Here we go again... no code this time though. Knock sensor or just paranoid?

[Kompresshun]

So i've had my car back almost a month after replacing the pre-cat. I've only been through a couple of tanks of gas since I don't drive it very much but i've only been getting about 13-15mpg around town. Last tank I only got about 200 miles before having to fill up, so today I dropped a new air filter in and some fuel system cleaner. I'm just wondering if maybe i'm jumping the gun on taking it to the dealer and let them do a diagnostic on it... i've been reading posts about knock sensor issues but performance wise it drives normal. Am I just being a little too paranoid or is it time to spend some more money on this thing again?

[Infinitiguy19]

Knock Sensors would not really cause such a drop in MPG. I think your problem lies elsewhere...Do you have any check engine light codes?

[Kompresshun]

No, I just got done fixing the P0430 code about a month ago(which was the pre-cat). No other CEL codes again yet though, I guess it's possible it may just need some routine maintenance. It has 130k on it and I have no idea what was done to it before I owned it so it probably has never had a fuel filter or plugs.

[rioredstang]

You should unhook the battery for about 20 minutes to reset learned fuel trim numbers and clear out computers memory.

[Markc]

Faulty MAF?

[Kompresshun]

You should unhook the battery for about 20 minutes to reset learned fuel trim numbers and clear out computers memory.

Probably not a bad idea since I don't think they did that while it was in the shop, i'll do that tomorrow.

Faulty MAF?

Wouldn't it throw a code though if the MAF was acting up?

[loystock]

A faulty MAF sensor will definitely turn on the CEL but a dirty one may not. Your poor gas mileage is indicative of rich mixture which may eventually damage the pre-cats and main cat (unburnt fuel may accumulate and ignite inside the cat). Clean the MAF sensing element with zero-residue contact cleaner or CRC MAF Sensor Cleaner. You might as well clean the MAF connector, O2 sensor connectors (3 near the radiator, 1 at the back, pax side) and then apply dielectric grease on the connector.

You can buy CRC MAF Sensor cleaner and dielectric grease from Autozone. While you are there, you might as well have a free scan just in case there are ghost codes in the ECM.

[Kompresshun]

A faulty MAF sensor will definitely turn on the CEL but a dirty one may not. Your poor gas mileage is indicative of rich mixture which may eventually damage the pre-cats and main cat (unburnt fuel may accumulate and ignite inside the cat). Clean the MAF sensing element with zero-residue contact cleaner or CRC MAF Sensor Cleaner. You might as well clean the MAF connector, O2 sensor connectors (3 near the radiator, 1 at the back, pax side) and then apply dielectric grease on the connector.

You can buy CRC MAF Sensor cleaner and dielectric grease from Autozone. While you are there, you might as well have a free scan just in case there are ghost codes in the ECM.

Thanks for the advice, I stopped by Advance today and grabbed some of the CRC MAF cleaner and dielectric grease. I almost bought some yesterday but decided to hold off until I heard some opinions first.

I have my own scan tool so i'll see if there's any ghost codes today too.

[TyH]

Hey guys. I know this thread a a couple months old but...I was wondering if the cleaning worked??? I'm having the same issue with my 07 3.5 Altima, it's only getting 12 mpg with premium yet the dealer says it's fine. Nothing had gone wrong with other parts -yet- but it does smell like it's running rich when it starts and the poor mpg is making feel like I'm driving a hummer.

Thanks in advance!

[loystock]

It's too early for the MAF to fail or be gummed up for a relatively new car. Do you have stock air filter? If so, just replace it annually and the entire air intake should remain clean for a while. But if you have oil-based filter from K&N and the likes, then it can really mess up the entire air intake - MAF sensor and throttle body included. You can use zero-residue contact cleaner or CRC MAF cleaner to clean the MAF sensor, if needed. If you have 'drive-by-wire' Throttle Valve, do not clean it. It's very sensitive and can get damaged easily (learned it the hard way on my 03 Q45).

Have the car scanned as it may have a 'ghost code' (fault code stored but did not turn on the CEL). A rich mixture for extended period will ruin the catalytic converters (very expensive).

Get a bottle or two of RedLine SI-1 or BG44K Fuel System Cleaner and add a bottle to a full tank of gas.

[Kompresshun]

The cleaning seemed to help a little but it wasn't causing the problem. The actual cause of the problem was: The shop that replaced my pre-cat didn't replace one of the exhaust gaskets when they did the repair, so it blew out and caused an exhaust leak, and there was a small split in the exhaust they had to weld up as well. So since taking it back to them my gas mileage seems to be much better and the car is running much better. Of course this seems unrelated to your issue, but I wanted to update so everybody knows what it took to fix the issue for me at least.

[msucurt]

hey A33, Ive got a similar thread, but im curious if you ordered you pre-cat online or did you go with a nissan/infiniti one? If you ordered online, where did you get it from (link?)

thanks

[maxhopper]

I've had good results with the ones I've gotten from www.sparkplugs.com (I bought 3 for my old 97). They are also a NICO sponsor, so they offer a discount to NICO members some times. I would recommend the OEM sensors that have the harness connectors on them .vs the universal sensors that you have to splice. I would give them a call (since I didn't see any specials in our Advertiser's Marketplace), let them know you are a NICO member, and see if they'll give you a deal.

[msucurt]

thanks max. I will check them out (sparkplugs.com). I have heard really good things about the Eastern pre-cats, so i was leaning toward them, and its less than $200 shipped to me.

Questions, If i get the pre-cat, and having my O2 sensors already checked out and are ok, should that take care of the code p0430 more than likely? Basically, it seems that the pre-cat should take care of it from what ive been reading, although i am looking into the O2sims and the anti-foul method.

Opinions and what would you do honestly?

thanks

[Kompresshun]

When I replaced my pre-cat, it fixed the P0430 code and I haven't gotten that code since then. That was over 2 years ago at this point.

I don't know what Scott thinks, but I would just order the pre-cat from the company he suggested above and have it replaced. It should solve your problem.