Runs rich, O2 says lean, ?bad ecu?

[bgim]

My A/F ratio reads off the scale lean at idle or constant speed, but goes to "stoich" when under full boost. The smell tells me it is very rich at idle. I don't know if there is a functioning catalytic converter, but I suspect not.

Here's the strange thing: When i disconnect the O2 sensor connector by the thermostat housing the AFR guage is still connected and it shows it is running moderately rich at idle or any time driving, which matches how the engine behaves. without the O2 sensor there is no change in performance, and the MPG is still about 17.5 either way.

Checked all the hoses, reinstalled the turbo and fixed some loose clamps, but to no avail, behaves the same. sprayed some starter fluid around hoses, at least away from the exhaust, and nothing changed the idle, so I gess there isn't a serious vacuum leak. I can't find a boost leak either.

I think it is the ECU. I tried swapping, but the ECU on the car is the older yellow connector, and my spare is for a newer car with grey connectors. Both ECU's are for auto trans. Is there more testing to evaluate if the O2 input to the ECU is bad or is this enough to prove the ECU needs to be replaced? I didn't think hooking up the ECU would change the O2 sensor reading if it was not bad.

This is an 87 turbo, ebay car I got for emergency parts and backup daily driver. I've spent too much time fixing lots of sloppy crap done to it. It's been tinkered with and I think someone threw their worst parts on it with a nice radio and AFR meter to sell it. I'd like to sell it too, but I can't do it in good conscience if it is messed up. I already got crapped on that way myself.

 

[Poodles]

Sounds like it has the light show AFR gauge that is hooked to the stock narrowband.

It's worthless to go by it...

 

[bgim]

Sounds like it has the light show AFR gauge that is hooked to the stock narrowband.

It's worthless to go by it...

yes, thats what i have alright. I don't want to put much into this car, but I'll need the wideband for my buildup anyway, so I'll get one. thanks.

Also, i see the standard tests in the manual for the ECU, so I'll work on that before i waste anyone's time. I'll get back on both results next weekend.

 

[grimreaper]

toss a narrow band out the window. you cant adjust what the ecu controls unless you take the ecu out of the loop, or max out (lean/rich) its ability to adjust fuel trims (bad).

check the vf at the diagnostic box to see what the ecu is actually reading....

and removing the o2 sensor should have changed a few things in the a/f ratio not to mention keep you in open loop permanently.

 

[bgim]

toss a narrow band out the window. you cant adjust what the ecu controls unless you take the ecu out of the loop, or max out (lean/rich) its ability to adjust fuel trims (bad).

check the vf at the diagnostic box to see what the ecu is actually reading....

and removing the o2 sensor should have changed a few things in the a/f ratio not to mention keep you in open loop permanently.

I just want this car running like stock to sell it, so a working ecu and O2 sensor would be ideal.

I will add your suggestion to my list of things to check.

With the O2 sensor removal not affecting performance or MPG or rich odor from the tailpipe, I'm reasoning it is the ECU, unless there is a partial short inthe connection from the sensor to the ECU. I'll get back on this in a few days. thanks.

 

[Poodles]

I have heard of only ONE bad 7M ECU in the years I've been around and in my research. And it was from someone hooking up a battery backwards with hacked wiring that fried it.

Don't think it's the ECU as it more than likely isn't.

 

[bgim]

I have heard of only ONE bad 7M ECU in the years I've been around and in my research. And it was from someone hooking up a battery backwards with hacked wiring that fried it.

Don't think it's the ECU as it more than likely isn't.

I got some help and I think your right. I had a fundamental misunderstanding of how the O2 sensor signaled to the ECU. I also RTFM and will work through the diagnostics like a good soldier. Thanks for the help.

 

[92TealSupra]

toss a narrow band out the window. you cant adjust what the ecu controls unless you take the ecu out of the loop, or max out (lean/rich) its ability to adjust fuel trims (bad).

check the vf at the diagnostic box to see what the ecu is actually reading....

and removing the o2 sensor should have changed a few things in the a/f ratio not to mention keep you in open loop permanently.

Man please sell that narrow band I had one, and it did.. [ NOTHING ] for me at all, i thought I was sweet, and then realized I was a stupid @ss, and made myself look dumb thinking I was getting actual readings.

 

[jetjock]

That's because you were using it wrong, as was the OP. Used correctly it's a valuable instrument...

 

[grimreaper]

agreed, as long as its not tunning wot

 

[bgim]

agreed, as long as its not tunning wot

These are tantalizing tips for a beginner, and i'm grateful, especially since it started with a dumb question.

If anyone feels inspired some night, a supra wiki article on theory of O2 sensors commonly used with this engine, narrow and wide, how they signal to the A/F meter and ECU, and how to interpret the readings on the A/F guage would be great. The regular wikipedia article is too general to be directly useful in learning to tune the Supra. A summary of other ECU inputs that affect A/F ratio, though available elsewhere, would be a nice contribution if it summarized under what operating condition one input or another would predominate. Again, the manual has a list of inputs, and test procedures, but no description of how it all comes together dynamically as the car is started and driven.

