Backfiring

[sethulrich]

Hi. I own a 1993 240, and it has been a good car....for the most part. I have noticed that it occasionally backfires. This almost always has happened when I free rev it (up to about 3000-3500 rpm) such as when I blip the throttle on a downshift to match revs. Is an occasional backfire NORMAL in a situation like that? Is it a potentially serious problem? Thanks for any help.

[NISTECH]

do a search on AIV you may find your answer there and some helpful hints on repairing it.

[sethulrich]

Well, I will check it out, but the symptoms listed under AIV aren't present. It idles fine. And not only that, I have a cold air intake, and the AIV is not connected to the intake. Instead, I have a breather filter connected to the AIV. Could the AIV still be the problem? I'll go ahead and remove the AIV anyway, just in case. But from what I read, it doesn't appear that the AIV is the problem.

[sethulrich]

Ok, so I removed the AIV, and plugged all the leftover hoses. It runs marginally smoother. But that didn't solve the backfiring problem. Today on my way to school, it backfired again. Same sorta situation, I was engine braking, and when I down shift, i rev it up to match the revs of the next lower gear, and it backfired. And it doesn't do it every time, just occasionally. Could it be something with the catalytic converter? Someone suggested that if I have a leak in my exhaust, then fresh air coming in could cause some unburnt exhaust to ignite....but I have no idea if that is right. :help

[p00t]

taken from http://www.ahooga.com/wwwboard...shtml

AfterfiringAfterfiring, sometimes called afterburning, often results when the fuel/air mixture is too rich. Overly rich mixtures are also slow burning. Therefore, charges of unburned fuel are present in the exhausted gases. Air from outside the exhaust stacks mixes with this unburned fuel which ignites. This causes an explosion in the exhaust system. Afterfiring is perhaps more common where long exhaust ducting retains greater amounts of unburned charges.Afterfiring can also be caused by cylinders which are not firing because of faulty spark plugs, defective fuel injector nozzles, or incorrect valve clearance.

Also check your timing, and check the exhaust manifold gaskets for leaks.

[sethulrich]

Thanks, for the help, p00t.

I don't think my car is running rich....a rich mixture would not lend itself to 37 mpg (which I have gotten on the highway on more than one occasion. I rarely get less than 30 mpg, even with city driving). If anything, I tend to run lean. Which, if anything, would suggest that it is backfiring, not afterfiring. I checked out the link that you posted, and if I understood correctly, it was saying that a lean mixture can cause backfiring (i.e. an explosion in the intake manifold) and a rich mixture could cause afterfiring. So, if I'm truly backfiring, then should I still check the timing? and if I'm running lean, I suppose there could be a whole ton of reasons....and I wouldn't know where to begin. Shoot, I was hoping this would be something simple...

[p00t]

im not tech and a few days shy of 19 so i have too many gaps in my knowledge about these problems, but here it goes:

One thing you also need to find out, is it actually coming from the intake or exhaust?

If you are indeed running lean then check your plugs for their condition. Heres a quick guide:

http://www.pdm-racing.com/features/plugs.html

Running lean can be caused by plugged up injectors, malfunctioning O2 sensor, bad fuel pressure regulator, vacuum leak, maybe even a MAF problem.

If you are running lean do process of elimination...check for vac leaks.... run injector cleaner.... then change o2 sens..... etc

[sethulrich]

Oh, I might add, spark plugs are NGK's, about 4 or 5 months old. I have no sign of a miss in any of the cylinders. Unless it just happens to miss in the situation where the backfire occurs, and no other time...???? and like I said, it happens when I free rev it, so a miss at that time might not be all that noticeable??? seems to be a pretty open ended problem!

[sethulrich]

I'll take a look at my plugs tomorrow.

I'm not positive, but I think it actually is in the intake, as it seems to be more under the hood than out the tail pipe. I replaced the O2 sensor....roughly a year ago? I cleaned the MAF with carb cleaner within the last year also. And I've also done the full seafoam treatment. (Although, I did not replace the O2 sensor after doing that, because I had just replaced it not long before then. Perhaps that's something else I should do.)...but I would think seafoam would clean out any clogged injectors....or is that not safe to assume?

[p00t]

*edit* errr i misread that you already seafoamed it.... all i can say is if some of the plugs are different then others you can almost garauntee its an injector problem and you will know which one is the culprit. You can also measure the resistances of each injector to see if its toast.

other than that i am out of ideas

[sethulrich]

Ok, well thanks a lot. At least it's a few more leads to check out. I'm gonna sign off. Take care.

[sethulrich]

I looked at the spark plugs today. They appear to show signs of normal wear, and not very much wear yet. They aren't very old plugs. They may very slightly exhibit signs of running lean, but they are all very consistent with each other, i.e. no single cylinder appears to be the culprit. My next line I think is to check the O2 sensor, since I replaced before doing the seafoam. Seafoam says it is "Oxygen Sensor Safe" but I have also read that you should replace it anyway, as it gets all coated with soot....??? Also, if the O2 sensor is malfunctioning, is there anyway to easily check that? or should I just replace it and see if that fixes it?

[p00t]

from the FAQs:

http://www.240sx.org/faq/articles/o2_sensor.htm