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Discussion Starter · #1 ·
A few months ago I bought a second Insight (2000). It was neglected in the past. I've been working on it, fixing a lot of stuff. The other day we got a P0172 (fuel system too rich) code. Looking at the factory service manual, I set out to do some troubleshooting yesterday. I tested what I could (EVAP canister vaccum and did a valve adjustment). The EVAP vacuum was fine. I made some adjustment to the valve clearances. I need to get a fuel pressure gauge to check the fuel pressure.

But, the manual also says to check the LAF sensor and confirm that the voltage doesn't stay below 0.3V or more than 1.0V.

I don't know the history of this LAF sensor in this beater Insight. I know that @*sean* would probably tell me to replace it and I may be heading that direction. But, I'd like to check the voltage on it first. (My son drives this car to work and school which is only about 5 miles each way, we won't be getting high fuel economy numbers in this car.)

Can the OBDIIC&C monitor voltage of the LAF sensor? I see in @retepsnikrep OBDIIC&C manual that it can monitor voltage of the 2nd O2 sensor, but I don't see any mention of the 1st one. If OBDIIC&C can't check the voltage, any other way to do so?

Thanks,
Bryan
 

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This article describes some methods for examining whether or not LAF sensors are working at all. But it is unlikely you will detect a degradation in performance due to contamination in an uncalibrated exhaust stream without a second good sensor in the same stream at the same time to compare against.


I can get LAF sensor voltage from my Autel MD802, and @retepsnikrep's OBDIIC&C has access to the same PID (I don't know if it he brings it out) and though it might provide subtle hints if lean burn performance is really bad, it doesn't tell you much other than it works or doesn't (in which case a code would be thrown).

The LAF sensor that I removed from my car, and replaced with a new one that immediately gave me more power in lean burn and let me climb slight hills in lean burn where flat ground was about all it could do before, threw no codes.

I spent a long time researching how one could truly test a LAF sensor for loss of performance.

The only way I could come up with required comparison against a known good sensor, using calibrated gas (which is available) compared to the same measurements with a known good sensor, or welding a second bung into the exhaust stream to hold the good sensor. Both of these techniques would require an external circuit capable of reading the sensor(s), similar to what's inside the ECU.

If you are not getting a code, and it is being driven by your son who is unlikely to hypermile, the car is probably going to spend little time in lean burn and therefore won't be used much in the sensor range where this matters most. As a result, the cost of the new sensor will probably take a looooong time to be recouped in fuel savings, so replacing it if it is not throwing codes is probably not worthwhile.

However, explaining to your son why you are not replacing the sensor may excite some interest in this "performance mod". He might noodle on it and decide to do it himself, and may take interest in other aspects of maintenance that improve MPG performance (EGR valve, EGR tube cleaning, valve adjustment, spark plug gap, etc).
 

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Quick and dirty way to check if the fuel pressure is to watch the short term fuel trim. If its in the negative at closed throttle and normal on the highway, wide open throttle, Its likely high fuel pressure. Either the regulator is bad or the return is blocked. Symptoms would also be it running till you reach stop light stop sign where it tries to stall and idles rough

I also have a 2000 5 Mt and had the same code.
 

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Discussion Starter · #4 ·
My son is definitely interested in hypermiling and has helped with every bit of repair work we have done, so far. My point was just that with a commute that is so short, he is going to have a hard time geting the fuel economy numbers up. What I hear you saying, Sean, is that if the voltage wasn't correct, I would actually expect to have some other DTCs, in addtion to the P0172? If that is the case, then I guess I can move on to the fuel pressure.

Thanks for the idea @Phantom Z3 for the suggestion for how to check fuel pressure. I'll give that thread a read!
 

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Most likely not relevant but yesterday I checked my LAF sensor voltage. It's a new NTK sensor that I just put in and working correctly now with no codes, at idle a few minutes after startup it was @ .53v

Tim
 

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@tmessenger how did you test that?
I used a Tonwon OBDII pro BLE4 adapter and a freeware diagnostic app called Car Scanner ELM OBD2 with my Ipad.


Tim
 

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My son is definitely interested in hypermiling and has helped with every bit of repair work we have done, so far. My point was just that with a commute that is so short, he is going to have a hard time geting the fuel economy numbers up. What I hear you saying, Sean, is that if the voltage wasn't correct, I would actually expect to have some other DTCs, in addtion to the P0172? If that is the case, then I guess I can move on to the fuel pressure.
Oh that's awesome! Not all kids want to get involved! You are very fortunate!

After re-reading your post, I think I steered you the wrong way suggesting you don't get a new sensor, because in fact if the factory service manual says that it may be the cause, it may, and checking the voltage as the shop manual suggests might tell you that. I was referring to more subtle reduced mileage issue caused by contamination. I apologize.

