"good" long-term fuel trim number for renix

[bval]

I've got an '87 Renix 4.0, 746 injectors, on 32s and stock 3.55 gearing. The bigger tires on stock gearing are not a recipe for great MPG numbers, but it's not absurd either.

 

Last year after the injector upgrade and a new NTK O2 sensor, I saw 15-17mpg (with a speedo gear that's just slightly off, so I believe it to be more like 16-18 based on GPS mileage) depending on terrain and cruising speed. Heck, I hit 17mpg climbing out of the San Juan Mountains in Colorado/New Mexico.

 

Over the past year it's gotten worse, and recently it hit about 12mpg. I also started noticing it holding a gear too long, which I narrowed down to a bad TPS, and replaced and reset to 17%. That all works great now. But, now I'm seeing it go lower and it hit 10.6mpg on the latest fuel up.

 

I've got a Renix Engine Monitor (h/t @NickInTimeFilms for the hookup). It shows good O2 heater voltage, the engine goes into closed loop as it should, and I see the O2 sensor swing correctly. CTS and IAT readings look normal. My LTFT (long-term fuel trim) is low, around 110-115, but not way low. I would expect something around there for 746 injectors, no?

 

So, is that a good LTFT number? And what other things should I check/replace to track down the MPG issue?

 

EDIT: Also running an '89-90 ECU and TCU.

 

Thanks,

 

Brandon

[Ωhm]

All mentioned parameters (REM) seem to show vehicle in running as it should. Possible fuel leak between tank and engine would cause loss of MPG's.

[bval]

Just now, Ωhm said:

All mentioned parameters (REM) seem to show vehicle in running as it should. Possible fuel leak between tank and engine would cause loss of MPG's.

 

I haven't thoroughly examined that yet, but I have zero dripping leaks, and it's too cold here for a vapor leak to be likely, but I'll inspect all of that anyway.

[bval]

Just now, bval said:

 

I haven't thoroughly examined that yet, but I have zero dripping leaks, and it's too cold here for a vapor leak to be likely, but I'll inspect all of that anyway.

 

Also, wouldn't I expect to see LTFT rich if it was losing fuel? It would add fuel to compensate, no?

[Ωhm]

Loss of fuel between tank and engine is never seen by the O2S.

[bval]

Just now, Ωhm said:

Loss of fuel between tank and engine is never seen by the O2S.

 

True, and I'll check. Just thinking that that would result in low pressure in the rail, lean conditions, and the ECU adding fuel.

[Ωhm]

Could be a fuel return line leak, if you have a leak at all. 110 - 115 LTFT shows your running rich and ECU is leaning INJ PW, but well within its range of authority.

[dvitha]

If you were 4.10 you would gain a bit, because you would be closer to original factory specs on ratio, for that tire size. Now that a side, did not see if on your info if you done a tune up (plugs, wires, cap rotor). Just wanted to say that. The return line are notorious for leaking, because it gets dried out. When checking for leak let every get nice and hot. The rubber fuel return line becomes more flexible when hot engine bay temps.

[bval]

5 minutes ago, dvitha said:

If you were 4.10 you would gain a bit, because you would be closer to original factory specs on ratio, for that tire size. Now that a side, did not see if on your info if you done a tune up (plugs, wires, cap rotor). Just wanted to say that. The return line are notorious for leaking, because it gets dried out. When checking for leak let every get nice and hot. The rubber fuel return line becomes more flexible when hot engine bay temps.

 

Thanks, it's had a tune up in the last year. I'll warm it up later and go over the fuel lines carefully.

 

4.10s would be awesome, but they're over the carrier break, so my re-gearing project has been on hold for a while.

[bval]

On 11/14/2018 at 11:05 AM, Ωhm said:

Could be a fuel return line leak, if you have a leak at all. 110 - 115 LTFT shows your running rich and ECU is leaning INJ PW, but well within its range of authority.

 

Thanks for the sanity check. Sho nuff, leak on the tank end return line. Now to decide if I'm going to fix as is or do a full replacement with AN and speedflex. 💰