Hey guys! I recently joined the club with a 1997 LS400 Coach edition, love the machine very much, But I'm suddenly having a pretty serious issue.

I have started to experience a misfire on cylinder 7 at idle, it happened about 2 days after replacing the plugs.

What is stumping me the most is that its sounds like almost a backfire through the exhaust on that cylinder, and I'm ONLY getting a code for that cylinder misfiring.
I pulled the plug back out to look at it and it doesn't have any fouling or strange appearance. I used NGK Laser Iridium plugs. The car also does have a fuel treatment in the tank (LiquiMoly) and a full tank of Premium.
Everything seems to be seated, and the misfire only seems to occur at idle, however I do believe that after 2k rpm the cylinder goes dead as well. Valve Cover gaskets are not leaking as I replaced them 2 weeks ago.

I did notice however that when it was colder this morning there was no misfire, it seems to be following outside temperatures. I drove it on the interstate on a cold morning and had no issues with my entire 3 hour trip (68f outside that day)

What could it be? What should I look at next? I just picked up some coils and will install them tonight.

 

Hmm. Not sure about the coil, not only that but make sure its oem quality as far as the coils go because these cars dont like cheap after-market coils.. the obd transition electronics are incredibly sensitive to resistance differences and you could be inviting even more problems. The issue have is probably traced back to the work you did, and probably not something outside of that.

 

That's what's really confusing me, I just installed the new coils and it has made the idle misfire intermittent. The only repair or modification I've made was the spark plugs themselves. (as in, if I rev it in park it stops misfiring for about 10 minutes before misfiring again.) However depending on rpm there is a deafening low "Helicopter" sound in the cabin, whether or not its in park or drive.

 

Where did you get the spark plug and coils? Beware of rampant counterfeits.

 

Local O'Reillys. NGK Laser Iridium's and Import Direct coils. I did find that both coils where original and had multiple cracks in the potting, so they def needed replacement, the Ignition wires look new but kind of strange, Should there be a brand on these tiny ignition wires? They where on the car when I bought it.

 

Try not to fire the parts cannon. In fact, I would try to put the original coils back on perhaps to reset the car back to how it was. Personally, I wouldn’t put anything other than the OEM coils on this car.

Start with the basics. Swap the spark plug with another cylinder to see whether the misfire travels. If it does, it’s the plug, if it’s not, it’s something else. You can check the spark plug wire resistance with a multimeter as one test to see if out of spec. If they look janky or unknown, might as well replace with NGK wires (in contrast I would run the original coils as long as possible, personally - I would only replace them preventatively with OEM). Misfire can also be caused by a fuel injector / delivery issue or in a worse case scenario compression issue although that doesn’t feel likely here. Don’t start just replacing stuff randomly (although if the wires look old or weird, it’s a low cost consumable probably due anyway)

 

I will try swapping the plugs around next, It had AutoLite copper plugs in it somehow, And had pretty weak power.

Im not sure about the OEM coils, There where very visibly cracked, After replacing them the misfire became far more random and intermittent, I installed on back in and the misfire became a dead cylinder.

Will report back with new distributor caps, Ignition wires, ECT sensor (Visibly crusty) and Rotary buttons.


Now here is important info I realized, since I bought the car The rear EGR Vent tube was split in two. To stop the leak, I bet the header side tube completely shut and didnt have any issues. I also found Silicone gumming up a vacuum line while replacing the plugs and cleared the line. Is it possible that the open side of the EGR tube against the firewall could cause a problem? This was 2 days after I bought the car, And had no issue with it until I did the plugs a week later.

 

Don’t think you can tell whether a part is still working based on whether it is cracked on the outside. Sure, they are probably due to be replaced eventually (with OEM!) but by replacing them with potentially sh-tty aftermarket you are confusing the issue. OEM parts for this car will last longer than you can imagine in some cases. Also someone can correct me if needed but based on the ignition system for this car which is not coil on plug, I would struggle to understand how a bad coil would ever manifest itself as a single cylinder misfire. Clearly this car has a lot of other stuff going on. Go back to square one and again, the most basic thing to test a misfire - move the plug. Test the wire to that cylinder. Make sure injector to that cylinder is in spec. Work from there and worry about the more complicated stuff after you’ve done the easy stuff.

 

Just for Sh*ts and giggles I replaced the ECT sensor before anything else...

It runs like a dream now, ofcourse its late at night at the moment so I wont be taking her for a rip (Don't want too wake up the neighbors, lead foot XD) But I will report back tomorrow after a a 40mi interstate trip to work. The old sensor was very corroded despite the water jacket looking clean and uncontaminated, along with the coolant, Which strikes me as very odd.

The only reason I decided too replace it was because my detailer said they get weak with age and it happened too all 4 of his LS's and 2 of his SC's. Seems he may have been on to something, even gave me a discount of some spot removal XD

 

EGR leaks should not cause any misfiring.

for the coils, you have two of them. swap between each other and see if the problem persists on the same bank or moves!

also check this out for spark plugs, plug wiring routing, and resistance checks!
http://planetsoarer.com/penski/ThePenskiFile.html

 

It appears that website is down or doesn't exist?

 

hmm works for me but it might be your browser settings as it shows on my side as "not secure" (i'm using google chrome). can anyone else confirm they can get to the site?

 

My PC (Linux and the browser Firefox) also says so warning that the data you submit to this site is not encrypted.

Regarding the misfire issue, I'd simply try to spark using a 12.5mm ( a half inch) spark gap in order to confirm the capability of the particular cylinder. The spark should travel even the gap is wider than 13mm in the air, you know.

 

Well I should mention that the ECT only fixed the issue for about one drive too work, before acting up again XD

I should mention that while i threw that code for one cylinder once, it never threw a code again. While still misfiring.

The easiest way too explain the misfire is by a sudden occasional loud, low pitch "boo" sound from the exhaust, accompanied by a lack of power until it comes back too life again. I don't believe its actually following any cylinders.
I replaced the ECU as the fact it wouldn't throw a code seemed strange, but it still misfires and still wont throw a code. Everything ignition wise checks out as far as I can tell.

Also the website started working for me 1hr later, I went ahead and archived it just in case

 

Hmm, well a fuel delivery issue may trigger a misfire code, that "boo" sound could be intermittent lack of fuel delivery for whatever reason, and bad spark plug wires? possibly, but I am leaning towards a fuel problem now .. of course only can go by the info.. and a guesstimation. Hard to say, interesting problem though.