OBD-1 and closed /Open Loop 3VZ-E 1995

Is there a way to stop OBD-1 from going on closed loop?

Y have this 4Runners that I can get it to run very good even on snap throttle while is on open loop, As soon as it warms up it just dies.

If I could keep it running while is hot without going on closed loop I could try to read the ECU parameters and find out which one/s are causing the problem.

I would suspect that temperature sensors (AFM or Coolant sensors) but then it dies I can measure as quick as I can and do seem to be doing the job calling for closed loop.
I do not have access to OBD-1 Scanner.

Any ideas?
Thanks

 

You do not want to stop the ECM from going into closed loop, although I can understand you pain in trying to understand what the heck is going on and why this ruddy system isn't working properly.

OBD-I is a rather crude standard and was the infancy period for what has become OBD-II. A scanner is not available like it is for OBD-II. The test port in the engine compartment would lead one to think otherwise. A DVM and understanding Ohm's Law is required to diagnose the 3VZ-E with the FSM. You can hook up an oscilloscope and read pulses too and I have seen another enthusiast do that but interpreting the signal/s is/are not a simple matter.

Open loop the ECM is delivering a canned algorithm for fueling and timing while the engine is warming up. It also does this when the AFM is swung wide open all the way at WOT the ECM delivers a canned fueling and timing algorithm to the engine and is what it is. But in between idle and WOT the AFM (with air temp sensor), the TPS, the Distributor sensing coils (engine position), the O2 sensor, the engine coolant temp sensor, and the knock sensor all provide input to the ECM so it can run in closed loop (to provide 16 MPG on the freeway)!..

I had a similar problem with my engine right after I rebuilt it. It didn't die but the second I got it warmed up (and it went into closed loop) it would fall on its face when I demanded acceleration from it. Also, as it turned out, it was getting too much fuel pressure and the new MagnaFlow CAT was smelling bad as it was digesting the extra gasoline. So I was desperate to get this sorted out. Initially I thought my new 4 hole injectors were crap and one or more were leaking. But I had other problems that were fuel related and it wasn't the injectors:

• The vacuum line to the fuel pressure regulator was clogged with rust in a metal part of the line. So when the throttle plate closed the regulator should see a vacuum signal and open the regulator as the engine speed is headed toward idle the fuel rail pressure is reduced as the injector demand is falling off. This was not happening and thus the suspected over fueling made sense once I had discovered this blockage of the vacuum line to the regulator.
• The new NGK oxygen sensor was not compatible with my 4Runner even though the eBay seller had advertised it as so. It threaded in and the connector was correct but the DVM readings were seemingly different from the OEM Denso. When I put the OEM Denso O2 Sensor that had been in the exhaust (which came with the vehicle in 2018) the stumbling problem went away. This was a last ditch attempt as it was the only thing left that was still different after the rebuild that had to do with external sensors. Denso is the only brand I will bother with for ignition wires and this O2 sensor. I might add this was after I re-engineered the couplings on my DT headers to have turbo flanges instead of slip joints in trying to make them air tight to the bung for the O2 sensor. If I had only realized that this thing is not a wide band O2 sensor I could have saved some time but the OEM exhaust is very air tight so it is probably not a bad thing to have done this to the DT headers with its 2:1 collector that is past the t-case.
• My EGR vacuum modulator diaphragm had a hole in it which was messing up how the EGR opens and closes. The tell tail sign is a black dot on the filters surface across from the port were the diaphragm's back side is vented. If it has a black dot it means the diaphragm is ruptured and exhaust is making its way to the port and puffing back carbon onto the filter. You can find this whitish filter under a plastic snap on cap on the top of the modulator at the front of the plenum on the NAS-PS of the motor.

The other thing that is crucial to the closed loop running for the ECM to be happy is the knock sensor that is located in the center of the engine block in the valley between the heads and underneath the intake manifold (a pisser to replace). If the piezoelectric sensor is not able to be connected to the ECM because of a bad connection or the sensor is broken then the ECM will not be happy and the closed loop running will not work. The connector is a two wire type that is above the intake manifold and plugs into the wire loom there but the sensor has only one wire that comes out of the housing the other wire is terminated inside the wires sheath. This would be an indicator as to why the grounds from the engine to the body's sheet metal, the frame and the battery are important and that they be sound connections with fresh copper wire and clean mechanically well connected ring terminals under washer faced screws and perhaps even a dab of dielectric grease having been applied to the exposed surfaces before they were secured. Three decades of time and oxidation can take a hold and get you into that voltage drop that just nixes the design requirement for that particular sensor to chit chat with the ECM in a robust manner.

Also as an FYI the distributor has signal coils and metal lobes on the distributor shaft that pass by the coils to provide crankshaft speed and cam position information to the ECM. When these fail the engine will not start and the check engine light will be on. But when they are tired and the signal is weak to the ECM the snap to started up will be very sluggish as was evidenced when my stock OEM Toyota distributor finally quit a few months ago. So even though the ignition is electronic without points and such the ECM is what delivers the signal to the ignitor that fields the coil and produces the spark to the distributor cap and rotor. But the static timing has to be set with the jumper in the test port to disarm the ECM from trying to adjust the timing on the fly when you adjust the rotation of the distributor to set the timing with a timing light. The timing mark on the harmonic balancer pulley is isolated from the pulleys hub that is keyed to the crank with a rubber sleeve which doesn't last forever. This can shift as some of the peripheral belt driven accessories like the alternator are on that pulley. I run my timing at 12-14° BTDC and the engine pulls much better than at 10°. So I suspect my pulley may be shifted, just an FYI.