1995 Toyota Previa • 165,000 miles

I have been chasing this problem for a couple months. I have had 5 Previas (two of them new and 3 of them SC's) so I am pretty familiar with their maintenance as I do all the servicing. Please read all of this. It gets really interesting. This problem is really elusive and defies explanation. The current status of my 95 SC is:
The engine idles very rough and the rough running continues into low throttle condition. The OX1 sensor (and OX2) is reading lean (0 - 0.05 volt typical continuous). It never indicates rich condition. Initially the P0171 showed up on a trip and the engine was not idling particularly bad yet. P0171 is set when the fuel trims are driven to their max values (computer trying to enrich perceived lean condition). MIL codes that I have been receiving include lean mixture P0171 and cylinder misfire P0300, 303, 304. After replacing the OX sensor and checking its wiring, I began testing, replacing and swapping things with known good components. I only use genuine toyota parts.
All fuses were checked and are good, battery is new and good.
Fuel filter replaced with new, no change.
Fuel pressure checked and is spec.
Fuel injectors replaced with new, no change.
Spark plugs replaced with new, no change.
Valve clearance checked and is spec.
Coolant temp sensor reads ~186 F when engine is warm (Is this warm enough for closed loop fuel control?)

Since I believed the OX sensor readings, I was falsely chasing a lean condition. I can easily look at a real time plot of the ox sensors and fuel trims thru the OBDII port with software that I have and compare this with my “good” 96 previa readings. When I reset the codes and the ECU, the ECU resets fuel trims to zero. Could not then get into closed loop fuel control and the engine began running rougher.

Swapped out ECU with two other known good computers, no change.
Cleaned the MAF sensor, no change.
Swapped out the MAF sensor with known good one, no change.
Cleaned and checked operation of the throttle body, ISC valve and TPS, and adjusted to spec, no change.
Swapped the throttle body/ISC/TPS with known good, no change.
Replaced distributor cap, rotor, spark plug wires with new, no change.
Checked distributor condition (was replaced new ~30k miles ago), no problems found.
Looked extensively for vacuum leak with none found.
Removed and checked intercooler for leaks, none found.
Suspecting the new OX sensor of also being bad, replaced it with another brand new sensor, no change.
Rechecked the OX1 sensor wire from the connector on the ECU (E6 pin5) to the sensor plug pin 3 and to the pin 4 on the D5 data link connector, good continuity, no shorts to ground.
Rechecked the OX1 sensor ground wiring from the sensor pin 4 to vehicle ground, all good.
Load tested the OX1 heater sensor wiring from ground to the ECU connector, all good.
Placing the vehicle in gear on jacks, I somehow managed to get the ECU to go into closed loop fuel control and both fuel trims marched up to their maximum values with the OX sensors continuing to measure constant lean condition.
The catalytic converters and the exhaust gas seemed very hot, so on a hunch, I disconnected the ISC connector and powered the ISC valve closed by directly connecting +12v to the ISC valve +B pin 2 and grounding the RSC pin 3, the engine immediately died from the rough idle. This indicated to me that there was no air leak, otherwise the engine would have sped up from the improved air fuel ratio. I confirmed that the engine was running very rich by opening the ISC valve (connecting +12v to the ISC valve +B pin 2 and grounding the RSO pin 1). The engine immediately sped up and began running smoothly at a high idle speed. The rich mixture was also confirmed by placing a cotton rag over the exhaust pipe during rough idle for 5 seconds and the hot exhaust gas (very hot from the extra gasoline delivered to the catalytic converters) ignited the rag.
I suspected bad gasoline may have poisoned the OX sensor, so drained and refilled tank with fresh gas, no change.
Verified the OX1 sensor is operating correctly by removing it from the 95 previa (where it always reads lean) and installing it in my 96 previa where it operates correctly (cycling rich – lean – rich – lean with the ECU controlling the fuel trim as it is supposed to).
Replaced the OX2 sensor for good measure, no change.

I can’t help but think that if I had an internal wiring diagram of the ECU that maybe I could find an errant supply or ground path related to the OX sensors.
Is there any condition that disables the ECU operation of the OX sensor drive circuits?
I am stumped! Any ideas on what to check next?
January 24, 2012.

Engine performance schematics attached.

Jan 25, 2012.
Thanks rivermikerat,
The first three schematics may be of help. They are for the SC, which is what I have. But the enlarged view is not fine enough resolution to make out more than the title and bottom caption. Is there a way for you to send me a higher resolution file or image of the first three? Thanks

New info: While performing a compression test (all 4 cyl good at 176 psi), I noticed that the #4 spark plug center electrode insulator looked gray. This was odd and different from cyl 1 and 2 plugs which were essentially still white with a small amount of carbon from the poor mixture control in earlier tests. I just installed these Denso iridium SK20R11 plugs a few days ago and the car has only seen testing on jacks with no road operation since. #3 has some gray as well. Since the poor idle has been accompanied most often by MIL code P0304 (cyl 4 misfire) and less often by P0303 and P0300, I am looking for something that would explain: Fouling of #4, less on #3.
Missing on #4, less on #3.
New OX sensors always read lean, but work fine in other Previa.
I had previously ruled out the EGR valve, but since it feeds into the #4 side of the intake manifold and opening at idle would cause fouling and missing mainly on #4, I am looking at it again. I removed the EGR valve and it is tightly closed with no gas leakage apparent. The EGR VSV operates correctly as well. My next step is to reassemble the EGR system with the egr actuation hose disconnected and plugged to see if that helps.