Or has someone already done this?

 

[buldozr]

very interesting, my car just started to do the same thing, I also have a narrow band but its slightly different, at idle o2 sensor wont cycle(NOT NORMAL), cruising it will go full rich(NOT NORMAL), under normal accel, o2 sensor cycles (NORMAL), under WOT, will go to full rich and not cycle(NORMAL). Swapped out the o2 sensor thinking it had an intermittent short and to no avail, same readings, I checked my bov, its fully seated and closed, checked clamps, everything seems to be in order. Under full boost car does have slight miss, checked and replaced plugs to no avail, all plugs were even in color and gap when pulled... Running out of ideas as well, any thoughts?

 

[grimreaper]

^^ how is your idle? do you have a wbo2 to get a number? does it miss?
do you have a boost gauge? does it respond quickly with even the slightest blips of the throttle?

 

[buldozr]

^ idle cold start is rough but most likely fuel pump not holding pressure overnight, warm starts are great, I had an epiphany that maybe my tps has a 'hole' in it telling the ecu I am WOT when I am 10-15% cruising, thats the only thing I can think of short of checking fuel pressures. I need to put a wideband on it but my funds are tied up tight with my Rx7. It does have a slight miss on WOT under heavy load(third gear up a slight grade), it does hesitate on tip-in sometimes and around 4"Hg-0psi, vacuum looks f'ing fantastic at idle @~22"Hg at around ~700rpm and holds boost all the way to my preset 10psi. I even tried running one step colder plug due to my 100+ degree climate thinking that was an issue, to no avail, doesnt matter if engine hot or cold.
So far my plans are:
1. check tps resistance for any 'holes' and adjust if necessary(I can bet money thats what it is)
2. check fuel pressure- if not in spec, new fuel filter, walbro hotwired with jpipe bypass/regulator/gauge 'the works'.
3. boost leak test for grins
Last resort 4. start looking for a another ecu or set car ablaze after removing my tools and grab my cd's out of the center console

 

[Nick M]

If anyone feels inspired some night, a supra wiki article on theory of O2 sensors commonly used with this engine, narrow and wide, how they signal to the A/F meter and ECU, and how to interpret the readings on the A/F guage would be great. .....Again, the manual has a list of inputs, and test procedures, but no description of how it all comes together dynamically as the car is started and driven.

Or has someone already done this?

Well, Toyota did.

It should be noted that 14.7:1 is done for the catalytic converter. Toyota does not use a wideband on this car. So the aftermarket devices are for tuning by the user, not the engine control unit.

I wanted to read this thread for the title alone. Have you checked for an exhaust leak yet?

 

[bgim]

Well, Toyota did.

It should be noted that 14.7:1 is done for the catalytic converter. Toyota does not use a wideband on this car. So the aftermarket devices are for tuning by the user, not the engine control unit.

I wanted to read this thread for the title alone. Have you checked for an exhaust leak yet?

Thanks, I believe you are right. I think the car is fine, but the A/F guage is not working with the stock O2 sensor. The O2 sensor output is approx 0.25-0.75V, and it fluctuates rapidly. I gather this is good. I could use a link to info on what the wideband sensor is for and how to use it properly. I thought it replaced the stock sensor and worked with the stock ECU, but I really have no idea. This is a practice car, but the other supra is built up to the point of needing decisions about what type of EFI mods to make, and I need to educate myself before I blow money on it.

i went through the EFI voltmeter diagnostics in the manual last night. I had a code 21 and code 51, which could be from the O2 sensor being disconnected for a while previously, and from other old conditions. I reset the ECU and got no new codes right away. I couldn't finish the diagnostics because I had an 1989 manual and it was a 1987 ECU with totally different pinouts. it looks like the 1989 non turbo connectors, but not all the wires matched. I will search for the right info later today. I know the TPS is working, but i couldn't get much further after that.

 

[Nick M]

I don't know about the aftermarket A/F sensor, often called a wideband. But Toyota's is not compatable with the car in any way.

It sends a signal at 3.27 volts to the ECU. And the voltage goes down when rich. There is nothing in common between a Toyota A/F sensor and old oxygen sensor. And that sensor is made to work with newer style cats. Yes, it has much better control than a standard oxygen sensor, so better fuel control in closed loop.

The O2 sensor output is approx 0.25-0.75V, and it fluctuates rapidly. I gather this is good.

It sure is. 450 millivolts avarage, 8-10 times in a second I think. At speed.

 

[3p141592654]

The Denso A/F sensor is biased at 0.4V and the resulting current is used to compute the air-fuel ratio. The attached chart shows the behavior.

This is a very modern sensor, with better performance and durability than the old 5-wire Bosch design used by most aftermarket widebands.

Read a paper on its development.

http://www.denso.co.jp/ja/aboutdenso/technology/dtr/v07_1/files/dissertation5-i.pdf