1997 Infiniti Q45- Lousy fuel mileage fix?

[MovieMaestro10]

Hey everyone,

Long time follower, first time poster. I love 90's cars and just purchased a super clean 1997 Infiniti Q45 (pewter green with tan interior). I should mention that I'm a car guy and am relatively familiar with how cars work and operate, but I'm not a mechanic (I live in an apartment so wrenching is tough to do), a big reason why I gravitated towards the Y33 seeing how many of you think it's the most reliable/easiest to maintain Q45.

My car was decently-maintained, picked it up for $3k/125k miles a month or so ago, I have some but not all service records- It was owned by a Marine who kept it garaged for 17 years. Last few years owned by a guy who had three other Q45s, would drive it a few miles every week and that's about it who I think let the maintenance slide a bit as he barely drove it. I've put in another $2k or so to make sure its a reliable long distance machine (new alternator, new power steering system, new tires, fluid changes).

I love the car but the gas mileage stinks, even for its type. Looking on other forums here I feel like I should be getting around 16 city, 21 highway. I'm actually getting around 13.5 city, 17.4 highway around LA. I don't drive slow but I'm not hoofing it either. I run on 91 premium gas, with overdrive on. I used to have a 2007 Ford Crown Vic ex-police interceptor, which had a 4.6L V8 with similar power and 4 speed auto on regular gas and was heavier, and even it got better mileage than my Q45.

I've already tried the obvious/cheap things- Replaced air filter, tires at full pressure, fuel injector/gas treatment bottles in gas tank. Anything else I can do, hopefully myself, to improve the gas mileage? I'm reasonably handy and willing to get my hands dirty but don't have a garage or full set of tools.

Car seems to run strong, smooth idle, but I see no records of spark plug replacement. Was also wondering if MAF cleaning/intake hose cleaning would be something I should look into. Can cleaning the throttle body be something I can do myself as well?

I'm not expecting to turn my mileage into Prius levels, but a few mpgs better would help a lot with LA gas prices, esp with how much I'm planning on driving this otherwise awesome car! Thanks everyone.

[94Q45Beast]

yeah it's baffled me many times, sometimes I get 200 miles a tank (free way driving) and then sometimes I get over 400 miles (415.7 miles to be exact) to a tank (free way driving) with Atlanta traffic....never got to the bottom of it. post if you do.

[goody90q45]

See if there's any stored engine fault codes. The knock sensor code will not illuminate the check engine light but will be stored in the ECU.

[3Q Jay]

post up your ca smog report.
other things: what is the true static timing at warm idle (with a light--not just what the grease monkey at the inspection station thinks)?
ks could affect dynamic timing and hurt mileage.
plugs--good idea. only use NGK double plats. be prepared to replace all 8 coils with genuine nissan parts.
other: bad alignment? dragging brake pad (can check temp of the rotors with IR gun after easy driving)?

best you can hope for is 23.5mpg on 110% (meaning ONLY x-c hwy 65-70 mph steady, no side trips) freeway driving.

[Q451990]

Couple of thoughts... if your car isn't warming up fully due to a thermostat that's not sealing completely, it may not be entering "closed loop." Also, your front (before cat) O2 sensors may be getting sluggish. I would invest in a cheap OBDII scan device that links with a smart phone or tablet. The ELM 327 scan tools are available on eBay for less than $5, and Torque Lite is free on Android devices. With these tools, you can see real time O2 sensor performance, check for stored codes, see your coolant temperature etc.

[MovieMaestro10]

UPDATE- I went ahead and cleaned out the throttle body and MAF. MAF looked brand new and clean but throttle body was a bit dirty, was harder to clean because of the open traction control butterfly valve that was in front of it. Will drive it another tank and see if there's any improvement before proceeding with further actions. After digging around the engine, I do feel like replacing the spark plugs and coils wouldn't be too hard for me to tackle.

Q451990...I did reset the battery after cleaning the throttle body (to reset the idle speed). Would hidden codes come to life after that point? No warning lights of any kind as of now.

