Horrible MPG Troubleshooting

[Kryllan]

Good Morning everyone. I'm new to the forums and I also just bought my first Z. Its a 91 300zx NA with minimal work done to it. I've been feeling it out for pretty much a month now and I seem to be getting incredibly low MPG. It has what looks like an Injen Intake and Greddy Cat Back, but other than that its stock. I can't figure out why I can barely squeeze 16mpg out of it. I drive 70% highway and I drive it strictly like a granny. Shifting at the appropriate times to save gas (never above 3000rpms) and I've never ripped on it while I've had it and I haven't been able to hit 300 miles per full tank. Any and all help would be appreciated.

Maintainence that I know was done so far is Spark Plugs and Fuel Filter. I haven't had a chance to check the air filters yet. Any suggestions?

[RicerX]

Welcome to NICO and congratulations on the Z purchase! You will love it.

There are several things that can contribute to poor fuel economy, but I'm going to go out on a limb and say that there is a possibility the car may have had more modification aside from the intake and catback. This could lead to an issue with the Air/Fuel Ratios of the vehicle. Running too rich or too lean can result from the modified airflow. This is typically fixed with an ECU tune to optimize those mixtures, but given your car is pre-OBDII, I'm not sure what the best course of action would be to fix it (there are guys that are more well versed in the Z32 builds than I - I am more of a Z33/34 guy).

I would definitely check the air filters as well. Considering the car is newly purchased, a compression test is another thing I would check into.

Hopefully one of the more experienced guys will check in soon.

[Kryllan]

Hey Xenon thanks for the advice so far. Any and all advice is more than welcome. The gentleman I bought the car from is a good friend who told me everything that was done to the car. The only things I have left out are suspension is dropped 2 inches and the lights are JDM. Nothing else I can think of has been altered in the vehicle.

[nissanfreak12]

MOst things that would contribute to horrible fuel economy are o2 sensors, coolant temp sensor, old plugs, air filter, fuel filter, poor flowing injectors, timing not set correctly, leaky o-ring.

The best thing you can do is read..
everything-you-need-to-know-about-the-z32-t508376.html

It shows pretty much everything you will need to do. Now, are you sure the odometer is not sticking giving you false readings? This is very normal in these cars due to the age and crap they put in the gears for the odometer. I thought when I first got my Z that I was getting bad mileage, ended up being the odometer.

How is the car running? Do you smell any gas? Different smell from the exhaust?

[Kryllan]

Hey Nissan,

The odometer seems to be pretty accurate. I know the exact distance to and from work and thats really the only place I drive right now. As for all of the sensors and such I'm slowly checking and changing out what I think needs to be, but on a budget its going to take a little time. (Damn holidays). I have a buddy whos going to help check the compression tomorrow and we're going to scan for codes today. The car seems to run fine. If I don't let it warm up before driving it, I do get some hesitation while maintaining speed. At roughly 2500 to 3000 RPM the car hesitates a little almost like a stutter. I used to smell a little gas when I started it up the first week I had it, which I've been trying to figure out, but it went away. The exhaust doesn't really smell any different than any other exhaust I'm used to smelling.

I really appreciate all the help so far guys. I love the car so far and just want to make sure shes running healthy. It might also be important to note that I passed inspections for emmissions perfectly.

[BigTDogg (MA)]

Hey Kryllan,

Welcome to Nico! I'll provide you with some free fish

First, http://www.nicoclub.com/archives/nissan ... ostic.html

Second, if your car has over 100K miles and the factory O2 sensors, they're likely toast. Bosch 13091s are excellent replacements, and cost less than OEM.

Third, get a timing light and check your electrical timing setting. 15° BTDC is ideal.

Fourth, if it has Injen hard pipes, those aren't your intakes. The intake is below the center nose panel. A K&N/JWT/Stillen cone is a great upgrade over stock, and will aid not only performance by also fuel economy.