New questions: Can an open EGR make new OX sensors always read lean (0 volts) at all engine conditions?
Is there an electrical condition that can open the EGR valve at idle and cause both new OX sensors to read lean (0 volts) with three different ECU's? (That's why I want a schematic of the guts of the ECU.

Gee whiz! I feel like Dr. House. I even have a bad knee and a nasty disposition. Are there any interns in this garage that can handle the pressure and regale me with their epiphany to save this patient. Well?

Jan 26, 2012.
No Joy on the EGR valve. The valve was reinstalled closed (leak tight) and the vac hose was left off (plugged). There was no change in operation. Here is a picture of the new plugs installed at the beginning of the investigation. Left to right - cyl 4, 3, 2, 1. Note deposits on #4.
When the engine starts cold, all cylinders are firing. As it warms up, missing gets worse.
Any ideas?

Jan 28, 2012.
The first picture of all 4 plugs didn't come thru. Here it is.

Jan 28, 2012.
Do you have power at the O2/snitcher?

Jan 28, 2012.
I leave you in the most capable hands of rasmataz. He's better at these performance issues than I am.

Jan 28, 2012.
No problema amigo-Do this below your seat -jumper Te-1 to E-1 and see if it will pop a code-

P0171: Lean fuel condition could be caused by one of the following below: 1. False air leakage

2. EGR and PCV valve.

3. Low fuel pressure.

4. Dirty fuel injectors.

5. Mass Air Flow Sensor.

6. Oxygen Sensor.

Jan 28, 2012.
Hello rasmataz,
Do you mean power at the OX sensor heater?
Yes the wires are good from ground to sensor and from sensor to ECU. In addition, the ECU completed heater system test and its status comes up as "complete" in my OBDII reader software (see image).
There are currently no codes stored. The ECU has been reset, so all fuel trims remain at zero until a rich condition is sensed allowing closed loop fuel control (which is part of the problem). Since trims do not change, P0171 will not be set because P0171 is flagged when fuel trims max out to the rich limit (ECU trying to enrichen a perceived lean mixture).
P0171 was present when problem first occurred on a trip from Oklahoma and came up again only one other time, when I placed the car in gear on jacks and raced engine repeatedly. I never saw OX1 go rich but it must have long enough to go closed loop, because the ECU went closed loop and started controlling the fuel trims and raising the short term fuel trim. The long term trim followed withing a few minutes and marched up to its maximum value as well (full rich), then P0171 was set. I watched the OX1 value on a real time chart the whole time and it never came off zero. I know in reality, the mixture at that time was way over rich because the exhaust pipe, cats and exhaust gas were all VERY hot. When I say VERY hot, I mean, I held a dry cotton rag at the tail pipe exit for 5 seconds and the exhaust gas alone ignited it. The cat was cooking a whole lot of extra gas and I've never seen exhaust gas that hot. However, the engine was cool as expected when running rich.
In subsequent trouble shooting, I reset the ECU and it is again stuck at zero fuel trims.
Other conditions that were chased: 1. False air leakage - Intake system verified tight with no leaks and all vac hoses and VSV's checked.
2. EGR and PCV valve - EGR is closed tight and its vac line temporarily disconnected. PCV valve was replaced with new, old one still operating freely.
3. Low fuel pressure - Fuel pressure tested and is at 40 psi, new fuel filter installed.
4. Dirty fuel injectors - Fuel injectors replaced with new from Toyota, old one's tested and found within spec, gas tank drained and refilled with "new" gas.
5. Mass Air Flow Sensor - old one tested and carefully cleaned, then also replaced with another known good one.
6. Oxygen Sensor - Upstream sensor replaced twice with new units from Toyota. First new unit was removed and placed into operation in my other PREVIA and is reading and operating perfectly. Downstream sensor also replaced with new unit.

After all this there is still no change in vehicle operation.
I am really grasping for ideas here. Any more suggestions?

I noticed that the OX sensor wires are shielded. This indicates to me that they are sensing a very weak current (the ECU sensor is a high impedance circuit). So I researched the operation of the OX sensors and they are electrochemical cells themselves and generate a small current and the voltage that the ECU reads (Neither the ECU nor the battery supply an excitation voltage as with other sensors). This makes sense and would make the OX sense wires very susceptible to current leakage. Even a small electrical leakage to ground could dump their feeble current and cause the sense wire to read zero volts all the time. This leakage could be tested by any high impedance ohm meter. A normal check for continuity (which is what I have done and what would normally be done by anyone looking to check this wire) could miss this leakage. I am going right now to check the OX1 wire resistance to ground in my two Previas with the E6 ECU connector and the H3 Ox sensor connector both disconnected.
. Hope this is not another dead end.

Jan 28, 2012.
Yup, dead end,
OX1 sensor wire resistance to ground is greater than 30 Mohm.
Will check ECU ground resistance and OX1 ground wire for bias next.

? Can the oil leakage from an intake valve seal cause the ox sensors to read lean?

? Can a head gasket leak cause ox sensors to read lean?


Jan 28, 2012.
A head gasket sucking outside air into the exhaust can.

Jan 28, 2012.
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