Random miss at idle, new plugs & coil packs

[SeaCruise]

1999 LX with 137k miles. I finally had my coil packs start to fail (two in three months Sept of 2022 - November 2022), so when I went to fix the second one I decided to just replace all of them, along with the plugs since they were due for replacement anyway. I used OEM plugs from Lexus, and purchased Denso coil packs from a third party for $60 each. I didn't gap the plugs (was told it's unnecessary when purchasing OEM), and the new coil packs look just like the old ones, have the Denso markings, etc. (I always try to stay OEM so I can avoid annoying issues like this, haha).

First week the truck ran like normal, then it started doing this annoying miss or hesitation at idle. It wasn't all the time (not anything like a misfire with a failed coil pack), and it isn't anything that triggers the CEL either. Okay I figured, let's start simple: I cleaned the throttle body and MAP sensor, and ran some BG 44K fuel system cleaner through it on a full tank of gas. Seemed to do great after that - definitely noticed better performance, along with seeming to cure the random idle misfire. Two weeks later, the random miss at idle came back. Since they were original, I went ahead and replaced the PCV valve and the hose, did a long test drive, ran fine. By the next week the random miss has returned, and it has me pulling my hair out now. There are only two other things I can think of doing:

1) pull throttle body and clean it more thoroughly (first time I did it in place), and replace the gasket
2) start pulling coil packs and replacing with one of my old saved ones one by one to see if perhaps one of my new coil packs is just not operating within spec

I did also order another can of BG 44K to run through it again next time I fill it up, but is there anything else anyone here would check or replace? It seems odd that this was never an issue until I replaced the plugs and coil packs, so given I did the 'easy' stuff (which needed to be done anyway, so no time wasted), I figure testing the coil packs is the only thing I can do, it's just going to be a pain in the ass.

 

[jerebear21]

tried looking at the camshaft position?

 

[SeaCruise]

No, I'm not sure how to check that. But would something like that explain the complete randomness of the issue? I've never had a camshaft issue on any of my vehicles, and the other two I have are double the miles as this one.

 

[Kamran Khan]

Inspect all vacuum hoses on passenger and driver side, check if they have cracks. Specially the engine ventilation hose always gets brittle that sit under air box on the right bank. It may have a vacuum leak that triggers rough idling.

 

[abuck99]

How substantial (or what magnitude) is this misfire ? Is it slight- just during idle, enough to shake the engine, does it stumble during acceleration? Does it cut out at any particular rpm’s or rpm range at highway speeds? What I are your fuel trims (short and long) looking like?

Drill down into the details a little more.

 

[Patassa]

I had some missing for a while too with no CEL, eventually my MAF went bad, cleanings stopped helping. Started hesitating and even dying at idle. New MAF and I was back in business. Never threw a code that indicated MAF was bad though.

 

[Mike NXP]

I'd say your focus of troubleshooting should be on the coils and plugs not fuel system/map/vac lines/ etc. Although something else could be the culprit the reality is you touched coils and plugs and within a few days you started having issues.
>>Where did you purchase the denso coils from, can you post a picture of the coil to confirm it's not a crap knock-off?

>> To follow Abucks comments we need more details. Maybe some audio of the issue.

>> Only at idle? What if you rev up and hold it at 2k rpms?

 

[nissanh]

Are the coils genuine DENSO? There are ton of fake Denso products for sale online.

Yeah, try the 2nd option.

 

[SeaCruise]

Thank you for all the replies thus far! To answer some of the questions that have been posted:

1) This is just a slight misfire, and it is NOT constant. It is not enough to shake, not like it did when it had a straight up failed coil (and CEL on). I mean the truck still runs way smoother than my nearly 300k 22RE pickup haha.

2) I have not experienced it cutting out at any specific RPM or speeds. I do recall once when I was coasting on the highway and when returning throttle there was some slight hesitation (this was before I cleaned throttle body and MAP too). But again, this is something mainly seen when idling, in drive, at a stop light, etc. And it isn't all the time - it may go a whole short trip without doing anything odd.

