Still Have Bad MPG in 1997 I30
11-14-2013, 03:48 PM
#1
Thread Starter
Join Date: Oct 2013
Posts: 16
Still Have Bad MPG in 1997 I30
Hi everyone,
Thanks for the suggestions in my last thread. So far, I have replaced the spark plugs, fuel filter, air filter, knock sensor, PCV valve, Seafoamed the engine with the spray, and cleaned the throttle body (as best I could with a rag and Seafoam spray; didn't have time to get new TB gasket in order to do a full cleaning). I always run 91-93 octane. My Torque Pro phone app also shows that one of my rear O2 sensors seems bad, but my other rear one and the front one both seem to have the correct voltage (cycling between 0.1 and 0.8 volts most of the times). Per my phone app, I only get about 23mpg on highway at 70mph. I have 16 inch wheels on the car instead of stock, so that might affect readings a bit, but from what I've read on here, I should be getting about 27mpg on the highway. Any other suggestions? Thanks!
Thanks for the suggestions in my last thread. So far, I have replaced the spark plugs, fuel filter, air filter, knock sensor, PCV valve, Seafoamed the engine with the spray, and cleaned the throttle body (as best I could with a rag and Seafoam spray; didn't have time to get new TB gasket in order to do a full cleaning). I always run 91-93 octane. My Torque Pro phone app also shows that one of my rear O2 sensors seems bad, but my other rear one and the front one both seem to have the correct voltage (cycling between 0.1 and 0.8 volts most of the times). Per my phone app, I only get about 23mpg on highway at 70mph. I have 16 inch wheels on the car instead of stock, so that might affect readings a bit, but from what I've read on here, I should be getting about 27mpg on the highway. Any other suggestions? Thanks!
Last edited by dutch300zx; 11-14-2013 at 04:23 PM.
11-14-2013, 09:01 PM
#4
What is your mpg after a full tank of gas? This is more important than some phone app giving you an instantaneous reading...
Sure it could be better, but 23 isn't too shabby for such an old car. I average 25-26mpg on a full tank. Getting that last 4mpg may prove to be difficult considering what you've done. New front O2's may or may not help.
You may want to consider trying adding TC-W3 oil. See here
http://forums.maxima.org/fluids-lubr...itive-new.html
Sure it could be better, but 23 isn't too shabby for such an old car. I average 25-26mpg on a full tank. Getting that last 4mpg may prove to be difficult considering what you've done. New front O2's may or may not help.
You may want to consider trying adding TC-W3 oil. See here
http://forums.maxima.org/fluids-lubr...itive-new.html
11-14-2013, 09:09 PM
#5
Thread Starter
Join Date: Oct 2013
Posts: 16
I'm not too upset with it either, just wondering if I can squeeze a few more out of it and was surprised it went down about 1 mpg after I cleaned the throttle body. My app has both an instant and average readout, and the average is what says 23mpg; haven't filled it up yet to measure actual to see how close it is. I live in Michigan, so I suspect some of what I'm seeing could also be caused by a switch to winter blend gas, which has less heating content than summer blend. I'll continue to monitor it and keep an eye on my O2 and MAF readings. Thanks guys (and please keep suggestions coming)!
11-15-2013, 04:38 AM
#6
Member who somehow became The President of The SE-L Club
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I suggest you fill the tank, reset the trip meter and drive until your almost empty. Next fill up, record the miles driven and the gallons on the pump. Do some basic math and report back.
11-25-2013, 11:38 AM
#8
Thread Starter
Join Date: Oct 2013
Posts: 16
Hi everyone,
I finally got through a tank of gas (cut it a little close actually, car was sputtering by the time I got to the pump). I went 290 miles and used 17.6 gallons per gas, giving 16.5 mpg, roughly 50% city, 50% highway. Seems low to me; shouldn't I be getting at least 16.5 in the city along? I do use the gas pedal once in a while, but don't consider myself an overly aggressive driver. Could winter blend gas be the culprit, or could something else be causing it? I had an EGR valve CEL, but cleared it and it hasn't come back. Only other problem is a pending vehicle speed sensor fault code. Any info is greatly appreciated! Thanks guys (and gals)!
I finally got through a tank of gas (cut it a little close actually, car was sputtering by the time I got to the pump). I went 290 miles and used 17.6 gallons per gas, giving 16.5 mpg, roughly 50% city, 50% highway. Seems low to me; shouldn't I be getting at least 16.5 in the city along? I do use the gas pedal once in a while, but don't consider myself an overly aggressive driver. Could winter blend gas be the culprit, or could something else be causing it? I had an EGR valve CEL, but cleared it and it hasn't come back. Only other problem is a pending vehicle speed sensor fault code. Any info is greatly appreciated! Thanks guys (and gals)!
11-25-2013, 12:16 PM
#9
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Join Date: Apr 2006
Location: Oakland, CA
Posts: 704
I'm not sure what it takes to turning a pending code into a CEL, but that might suggest your speedometer isn't always showing true speed, and it may not be registering all the miles you travel. This may be obvious, but have you noticed the speedo stuck at 0 occasionally, or fluctuating at all?
