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Anyone know why my car would run great one day and feel like it's gonna die the next? Would the weather make that much difference? It's really humid all the time here.

 
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I have noticed that too...... but you got me on that one. I have NO clue.

 
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My car has good and bad days too. There's no rhyme or reason to it.

 
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^^^ I agree with ya'll~~ haha

 
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This goes into the vague and obscure category. What do you mean runs bad? Bad acceleration, an engine that misses, an rough idle?

 
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Well my idle is rough all the time, but that's a different story. I mean some days it will be very peppy and accelerate fast. Other days I mash the gas and it struggles or goes up to speed slowly. Very strange.

 
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Perhaps the best thing you can use to diagnose this is an OBDII data logger. This will confirm if it is your imagination or not. This monitors your sensors and your acceleration to see if there are differences or if there are sensors which are starting to fail and not set a code.

 
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My old cars are way more consistent, so my guess would be that the computer has something to do with it. Maybe it doesn't continuously adjust as perfectly as it was intended to.

 
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It could be that your o2 sensor is going. But that is just one of many things it could be. My friend had the same sort of problem(not a civic though), he replaced the gas tank, filters, o2 sensor,etc. over 2 grand worth of parts and it still does it sometimes.

 
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Well the dealer keeps telling me everything is fine because there is no check engine light. That's not cool.

 
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Okay, the dealer may be telling you only half the story. The check engine light only comes on if it stores a code. There are pending codes and intermittent failures which would never log a code. Those can only be picked up with a data logger or a scope.

 
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Well I did have an ODBII diagnostic done and it said system was okay. Is that what you mean?

 
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That depends. Cheap OBDII readers only read logged codes, not pending codes nor data. True data loggers actually record the raw data from the sensors so you yourself can determine if they are beginning to fail.

 
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Oh okay. The guy said the one he used was like $500 or something. But he just ran a test and it said there's no problem. Maybe I should see if he can get the sensor input.

On a side note, what happens if the car is running lean? I know there's a cel for it, but not sure when it actually turns on. I need to find a way to measure air/fuel ratio. Gauges don't really work well, and a dyno is far away.

 
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Yep, a $500 scan tool, depending on the model will do the trick. Now, the interpretation takes a real skilled mechanic. If he's telling you there are no codes stored, then he's not lying to you, it's what he sees. Interpretation of sensor graphs to show an intermittent condition takes skill. Regarding a lean a/f mix, any decent scan tool will tell you sensor voltage in real time (and thus the fuel trim). If your car runs lean, everything will be very hot. You will shorten the life of your spark plugs, catalytic converter, among other things. Everything that sets a check engine light is based upon thresholds. If it exceeds a certain threshold, like a sensor on for too long, too short, too high, too low, it sets a code. However, there is everything in between that doesn't set a check engine light.

 
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what causes the car to run lean or rich?

 
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There's endless reasons. Automotive engineers try very hard to balance air fuel mixes. A faulty MAF sensor can cause it to run rich/lean depending on how it fails, clogged air filter can cause it to run rich, a so-called performance chip will cause it to run rich almost all the time, a faulty upstream oxygen sensor will cause it to run rich... Umm.. let me count thy ways a/f mix can go wrong. There are probably a lot more which I have missed.

Edit: I should point out that the PCM/ECU on the car learns the fuel trim as you drive, so frequently disconnecting the battery will wipe these settings so your car has to relearn them again.

 
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My ported head work. I actually tried to put regular copper ngk plugs in and they fouled out after a day. White stuff was all over the ground electrode, probably indicating a lean condition, advanced timing, or improper plug heat range. I'm using iridiums now and so far so good. I really need to know the air/fuel ratio though so I can add more fuel with the vafc. I've already added some, but don't know if it's too much or too little. The acceleration through the power band still isn't very smooth.

 
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Oh boy... Looks like the custom work threw something off. You can tell the a/f mix by putting a scope onto the upstream oxygen sensor, but then again, there isn't an easy way to fix it on this car. You can't really mess with the timing nor pulse width of the injectors.

 
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Well I'm playing with the vafc by adding fuel under different rpm conditions, but I can only guess what it's doing by driving around. My timing may be a tad off just from the install. Still don't know, but a compression test is coming soon when I get my service manual tomorrow hopefully.

 
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White stuff on your plugs are additives from the gas.

Too rich - black plugs, or lots of carbon crusting.
Too lean - the plugs look scoured clean, maybe some spots, eroded electrodes.
The plugs should be a light tan color, very clean, but not scrubbed looking.

Also, do all 4 plugs look exactly the same?

 
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OK, not to thread crap, but I've got a related question. What would a plug completely caked in white stuff (to the point where I couldn't even see the electrode without chipping some of it off) indicate?

 
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I'll post a link with pics when I get home for plugs. Oh and yeah, all four plugs were the same. White dust over the ground electrode. I did have fuel injector deposits, but those were green and just wiped off.

 
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http://www.sa750.com/plugs/plugs.htm

 
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Oh, cool. I forgot where that link was. Much simpler. Thanks Gearbox.

 
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Would slack in the timing belt cause rough idle?

 
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if your concerned about a/f ratio id just buy dyno time with an exhaust sniffer. the way i see it, if a set of plugs fouls out after a day, and iridium magically fixes it, the problem might not be fixed.. it couldve just had a band aid put on that could be covering up a potentially serious issue

edit..
heres a shop i know of.. really good guys.

http://www.automotosports.com/dyno_tune.asp

 
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Thanks for the link. I've heard about them before, but I work all the time and can't just drive up there for a day. I really need something close to Urbana here, and so far no luck. But a dyno will certainly answer alot of questions.

 
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No. Only if you slipped a gear.

 
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according to these guys white stuff on the plugs indicates a coolant leak

http://theoildrop.server101.com/ubb/...;f=24;t=001181