Mechanics

OXYGEN SENSOR FAILING TO READ, 95 LE SC

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?
Avatar
Drocketman
January 24, 2012.



This snapshot shows the transition from open loop to closed loop control. About a minute after engine cold start on the 96, I revved the throttle slightly and the ECU momentarily jumped the long term fuel trim (blue plot) off scale high to enrichen the mixture. Less than a second later, both OX sensors showed the momentary rich condition then returned to lean. At this moment the fuel system went closed loop and the short term fuel trim (red plot) began to be changed by the ECU. Short term fuel trim was raised until OX1 indicated rich again, then short term fuel trim was lowered until OX1 showed lean. And so on for the closed loop control of mixture using the OX1 sensor. The 95 Previa will not go into closed loop and the fuel trims and OX sensors all remain at zero values.


Tiny
Drocketman
Jan 30, 2012.
This stapshot shows the 96 at idle in closed loop, several minutes after engine start. The short term fuel trim (red plot) is raised and lowered in response to the OX1 value to control the mixture. The long term fuel trim (blue plot) has yet to be changed by the ECU from readings of the average short term trim, and remains at 3.1%.


Tiny
Drocketman
Jan 30, 2012.
Here is a snapshot of the 96 Previa engine parameters idling properly in closed loop. This was taken just after the last plot. Note the engine is still warming up and the coolant temp is at 161.6 DegF.


Tiny
Drocketman
Jan 30, 2012.
Perhaps it is wishful thinking, but because of the good compression test, I am leaning toward the idea that my head gasket is not blown, and my valves and valve seals are OK. So I ask, can spark timing or injection timing cause my symptoms? I think so.

I read some on the OX sensors. They are just that. They sense FREE Oxygen level. They do not sense fuel mixture ratio or fuel directly. So I ask, if a cylinder with a good mixture does not get ignited (misfires), what will an OX sensor read. I think the answer is TOO MUCH OXYGEN because it will sense oxygen in the exhaust that should have been reacted but was not (free oxygen), so it will output a lean signal (zero volts). That could explain my problem. A faulty crank position sensor is my best guess for now. If it fails in a way that makes the spark occur at the wrong time or not at all on cylinder 4, then that cylinder would carbon up and OX1 would read a lean mixture. Checks on the ignition and timing signals are my last hope of finding the problem.

So I am anxiously awaiting the arrival of my scope so I can see if I have an ignition or injection problem.


Tiny
Drocketman
Jan 30, 2012.
Forgot to include this last snapshot.
Here is a snapshot of the 96 Previa engine parameters idling properly in closed loop. This was taken just after the last plot. Note the engine is still warming up and the coolant temp is at 161.6 DegF.


Tiny
Drocketman
Jan 30, 2012.
FOUND THE MAIN PROBLEM!
Intake manifold leak!
I missed it on the earlier checks. After eliminating ALL the other alternatives, I started going back with more thorough tests. A manifold pressure reading was way low (idling down to 11 and 10 inches Hg, when should be above 19). To check for any manifold leaks, I wanted to eliminate vehicle noise when looking for a leak. So I removed the throttle body and made a special test fitting (piece of plywood with a 5/16" hose fitting on it), and bolted it in place of the throttle body. With a friend maintaining ~10psi on the hose, I went right to the leak. The rubber boot just above the bypass valve has a 1" split in it.
This boot is called the No. 1 Intake Air hose, Toyota part number 90904-45003. It is more of a boot than a hose. The old one and a new one are shown in this picture. The split looks like classic dry rot (cracks) on the outside. The rubber has been softened by oil mist or other intake fumes and is much easier to stretch than the new one. Note the bulge of the old one.
The really embarrassing thing about this is that I had the same split in this boot on my '96 previa six or seven years ago but didn't think of it here. But in my defense, the leak on the '95 was on a different side of the boot and not as easy to detect. In addition, the '96 never showed lean codes like the '95 has.

Moral of story: Go old school. If poor idle, first put a manifold gage on it.

It may be worse in Texas heat, but judging by my two failures on two Previas, I suspect this is a common problem and may have caused more than one unnecessary engine removal or Previa sale.

I am doing this vehicle's second SAD shaft coupling replacement while working in the same area. First replacement only lasted 40k miles. I am using a genuine Toyota part this time instead of the ebay special. The bolts in the Toyota kit are definitely better quality than the ebay kit, and are exactly like the original equipment. And I suspect the rubber is a better mix as well. I will also pay closer attention to the shaft alignment as well. If anyone wants pictures of the new or the failed original equipment couplings (130k miles), as well as the new or the failied ebay couplings (40k miles), I have pictures of all of them and will post on a new thread.


Tiny
Drocketman
Feb 11, 2012.
Well, old school wins the day. Imagine that. Damn glad you were finally able to figure this out. All this hassle for a simple vacuum leak. Sucks, don't it? Well hey, at least it's fixed and now you know to go back to basics quickly.

I'm going to go ahead and close this thread now.


Rivermikerat
Feb 11, 2012.
Drocketman, I have a 1995 Toyota Previa that is experiencing the same problems you have outlined in your postings. Thank you again for all your postings, and if it's that part, you will have saved me hundreds of dollars and lord knows how much of my time. Thanks!


Tiny
Robertfmorrow
Apr 1, 2012.
Drocketman, yep found the location of the No. 1 Intake Air hose, and I can visually see a hole.


Tiny
Robertfmorrow
Apr 1, 2012.
Glad to hear it robert. Knowing all this work has helped someone avoid it makes it more worth it.


Rivermikerat
Apr 1, 2012.
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