Anyone had an EFI Samurai that mileage went in the toilet on?  I had the ECU go out and after repair it ran okay but the mileage went from 24 before to about 10 mpg now.  I since swapped another ECU and still lots of carbon from tailpipe and poor mileage. Mechanic said cylinder pressure was okay and it has newly changed cap, rotor, wires and plugs. Recently changed fuel filter and cleaned checked tank and sock on fuel pump. The check engine light does work but not coming on and no problem codes.  Checked resistance vaues on TPS and all within spec of repair manual.
  I previously changed the 02 sensor and plugs due to the carbon on them but no better mileage.  Ive checked all the vacuum lines and find no leaks.  Swapped the CTS sensor and no change....Any ideas?
Mileage says 99,700 on odometer. Original 1.3 engine with pacesetter headers and stock EFI. Has a recently replaced cat and exhaust system.

Where is the O2 sensor located on the exhaust manifold? is it a on-wire sensor or a 4-wire?

Hey Rhinoman,
   THE o2 sensor is located on the upper end of the pacesetter header. The headers came with a factory prewelded bung for the sensor and it is a one wire on this EFI Samurai.  The mechanic that worked on it previously swapped the O2 sensor with a cheap one wire replacement due to excessive carboning and a high volt reading on the original sensor.
Is there any sensor that works better on these than another?  Thanks for any ideas.

It's possible that not everything got fixed in the ECM.  Only way to know for sure is to swap in a known good ECM and see if your MPG comes back.

I would check that the O2 sensor is earthing correctly through the manifold. It sounds like the O2 sensor is in the right place, the one-wire sensors need to be much closer to the exhaust ports than the 3 or 4-wire sensors. if you are near a main dealer then they may still have a scan tool that will be able to check the sensors and actuators, it should work out much cheaper than randomly changing stuff.

    I will try to check the ground on the O2 sensor.  I did swap the ECU with no noticeable difference.  The header seems to leak some in a unwelded-poorly sealed connection just below (downstream) of the O2 sensor. (Pacesetter quality control must be really bad) I think it was the same prior to the mpg degraded.
    Am I correct that much of the lean rich input to the ECU is via the readings on the O2 sensor and MAP?  I dont think there is a dealer in my area.  If anyone has schematic or input values to the 90-93 ECU I could check voltage-resistance values to see if anything looks out of spec.

    The header seems to leak some in a unwelded-poorly sealed connection just below (downstream) of the O2 sensor.

Good chance this is your main problem. Any leaks prior to the cat will cause the o2 to read high, & the ECU will compensate by enriching the mixture.

  I checked the ground from 02 sensor to neg battery post, firewall, and engine block is good.  The voltage reading off the sensor is .93vdc after warmup. Haynes book says .45v is normal.  I'll get the downstream leak on the header welded and see if the mileage improves. Thanks to all for your help and ideas.

 I checked the ground from 02 sensor to neg battery post, firewall, and engine block is good.  The voltage reading off the sensor is .93vdc after warmup. Haynes book says .45v is normal.  I'll get the downstream leak on the header welded and see if the mileage improves. Thanks to all for your help and ideas.

If once the engine is warmed up, the O2 sensor reading is staying above .45V, then the AF (air fuel) mixture is rich).  The reading should deflect between above .45V and below .45 V, as the ECM changes the AF ratio.
If the O2 sensor is good and the wiring between it and A19 on the ECM is good, then you need to check the TPS (throttle position sensor), the pressure sensor and hose, the ATS (air temp sensor), the WTS (water temp sensor), and injector.  If all of those check out, it says to look at the ECM.



Here are the things that the FSM (much better than a haynes or chiltons manual) says could cause poor MPG....
high idle speed
fuel pressure out of spec and/or fuel leakage
faulty EGR system
defective injector
defective TPS, WTS or pressure sensor
faulty ECM

If you can find a copy of the 90-94 samurai supplementary service manual, I would download it.  It is much more detailed and has much better info in it than any store bought hanyes or chiltons manual.  If you can't find the manual online to download, I might be able to email it to you.

As was mentioned, if you have any exhaust leaks, that can cause all sorts of running issues and MPG issues.  I'd definitely get those fixed, before I would worry about checking into anything else.