Hey guys,

This is on a 98 Z28 with basically full bolt ons, rear O2's are deleted and has a mail order tune by frost. It also has a 2002 PCM in it, other mods are listed in sig. I have been getting terrible gas mileage lately and I'm trying to figure out why. I'm not sure what these readings are supposed to be, but I'm assuming they should be the same.

So what should they be?

Is one of them within spec?

And what does bank 1 and bank 2 mean?

I'm asssuming bank 1 is driver side and bank 2 is passenger side?

Thanks!

Oh, and these graphs were generated while I was holding rpm's at about 2500. Not sure if that's what you're supposed to do or not?

 

I switched front O2's and the fuel trims stayed the same. I'm going to be replacing the O2's anyway because I already ordered new denso ones, even though I don't think that's the problem anymore.

 

Anybody out there...??? And btw the car runs just fine still. Just getting terrible gas mileage.

 

Those are short term fuel trims. They are the fuel trims that happen within the long term fuel trims. The long term trims can be adjusted through a tune, the short terms are adjusted by the computer only.

Bank 1 is Drivers, Bank 2 is Passenger side.

The trim % you are showing is nothing to worry about and certainly wouldn't cause your issue.

A car can smell rich, but not be rich at all. Especially if you no longer have cats.

Crappy fuel economy has a lot to do with driving habits as much as vehicle running condition. Did the fuel economy suddenly go to poo? If so, check for exhaust leaks first. Exhaust leaks before your O2's and even shortly after can tell your O2's that you are running lean and it'll richen up your AFR.

But, if you suddenly changed your driving conditions (new bolts on's and your enjoying them or maybe you got a new job closer to home) that can effect your fuel economy more than most mods ever will.

 

Haha, you nailed it. I did get a job closer to home where I only drive 5 mins through town to get there compared to 40 mins on the highway.

But still I noticed my mpg's suffering on the highway too, about 18 mpg on the highway when it normally gets 25 mpg. Now I knew my mpg would go down a lot driving in town only, but this last tank I only got 8 mpg. I guess I will try driving like a grandma and be a lil lighter on the gas and see how much it changes.

Oh and I just noticed on those graphs I posted that the vertical axis values are different, so it looks like the fuel trim % may actually be the same on bank 1 and 2. Oh boy, I may just be worrying all over nothin, but really 8 mpg?? It just doesn't seem right to me.

 

Here is some advice from a non tech person.

I was told by Frost b/c I have a similar situation "the front 02's control gas mileage and they can be running rich at cruise". He also said to "check for exhaust leaks". Imo first put in the new 02's and record the mpg so you have a base line where the more experienced people on here can help.

You need to find someone near you w/ hp tuners or ls1 edit to look at the air fuel ratios. A phone program will not help you.

410's are not going to be very mpg friendly lol. Hope this start helps.

 

Yeah I was a little questionable about the free app, but I mainly wanted it to check for codes. Then I realized that it could check fuel trims, O2's, maf sensor, and all kinds of crap. Idk seems to be pretty legit so far, I just need to learn how to read the graphs lol. I'm going to check the long term fuel trims and see what those show.

 

Good deal.

 

If you have an exhaust leak, replacing O2's will not make a difference assuming the only issue is the exhaust leak. Fix the leak first. The front O2's adjust the long term and short term fuel trims (AFR and gas mileage). The rear O2's read how well the cats are working.

An O2 sensor reads how much unburned oxygen is in the exhaust. It'll then add or remove fuel to target a specific voltage (tuneable, but factory set to ~14.7:1 AFR). If you have an exhaust leak then air (oxygen) will enter the exhaust after the combustion process but before the O2 sensor (this can happen post O2 depending on exhaust mods and cam) which will give a false lean condition. The false lean will richen your AFR and fuel economy (along with power) will drop. The O2 sensors know not there's an exhaust leak, so no codes will be thrown and pulling data will not show they are reading wrong.

In any case, when checking fuel trims look at the long term % not the short term. What does your long term fuel trims look like?

I once had a job that was 5-6 minutes to and from home. My commuter car was a 2004 Saturn Ion. At my previous job I'd routinely turn in 26-28mpg for my daily commute and 36+ when all highway (routinely besting 40+). Then once I started driving my new daily routine for my new job my car would only get 16mpg. Crazy bad, nothing but my daily commute changed.

What needs to be considered here is the car needs to warm up. It may be worth letting the car idle for 5-10 minutes to get it into closed loop. Try that for a couple weeks and see how it does. It's not just all city, it's also when the car is driven. 5 minutes is hardly enough time for the engine to settle in. Driving for only 5 minutes at a time is very very very hard on any engine.

