Dear friends, I'm facing very poor gas mileage in my maxima 2008 I always use 93 octane gas and no matter how soft /smooth I drive I never get anything above 8.7 mpg. In real life I guess it is even worse. I replaced spark plugs, maintain correct Tyre pressure and also Cleaned mass airflow sensor. There is no check engine light. Although car knocks /ping at around 1200 rpm, specially when not fully warmed up. But disappear if I accelerate. car total run 140k. Sometimes I smell rotten egg smell indicating the catalytic conv maybe little bit clogged. I don't know if all 02 sensor are ok. But I don't get no check engine light. That's why I can't figure out why it would be so poor on gas. Please anybody experienced similar issue? any idea how to resolve please? Thanks in advance

 

First of all, is this calculated mpg's by you, or are you relying on the trip computer?

 

Thankz for the reply. The 8.7 mpg is calculated by trip computer however I myself checked and it is even worse. I could drive anywhere 80 to 85 miles with 10 gallons.

 

Sure sounds like a typical clogged Cat. More pedal pressure to get up and go, really poor gas mileage and the coup de grace, sulfer smells. Maybe both cats. But agree you should have a CEL or perhaps you bought this used and someone did something to bypass the P420/430 you probably had at one time? But it really acts like clogged exhaust.

 

As LtLeary pointed out, I would start by diagnosing and looking for a clogged cat.

 

yes I also kind of narrowed down it to clogged cat. but wasn't too sure as the cel light dosent lit up. I guess perhaps the privious owner did something to bypass the code. I once got cel but that was for 6th cyl missfire, I changed the plug and since then the missfire never happened and cel didn't lit.
you are right about the smell, I get that specially when driving highway, or if the rpm goes above 3k. Should I go for both cat replacement or use any kind of cat cleaner? does this stuff work? please if you have any suggestions I highly appreciat.

 

Thank you. should I Start with any kind of cat cleaner. or is there any way I could clean the cat physically accessing it? thankz once again for the reply.

 

I know of no "Cat Cleaner" that really works. Based on your mileage, I am afraid it wouldn't help anyway. Now, of course we are saying clogged Cats. But you COULD be suffering from a "Beverly Hills Cop" "Banana in the tailpipe" too (read, clogged exhaust) but it is more probable the cats are clogged. The only way to be sure is (unfortunately) to take them off and see.

 

I think I may have a similar problem with my 97. After I took it to a mechanic to tighten up my heat sheilding, it started to run very poorly at times almost stalling. No even throttle at parking lot speeds. Very poor fuel economy, exhaust smell in the car. All this time I thought my trans was going. Looks like a trip back is in order. Let us know if this was the case with yours.

 

Im experiencing pretty much the same issue except for the stalling and poor trrotle response. In my case, the car would run fine, crispy smooth acceleration, and no hiccups or lack of torque. However it sometimes does ping/knock at 1400-1800 rpm although it neglegible. I have tried cataclean and also tried seafoam spray based on reveiw. Nothing seems to improve mileage condition however. Ordered an OBD2 scanner. Going to check for O2 sensor reading. If no reading found I'm pretty sure the previous owner bypassed the system and that will explain why I never got CEL. if Redding found then I guess it might be any damaged injector. I will update on the matter.

 

Pull your precats, hollow them out, buy these: and call it a day.

 

never knew about these and I'm already impressed reading the reveiws! surely going to try these. thanks a ton.
Update as of now : used seafoam spray through intake as per manual and also put cataclean while tank was 1/4th filled. Drove for 15 miles then refilled the tank full. now as I drive, the low end torque surely has improved and pinging is almost non existence. No improvement for the milegaue issue and burnt gas smell makes way to the cabin. Will update further...

 

Do the steps I posted and you'll be good to go. I don't care what kind of magic liquid in a bottle you use, they are all a waste of time. No car rolls out of the factory needing any of that crap to run properly. Also, the longer you wait, the more risk you introduce of substrate ingestion and piston ring failure. = Bye bye motor.

 

as he has no cel, i am thinking some already installed the "anti foulers" in at least the bottom (rear) O2 sensor. My preference is to replace the precats and restore it to factory but sometimes "ya gotta do what you gotta do."

 

Point noted. A hollow Cat makes more sense. I believe there are two cats for our vq engine. So I might need pro help to do that. Too bad in NY they want to rip off for even untitghen a bolt. Also I was looking for catalytic conver but everyone of them is marked for not sale for NY and CA. so gutting the exhsitng cat and try your method with O2 sensor is what makes more sense. I also will post a picture of 2-4-6 plugs. Maybe you will have a better idea if combustion is getting hayware. Thankz for the reply

 

I suspect the same. The weather turned too bad to go under the hood I will soon find out. I bought the car from a guy who sells old car makes profit. I was in need of a car asap and this one looked clean from outside!! I only looked through the engine and bottom of the engine, looked very dry, no sign of oil dripping. the oil level looked OK as well. The engine revved very crisp. All lights and switch was working as should. ac was blowing chilled air . it was a hot summer day and I headed straight to dmv.
When I returned home with plates, I saw the outside temp sensor is not present, radiotor missing a rubber clamp, the side mirror is put form another car, the door trim was loose, the AC was short meaning the switch wouldn't turn the compressor off. The car was simply put together to look good at first glance. And now as you mentioned I believe it is very much possible they played with the Sensors and cats too. The reason why I'm writing these never buy a car in a rush and always beware of professional car sellers. And thanks for the suggestion again.

 

Certainly possible. God knows what's been done to that car.

 

I get about 23 mpg average on 93 octane. Mostly highway miles. 185k on the clock.

 

Well my exhaust came apart at the resonator the other day and I haven't seen a change in how it drives. So I need to find a quick and dirty way to patch it together for now because the noise rattles my soul when driving. This also leads me to conclude that my muffler isn't plugged up so it's either an issue with the cat and up or something else entirely. My issue is mainly based on driving at steady speeds. My rpms go up and down when driving through parking lots and rpms hesitate and dips from a stop before I give it the beans, then it seems ok but still feels constrained when giving good throttle.

 

This is the data I pulled after warming up the engine at idle. The obd2 data shows everything is within safe parameter yet dosent explain the poor gas mileage
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It's been a month. What have you done in that time?