[MovieMaestro10]

UPDATE- Got the code scanned and came up with bad knock sensors, both need replacing.

Seems to make sense with everything that's going on...And apparently these do not trip the check engine light, so they were indeed hidden codes.

Been looking for more info on how to replace them...Does the entire intake manifold have to be removed to get to them? Wondering if that job is worth it for a few MPGs.

[Q451990]

It can apparently be done without removing the plenum.

y33-97-q45-knock-sensor-replacement-t166615.html

Bad knock sensors send the ECU into a "max protect" mode where it works under the worst case scenario fuel map... so it sets your engine timing and fuel ratios as if it's detecting knock. So yes, it could cause lower MPGs due to the rich fuel map, but you're also losing performance. A technician once told me that the description that he most heard from owners who'd had the car for a while (back when the sensors were good) was "something just doesn't feel quite right"

So you'll be rewarded with more than better fuel economy if you replace them. I would avoid aftermarket or cheap sensors of unknown origin... there are a lot of knock-offs floating around out there that I suspect are just cheap resistors encased in plastic.

[94Q45Beast]

what I don't get is how a genuine nissan Knock Sensor is sold on...lets say Ebay...for 30-60 bucks when the Dealer sells them for around 340 bucks. which means someone had to pay the dealer price to sell it for 30-60 bucks, that's what I don't get. why would they sell it for that cheap. btw. I bought the 35 dollar OEM Nissan knock sensors and they only lasted 8K miles before I got a code for it.

[3Q Jay]

UPDATE- Got the code scanned and came up with bad knock sensors, both need replacing.

Seems to make sense with everything that's going on...And apparently these do not trip the check engine light, so they were indeed hidden codes.

Been looking for more info on how to replace them...Does the entire intake manifold have to be removed to get to them? Wondering if that job is worth it for a few MPGs.

The entire intake manifold (i.e. upper and lower plenum) *should* be removed to do the job properly.
Not saying the KS can't be replaced with the plenums on, but you don't get the full benefit.
by removing the upper plenum you also have easy access to the EGR pipe which undoubtedly needs cleaning. Removal also makes changeout of the DS vcg a snap and i recommend that as well, you were already talking plugs and coils so once those are removed then there really is no reason not to go ahead with the vcg.
by removing the lower plenum you have easy access to the KS, you can change the y-pipe hose, you can clean the lower plenum runners to spotless.

Having done both the G50 and the FGY33, I can tell you that the FGY33 is cake by comparison.
one more thought:
nissans (maybe other cars too, but especially nissans) LOVE to be taken apart. removing layers of the onion is usually not that bad and you are rewarded by gaining easy access. There are many instances where i've pissed away waaaaay too much time by trying to take a shortcut only to end up removing the interference anyway. re-assembly (for me anyway) is typically half to no more than 2/3 the time to tear down. they go back together very easily.

[MovieMaestro10]

3Q Jay...What do you mean by DS VCG? Not familiar with that.

Would love to know the best deal for getting new knock sensors. I'm going to have a trusted mechanic change the knock sensors as its beyond my capabilities/tools, but they source from dealership so knock sensors are 380 bucks for two of them! So they said I could bring in my own parts, they just couldn't guarantee the parts obviously. So I'd rather not buy knock sensors that are going to fail soon considering I'm paying them about $300 in labor.

Any thoughts for where to get good value knock sensors (and the harnesses while I'm at it)? Thanks!

[3Q Jay]

vcg=valve cover gasket
ds= driver side.
the nissan part number for the ks is 22060-30p00---they fit a LOT of engines over the years, so you are not limited to your local infiniti dealer.
There are a LOT of second rate parts out there. I can't swear that a genuine nissan part (how you would be assured of authenticity is arguably an issue as well) won't fail, but the heartache of an 'OEM second' is simply not worth it given the amount of labor involved.
when IoS was THE source for OEM Q parts, many of the things I ordered came not only in genuine nissan packaging, but would have a second part number sticker on it with IoS address, and "ASR"
ASR-automatic stock replenishment. Due to the volume of Q parts sold, they pretty much had a fresh inventory. the ASR gave me additional confidence that the parts were coming from a nissan value chain (no third party counterfeiters).

