SOLVED - Intermittent misfire at speed / rough idle
 

So I have been trying to track down a misfire on moderate / hard acceleration for about 3 months now. The problem is, I couldn't isolate it as it only happened under hard throttle under load.

I got no related check engine light (have one for some other stuff) so that wasn't helping either. OHM'd the coils and injectors - nothing noteworthy.

Finally I started to get very slight misfire at idle as well!!! Noteworthy, the plugs, fuel filter, air filter, knock sensor are fairly new. I have cleaned and re-soldered the MAF.

So going one by one, I unplugged each coil at idle. By listening, they all made the engine RPM drop - but one cylinder slightly less affected the idle than the others. Luckily, it is the very front most one on the passenger's side (think this is number 2).

First thing, swapped coils with neighboring cylinder - misfire stayed, did not follow the coil. Damn - not the coil. Pulled the front 3 plugs (about 20K on them, all good). Ohm'ed again front injectors, all about the same. Damn.

SO I know I have number 2 not contributing as much. I know it isn't the coil. I know the injector's electrical side should be good. I don't know how well the injector is actually firing, and I didn't have a compression gauge handy so that test would wait.

I went to autozone for some snake oil - why not. I left with sea-foam. I used to clean parts and carbs with this stuff - it's an old car, why not.

About 10oz of sea-foam half a tank of gas. Drove around swerving like an idiot to mix it up. The other 6 oz's very slowly drizzled into a vacuum line into the intake plenum while it was running.

Waited 20 minutes. Started it up - ran like crap - lots of smoke for a good 10 minutes. Smoothed out.

Now about 100 miles later I'm almost out of fuel (the sea-foam tank) and no more hesitation, no more engine miss at heavy throttle and no more rough idle. Call me crazy but 11 bucks (had to buy a funnel) appeared to fix it. I plan to fill up with a fresh tank of premium in a day or two and see if this continues, but it appeared to work.

It is a 95 max with 220k. I keep up with maintenance but it is an old car- I'm not going to throw new injectors, coils etc at it unless it dies completely. This is a great fix to try before going shopping if you have some roughness but not enough to set a misfire code.

J

Glad it worked, but I think you're celebrating too soon IMO. Report back after 1000 miles and tell us if your intermittent misfire is still gone.

Your tests seem to confirm that one of your injectors was sticking. TCW3 would have smoothed things out and its chemicals would be more gentle to your engine internals (its a motor oil and doesn't contain alcohol)

~Whats tcw3 two stroke oil?

Why yes it is..........

https://maxima.org/forums/fluids-lub...itive-new.html

I have run TCW3, blue velvet in my I30 for a year now. My emissions were so low I thought they misprinted the results. My NOX was 2, down from 235. This stuff makes your engine run smoothly, like velvet. It probably prolongs the life of the fuel injectors. I wish it could prolong the life of my CV joints.

I just fixed an intermittent misfire/rough idle problem on my '96 GLE with 90K miles. When the engine was cold it ran fine, but after getting up to normal operating temperature, it would start with the occasional misfire. It was very apparent when stopped at a light, you could feel it shake. No CEL, no codes. According to the service records, the same problem happened about 10K miles back. The shop replaced the #4 and #1 coils at that time. I found a reasonable price on a new set of coils so I replaced all 6. Now the engine runs smooth as silk regardless of temperature. It drives great for an almost 19-year old car.