Hi everyone, lately I've been having issues with a 1994 Australian delivered Camry with the 5SFE engine, on ~230,000 km and am running out of ideas after much google and youtube based troubleshooting.

Symptoms:
The car will sometimes run fine, but has over the past few months more and more frequently been experiencing issues with black smoke and rough idle - sometimes the black smoke is only during acceleration - this is when idle is smooth, but then other times when there is fluctuating idle (where it tends to drop quite low) there is also black smoke while idling. Also when the idle is rough, sometimes it will drop so low it will stall and require holding the throttle to keep the engine running - this "forced" method of keeping it running causes a lot of black smoke. I know I've said "sometimes" a lot and that is perhaps what is most frustrating as the symptoms don't stay consistent.

Check engine light will be on sometimes but is also inconsistent (occasionally even when running smoothly it will be on - and sometimes when running poorly it won't be on) I have tried many times over the past few months and always gotten the same code 31 - MAP sensor, however I have tried replacing with two used ones of ebay and also tested all three MAP sensors (including the original one) with a vacuum pump and multimeter (following various youtube tutorials) and they all appear to be functioning properly. I note however that when the idle is rough and the car is making black smoke, disconnecting the electrical plug for the MAP sensor will make the car idle nice and smooth without smoke around the 1000 rpm mark. However adding throttle to rev the engine will cause black smoke again since the sensor is disconnected (unsurprisingly).

Over the past couple months have tried replacing the following but have not been able to fix the issue:
MAP sensor (as noted above)
New spark plugs
New spark plug leads
New distributor
New idle air control valve

P.S. I also tried putting smoke through the throttle body with the throttle plate held wide open to detect any vacuum leaks and no smoke came out anywhere which I guess is good that there is no leaks. However strangely, even when I removed the thin hose that leads to the MAP sensor and took that off the throttle body to provide a hole for smoke to escape, no smoke came out of that hole - could it possibly be something clogged around there?

Any ideas from the community? Thanks

 

Code 31 is an open or short in manifold absolute pressure sensor circuit wring.

Possible problem areas are:

Open or short in manifold absolute pressure sensor circuit wiring.

Bad MAP sensor.

Bad ECM

Check wiring between MAP sensor and computer.

There should be 3 wires going to the MAP sensor. The wire color may be Red, Black/Yellow and Brown, Disconnect MAP sensor wiring plug. With engine off but ign switch on check voltages at the plug connectors as follows:

Between Red and Brown wires, should be 4.5-5.5v

Brown/Yellow and Brown, should be 13-13.9v

MAP sensor vacuum hose goes to throttle body. If you disconnect the sensor and blow into the hose, air should pass to the throttle body port.

 

Thanks, that's really useful information, I'll give that a try in the coming days and provide an update

 

@toyomoho
I've now measured it - between the red and brown wires is 5 volts, between the black and brown wires is 10 volts - based on the numbers you provided that suggests the voltage between black and brown is too small - what fix/next step should I be looking at?

Thanks

 

Revise Brown/Yellow and Brown wires to Black/Yellow and Brown wires. Revise the voltage from 13-13.9v to 13-3.9v. Based on this your voltage is OK.

Check voltage at the computer for the Black/Yellow and Brown wires. This will bypass the wiring to the MAP.

Here the issue gets complicated as don’t have a wiring diagram for an Aussie car. Might assume the car is US Federal emissions and use this wiring diagram.

With the top of the computer up and the 3 harness plug horizontal, face the 3 plug terminals of the computer towards you with the center plug removed. The two connections are in the center plug terminal. One is on the top row of terminals, 2nd from the right. The other is on bottom row, far right terminal.

If the voltage is the same and within the specified range (now revised) the diagnostics states the computer is bad.

 

@toyomoho
I've tested the voltages at the ECU terminal - the voltages are within the specified range. Between Black/Yellow and Brown is hard to say whether it's the same on the ECU end as the MAP sensor end as the voltage keeps changing - and I can't test it at the same time with just the one multimeter (though it seems to be similar enough when I quickly change over to test the other end). Between Red and Brown was the same voltage.

One thing of note - when I try resetting the ECU codes by disconnecting battery for 15 mins, the code 31 comes back immediately once the ignition is turned on (without starting the car) after connecting the battery back up. I am fairly confident the code is being cleared and coming back instantly since when I was unplugging stuff from the ECU, other codes came up (I assume due to other sensors therefore being disconnected) and those other codes were successfully cleared but code 31 remained. Just want to check with you - does the fact this code seems to appear without the engine being started support a faulty ECU? I can only find 2nd hand ECUs available and it would be a relatively pricey replacement that I want to avoid until I'm as sure as possible it's really an ECU problem, considering I have no idea if the replacement ECU I buy will work properly either.

 

Are you checking voltages with engine off?

Red wire to MAP sensor is the voltage standard, in this case 5v plus/minus.

Black/Yellow wire from MAP sensor is the signal wire to computer.

Brown wire goes to ground located inside computer..

As to the code appearing without starting the engine. The computers test criteria is to look for an open or short in MAP sensor wiring that has a duration of 0.5 seconds or more. The test criteria states nothing about the engine running.

Do your tests show the computer to be OK in terms of the voltages? The range was 13.9 to 3.9 volts.

If looking for a replacement computer get the part number off the old one. An example part number is 89661-3339084. Your part number may be different but should have the same format..If you find a computer with the same part number (or its replacement that superseded it, you should be OK. Try Ebay for computers. Computers sold online typically work. There is a good business in selling used computers.

Your possible problem areas are the MAP sensor, wiring between sensor and computer, and computer. You should be able to rule out wiring. Find a junk yard for a used MAP sensor. They seldom fail.

 

@toyomoho
Thanks for all the help mate, I've already got three MAP sensors (the original plus two extra from a wrecker) that all seem to work from testing with a vacuum pump. I've pulled the trigger on buying a used ECU from an ebay seller so fingers crossed that works and fixes the problem. I'll post an update once it arrives and I've fitted it.