Misfire still present after coil/plug replacement

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RyLS

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Hello everyone,

I'm back again here with another round of s*it gone wrong with my LS. This time, I developed a misfire on Cylinder 3, which quickly turned into a random miss and 3 & 4. We replaced the valve cover gasket, and all 8 plugs and coils (with NGK & Motorcraft parts, so, good s*it). I literally just got the car back from my new shop and I can feel her still miss at idle (clears up as soon as I get my foot on it) and I can smell raw fuel in the exhaust. She still hasn't thrown a code at me, unfortunately. So I am left to guess what could it be causing another misfire.

All help is greatly appreciated.

Thank you,
Ry
 
Unfortunately, there are many possibilities here.
My not entirely random guess based on your described symptoms is a possible small intake leak, like maybe some of the o-ring seals between the plastic and metal parts of the intake manifold. If you look at the short term and long term fuel trims using Forscan, that may help point towards or away from this.

If the miss was there all the time, I'd say to look at the connectors to the coils. Maybe still have a look at them since they are old and brittle and easy to damage.
 
joegr said:
possible small intake leak, like maybe some of the o-ring seals between the plastic and metal parts of the intake manifold.
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These are the same guys that did my T. Stat Housing (Jaguar part upgrade, so no mo' problems with that) and they told me that they had to take off the intake manifold for that, which I found odd because it's right there when you pop the engine cover off, but they're the wallet rapists, I mean mechanics, so I didn't question them. Which means that your educated guess has even higher chances of being the culprit.
 
joegr said:
If the miss was there all the time, I'd say to look at the connectors to the coils. Maybe still have a look at them since they are old and brittle and easy to damage.
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Bingo. That would be my first guess. The locking tabs on the harness for the coils and injectors get brittle and break. Then they may come off on their own. Sometimes just enough to disconnect the electrical signal.
 
RyLS said:
These are the same guys that did my T. Stat Housing (Jaguar part upgrade, so no mo' problems with that) and they told me that they had to take off the intake manifold for that,..
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They didn't have the correct crow's foot wrench.
 
04_Sport_LS said:
I think you need to find a different (better) shop to do the work on your car.
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The guy that actually does work on cars actually impressed me. He had the whole intake assembly from the throttle body down to the airbox out in one piece in like, 2-3 minutes.

And they didn't dirty up my car. And they also did the 8 plugs/coils, Valve Cover and T. Stat housing jobs for ~$1450 out the door IIRC.

We'll see how they treat me when I bring it back to them tomorrow with the random miss. Hopefully they'll figure it out.
 
As Joe said... it's also possible that an intake gasket is leaking (sucking air). Especially if they re-used the old gaskets.

They can flatten out and leak... even when torqued to proper specifications.
 
Well, got hooked up on FORScan, and wow. She's not happy.
1632533875133.png

At the very least, she knows she's missing so I'm hoping that the PCM does some corrections to avoid engine damage, but I don't know if this car is that smart.
 
I would go back and check every electrical connector the mechanic unplugged during his nascar "pit stop" removal of the intake.

Especially the coil and injector connectors on cylinders 3&4.

Check all the connectors he disconnected... to make sure a pin didn't get bent or broken... and that all locking tabs actually lock.

Also... there are a couple ground wires that go to the intake. In his haste the mechanic may have missed these.

Lastly, (as a possibility), how old is your battery? As batteries age... they can randomly/suddenly develop an internal short... which can play mind games with computer systems.
 
Very likely a vacuum leak. Easist to find and most overlooked cause of misfires. Will usually get worse as it warms up as the cold enrichment covers up the lean condition.
 
Vee8Guy said:
...Will usually get worse as it warms up as the cold enrichment covers up the lean condition.
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Except for some small ones that will seal off when things warm up and expand...
Fuel trims should tell the story.
 
I have recorded about 30 minutes of driving data. I'll leave the links here if anyone has the time to review it and let me know if anything looks weird.

Just about 5 minutes of idling:


A short drive to my favorite gas station in town:


The drive back home:

 
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In my opinion, the fuel trims look ok...not great. But did you following the directions in the article Joe posted?
"all fuel trim values should be taken at a minimum of at least three steady engine speeds, these being at idle, at about 2500 RPM, and at about 3500 RPM. Note also that fuel trim values should only be taken when the engine had been running at the above speeds for at least 30 seconds."

Taking the car out for a drive probably not a very good way to conduct this type of analysis.

That last graph shows long and short fuel trims in a similar pattern (on the + plus side), are your cats OK? Probably wouldn't hurt to check your air filter (make sure it is clean) and clean the MAF sensor. Have the O2s been replaced or are they still original to the car? When was the last time you used a fuel injector cleaner?

It has been a while since I had to diagnose the fuel system on a car, but for some reason I keep thinking too much air flow (running lean). So, a vacuum leak is plausible. I maybe wrong (and probably am). I am not a professional mechanic, so take this as an educated guess based on my experience. I hope a few others on this forum can provide you with their opinions.
 
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jmeecer said:
In my opinion, the fuel trims look ok...not great. But did you following the directions in the article Joe posted?
"all fuel trim values should be taken at a minimum of at least three steady engine speeds, these being at idle, at about 2500 RPM, and at about 3500 RPM. Note also that fuel trim values should only be taken when the engine had been running at the above speeds for at least 30 seconds."

