00 Q45 a/e bad gas mileage 150 per tank

[Q45-pj]

My 00 Q45 has been getting bad gas mileage. I've tried techron, marvel mystery oil, and nothing has helped I'm getting 140 miles per tank. I know its a V8 (a pretty sweet one I might add) but at $60 a fill up its insane. What do you guys think is causing this?

[Skibane]

Yeesh - 140 miles per tank would be around 7 miles per gallon!

Has the mileage always been that bad, or did it change suddenly?

How well does it run? Smooth and plenty of power?

Is the check engine light on? Any trouble codes?

Does it smell normal? No raw gas smell in the engine compartment, or in the exhaust?

[Q45-pj]

The codes I got were for the coil packs, o2 sensors. I ordered the coil packs and sensors along w/an air filter, fuel filter.i need to change my valve cover gaskets and I'm on the fence about injectors. On two occasions I've noticed a faint scent of gas when I turned on the heat. It lasted a few mins then dissipated.
When I accelerate my temp gauge goes straight to cold, and I need to get new shocks.
The car runs great but I certainly feel power being withheld. Especially during takeoffs. The previous owner treated the car like crap. But I love it! On cold starts it fires up like a beast, but when it's already hot it takes a few seconds to start almost like it's not going to start at all but always does. A few times I start up I get that sound you'd get if you turn the ignition switch while the car was running. But then it starts right up. I know it's a lot to read. Thanks for looking.

[maxnix]

TB cleaning? Both sides of both? EGR tube and valve? Fuel lines? Fuel filter replacement schedule? So much left unstated with regards to maintenance.

[Q45-pj]

I just picked up the car so there hasn't been much done in regards to maint. I'm still gathering parts (I have a small stock pile in my trunk) two oil changes so far. The oil came out very dirty after just two months of very light driving. What's TB cleaning? I have a napa gold fuel filter. Not yet installed. I'm not sure about fuel lines. And just looked up egr tube and valve.

[Skibane]

When I accelerate my temp gauge goes straight to cold

Sounds like your thermostat is stuck open.

With the engine being constantly overcooled, your fuel economy will go to crap.

Fortunately, that's a cheap, easy fix.

[OwnerCS]

If it has over 100,000 miles, you'll be surprised at the mileage and performance boost that will come from changing the O2 sensors and cleaning the carbon buildup from the intake, EGR system, and IACV. I have a 99 Infiniti I30 with 265,000 miles, a 99 Maxima with 150,000, and had another 95 Maxima with 155,000 miles. [I guess you tell I like the DSC4. Ha!] All cars were running poorly when I got them and using too much fuel. After cleaning out the carbon buildup on the intakes, EGR, IACV, TB, and replacing the O2 sensors they all run like new cars and deliver the high end of expected mileage.

I have a Honda Ridgeline with 130,000 miles that was eating gas like it was going out of style. I changed the O2 sensors with new NGK/NTK OEM replacements at a cost of ~$325 from RockAuto.com. Gas mileage and pep has returned to like it was when it was new.

If you think O2 sensors for an Infiniti are expensive, try buying a set of 4 for a V6 Honda. Ouch!

[Q45-pj]

Thanks guys!
So skibane, does that mean the thermostat is not seated properly? Or will getting a new one solve it?

Owner CS, is there a guide on how to clean the intake, egr, and iacv? Your reply made my entire day a lot smoother. Thanks bud!

[Skibane]

Thanks guys!
So skibane, does that mean the thermostat is not seated properly? Or will getting a new one solve it?

It could be stuck open all the time, allowing more coolant to flow than is necessary to cool the engine in cold weather. Typically, you can also see it on the temperature gauge - The gauge shows a lot cooler temperature out on the highway than idling or in traffic, because highway speeds force a lot of cold air through the radiator - and a stuck thermostat has no way of reducing coolant flow to compensate for it.

It's also possible that a previous owner removed the thermostat entirely, in the mistaken belief that doing so would make the engine run cooler during the summer.

[Q45-pj]

Great news! So yesterday I had the coil packs changed (all 8) and the power is insane. I drove it today and it was AMAZING! Now for the bad news. Since the coil packs were changed the car started to idle very rough. But it doesn't do it the whole time. If it's in park it's smooth then it's rough and smooth again. Almost like the stutter is going to continue and shut off the car. But then it's smooth again. At a stop light or in park it's the same. When I put the car in reverse as I ease my foot off of the brake it' does it a lot. Like consistently. One of the coil pack plugs was damaged pretty badly. So badly I had to help the guy changing the packs. He used some WD40 to get it off and sprayed some inside the connector afterwards. The air filter is also in. The guy changing the packs suggested I get a cold air intake. It looks like a pipe found in a honda. Wasn't too sure about it. Since changing the air filter the temp gauge has been better. Much less bouncing to cold side.
The oxygen sensors are next on my list since it's going to be cheaper than the valve cover gasket. I found a local mechanic that will change the four for $75. Is this something I can trust any mechanic to do? I was also told to just go ahead and do the lifters too. Since it's making noise and 15w50 oil isn't a good idea for New York City winters. What do you guys think?
On a much lighter note. I've been using the leather conditioner called Lexrol and steam cleaned the engine. Car is so fresh and so clean! I can't wait to have the rust dealt with.

[Skibane]

The guy changing the packs suggested I get a cold air intake.

The factory-designed air intake is superb. An aftermarket replacement won't improve performance one bit - and that money could be better spent on getting the car running right.

[Q45-pj]

Thanks Skibane! That's what I thought too. What do you think about the O2 sensors, is it something I can let any mechanic do?