Could fouled O2 sensor cause a rich condition?
 

I'm trying to see if I need a new one. My GSL-SE is definitely drinking a lot of fuel... I'm still on the break-in, so I don't go full throttle and I say below 4K RPM, and yet still I'm going through fuel like a Russian through vodka. I put in 3/4 tank only a few days ago, drove maybe 40-50 miles fairly conservatively and now I'm at almost 1/4 mark.

I took a voltmeter to the O2 sensor wire, and I'm reading about .9V at idle, which I guess is irrelevant, since O2 sensor isn't used at idle, but then I hold it at a steady cruise RPM and it goes up to 1.01V or so and just kinda stays there. That's odd, it should be around 0.5V if the mixture is right... When I let go of the throttle and it drops to idle, for a second it reads 0.3V or so, and then back to 0.9V.

I have a new coolant temp sensor and it gives appropriate voltage.

interesting. the FSM has the specs. off the top, the 02 should switch between high and low V from about 1500-3500rpms.

basically without the O2 sensor, it runs at like a max power mixture (mid 12's), and the o2 sensor pulls it lean to 14.7. as you can imagine, fuel economy is better @14.7 than 12...

at idle it should be pegged lean, due to the PORT AIR.

couple things to look at; the o2 might just be bad. the water temp sensor in the bottom of the radiator that turns the O2 feedback system on.

Don't have port air. All emissions deleted. So at idle it should be reading the actual mixture, undisturbed by port air.

I thought the sensor at the bottom of the radiator was mostly to determine if radiator coolant is overheating, and the sensor that's actually used for engine temp based mixture is the one in the back of the waterpump? It would make sense that the No 2 coolant sensor is bad and causing poor fuel economy. I haven't replaced it in a long time...

So if my O2 sensor is toasted, I should be able to drive without it and not notice any difference, since ECU should just default to limp mode or whatever. Does that sound correct?

[edit] Ok, looking over the FSM, it seems the water temp switch a the bottom of the radiator should be ON (whatever that means) at above 15*C. Should be easy to test.

You are right that the water temp sensor in the pump housing us the one that sends the signal to the ECU. When I got my 7 back on the road, I just automatically replaced it along with the O2 sensor. I would say that if it is running extremely rich, then most likely it is the water sensor. How does it idle? My SE ran so rich before I figured out what the problem was that the gases would literally burn your eyes. At idle it stumbled pretty bad. It ended up being loose/corroded ECU/EFI fusible links. Once I snapped them in place, the car smoothed right out and my gas mileage doubled.

Well, it idles acceptably once warmed up.

I tested the radiator thermo switch and it seems ok. So I swapped in a generic O2 sensor. So far I can't tell if there's any difference. The voltage from the O2 sensor wire at idle is about 0.7-0.8V now, and holding it at cruise RPM it goes up to about 0.9V. I feel like I should see the voltage bounce around, as the ECU adjusts the mixture and all that, but it doesn't. It's like the ECU just goes "Well, here's the mixture and that's it" and doesn't actually bother tweaking it based on the O2 sensor input.

I even drove around with O2 sensor disconnected completely and still felt nothing.

yeah without the emissions the O2 should read high at idle. since the PORT AIR actually goes INTO the engine, without it will be rich

you may want to check the water temp and o2 at the ecu, i think the o2 sensor isn't turned on if the water temp 2 isn't also on

Hmm. I've already tested O2 at the ECU, its' the same as at the initial wire, but I'll double check the water temp sensor.

since there are some good responses, i'll chime in with this. did you send the injectors out for a cleaning and flow test during the rebuild? you oculd have a poor or excessive spray pattern. do you notice the engine smelling rich and have you inspected the plugs to see how rich the engine is actually running. 40-50 miles on 3/4 of a tank is near impossible unless you have a fuel leak. you figure thats's roughly 12 gallons in say 50 miles...which equates to a hair over 4mpg..not really possible with a stock port SE engine. are you sure the gas gauge is working properly? best thing to do is see where you are at now on the gauge and fill the tank back up and see how many gallons it actually takes.. for reference, the tank capacity is 16.6 gallons.

Yeah, injectors were cleaned and serviced. I can't really smell fuel, so I'll assume I don't have a fuel leak. Plugs don't look too bad. Fuel gauge isn't accurate - goes up and down a bit based on which way the car is leaning, but I can kind of average where it's at.

It's not 50 miles on a 3/4. It's 50 miles on a half, from 3/4 down to 1/4, or close to that. More like 10 mpg, I guess, which is still silly though. It's a mild street port, also. I'm gonna fill it up and measure again.

Knowing that water temp sensors (the one in water pump housing) seem to go bad over time, I would just replace it and see how it runs. Many SE owners drive around with bad WT sensors and just assume that the roughness and poor gas mileage is due to a wore out engine or other issues.

Mine is only a few months old, and it made no difference when I put it in.

I checked the coolant sensor voltage, and it's within spec.

#1 or #2?

i have a thought, you might wanna try intentionally leaning it out, and see what happens.

not quite sure how you'd go about that, of course, maybe pop a vacuum line off while its running or something, and see if the o2 signal changes?

#1, the main one, in the back of the water pump. #2 also seems fine.

I can lean it out with vacuum line from brake booster. I'll try that.

Update. Ran a full tank and ended up with just under 16 mpg. Is that reasonable?

its common, but not reasonable.. my gsl se gets that on a regular basis.. yet i get 26 mpg going to sevenstock

90% city driving, though. I guess there's a lot of hills and such...but I don't drive it aggressively, mostly quarter to half throttle. I still want to take it to a dyno and have my cruise mixture tested.

seems low to me. i used to get 20-22 with mine, but we have a lot of 35-45mph roads.

as an alternative to the dyno, you could buy a cheapie air fuel gauge, or just use a voltmeter

What RPM do you usually cruise at?

I think that 16mpg could be reasonable if it is all or mostly city driving. I know I can easily get 26 mpg with my SE driving it on open roads. I think I got 28 mpg once going back and forth to work, but I do have an 2gen tranny.

me? probably 2500-3000

Alright, something doesn't add up then... I drive around 2500 - 3000 as well.

i found that if i'm in traffic or (street)racing its about 16mpg - 20

so usually i just drive for fun since it'll use up the same amount of gas lolz (maybe)

This has been bugging me so much that I finally decided to get a proper test. Took the car to a local dyno shop and watched the mixture under simulated driving conditions with minor load. It's a bit rich at idle and below 2k, but above that with light to half throttle the mixture is pretty close to ideal.

So the combustion is at least optimal, and 16 mpg is likely close to norm, given the terrain.

The Se runs rich at idle and 16 MPG with porting on the Engine and 90% City driving is very normal.. The Se does not get very good mileage in stop and go driving.

I have pulled over 30 MPG on my Se on the highway.

Reference thread.

https://www.rx7club.com/1st-generation-specific-1979-1985-18/fuel-economy-report-since-recent-mods-se-656700/

If you are not running the DLIDFIS or 2nd gen leading coil system or then you may want to consider it.

The Se has a hard enough time with it's huge injectors and antiquated ECU adjusting the fuel as it is but with a ported engine it is even more challenging for it. A Mega Quirt system would be better to keep the fuel system where you want it.

Are you running a stock air filter or a KN?
Electric fan helps with improving fuel economy also.
Read my attached link for some more tips.