1995 Toyota Previa Repair Question
Oxygen Sensor failing to read, 95 LE SC
I once had a head gasket blow direct between cylinder and outside in an old Rambler inline 6, and it made a loud snapping noise. No such noise here. Where could the leak be and how could I detect it?
All three ECU ground wires (EO1, EO2, EO3) are less than 0.1 ohm to vehicle ground. I checked the distributor sensor and crankshaft position sensor wiring and all good. The sensor coil resistances are 2070 ohm for the crankshaft sensor and 165 ohm for the distributor coil and are in spec. I will do a few more checks of voltages between the ECU ground wires EO1, EO2, EO3, E1 and the OX sensor ground wire to see if any bias exists with the engine running. I don’t expect to find any.
Other than that, I am out of ideas to check without a scope. I need a scope to check the injector timing and durations, the crankshaft position sensor signal, the distributor signal, and the igniter drives from the ECU.
I want to make sure I don’t tear down the engine unnecessarily.
I'm stumped and still open for suggestions.
My dad has an old Rambler Marlin. heh. What I was talking about with the head gasket sucking air is if the gasket is blown it can suck extra, unmeasured air into the combustion chamber(s).
You're hunting a lean condition. You've checked just about everything there is to check. Except injector pulse width.
Have you checked compression? That will tell you if the head gasket is blown on a cylinder. It can also tell if you if a valve that isn't opening and closing correctly. You could also pull the valve cover and watch the valve action from the top of the head.
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I have the same year Toy Van-no SC and still kicking like new
Have you checked all the items I've given you earlier and rule it out as the problem?
Check engine compression and injector for those cylinders that are misfiring-
*** Coolant temp sensor reads ~186 F when engine is warm (Is this warm enough for closed loop fuel control?) Should be 195-205degs
Read the sensor's resistances hot and cold- The CTS controls the loop while in open mode when it switches the O2 sensors takes control of the fuel demand-
2 adjacent cylinder misfirng could be a blown headgasket
Oh, my dad's Rambler Marlin is a 65.
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Who knows it could be leaking thru the injector's seal. I remember that car rode on it before so as the Gremlin, Nice cars though
In college (circa 1974) I bought a 57 Rambler SW that was a high school student’s labor of love (see pics). 196 cu inches of inline power, ha. Called it the green hornet. Green metal flake paint, black and white faux leather interior, bullet muffler, 3 spd manual on the column with a 2 spd automatic overdrive, chrome slots in the back and chrome reverse in the front. A memorable car. The head gasket blew out on the firewall side of the rear cylinder and made the snapping noise.
I checked the Previa’s compression with a cold engine and all four cylinders measured even at176 psi peak. I bought this Previa at 47k miles and it now has 165k. I’ve run only Mobil-1 in it and the valve clearances have never gone out of spec and I checked them after this problem started.
Also for good measure, I bought another spark plug and replaced the black #4 plug in the picture above. No difference.
Watching it run with the valve cover off is not an option in the Previa. The engine is laid over more than 70 degrees and it would be a real mess with oil going everywhere. See the picture here of the valve cover seen thru the passenger door.
***Have you checked all the items I've given you earlier and rule it out as the problem?
Yes, and I have gone over some again.
I just ordered a PC based oscilloscope and will check the injector timing and durations, the crankshaft position sensor signal, the distributor signal, and the igniter drives from the ECU as soon as it gets here. That should take about a week and I will post images of the scope traces here.
***Coolant temp sensor reads ~186 F when engine is warm (Is this warm enough for closed loop fuel control?) Should be 195-205degs
I had checked the cold temp resistance of the coolant temp sensor and it is in spec there. However I did not remove it and it may be lagging 10-20 degrees below the actual warm engine temp due to deposits. I will pull it and test in boiling water to see if it is biased out of spec there.
***Read the sensor's resistances hot and cold- The CTS controls the loop while in open mode when it switches the O2 sensors takes control of the fuel demand-
Yes that is what I have seen, however there seems to be a third mode that keeps the engine safe when the ECU is first initialized, and my 95 SC is stuck in that mode. When the ECU initializes from power loss (battery disconnect), BOTH the fuel trims are set to zero. And they will not read off of zero and the ECU will not engage the SC clutch until some conditions are reached. I’m not sure what those conditions are but think I have read that the OX1 sensor must read rich at least once. I can see how my good 96 Previa SC behaves different from my “stuck” 95 Previa SC. Here are some snapshots of the engine data of the 96 Previa on a cold start.
Ignition switch on, before startup:
Note that all system monitoring is complete, fuel control is open loop, coolant temp is 98.6 DegF, and the long term fuel trim is at a value of 3.1%.
Here is the plot of the fuel trims and the OX sensor voltages also with ignition on, before start.
The long term fuel trim in blue is holding constant at 3.1%.
The short term trim, OX1 voltage and OX2 voltage all are steady at zero.
(The vertical lines are ~2 seconds apart so the charts show the last
This snapshot was captured about 20 seconds after engine start.
The OX sensors have begun to warm up and are still lean, but rising. OX1 is at 0.18 volts.
The red and blue fuel trim lines are hard to see, but the long term trim (blue) is holding constant at 3.1% and the short term (red) is holding at 0.0%. This is where the 95 Previa gets stuck. Its OX sensor values start out the same as the 96 Previa, but go to a solid 0.00 volts as it warms up and starts missing. If I rev the engine on the 95 in the first minute or so after a cold start, I can sometimes get OX1 to show a little blip above zero. But once it is warm, there is no getting it off of 0.00.
(The vertical lines are ~2 seconds apart so the charts show the last 10 seconds of data.)