After I left that close commute job and my new job was a 25-30 minute drive, my fuel economy returned back to 26-28 and 40+.

 

Okay this may get a bit pic heavy, but here are my O2 readings and long term fuel trims.

These were all done while holding 2500 rpms.






First LTFT reading






Second LTFT reading






And this is what my temp was after holding 2500 rpms, sitting still, for the last 3 mins or so.... oops






So basically, at this point, I guess I'm wondering if all these readings are within spec?

From what I have read online it seems like they are all okay.

Thanks again

 

Here's how this all works. Your engine takes in air as read by the MAF and Map sensors and then calculates a fuel mixture based on that. After the fuel is burned and goes into the exhaust the o2 sensors take a reading. They do this hundreds of times a minute. Perfect AF for the o2 sensor is about .450 mv which is stoich at 14.68 for regular gas anything less than .450 will be seen as a lean condition by the engine and the pcm will add fuel, anything over .450 will be a rich condition and the pcm will subtract fuel.

With this new data the pcm will respond accordingly, the negative STFT fuel trim you are seeing is a response by the pcm to the o2 sensors it is subtracting fuel becauuse the o2's posted a slightly rich condition. Over time these STFT's are learned and will be stored as LTFT's in the pcm, the the pcm resets and begins logging stft again.

The problem with an exhaust leak before the o2 sensor is that it will cause the o2 sensor to see a lean condition which will make the fuel trims positive and dump fuel into the motor thus killing gas mileage. The effect is similar with a vac leak after the maf.

Now since that is cleared up, you cant look at LTFT's while the car is idling as they will be all over the place. You need to drive the car and log them for about 20 minutes and take a look. In general you will never be perfect so ltft's in the +-5 range is ok.

In general though I dont put much stock in ltft's as they can be pretty unpredicatble at times because of extended idling or just varying conditions. If im tuning without a wideband I'll log maf vs short term fuel trims and turn the lt's off and adjust the maf that way. I do this until I get a stft/maf error i like then I just turn on the ltft's and never look back.

Hope that helps give you a better understanding of what you're actually looking at

 

The job of long term fuel trim is to keep short term at 0. So lets say you have a long term of 5 and a Short term of 0 , That means at some point your short term was at 5 and your long term had to be adjusted to 5 to make short term 0 again.


So with that said , If your long term fuel trims are within 5 Plus or minus then you are doing good. If your short terms are anything other then 0 , then you have an issue that the long term has not adjusted for yet. Now keep in mind that long trims are decided based off the information the computer has in a "table"
For instance from the factory the computer is programed so much air , at a certain tempature with certain outside pressure will result in the fuel trim being such and such , and when it does not match thats how you get plus or minus any givin number.


Anyways your fuel trims look fine.

can you provide your Long term and Short term trims @ idle please ?

 

Yeah, I will record short and long term trims at idle tomorrow. I'm at work till 6am right now.

 

Is anyone else concerned about the 233F coolant temp from just idling at 2500? There should be plenty if coolant flow, and both fan stages kicked on.
I don't have my laptop in front of me, but I remember there is an ECT fueling table that will add fuel when temps get too high in order to bring combustion temps down. I'll take a look at work tomorrow unless someone else shoots me down before then. :-)

 

stock fans will kick on at 226-230 depending on the tune. if its hot in his area im not shocked at all. I run a 185 tstat meziere ewp and fan settings at 185 and my car will still creep over 210 in a hot day and bad traffic

and op as long as the ltft's are in good shape you are probably ok. stft's will bounce around quite a bit so it will be difficult to actually see them.

 

I agree with what everyone else is saying. All that looks good and it's pulling a little bit of fuel (means no exhaust leaks causing issues). If anything I'd say warm up your car before you drive to and from work and use a little less right pedal. LOL.

 

Okay guys here are short term and long term fuel trims at idle

 

I also checked o2's at idle, and this time it looks like driver side o2 isn't switching back and forth quite as good as passenger side. So I think I will recheck all this stuff after I replace o2's

 

all this means is your running abit rich at idle, equally on both banks so tells me it's likely not a problem most likely caused by your upgrades, the minus 10 means it's pulling fuel personally still looks ok to me

 

No offense op but have you read through our replies and tried to understand what the graphs mean? Instead of just posting snapshots and asking for evaluation? We explained it pretty thoroughly.

Takinga snapshot doesn't really do much as the change so frequently. You need to understand what is acceptable and what's not per the posts above then interpret your results. Also the idle trims don't mean much you have to see them under load while crusing at speed