Lastly, cleaning your MAF sensor with some MAF cleaner never hurts. Just be sure it's fully dry (gently shake it out and let it dry 20 minutes or so, NEVER use compressed air) before restarting the car.

Hope that helps!

-Tony

[Kryllan]

Wow you guys really are the best. I work in the aftermarket automotive industry, but just recently started trying to work on and fix up cars. I sit next to a guy whos on this site and he reccommended it once I bought my Z.

TDogg I will definitely be checking everything you talked about. I can get all of the necessary parts for cost through my job.

[BigTDogg (MA)]

Nissan OEM, Bosch or NTK only for the O2 sensors. Trust me.

[Kryllan]

Yeah, I get the Bosch for cost from my company. I'm looking at $60 a piece which isn't bad.

[BigTDogg (MA)]

+1, Well done.

[Kryllan]

I should have an update on the ECU Scan in a little bit. I'll throw up an update once my buddy is done checking it out. I'm really hoping its something simple like the Sensors. I don't want to/have the money to rip her all down and fix a major problem.

[Kryllan]

Ok, so my buddy forgot he didn't have the right port tool for a Nissan. Hes a domestic guy so he only had domenstic ports. I'm going to have to do it the manual way and I won't have time to run the ECU Diag on my lunch. Maybe this weekend I'll have a chance to get to it. In the meantime I'll check my air filter.

[BigTDogg (MA)]

http://www.nissandatascan.com/

[Kryllan]

I'm definitely going to buy a scanner for the car down the road, but for right now since I'm sort of broke for the holidays I'm going to man up and do it the old fashion way. What better way to learn and become more accustomed to your car then to get dirty. I did check the air filter on my lunch break and I definitely have to say my buddy isn't the brithest when it came to picking a filter. It looks like a cheap crappy cone filter with a foam filter or pre-screen thats nasty and dirty as hell. Thankfully I'll be grabbing a brand new K&N or Injen filter for about $20 from work. Trying to do this car the right way.

[300ZXttZMAN]

Welcome to NICO! Looks like your already on your way to fixing the issue.

[Kryllan]

Hey guys/gals, So I haven't gotten to Diag anything yet, but I did come across some interesting info this morning at the gas pump. My needle was at roughly 1/8 of a tank or less and I filled her up. Now with an 18.7 gallon tank it should have takn roughly about 16.4 gallons or more. Lo and behold it took 13.4. I still had a little over 5 gallons left. So it looks as though my needle is off, but I definitely still will be looking into fixing any possible problems. My first priority with ANY car is to make sure its running as healthy as possible. Can't go fast if you cant go at all. Thank you so much for all of the help so far.

[DCaff300ZX]

That's a good point as my '90 NA gauge and the one in my '93 TT definitely operate differently. The NA light only comes on when the tank is actually nearly empty (16-17 gallon fills) while the TT lights up but only takes 13-14 gallons to fill.
If you want an accurate MPG reading wait until the fuel light comes on, fill an exact amount of gas (say 10 gallons) then check mileage when the light comes on again.

[RubyRed300ZX]

Gas gauges are notoriously far from accurate; and, that's irrespective of the car manufacturer. To properly check your fuel mileage, fill the tank full, set your trip odometer to Zero. Drive 200 miles or so. Refill the tank. Note the number of gallons. Take the mileage from the trip odometer and divide it by the number of gallons you just added. My own experience has been around 17 mpg combined, and never any better than 26 mpg on pure highway cruising (meaning open 4-lane highway for several hundred miles at a steady speed).

[Kryllan]

Thanks everyone. Yeah, I plan on checking on my next fill up what I am getting MPG wise. I just bought a new Injen filter. $23 shipped to my door. (Did I mention I love my job) So many aftermarket goodies at the palm of my hand. Got a set of 18" SSR Integral A2 Wheels for free from a buddy. Not in the best of shape, but I plan on refinishing them and everything. Big cosmetic plans for the Z. Its in rough physical shape right now, but I see it on the bright side. More things to have fun fixing up.