[3Q Jay]

you should have your mechanic change out your fuel hoses as well. genuine nissan hose is A6440-N7686. sold in 5 meter (3 meter?) lengths. pricey.
never leaks. correct fuel hose clamps (not the worn out tension type you have now) is NISS # 2xxxx-53A00 you will need 8 of them if i remember right.

[MovieMaestro10]

Help me guys- What the heck does the wiring harness for the knock sensors look like? I'd like to replace them if I'm going to the trouble of digging down there.

Checking online I get a million results for something that goes one connector to another connector- http://www.ebay.com/itm/Knock-sensor-Wi ... 1156001695

But the Infiniti dealership is saying it goes from one connector that splits into two (which I cant even find online). I feel like they gotta be confused.

Anyone actually know?

[Q451990]

what I don't get is how a genuine nissan Knock Sensor is sold on...lets say Ebay...for 30-60 bucks when the Dealer sells them for around 340 bucks. which means someone had to pay the dealer price to sell it for 30-60 bucks, that's what I don't get. why would they sell it for that cheap. btw. I bought the 35 dollar OEM Nissan knock sensors and they only lasted 8K miles before I got a code for it.

It's not that difficult to created counterfeit packaging and parts. My guess, and this is just a guess, is that you might be OK buying Hitachi parts off of Rock Auto. I would hope their channels are vetted enough to be sure you're getting the real deal. I know that Hitachi alternators from Rock Auto come directly from the Hitachi facility in KY.

[MovieMaestro10]

UPDATE- Knock sensors were fixed (also replaced harness, it was the connector that splits in two like a Y). Immediately noticed a performance gain, but mileage is still pitiful (12 mpg or so in mostly city driving).

Couldn't find any records of the fuel filter being changed, so when ahead and had that done...Apparently it was long overdue, possibly even still on stock fuel filter, as the gas that came out of the old one was tarnished black.

Gas mileage still in the 12 mpg range, so on to spark plug change next, a job I'm going to tackle myself. Again, can't find any records of it ever being done, so curious if these were long overdue for a change as well. Any way I'll know if the plugs I pull out were from the factory?

[94Q45Beast]

check your FSM or go to your local dealer parts website and look at the model number, should be an NGK something.

https://infinitiofsouthatlantaparts.com ... code=22401

[MovieMaestro10]

Update- Looks like the old spark plug (correct NGK's) and the coil packs (Nissan OEMs) still looked fresh when I pulled them out. So I didn't end up swapping them.

My engine threw a code for the EGR system. There were a few leaky hoses, as well as a leaky MAF hose that I wasn't aware of. Got those fixed up. Going for a long drive this weekend, hopefully mileage has finally been improved!

[sheikhnbake]

Would love to hear your mileage update. The '97 Q45 I had was always 16-17mpg in the city and 24 mpg highway without fail. I think it's still the same, although my ex-wife now drives it, with 160,000 miles on it. I drove it until only about 75,000.

[HansCC]

Just went through this with my 1999 with 50k - Had to replace all coilpacks, plugs, clean fuel rail, clean injectors, clean throttle body - last tank with 25.1 MPH.

[BadQ45t]

It is funny, I live in the Bay Area and when i take my Q to LA the gas mileage always takes a drive. I have big "19 rims, sticky Michelin YR tires which for sure cut mileage by a bunch.

I used to see 350-380 a tank, but now 280-300 seems like the best although my gas gauge is not very accurate anymore and I usually have more gas than I think I do. I get 17-18 on good days, i will see 23-24 on some drives where I really keep it 70mph, but I am lead foot so I get what I pay for...bad mileage. But even hyper-miling this doesn't do much. We're screwed by the 4 speed transmission. My bros old 96' LS400 did 24-26 due to 5 speed I suspect.