TBI fluctuating mileage?

HI guys I have a mileage question for you. I have a 1980 scout with 304v8, 89 gmc truck TBI, custom chip, vss, self heated O2 sensor, tuned by myself, with lean cruise initiated thanks to Mark works great. I have developed a pattern in mileage that is both frustrating and rewarding I've had this system for about 4 years now and the last 2 years i have had the custom chip installed, since then my mileage has increased greatly. For the last 2 years every time I go on a trip which is only a few times a year I get great mileage leaving home or after the scout has sat over night and is cold I will get 18-19 mpg's 5 times now using GPS to track mileage. This is going 60mph at 2400rpm on the interstate from gas station to gas station no driving around idleing. NOw on the way back I will fill up my vehicle to check mileage/ the scout will have only sat for say a couple hours while shopping and I will do the same on the way back go fill up and go directly to the next gas station to check mileage same route on the way back. I have done it 4-5 times now and one the way back it always drops to around 12MPG!!! I only have theories on this no real proof why this is and I would like to get to the bottom of it as it's driving my nuts....... Finally my most recent trip was a 3 hour route to hot rod power tour which is totally different route than i usually take to check mileage and the same thing happened 19mpg on the way down and 12 MPg on the way back!!! So now that I have nailed the same numbers on a different route using different gas stations to fill up I feel my numbers are correct.........
-Can someone please give me some ideas on what to look at that may cause this issue?


-Currently I have only used winaldi for data logging and it has been about a year since I hooked it up so my skills are rusty.... Is there a specific area to look at in a data log that would show a difference if I logged a trip down and back to see the difference?


-Is it common for a bad O2 sensor to cause an issue like this?? If so how can I test it?



-This has been driving me nuts for years now and I feel I must ask for help so any thoughts are appreciated!!!!!!!!!!


Thanks Brandon

could be more than a few reasons actually... wind patterns, elevation in either direction, fuel at the locations, road surfaces...

yeah but still very strange he has a pattern of return trips that drop significantly in MPG? :confused:

Unless he's just glade the trip is over and got his foot to floor to get home? :laugh:

Wind patterns do it for me whenever I do a trip west of where I live. When I head west I am into the wind, on the way home is always better MPG as I'm going with the wind currents...

Quote:

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Originally Posted by EagleMark

yeah but still very strange he has a pattern of return trips that drop significantly in MPG? :confused:

Unless he's just glade the trip is over and got his foot to floor to get home? :laugh:

Wind patterns do it for me whenever I do a trip west of where I live. When I head west I am into the wind, on the way home is always better MPG as I'm going with the wind currents...

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Happens to me all the time driving vehicles shaped like an aerodynamic brick.

Yep it is weird at first I thought it was elevation but I live in wisconsin I get the good mileage when I am heading south. I live at 1,473 feet the usual route I take leads me to wausau wi 1,195 feet. Finally the trip I just took lead me to 909 feet so I am dropping each time I leave but not by much I don't think? I had my dad drive his 02 chevy silverado with 5.3 liter as I am thinking of doing an engine swap for better mileage. He drove the same route same speed 60mph and got 19mpg there and 19mpg on the way back filling up at the same pumps! I am just half way there, drives me nuts. Originally i was guessing the 304 just doesn't have any extra power to take on the elevation change without sucking the gas. It does not feel like it is working harder on the way home though. My engine has great compression it's not tired so it is a good one to do this testing on.
THrough my reading I came up with 2 supposed common problems for mileage, however I think it would happen all the time?


I was reading a faulty O2 sensor could lead to this and testing it wouldn't necessarily reveal it as bad. the other idea I came across is a faulty temp sensor not sure how to test that one other than replace it.


P.S. I always use the cruise control to help keep the lead foot from happening....

Brandon

That's really strange Brandon? Really not much change in elevation, it's not like you've got a good downhill one way and a clinb the other?

You can test the CTS and it's always good to have a fresh quality O2 sensor. But neither explains why your MPG is so specific to return home trips?

Here's a link to CTS, but you could see it while looking at data too:
http://www.gearhead-efi.com/Fuel-Inj...erature+Sensor

The IH motor has the perfect spot for the CTS under the thermostat so that is not going to be an issue.

Spending some extra $$ on good O2 sensor is worth it. Old AC/Delco made in USA were good but since they were sold and now some boxs are from China? Who knows... might just be another cheap part. The new factory replacement is Denso or Delphi...

Where did you place the O2 sensor? It's not your return home MPG issue but if you move it down stream there is an adjustment for that.

My O2 sensor is placed at end of the header before it bolts to the exhaust pipe about 3-4 inches based off my memory. I was thinking of taking it on the route and recording a winaldi log and comparing it there and then on the way back. I am not sure how big of a file I would create on a 1 hour trip, I would think that is almost too much data to look through...Problem is I only have so much cash wright now and can't just throw money away so I need a reason to drive that way.....
P.s. thanks for the link....
Brandon

too much data? probably not....

i've recorded ~4 hour driving sessions at 8192 baud(so something like 145,000 samples), there really isn't such a thing as too much information, if you can relate it to real-world events.

Geez, move up to some new software and get TunerPro RT running on your laptop. It's like having a modern tool to work with and if you want to find small issues or glitches it is the tool of choice. The 1227747 $42 files are available and I've got a lot of time into them to make them better!

I'm not sure if an entire hour of data is needed and wow what a big file

So your O2 is a few inches behind the header collector? That's a good spot and better then in header as sometimes there, they will pick up more exhaust from just one cylinder. But that's a long move from stock which on most of the donors you listed are in manifold. So the "INT - Delay vs. Air Flow" would need about 400% added to it. This can make a big change in what you see for BLM data readings!