PNWjeeper

I didn't want to hijack the thread where this conversation started here so I stared a new one.

At this point in time I'm not looking for suggestions on how to improve or why I lost MPG.

Before I swapped to the Bosch 746 injectors for Renix era XJs I got about 13 MPG not the best but not the worst either. After the swap I clocked in at 11 MPG for those keeping tabs at home that's a 2 MPG loss.
My explanation for this, which I'm currently testing, is due to a crack in the exhaust manifold causing false readings at the O2 sensor. While inspecting the cracked manifold, as I feared, my O2 sensor became inoperable as I don't have the money to replace one at this time I'm going to run through a tank of gas and see if the exhaust leak is the cause of the loss.

Info on my jeep is as follows:
Stock suspension
Stock intake
Upgraded 746 injectors
Slightly larger then stock tires (235 VS 215)
Stock exhaust setup

Will update as my test finishes, which may be a while as my jeep doesn't get driven much. This last tank of gas lasted me two months.

Martlor13

How did you calculate?

PNWjeeper

186 miles on the odo from the last time I filled the tank divided by 16.27 gallons to fill up this time.
As for the 13 MPG before it was more rough estimation as I got around 250 miles on a full tank before the swap.

andrewmp6

Driving habits and even the gas you use can case that lost of 2mpg.If your gas station has switched to the winter blend of gas that could do it right there.

cruiser54

Only use NTK replacement O2 sensors.

All bets are off until that's fixed.

Fred/N0AZZ

I think your right on track with what you found on the exhaust.

PNWjeeper

I had been driving a little more liberally testing out the new injectors but nothing that I would expect to cause that much of a loss.

mschi772

Whenever you change a component, especially one so directly involved in air/fuel mixture, you have to allow the computer time to "learn" and reach a new equilibrium. You can't just test mpg off the first tank of gas after a mod and think that you're being scientific/accurate. Give the computer some time to "learn" the new situation then take data from multiple tanks of gas and average them. One tank of gas is just one data point; drawing conclusions from that even if you hadn't changed anything recently is just bad science.

Have you changed your speedo gear since getting 235s? If not, are you mathematically accounting for the larger diameter tires (235's are about 1" larger--29" vs 28")?

How exactly are you going to do this? Did you repair the crack? If it is still cracked, I don't understand how you'll be able to look at your data and know "Yep, the crack caused this." If you repaired the crack and the numbers improve, OK.

You should repair that crack and replace O2 sensors (NTK only as Cruiser said) if you haven't.

You may have winter blend gasoline running now. Winter blend will always yield worse MPG than non-winter gas.

bad_idea

PNWjeeper has Renix. It's my understanding the Renix have no long term memory. No relearning to do.

cruiser54

You are correct.

s346k

so the crack in the manifold didn't matter with the stock injectors? this doesn't make any sense.

changing variables and claiming a pre-existing condition caused the result isn't logical.

edit: there has to be other issues. 11-13 mpg with a stock jeep? even with the magical fuel dump monster known as the manifold crack my jeep never dropped under 20 mpg unless the weather was cold.

mschi772

Ugh, and I knew that. I'm just so used to reminding guys of it that it's a reflex now.

PNWjeeper

No I did not account for the 1" of extra tire for two reasons 1. I don't know how to and 2. It was a small enough difference that I didn't expect it to drastically effect my readings by say 8MPG.

I have not fixed my cracked manifold but with my O2 sensor being temporarily inoperable, by means I cannot explain per forum rules, I hope that the default setting from my ECU with a non-working O2 sensor will cause less fuel compensation.

So in short by artificially "fixing" my cracked manifold (inoperable O2 sensor leading to a default setting) I hope to find that my mileage increases.

Also, while yes one tank of gas is not a good series of data points. I failed to state in this thread that my work provides me a vehicle so one tank of gas in the jeep can last up to 6 months or possibly even longer depending on events.

Hope I didn't miss any other questions or flaws in my plan or scientific method.

Martlor13

6 months!!?? Dang!! Isn't that kind of bad? I vaguely remember something of the sorts. Probably wrong though haha

s346k

the poor fuel mileage would be a direct result of no o2. if the ecu gets no feedback from the o2 it will go back to open loop. open loop is a fuel rich state by default. i'm not sure how you can expect to get accurate mileage results given all of those variables. the manifold crack will have very little or no effect on your fuel mileage, especially at this point.

if the injectors are rated the same as stock i wouldn't expect to see a drop in mpg after the swap, operable o2 or not.