Taking the car out for a drive probably not a very good way to conduct this type of analysis.

That last graph shows long and short fuel trims in a similar pattern (on the + plus side), are your cats OK? Probably would hurt to check your air filter (make sure it is clean) and clean the MAF sensor. Have the O2s been replaced or are they still original to the car? When was the last time you used a fuel injector cleaner?

It has been a while since I had to diagnose the fuel system on a car, but for some reason I keep thinking too much air flow (running lean). So, a vacuum leak is plausible. I maybe wrong (and probably am). I am not a professional mechanic, so take this as an educated guess based on my experience. I hope a few others on this forum can provide you with their opinions.
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I assume my cats should be ok, since I've only put 93 on it since I've gotten it and it also only has 109k, but, who knows. It's also kinda weird that they would go out at the same time my plugs and coils went. I dont know if O2 sensors have ever been done in this car, but I think that would throw a code if that was the problem. Last time I put a bottle of concentrated Techron was the last time I filled it up fully (about 3 weeks ago). I should probably put another bottle in it, see if it helps.

I just tried getting fuel trim data after revving to 2500 and 3500. 2500 she did fine, but I know the LS has a rev limiter at 3k, so I couldn't do that one. But here is a minute clip of me holding it at ~ 2500 for 30 seconds and another 30 seconds of idling:

 
So that is the fuel trim levels at 1472 average rpms. You need to do two separate tests, no need to worry about the 3500 rpm test due to rev limiter. One test at idle for 30 seconds then record the fuel trim data. Then the second test at 2500 rpm for 30 seconds then record that fuel trim data for that test. I think you will probably get similar numbers to what you posted previously.

No one mentioned the famous cowl leak, does it seem to miss more often when it is raining outside? Intermittent misses are tough to find. Did the car originally have a miss only on 3 and/or 4? Could be a bad new coil or a bad new spark plug, rare but it does happen. Cracked spark plug boot could also caused problems.

"I dont know if O2 sensors have ever been done in this car, but I think that would throw a code if that was the problem." As far as I can remember, the O2 sensors have range that they operate in, they will only throw a code if the computer sees a range outside of that. They can be clogged and still working barely and not throw a code.

I have had pretty good luck find small vacuum leaks using a spray bottle with soap in it. Spray around the intake and any other parts the mechanic took off. I remember my uncle using starting fluid to find vacuum leaks, it worked, but man I am not that crazy.
 
jmeecer said:
No one mentioned the famous cowl leak, does it seem to miss more often when it is raining outside?
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Cowl leak? I dunno nothing about that. What I do know, it's that it seems to do it when it's cold. Once it warms up, it barely does it, but I can still feel that she's just not running right.
jmeecer said:
Did the car originally have a miss only on 3 and/or 4?
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Car developed a misfire on 3, then it went to 3&4, and 6. Which pretty much gave it away that it was a valve cover gasket leak along with bad coils and plugs (last record of a change was 85k on the CarFax)
 
2 weeks later, and oops! She did it again!

I have numerous codes for an EVAP leak, even though my gas cap seems fine. Could this be the reason for my random misfire? (Sidenote: This llittle light o' mine went away after a couple times of me doing a Control Alt Delete and some driving, but I can't help but feel that it's related to my misfire.)

20211020_224311.jpg
 
You may be looking at an evap issue. The fuel system is a closed loop for vapor and there is a solenoid that opens air flow to vent the fumes into the running engine. The system runs tests to confirm that the loop is closed and any rusty fuel piping could release fumes, failing this test. The problem could be corroded lines and a faulty fume release (evap) solenoid. Anything in the piping could be suspect, including the fuel filler piping, or even an unsealed fuel tank (think corrosion again around the pump/sender assembly fittings. Come to think of it, as many times as I have had my V6 opened up under the hood, I can't say that I have ever seen this solenoid in my LS. I have successfully diagnosed and repaired this problem on 2 different subarus. There are some good videos on youtube that show how to diagnose this evap problem on a generic basis, not the LS.
 
Well, it seems that she got tired of the guessing games and finally decided to speak her mind!

1634848718247.png


Back to the dudes that did my coils and plugs she goes!
 
RyLS said:
Well, it seems that she got tired of the guessing games and finally decided to speak her mind!

View attachment 828576597

Back to the dudes that did my coils and plugs she goes!
Click to expand...
Okay now you're getting some where ...I would swap out a #2 plug and COP with the misfiring #1 plug and COP ...does the misfire change to cylinder #2? If so you solved the issue the simple way ...now you'll know it's a improperly gapped plug or bad ignition coil ...if the misfire stays in the #1 cylinder you can go down the other road ...In the past I've kept it simple as possible when trying to diagnose a misfire in any of my vehicles
 
Well, got her back! She's now running smoother than she's ever had during my ownership period. Turns out that water was somehow getting into Coil #1 and fouling it out. My drains were probably clogged because the only spot to park in my driveway is all covered in trees :/.

Now all I gotta do is LCA bushings, brakes, diagnose and fix AC issues, and some cosmetics (mainly reattaching the stupid little headliner clips that came unglued when I had the sunroof open on the highway) and I'll have a mint LS that I know will last me for a while (knock on fake wood).
 
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