Sc400 Intermittent Misfire / Rough Idle

[lexsc92]

Hi I posted this on clublexus too, but trying to get some more opinions. Ihave a 92 sc400 without about 130k miles.

It runs super strong 95% of the time, but I've had an intermittent misfire issue. The idle is rough enough to feel the car shake. Also I can get it up to speed okay, but the car will fluctuate between bucking/hesitating, with random surges of power.

It seems to do it the most often when the car sits for a week or so. If I drive it regularly it doesn't seem to happen as much. However today it randomly decided to do it. But then I go back outside an hour later and it took it for a drive to have it run full power again.

It's had a full tuneup: plugs, wires, caps/rotors, coils, etc. The injectors and fuel filter are also new. Good compression. I'm stumped, and not sure where to go from here. There is also no check engine light.

[UCF3]

Couldn't say exactly what might be going on, but my neighbor mentioned a similar hesitation when he drives.

there are a number things you should try out.

-check out the timing belt.

--check for cracked hoses and belts.

-check out the fuel pump and the pressure.

-check out the torque converter.

-check out the shifting solenoids in the throttle body of the transmission

-check out the spark plugs and wires... (I know you mentioned you got them replaced. Hopefully they're the correct type).. Look for spark.

-check out if there are any check engine codes, not being displayed or displayed.

--there is a possibility it might be the Throttle Positioning Sensor (TPS) might be malfunctioning.

-the problem my neighbor had was a bad ignition coil; got it replaced and it worked out for him.

[lexsc92]

Thanks for the response.

I know the timing belt has been changed, and if it jumped timing wouldn't that stay running horribly?

It's not transmission related as it does it in neutral, park ,etc. Transmission shifts great, and actually the engine runs great until it decides to act up.

It's tough to really even have someone look at too, because by the time I can take it in somewhere it runs good again.

I actually wish the car would throw a code and make things a little easier.

[UCF3]

Thanks for the response.

I know the timing belt has been changed, and if it jumped timing wouldn't that stay running horribly?

It's not transmission related as it does it in neutral, park ,etc. Transmission shifts great, and actually the engine runs great until it decides to act up.

It's tough to really even have someone look at too, because by the time I can take it in somewhere it runs good again.

I actually wish the car would throw a code and make things a little easier.

Oh, I've been there. You could try dropping the car off at a shop the night before and let the Tech look for it in the morning.

So let me get this right>

You go to start the car, turn the engine, and then ...

[lexsc92]

It fires right up, acts like it's going to idle fine for a second, but then starts idling very rough.

You can really feel it once you put it in gear and there's load on the engine. Feels almost like a big lopey cam v8.

I can get it up to speed but it hestitates/bucks through different rpm ranges, but randomly surges full power sometimes.

Not sure if related but I did notice when I've driven somewhere and start the car again later the engine turns over a few times before it starts. Where typically it fires right up.

This is only when it acts up though. When it runs good, which is most of the time, it runs like a champ.

[CASS SC4]

same problem...check your sub O2 sensor circuits...not just the sensor its self...check for a fray in the connection or opens in wires before and after the actual sensor...any mods done? also check mass air flow sensor and engine coolant temp sensor...all of these things will cause a rich running condition and even lean at times too...i found out first hand on my 1993 sc400...swapped out stock air intake for INJEN short ram...ruined everything..

[lexsc92]

same problem...check your sub O2 sensor circuits...not just the sensor its self...check for a fray in the connection or opens in wires before and after the actual sensor...any mods done? also check mass air flow sensor and engine coolant temp sensor...all of these things will cause a rich running condition and even lean at times too...i found out first hand on my 1993 sc400...swapped out stock air intake for INJEN short ram...ruined everything..

Would an 02 sensor issue be possible right at start up? I thought the MAF is used until the 02 sensors heat up?

Engine coolant temp sensor..that's one thing I've heard can cause some random issues. It's cheap too, so I may try that first.

Don't really have any mods besides wheels/suspension/mufflers. How do I actually check the MAF?

[lenore]

What kind of plugs did you put in? I hope not Bosch.....

[UCF3]

It could be your MAF that's not opening up.

Are you loosing throttle pressure (drops below 8-1000RPMs)at idle?

[lexsc92]

It could be your MAF that's not opening up.

Are you loosing throttle pressure (drops below 8-1000RPMs)at idle?

Now that you mention it, the idle does seem a little low. When warmed up its idling closer to 500 rpms.

By losing throttle pressure, do you mean it feels like the response is a little "soft" and have to give a decent amount of throttle to get rolling?

Oh, when I bought the car the previous owner had removed the stock intake box and put a small cone type filter on. I kept it on for a while, but bought a stock intake box and put it on a couple years back.

[lexsc92]

What kind of plugs did you put in? I hope not Bosch.....

Nope NGK, and the problem started before I changed those out. I just figured I'd do a full tune up to see if it would take care of things.

[UCF3]

It could be your MAF that's not opening up.

Are you loosing throttle pressure (drops below 8-1000RPMs)at idle?

Now that you mention it, the idle does seem a little low. When warmed up its idling closer to 500 rpms.

By losing throttle pressure, do you mean it feels like the response is a little "soft" and have to give a decent amount of throttle to get rolling?

Oh, when I bought the car the previous owner had removed the stock intake box and put a small cone type filter on. I kept it on for a while, but bought a stock intake box and put it on a couple years back.

Loss of throttle pressure means it drops below the normal 700-800 RPMs and starts to shake and stalls.

If you're car isn't stalling out it could be the TPS, fuel injector, or spark plugs.

Anybody else have any opinions.

[lexsc92]

Well I've pretty much confirmed it only runs rough if it sits without being driven for a long time..a week or so.

Been driving it this week and hasn't acted up once. Drove it somewhere an hour each way and never missed a beat.

Maybe the fix is to just drive it more lol.

[UCF3]

Well I've pretty much confirmed it only runs rough if it sits without being driven for a long time..a week or so.

Been driving it this week and hasn't acted up once. Drove it somewhere an hour each way and never missed a beat.

Maybe the fix is to just drive it more lol.

:) Maybe that is the fix.

[raytbo]

Is it possible you had a bad tank of gas? Maybe some with water in it? Try the Seafoam gas additive and see if that helps the problem.

[lexsc92]

Thought I would update this. The weather has started to cool down here a bit, and seems to have made the rough idle issue appear more often.

Basically now I start the car when cold and it fires right up, then rpms chug and idles poorly, and puffs some white or greyish smoke for a bit. However I've realized all I have to do to make the rough idle go away, is simply rev it a couple times. Then the car smooths out like nothing happened.

I did decide to change the Engine coolant temp sensor, and it did not fix the issue. However the car does seem to have picked up a bit of power. The car runs great! I just need to figure out this cold start idle issue.

My next things to check are the EGR and IACV for carbon blockage. Any ideas/advice?

thanks

[lexsc92]

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?

[lexsc92]

Pulled the IACV to clean it today. Decided to do a resistance test first.... and the resistance between the specified terminals was 0 ohms! I believe the manual says they should be around 10-30 ohms depending on temp.

I think I found my problem?? hopefully...