3) Not sure on the fuel trims - I'm good at basic maintenance and part replacement, and even troubleshooting generally, but that is not something I have checked on a vehicle before (I'm assuming it requires a special tool).

4) Coil packs were purchased from 'densoproducts.com'. They have the Denso stamp on them (don't have truck now but can send pic when I do).

A check of the vacuum lines under the air box seems easy enough - I may just replace those ones, like I did the one for the PCV (that one was cracked on the ends). I have to remove it anyway to test any coils on that side. Wife has the truck today but I can try to get a picture of the coil pack along with an idle video once I have it here.

Thanks again for all the responses so far, this is a great community!

 

[Mike NXP]

You could get a clamp on or in-line style spark tester and save time in removing and replacing coils. Still need to pull the coil for in-line but it would be nice to visually see spark. The cheap testers don't take much, get one where you can adjust the gap if you go this route.

All your symptoms do sound more in line with a vacuum or other issue but since you just replaced the coils I'm still leaning that way


Any chance it's always sort of done this and your just noticing it more now after the recent coil / spark plug swap? I also have days where my cruiser likes to grumble at stop lights and other days it purrs smooth like a kitten.



It doesn't sound like you are misfiring based on the info you just shared.

 

[Patassa]

Also, one of my sparkplugs wasn't firmly seated in the rubber gasket and kind fell the last couple of inches down the hole when installed, it was enough to almost close the gap and that caused some misfiring and hesitating until I fixed the gap.

 

[TRDdrew]

I would also confirm that the motor is still in time, i had an lx470 that jumped one tooth and the truck would misfire only at idle but the second you would start driving everything felt fine. i set everything back to TDC when i did the timing belt and it was perfect after that

 

[abuck99]

The added detail doesn’t point to misfire but all of this is subjective for the reader since we can’t observe the condition in person.

Know that the engine at cold idle(first start of day) runs open loop and will idle a little rougher. Once warm it should be idling smoother in closed loop mode.

If it’s not throwing codes- you might do a compression check and leak down test to identify any issues contributing to a rougher idling engine- could be carbon build up, could be nothing.

 

[Mike NXP]

The added detail doesn’t point to misfire but all of this is subjective for the reader since we can’t observe the condition in person.

Know that the engine at cold idle(first start of day) runs open loop and will idle a little rougher. Once warm it should be idling smoother in closed loop mode.

If it’s not throwing codes- you might do a compression check and leak down test to identify any issues contributing to a rougher idling engine- could be carbon build up, could be nothing.

Sounds like it needs an Italian Tune-Up. Little 91 octane and high rpms . Let her rip.

 

[abuck99]

Sounds like it needs an Italian Tune-Up. Little 91 octane and high rpms . Let her rip.

Exactly!

 

[Biga27110]

My cousins LC had a misfire it ended up being the injector not the coil.

 

[SeaCruise]

I had to leave town after posting the thread and reading the initial responses, but I was finally able to get a video, and wanted to share - however this site will not allow me to attach anything but an image. Is there a special way to attach a video?

 

[Mike NXP]

I had to leave town after posting the thread and reading the initial responses, but I was finally able to get a video, and wanted to share - however this site will not allow me to attach anything but an image. Is there a special way to attach a video?

Some people post to YouTube but I usually upload to Google drive and share a link . It shows up as a video here , sometimes , I think .....

 

[SeaCruise]

Okay, I think I was able to get a link to Google drive to work. Turn your volume up to make sure you can hear the idle good, the 'stutter' can be heard / seen at the 8, 11, and 13 second mark. I referred to this as a 'miss' for lack of better description, but as some other responders have noted it may not be a 'miss' - it definitely is not as rough as when a coil permanently goes bad, however it's definitely not normal. For reference, truck was in gear, at a stop when I took the video.

If you have any issues accessing the video, please let me know!

LX idle - Google Drive - https://drive.google.com/drive/folders/1U1Z0-bm4JfQpfnAg1LjblpKmDE5qE4QY

 

[bozncsurf]

Bump... any input would be greatly appreciated