Does my car know something I don't?

October 6, 2007 8:52 PM

Shouldn't my car run without an air filter?

I replaced the stock airbox on my 2001 Maxima with a K&N cone filter, using a cheap plastic ebay adapter to mount it to the MAF. Not too surprisingly, 6 months later, the cheap plastic adapter breaks and the cone filter falls off. No other components appear to be affected, and no wiring harnesses got knocked loose.

However, now my car won't run. I'd like to limp to the house where my factory parts are, so I can get the car running again. But as soon as I start it, it revs up normally and sounds fine, then immediately stalls out. No amount of throttle can stop it.

Why would my car need an air filter to run? Does it depend on the partial vacuum on the intake? (that doesn't make any sense to me) If I can't get it running, I'll have to get someone to bring the parts to me instead. (Not the end of the world, but this is annoying.)

There is a steel mesh on the intake side of the MAF, so it looks like I didn't accidentally suck in a pebble or a chunk of that plastic adapter.

posted by knave to travel & transportation (12 comments total)

I can't speak for a 2001 maxima, but I know that on motorcycles, no airfilter means no running, but then my bikes have always had vacuum carbs.

If I had to guess, I'd say there's a sensor, maybe even the mass-air-flow that's detecting a lack of filter via completely unrestricted air flow (waaaaaaaay too much o2 means waaaaaaaaaaaaay too much gas in the cylinder) and is shutting it off to avoid damaging the motor.

Just a guess.
posted by TomMelee at 9:06 PM on October 6, 2007

It doesn't make sense to me, either, but you can get a baggie and a rubberband, cover a fraction of the cone, and see if it runs or limps wherever you want to take it.

In other words, if it needs resistance in the intake for some reason, give it resistance.
posted by Kwantsar at 9:13 PM on October 6, 2007

If you can get a code reader to the car, you can probably find out if the problem is as TomMelee suggests and then proceed from there. A way out of whack reading shutting the engine down would be my first thought too.
posted by ssg at 10:02 PM on October 6, 2007

Let's see...problem upwind of the MAF, causing the air filter to go byebye, and now the car starts, revs, and dies?

The MAF isn't functioning properly. You say no wiring got knocked loose, and you say that there was no other damage. That leaves dirt and oil at the top of my list to check; cleaning the wire on the MAF is a delicate procedure not to be taken lightly. Trouble is, the behavior your describe really does indicate a busted MAF, not a dirty one -- although folks who over-oil their K&Ns often end up with oil on the MAF wire, and then it's as good as busted (unless you have the mad cleaning skills.)

Why don't you reinstall the stock airbox, to rule out anything having to do with resistance or whatnot? Then you can worry that the impact of the air filter coming off the car knocked the MAF and killed it, even though there is no visible damage -- they're not hardy pieces.

Final word on it, though: don't replace the MAF unless you've tried everything else, because they're pricey.
posted by davejay at 11:00 PM on October 6, 2007

It doesn't make sense that no air cleaner is causing it to stall out. In most cars the tube the MAF is mounted in is the point of restriction. I've never heard of an open intake causing this problem. I'd look really closely down stream of the MAF for a vacuum leak as maybe the cone filter breaking cracked something else.

Also if the mesh is down stream of the MAF maybe the MAF itself is damaged. The engine starts with settings pulled from a lookup table. If it attempts to read the after startup MAF and gets a bogus reading it might shut down at that point.

If it is indeed the lack of a filter some duct tape would hold everything together to let you drive home.
posted by Mitheral at 11:03 PM on October 6, 2007

Also if the mesh is down stream of the MAF maybe the MAF itself is damaged.

The mesh is on the inlet side of the MAF itself, hopefully protecting it.

although folks who over-oil their K&Ns often end up with oil on the MAF wire

I've only had the filter on for a short while, and I've never oiled it. (Comes pre-oiled and I don't think I'm supposed to touch it for 50,000 miles.)

Thanks for the thoughts, I'll try some of the suggestions tomorrow.
posted by knave at 11:13 PM on October 6, 2007

Owner of a 2001 Maxima SE here...

Without a filter of some sort to guard against dirt, oil, bugs, water, etc, you've probably seriously borked the MAF. You might be able to clean it (there are commercial cleaning sprays especially for MAFs) and get things running again, but I wouldn't do it without a filter in place.

Just for the record, simply installing a CAI like you did won't realize any sort of real performance increase (if that's what you were after.) You need to do some further re-engineering (headers, exhaust, chipping, supercharging, etc) to get any real performance gains. The stock VQ in your Maxima is pretty well developed as it is. A simple intake mod just isn't going to accomplish anything, except for some added noise.
posted by Thorzdad at 5:12 AM on October 7, 2007

I suppose it's possible that something relying on intake vacuum isn't getting that vacuum now. But I bet Thorzdad has it: you messed up your MAF sensor, unfortunately*. The reason it starts fine is because starting doesn't rely on the MAF sensor (the engine has to be running or at least air needs to be moving through it to provide a signal). Once the computer recognizes the engine is running then it switches to normal mode where it looks for a MAF signal to calculate how much fuel to inject, and it promptly dies because it's not getting that critical piece of information.

*It's a $170 part after getting the core deposit back at Autozone, at least in Denver. But first go to Harbor Freight and buy this engine code reader, at least if you have any inclination to want to do your own repairs or even just want to know what's going on. Then you can check your car for the code to verify what the problem is before you spend money on a new MAF sensor.

And stop trying to drive or even start your car.
posted by 6550 at 9:34 AM on October 7, 2007

It is always worth pulling the plug from the MAF filter (as a last resort) being as it is borked anyway. It is possible that the ECU may have a 'limp-home' setting if it can't read the sensor at all, rather than getting a 'bad' reading like it seems to be now and so is shutting the engine off/giving it a fuel mixture that the engine can't sustain.

Older ECU types had a default map for faulty senders and the like (from temp to airflow to all sorts) but the modern ones may be too clever and end up crying and farting all sorts of odd readings if they aren't plugged in, but it's worth a go.
posted by Brockles at 3:03 PM on October 7, 2007

And stop trying to drive or even start your car.

You'll only potentially damage the MAF sender by running it without the filter (unless you drive through a large cloud of concrete dust or something for a few hours) and as it sounds like you are only going to use it to get home, then I think you'll be fine. Air filters are not essential, it's just much better to have them for longevity of the engine. Just don't drive t for weeks while you piss about trying to clean you MAF filter with a toothbrush and whiskey to save you $175... ;)
posted by Brockles at 3:06 PM on October 7, 2007

You guys were right. The car stored a MAF trouble code (P0100). I put the stock airbox back on, tried cleaning the MAF twice with "Mass Air Flow Sensor cleaner" from Pep Boys, got everything together, but it's still throwing a MAF code. (If I erase the code, the MIL goes out for about 2 seconds, then lights right back up.)

The car runs, but it's in limp-home mode (rev limit at 2400, only about 50% throttle allowed). Otherwise it seems to run fine. There is an Autozone with my MAF in stock so I'm going to pick it up tomorrow and swap it out.

Lesson learned: $10 car parts can be expensive.
posted by knave at 6:41 PM on October 7, 2007

Ok, all set... For some reason, the car kept refusing to run with the bad MAF, but Brokles's trick of unplugging the bad unit got the car running in limp-home mode consistently.

Today I replaced the MAF and everything is peachy.
posted by knave at 2:54 PM on October 8, 2007