Oxygen Sensor failing to read, 95 LE SC

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.

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%.

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.

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.

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.

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.

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!

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

I have a 1995 Toyota Previa minivan S/C w/ check engine light that's been in the shop numerous times. They see a P0171 code, and we have replaced the MAF and it still hesitates when going up steep hills for a long time ( I live in colorado at 9,300 feet - there's a lot of hills). I Am NOT a competent mechanic -- rather, a non-mechanically inclined woman, but the shop doesn't seem to fix my car.
Could it be the ECU?
How do I find out?

Thanks, sibylle

I have exactly the same problem, but where do I locate the rubber boot or the bypass valve?