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Discussion Starter · #1 ·
I plugged a scanner to my car and I saw two odd things.

First....primary 02 sensor voltage was bouncing around like crazy at an idle (warmed up idle at 670 rpms). Going from ~.2 to ~.7 and back almost like clockwork....I know the sample rate is slow on most scanners but it's odd that it bounces back and forth.

Second....the secondary 02 sensor voltage was completely static at 1.275v. Didn't matter what I did (revving), it would read a fixed voltage. Is this right?

I don't have a CEL but I have been battling a knocking issue for a year or so at low engine speeds, high load like the PCM isn't retarding the timing enough so that's why I put the scanner on there to check timing under certain circumstances as well as see if I'm getting a weak MAP signal. The base timing is perfectly at 12deg though.

Any thoughts are appreciated.
 

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Discussion Starter · #2 ·
Ok so I learned that until the O2 sensors hit 600 degF the PCM runs in open loop so I guess the voltages are meaningless anyway. I had the scanner plugged in and as I drove around the primary voltage was right (.8-.9 under load, .1-2. at low load) and the secondary voltage was actually changing. I guess when I first checked the sensors were not at operating voltage.

I guess I need to get a vacuum pump to check the MAP sensor and/or record some data while the car is making the knocking.
 

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The voltages are important. The primary sensor in the header pipes is supposed to bounce around. From .2v (02 sensor reads lean) to .7 ( 02 read rich), and .5 being the midpoint. The secondary 02 after the cat is supposed to read a bsaically steady voltage, well more steady than the primary but it should still cycle in a smaller range.

Since your car is obd2 your sensor should be heated 02 sensors. Meaning they dont take long to warm.

Another thing about 02 sensors is they send back the signal to the comper telling the ECU how the car is running. So if the car is running lean X<.5v then the ECu will command more fuel. If the car is running rich X>.5v , the ECU will take away fuel.

But your problem is knocking at low engines, but what happens in the upper rpms?

Just read the vacuum on the scanner. MAP readings at idle should be around 3"hg-5"hg or somewhere around 3v, i think.
 

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Discussion Starter · #4 ·
Thanks for that explanation.

My knocking is usually in conditions of low revs high load.....example: short shifting into third only to discover that you're coming to an incline and you are too lazy to downshift (or drinking coffee). So you are at say 2300-2400 rpms and you start giving it some throttle...at that point the engine is under a lot of load for a such a low engine speed so ideally the timing needs to retard, but it seems that it doesn't retard enough and I get a little crackling sound that I take as knocking until around 3500-4k rpms and it goes away.

If I run 91/93 octane gas it goes away. Most people tell me that I probably have bad carbon build up and to run this or that through the gas or that but my pistons are so clean I can read the piston code looking through the plug hole at TDC. My car has run on 87 since mile one and I see no reason it should need any higher...I have no mods and I drive like a grandmother for the most part :). Could gas quality or it's anti knocking qualities really declined that much over the years?

Again, no CEL, perfect idle, I maintain it very well.....OEM plug wires, NGK not platinums that are only a few months old, new cap/rotor, new fuel filter.

I mentioned the MAP sensor reading because I've read a few stories about 'weak' signals from MAP sensors and since the MAP is responsible for telling the PCM about load conditions for advancing/retarding timing, I thought it a good candidate to investigate. So I want to take it off and put a vacuum pump on there to make sure that the input = output over the full range of values. With the car off it reads an almost perfect 100kPa but that doesn't imply it reads correctly across the range.

I guess I should mention, while we are talking about O2 sensors, that my primary's heater circuit went about 4 years ago and I was in college and too broke to pay $300 for an OEM NTK so I bought a Bosch (not universal....it had the same connector) but it was only $80 or so. I know Bosch spark plugs are shit...could the same be said of their O2 sensors and maybe could a bad reading on an O2 sensor cause this issue?

Any other thoughts would be appreciated.
 
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