I don't have a lot of experience troubleshooting P0172 and following the factory service manual is probably best. Your post just now reminded me that I did have a LAF sensor failure in my HCH earlier this summer and it was quite obvious from the OBD2 data. I do not recall if the code was P0172. There are other codes related to the LAF sensor. I just replaced the sensor with a spare and it was such a non-event that I completely forgot about it until now.

As @tmessenger points out by indicating he used a generic OBD2 scanner, I thought I recalled that the sensor voltage is one of the core OBD2 signals that can be monitored, so that should be a quick test with any cheap OBD2 reader.

So I grabbed my reader and put it into OBD2 mode. Unfortunately neither of my Insights are running at the moment so I could only check on the Honda Civic Hybrid. The live stream showed voltage for the rear two sensors but only current for the LAF sensor. This kinda makes sense if you study the link I sent previously.

Perhaps a voltmeter is all you need to use for that step in the manual.
 

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Discussion Starter · #9 ·
Thanks, Sean. Anybody know which pins on the LAF sensor should be checked for voltage? I'm confused. There are 5 wires but 7 pins. Not sure which ones to check with a multimeter.

Thanks!
 

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Discussion Starter · #10 ·
Thanks, @tmessenger. I may have to look at one of those. Hate to not use the OBDIIC&C, though, if possible. I suppose I should post this question in the obdiic&c thread.
 

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Discussion Starter · #11 ·
Well, I swapped in the LAF sensor from my good Insight. That sensor only has about 5,000 miles on it. But after swapping in my good LAF sensor and also verifying that the fuel pressure system is within spec, the P0172 came back after about 10 miles of driving. According to the FSM, all that remains are the injectors. :-(

I'll have to look into my options there.
 

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Have you tried the Spark Plug Anti-fouler under the #2 O2 sensor?

Sam
 

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Discussion Starter · #13 ·
@Rainsux can you expound upon that idea some? I'm open to trying that, but I'm confused a little bit. Problems with the second 02 sensor and/or the cats aren't mentioned in the service manual's P0172 troubleshooting.

Thanks!
Bryan
 

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Here is a thread. There are plenty of others here.


It's cheap and easy and if it doesn't work you haven't lost much.

Sam
 

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I have the same question about how the 2nd O2 sensor factors in. Also, have you looked at @Phantom Z3's suggestion about the fuel pressure regulator and looking at the fuel trim? The regulartor normally adjusts the fuel pressure in sync with the intake manifold vacuum, to keep the actual fuel flow constant regardless of how hard the vacuum was trihng to suck it out (more vacuum, less rail pressure and less vacuum, more rails pressure = net constant fuel pressure at the injector tip where these two forces meet. So arguably, if the inake pressure sensor (MAP) is not accurate or the fuel rail pressure is unchanging, you will ahve a different amount of fuel metered than the car expects. (I suppose that if the MAP sensor was actually broken the ECU would detect thsi and throw a different code.

Again, take this with a grain of salt since I have only examined the effect of changing fuel rail pressure once (with very interesting results), and never had to diagnose an issue related to this.
 

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Discussion Starter · #16 ·
Well, I actually tested the fuel pressure, by hooking up a fuel pressure test kit, and the pressure was in the proper ranges -- both with the FPR vacuum line detached and clamped, and with it reconnected, so I don't think that is my issue. I've done everything that the factory service manual says to do, short of replacing the injectors (Well, I swapped in a good LAF sensor with only 5000 miles on it, rather than test the voltage on the LAF sensor.) It looks like getting the injectors cleaned/refurbished isn't too expensive.
 

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I'm interested to hear about the injectors cleaning. I have wondered if the not-so-great mileage I'm chasing is due to a poor injector spray pattern, deposits in the cylinders, something else, or a combination of things. (I wish I had an old smog check dyno to run tests with to measure the effect of certain changes.)
 

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P0172 is too rich, injectors clogged would be too lean. Could be a stuck open injector. Theoretically, a bad spray pattern could maybe cause inefficient burn and misfiring, which would cause it to be too rich. But you'd have misfiring.

I'd give the map a check. Maybe EGR? Computer thinking it's putting in less EGR gasses into the intake than it really is? That would do it. Has the EGR plate and passages been cleaned?
 

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So ahead and try the 2nd sensor spacer and see what happens.
 

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Discussion Starter · #20 ·
UNCLE! I still don't understand the connection between a P0172 and problems with the 2nd sensor. But, I agree it is an easy thing to try. The next time the P0172 comes back, I'll add the spacer. Sure wish somebody would explain the connection between these. (I'm not getting a P0420, by the way.)
 
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