I sure did. I reset the ECU when I replaced the ECT sensor.

 

Alright so I let the car sit for a few days because that's when it seems to act up the most.

It started rough and poured some white smoke as expected. I was able to rev it for twenty or so seconds to get the idle to smooth out. However 5 mins down the road it started acting up again.

I was able to figure out the exact symptoms...

-idle is fluctuating randomly (no more than 100-150 rpm difference) but you can feel a random misses here and there
-coasting without any throttle runs smooth, however barely touch the gas and it bucks
-give it a decent amount of throttle and it smooths right out , especially if you give it enough gas for the transmission to kick down a gear it takes right off... once you back off the throttle a bit it bucks again, let go completely and it runs smooth. Once you come to a complete stop at idle you can feel the miss again.

This is starting to sound more like a MAF issue, or possibly TPS. But would it be that random?

One other theory... I noticed the wire harness that plugs into MAF runs along the block, on that passenger side there's some oil seepage from the valve cover. I could smell oil burning off of it today. Is it possible that when the car sits for a few days the oil leak is causing interference with the harness, and the car runs good when driven often because oil isn't given the chance to accumulate?

 

(small) Blown Head Gasket?

 

I have a 96 ls400 and have the exact same issue......It is so random, that I told my wife it is only when she is near the car. I have changed the plugs, the plug wires. still random misfire that comes and goes and seems to disappear with acceleration. I put a new fuel filter on today and it was running awesome. Not one misfire. Then I decided to take my wife to eat some pizza. On the way back.....Hello misfire. Reading some of the other posts, I will try these next FUEL PRESSURE REGULATOR - FUEL PUMP ECU. Then I guess all I have left are the actual injectors or wiring to the injectors. I do know that #7 and #8 are misfiring.

 

....................

Does colder weather seem to cause to it appear more often?

 

Well the it has been colder the last two days and it has been constantly misfiring. It has yet to stop in the last 24 hours.......and I have been driving alot to see if it will stop. Are #7 and #8 on the same ignition coil? I sometimes smell a strong gas smell when it misfires.

 

Yeah here's the odd thing for me.. Over the summer as long as I drove it fairly regularly it ran great with no issues.

Now that its been 40-50ish degrees it seems the issue has been starting to surface whenever.

Today I started it and it sputtered a bit, then fired up and idled fine. Ran great, then started running rough 10 minutes later. It seems ambient temperature affects the issue somehow too.

I ohm tested my MAF and resistance was within spec. I need to check the voltage coming into it though.
Still running rich too. Inside the MAF housing smelled like fuel as well, i dont know if that's normal.

I let it sit for a while and idle, and the miss you hear is so random... smooth for 5 seconds, a couple misses, smooth for 3 seconds, another miss, etc. Rpms only drop 100 or so rpms when it misses at idle.

 

I bridged the diagnostic ports and was able to pull a code.

it came up with a code 24 which is Intake Temp Sensor Circuit Failure.

However I cleared the codes, then drove the car again and it didn't come back. So I think that was just from unplugging the MAF momentarily

Definitely seems like cold ambient temps play a part in whatever is causing the issue though. It stalled a couple times and the CEL light flashed for a second, but didn't store anything. I can also manually get it to misfire by quickly..but barely tapping the gas pedal at idle...it bogs and rpms drop a couple hundred rpms.

 

Well, hopefully I found the issue.

I pulled the IACV today to clean it. First I decided to do a resistance check. Testing all the terminals as instructed by the manual showed resistance to be 0 ohms!

Is it possible for the IACV to be partially working, causing the intermittent issues?

 

Iacv can cause intermittent miss but I have seen it usually more consistent. Pull the plugs and see which appear to be burning richer, you know darker and smell like good ol fuel. That way you can hopefully isolate the issue

 

Ha ha miss. Tired from working on house all day

 

Had the same issue , replaced the fuel pressure regulator and no problems ever since

 

Well, I replaced the IACV, cleaned the throttle body, rechecked vacuum hoses, etc.

Started the the car after not driving it for a week, and it ran extremely rough. I couldn't "rev" the problem away like in the past. It's definitely an issue that seems to be worse the longer the car sits.

I go back out and start it a few hours later, and it idles super smooth. I did feel a couple occasional misses at idle down the road. But it's amazing how much better the car idled just starting it 3 hours later.

@IanWatson you had the same intermittent issue? How did you verify it was your FPR?

 

last time someone had this mysterious weather issue, it was actually the area next to the ecu on the passenger floorboard was getting wet. maybe do a visual check on your ecu make sure its nice and dry. sometimes passengers can spill stuff there and not mention it, or even bad weatherstripping can let water in that window.
long shot I know, but worth a look.
I have also experienced something similar when using a rotor/cap I bought from an auto store, started misfiring randomly. went and got Toyota ignition rotor/cap and it cleared up my problems.

 

I vote blown head gasket too. everyone seems to think that the give away is oil and coolant mixing. the truth is you can nuke a headset and rather than the oil and coolant mixing the coolant is pushed outta your exhaust valves. does the white smoke (a good indicator) smell sweet at all. is say the gas smell is unburnt fuel resulting from the missfire