Bad computer or vacuum sensor?

Tiny
FULLMONTE44
  • MEMBER
  • 1987 NISSAN SENTRA
  • 80,000 MILES
'87 Sentra, with feedback carb, and duty cycle air/fuel mixture ratio solenoid inside carb. Finally got it running good, good power, good response. But it is now blistering the spark plug insulators white. EGR valve is working, timing at 7 BTDC as called for by hood sticker. Not overheating, not losing coolant. Carb has been professionally rebuilt. New oxygen sensor. I was checking the vacuum sensor (just like a map sensor). Chilton says should be 4 volts to computer for vacuum signal. Mine says almost battery voltage, very close within two tenths of volts. Pulled connector off sensor, key on, I have again near battery voltage from computer to sensor? Chilton is no help further. Question is, does computer send 4 volts to sensor, or battery voltage and sensor is supposed to cut it down to 4 volts? If signal to sensor is wrong, bad computer. Or if battery voltage to sensor is right, bad sensor. Please help. Can anybody find the specs for this. And is this the reason my plugs are blistering white.
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Saturday, May 18th, 2013 AT 4:46 PM

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Tiny
KHLOW2008
  • EXPERT
Information is lacking in our database but I will try to see if I can be of any help/.

Is it California or Federal specifications?
For Federal specifications, one of the O2 sensor wire is grounded whereas for California specs, 2 wires goes back to the computer.

The ignition timing seems to differ from our specs.
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Sunday, May 19th, 2013 AT 2:14 AM
Tiny
FULLMONTE44
  • MEMBER
Thanks. This is federal emissions. I have seen 2 degrees after for this car also, Under hood sticker says 7 BTDC, so I tried it both places, but seems to run best at 7 BTDC. Still blistering my plugs. Checked for air or vacuum leaks-none. Vacuum gauge held steady at 17 inches hg. At idle, and my altitude is about 2500 ft. Above sea level, if that matters.
My new O2 sensor has a 3 wire connector. I have not tried to test it since installed, about a month ago.
I was checking vacuum sensor operation per Chilton: voltmeter on terminal 9 to ecu should show 4 volts with no vacuum applied. I had near battery voltage-didn't write it down, just noted "batt voltage?". So did not proceed to applying 25 in, vacuum and checking for 0.5 volts. Took a coffee break, had a few smokes. Do I have a bad vacuum sensor? I checked voltage into the firewall mounted sensor (it has 3 wires: ground, signal from, and signal to computer. Well, that only middle wire had voltage and that was near battery voltage, like 12.0 volts, if I recall. Would the ecu send battery voltage to this sensor? When it is supposed to measure about 4 volts ranging down to about 0.5 volts as a signal to the ecu? So I'm at a loss, bad sensor or bad ecu?
Last question, if I check sensor again and note voltage with no vacuum applied, and then apply vacuum to sensor and note a voltage drop of about 3.5 volts, can I assume that sensor is working right, and that I have a bad ecu? Kinda hoping so, cause a remanufactured control unit is about $200, but their vacuum sensor is $420. And I only paid $300 for this 71,000 original miles Sentra, not running. Are map sensors critically calibrated, or can an old gm map sensor work just like my "vacuum sensor"?
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Sunday, May 19th, 2013 AT 3:28 AM
Tiny
KHLOW2008
  • EXPERT
Of the 3 wires for the vacuum sensor, one (Green/White ) should be battery voltage. What you need to test for 4 volts would be the Red/Yellow.

MAP sensors are vehicle specific and alternatives might not work correctly.
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Monday, May 20th, 2013 AT 10:00 AM
Tiny
FULLMONTE44
  • MEMBER
Okay, thanks, KHLow. Good to know about the sensor. Yes, I was checking the red/yellow wire and had near battery voltage, so looks like I need a new sensor. I have a wrecking yard putting out a nationwide locator for one. Any suggestions where else I might look for one? Guess I will try a few sites online.
I took a one month subscription to Mitchell 1 yesterday. Now I see the green/white wire does send battery voltage from the ECC relay. Chilton had me all confused, with only partial diagrams. I just hope I didn't ruin my control unit with my haphazard tests run to date. I didn't understand the import of disconnecting battery before taking connectors off control unit. In fact, battery off, connectors off, remove pin retainers from connectors for back probing, reconnect control unit, then battery, then key on-to run any voltage checks on the computer. I was going at it cowboy style, just pulling and checking. With Mitchell's help, now I can run a full and thorough testing to see where I'm at. Do you suspect it is a lean running condition blistering my spark plugs? I just want to see if I can get it running right before replacing the timing belt- it is probably the original one with only 71,000 miles, and been on there since 1987!
Thanks for your help. It is great to hear from experts, you guys have a great site with invaluable help. I will donate soon when I can.
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Monday, May 20th, 2013 AT 4:03 PM
Tiny
KHLOW2008
  • EXPERT
What I have is from and I see you determined to get to the bottom of this. Lets hope we can pull this through.

Blistering white is a symptom of lean running conditions or engine is operating extremely hot.

Other factors contributing to this can be
1. Valve clearances. Ensure they are correctly adusted.
2. Ignition timing. Ensure distributor vacuum advance mechanism is working correctly.
3. Check exhaust for partial clogging and a failing catalytic converter.
4. Carburettor. Is there any possibilities the jets were of the wrong specifications?
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Monday, May 20th, 2013 AT 6:00 PM
Tiny
FULLMONTE44
  • MEMBER
Test test test cannot post, test
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Tuesday, May 21st, 2013 AT 5:30 PM
Tiny
FULLMONTE44
  • MEMBER
Tried a fourth time to post long response, didn't work again-will try tomorrow.
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Tuesday, May 21st, 2013 AT 6:51 PM
Tiny
FULLMONTE44
  • MEMBER
And now, even another time-I may have a connectivity problem today or some time-out voodoo.

