmed up.

Pulled 2 codes: P0356 and P0306

Coil primary/secondary circuit malfunction cyl 6
Misfire on cyl 6

Pretty sure I found my problem. Gonna get some new Motorcraft OEM coils.