11-25-2013, 12:18 PM
#10
Member who somehow became The President of The SE-L Club
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Seems a little low. Driving habits can play a big part in MPG and you do admit that you use the gas pedal once in a while. The EGR code is probably due to a lack of flow which can also slightly decrease your MPG. If the code comes back, clean the EGR passage and EGR tube which is located under the throttle body.
Short trips in the winter will also greatly impact MPG. The engine is going to run an enriched mixture until it's up to operating temp. Warming it up by idling is a waste of fuel also. Have you checked tire pressures? As the temps go down, your tire pressure also goes down. Adjust it for winter.
If you really want the best MPG, go on a long trip, use the cruise control or drive with a steady foot and keep it below 80. I have gotten upwards of 30MPG on trips easily but our day to day commuting doesn't come close to that.
Short trips in the winter will also greatly impact MPG. The engine is going to run an enriched mixture until it's up to operating temp. Warming it up by idling is a waste of fuel also. Have you checked tire pressures? As the temps go down, your tire pressure also goes down. Adjust it for winter.
If you really want the best MPG, go on a long trip, use the cruise control or drive with a steady foot and keep it below 80. I have gotten upwards of 30MPG on trips easily but our day to day commuting doesn't come close to that.
11-25-2013, 01:28 PM
#11
Thread Starter
Join Date: Oct 2013
Posts: 16
I haven't noticed anything funky with the speedometer. I did have to tighten the circuit board screws on back to make my cruise work again, and the cruise still doesn't always work, so I suspect I may need to do some soldering of the connections in the future. Perhaps the speed sensor could also be causing it to not work all the time, but it doesn't show any symptoms on the speedometer itself.
A lot of my city trips are short, and I don't often have time to let it warm up (which also used fuel, like you mentioned); it is also about 30 degrees here. This could all be giving me some crappy city MPG. Still wonder why my Torque app showed only 23.5 mpg at 65-70 mph on highway, but I hadn't reset the ECU after I cleaned the TB so it could've still been adjusting to new air flow values. And, as always, the crappy winter gas probably isn't helping either. I'll keep an eye on everything and try not to give it too much throttle (but it's such a smooth engine!), and report back if anything out of the ordinary pops up. Thanks everyone!
A lot of my city trips are short, and I don't often have time to let it warm up (which also used fuel, like you mentioned); it is also about 30 degrees here. This could all be giving me some crappy city MPG. Still wonder why my Torque app showed only 23.5 mpg at 65-70 mph on highway, but I hadn't reset the ECU after I cleaned the TB so it could've still been adjusting to new air flow values. And, as always, the crappy winter gas probably isn't helping either. I'll keep an eye on everything and try not to give it too much throttle (but it's such a smooth engine!), and report back if anything out of the ordinary pops up. Thanks everyone!
11-25-2013, 01:34 PM
#12
If your cruise control stops working, then it also means your odometer isn't rolling either at that time. Thus, there is no way to get accurate mpg's until you fix your vehicle speed sensor and go a full tank. Then report back.
11-25-2013, 05:55 PM
#13
Your MPG does seem low. I get about 25mpg average, and up to about 28-29 on the highway. Anyway as a 'think outside the box' option. Make sure all your wheels / brakes are in good working order. I remember stories of people complaining of bad gas mileage, and come to find out one of the rear brakes was almost seized up. May not be it, but worth taking a look at.
11-25-2013, 07:34 PM
#14
Thread Starter
Join Date: Oct 2013
Posts: 16
I understand where you're coming from on the speedometer, but I don't think that's the case. My trip odometer definitely seemed consistent with my actual traveled mileage. I have also read on here that the speed sensor code could be from the bad connection on the back of the gauge cluster. From what I understand, the speed readings go from the sensor to the cluster to the ECU, an I think the connection from the cluster to the ECU is my problem. I will definitely keep my eye on it though and thank you for the suggestion.
As for the other options, I'll take a look at the brakes, including the e-brake, which I used for the first time in a while (fresh snow here). I know my fronts are getting due for replacement, perhaps they are sticking as well. Thanks for the suggestions!
As for the other options, I'll take a look at the brakes, including the e-brake, which I used for the first time in a while (fresh snow here). I know my fronts are getting due for replacement, perhaps they are sticking as well. Thanks for the suggestions!
11-25-2013, 09:18 PM
#15
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Well dutch, if this makes you feel any better (it always cheers The Wizard up when he's down), my 18 year old, 250,000+ mile'd, stinky, rough-idling, 87 octane'd, '96 GLE gets over 400 miles per tank and 26-28 mpg consistently every single time. 
11-25-2013, 10:11 PM
#16
Your O2 sensors work best when they are able to rapidly change voltage readings, and just because they measure well does not mean they are performing optimally.
O2s do get tired and will not throw a code.
On my 98 I30, I replaced the front primary O2 chasing down some funny behaviour...but my mileage wasn't bad (23-24 mpg). Once it was replaced though, my car became more peppy. My problem eventually turned out to be a tired FPR.
Assuming the oldness of your O2s, I would replace the front primary.
O2s do get tired and will not throw a code.
On my 98 I30, I replaced the front primary O2 chasing down some funny behaviour...but my mileage wasn't bad (23-24 mpg). Once it was replaced though, my car became more peppy. My problem eventually turned out to be a tired FPR.
Assuming the oldness of your O2s, I would replace the front primary.
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