The valve clearance check is as of now a top priority.
Do not believe it is running too hot.
Timing does advance by computer, the peak on my timing light was between 20-30 BTDC.

Exhaust was checked, if it leaks between exhaust and converter where once disconnected, would that cause engine management problem? Will be checking that seal tomorrow or soon. Previous owner had done that and said it made no difference in the engine running condition-(he had a dead spark plug, his converter was getting red hot, running rich, the oil sump had lots of gas in it).
The carb is adjusted right is all I can say about it. PO had it rebuilt by a shop in Kalispell, Montana. He said he had receipts to prove it. I just took his word on that. Fuel bowl level is on the dot, looks good, runs good, idles good, resistance check on mixture ratio solenoid good.
I will be getting into tests and diagnostics since I finally found the red and green LED lights on the control unit. Will let you know when I find anything. I'm slow in the head and slow on my feet.
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Tuesday, May 21st, 2013 AT 9:16 PM
Tiny
KHLOW2008
  • EXPERT
Any leaks in the exhaust system can provide the exhaust sensor with incorrect readings and when this does not match what other signals are sending to the computer, it would cause problems.

Excessive blowby in engine indicates weak piston rings.
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Wednesday, May 22nd, 2013 AT 6:45 AM
Tiny
FULLMONTE44
  • MEMBER
No, it wasn't blowby. Previous owner had put new spark plugs in, but one was defective-dead. Found that when I did a compression test (for very weak power-I didn't even realize the engine was missing when I got it started. Noticed a strange burble in the exhaust - I thought that was normal Nissan exhaust sound. Tried a different plug from old Datsun truck, and suddenly instant power, incredible power, and no more "burble".) Compression was 155-165. I assumed the gas from number one cylinder had washed down the cylinder walls into the crankcase-looked like it may have had about a quart or 2 of gas with the oil. The engine and exhaust really smoked the first time I got it started. But then it all straightened out and settled down, but that is when my spark plugs started blistering white.
About all I have put on it is a new air filter and a new set of plugs, besides changing the oil and filter, well two oil changes now. The plugs that were in it were a cold range plug, and I put standard plugs in. Funny thing: when I pulled the plugs for a compression test, number one, the dead plug, was black and soot covered, but number 2, 3, and 4 plugs were normal-not white, not soot covered or black, but normal looking with a light brown color.
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Thursday, May 23rd, 2013 AT 4:12 AM
Tiny
KHLOW2008
  • EXPERT
The gas in the oil could be the cause of your problems as the PCV would allow the crankcase ventilation to go back to the intake manifold and since additional gas is detected by the O2 sensor, it could have signalled the computer to lean out the mixture. The problem might get resolved after some running when gas in the oil are evaporated.

As to # 1 spark plug being sooty, that could mean incomplete combustion. Try swapping the spark plugs a d see if it makes a any differences.
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Thursday, May 23rd, 2013 AT 6:18 AM
Tiny
FULLMONTE44
  • MEMBER
OK, I put new plugs in and changed the oil when I did the compression test. I have not been running the car except for tests or any little thing, after finding my blistered plugs.
Well, my vacuum sensor must be ok, took it off car and with battery voltage applied, the signal said 3.6 volts, then with about 20 in. Vacuum, it said 0.9 volts. So put it back on and checked at ecu-when I said I had battery voltage, I must have been seeing 1.1 or 1.2 volts like I saw today-Dumb! So ran the systems check per Mitchell- and the red LED was off, and green on steady at step 5-meaning system is good, no problems.

Next checked coil and power transistor. I need help with this: the coil primary spec is 0.82 - 1.02. Mine said 1.1 ohms. My secondary check was good, 9600 ohms-within the spec.
Then the power transistor which (?) Amplifies signal from ignition module, Is that so? Says to check with coil, the transistor can't be tested by itself. And says to use ohmmeter to run 4 checks. I did. The two checks where I was supposed to show no continuity, both were very close to same, at approximately 5 mega ohms. The two checks where I was supposed to show continuity- I did have continuity, bot both were at approximately 1 mega ohms. I checked several times, not like my bunk vacuum sensor testing originally. Do I have a bad power transistor and/or coil? Could THIS be why my spark plugs are blistering?
Need Help!
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Tuesday, May 28th, 2013 AT 11:48 PM
Tiny
KHLOW2008
  • EXPERT
The main function of the transistor is to provide the sparking signal and has nothing to do with the spark plugs problem. Since you have not been runing the engine much, you might be reading the spark plug condition wrongly. It requires quite abit of running to get the correct mixture and most computers require a little relearn procedure which requires the engine to be operated for a certain period of time.

I would forget about everything for now and go about enjoying my ride instead of worrying about something that might not be a problem at all.
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Wednesday, May 29th, 2013 AT 5:55 AM
Tiny
FULLMONTE44
  • MEMBER
Okay, thank you. I will. Rrrrroadhouse, lol.
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Wednesday, May 29th, 2013 AT 11:40 AM
Tiny
KHLOW2008
  • EXPERT
Enjoy the ride and good luck.
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Thursday, May 30th, 2013 AT 7:00 AM

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