Man oh man this is a difficult DTC to troubleshoot! I have tried every test in the service manual and the updated modified tests sent by Princess (Tech Bulletin) and so far no luck.

Here's what I see. The fuel tank system holds vacuum and pulls easily to 2.1 Volts as measured on the FTP (Fuel tank pressure sensor) when I apply vacuum with a hand pump to the canister side of the 2-way valve and manually open the bypass valve as instructed by the TB. This vacuum easily holds for the required 20 sec test interval as specified. If I release the gas cap the vacuum does go away immediately, so there are no leaks anywhere. Good.

So I reset the code and go out for a ride with my PC attached and after driving for about 10 minutes I see p1456 pending again! (No MIL yet, just pending. This is a two-drive code for MIL.). At the same time I left my DVM connected to the FTP terminals of the ECM so I could constantly measure the fuel tank pressure while driving. It is 2.4 V when open to atmosphere. I never saw it drop for the diagnostic to run. I also never saw it increase much! This is the interesting part. According to the service manual the FTP should read about 3V after the car has been running for a while. Mine only increased from 2.4 to 2.55 max. (If I open the tank drops back to 2.4 as expected) What is limiting max pressure?

So, my conclusion so far is that the tank system holds vacuum, but not pressure. I am further assuming that the diagnostic first runs a pressure test before opening the bypass valve, applying canister vacuum to the tank and running a vacuum test. However, it is impossible to know these things for sure without talking to a Honda engineer. Any out there?

I did notice that the 2-way valve is a rather complex thing. Apparently it is designed to permit a certain minimal vacuum or pressure from the tank before opening. The amount is so small I found it hard to measure on the standard vacuum gauge. .2-.6 inHg. I suspect that there is some possibility that this threshold is too low in my valve and it is opening under too little pressure. For anyone who's studied this system you know that this valve connects the tank side of the system to the canister side of the system. The canister side is pretty much always under vacuum. The tank side is supposed to be able to achieve a pressure reading of 3 V.

Going out on a limb here, but my guess at this point is that the 2 vay valve is breaking FT pressure too early and only allowing my tank to reach 2.55V rather than 3V. Think I'll buy a new one.

Looking at mode 6 on the scanner I see one test failed (Test number 39). The reported value is 1 and the minimum is 10.

Anyone know what #39 is testing on Honda?


Thanks for any help, and hopefully this will be of help to some other poor souls suffering from the ever elusive p1456.

Rich

(Car is 1999 Accord 4 cyl automatic)

Interesting. I found this excerpt from the following doc.

http://techinfo.honda.com/rjanisis/pubs/MO/AMO27070.pdf

If I convert test id 39 to hex it is 27. And lo and behold $27 is a non continuously monitored test that does cause p1456 to set. Seems to describe my condition.

---------------------------------------------
Test ID $27 Test Limit Type and
Component ID $81
DTC P1456
Test Description
Monitoring the fluctuation of fuel tank pressure sensor output value. It should be monitored
before and after the two-way bypass valve is opened, after fuel tank pressure is checked in
certain time after engine starting with cold condition.
Store Timing Normal judgement/Failure judgement
Conversion to Engineering
Units
Measured value: Output value (Decimal) x 0.488281 (mmHg)
The lowest limit value: Output value (Decimal) x 0.488281 (mmHg)
The highest limit value: Not applicable

I no not much about your problem, but have you seen the Non-cont $05 test?
I can only hope I never have your problem...

I am very happy to report that the problem appears to be fixed now. Finally!

Today the readiness indicator for the evap test went green indicating ready and no codes are stored or pending. I am so relieved. I was starting to think it would never go away.

So in summary, the problem causing the p1456 code was a leak at in the ORVR valve (the one that is mounted on top of the fuel tank. The top popped off the little can in the front part of the valve where the hose from the two way valve connects. Inside is a steel ball and a spring. I lost the original ball and used a replacement 1/8" inch. I originally glued the top back on using general purpose cement and it held under vacuum but not under pressure and still caused p1456. I then opened it up cleaned it off and put it back on with epoxy and it held vacuum and pressure. After that the diagnostic passed. No more 1456! (btw, I accessed the valve by cutting a hole in the trunk floor near the fuel pump opening because dropping the tank looks really hard)

I did notice from the layout or the vacuum circuitry that it must be necessary to pass p1456 before the car can test p1457 since the latter involves a vacuum on both tank and canister sides, which can only be interpreted as a canister test if the tank side has already passed.

I learned a lot from this adventure. For what it's worth, 2 way valves open when the pressure evap system pressure reaches about 2.98 V on the FTP, and ORVR's open when the vacuum reaches about 1.8 V on the FTP. In case anyone's ever interested. If you tee in a hand vacuum pump in the line between the 2 way valve and the ORVR you can perform both these tests.

-Rich

Cong-grads!

(btw, I accessed the valve by cutting a hole in the trunk floor near the fuel pump opening because dropping the tank looks really hard)


I was wondering you got to it. Did you ever smell vapor while you were having the problem? My car stinks if it's out in the sun and I think there is an issue with the ORVR since I can trace the smell to that area if I use a hose to sniff around. I have no idea what mine looks like yet because I'm afraid to cut into the trunk (sparks + fuel vapors and such).

I was wondering you got to it. Did you ever smell vapor while you were having the problem? My car stinks if it's out in the sun and I think there is an issue with the ORVR since I can trace the smell to that area if I use a hose to sniff around. I have no idea what mine looks like yet because I'm afraid to cut into the trunk (sparks + fuel vapors and such).

Yes. There was a faint hint of gasoline inside the car/trunk before the fix. Gone now. I used a good pair of sheet metal sheers to make the cuts. There are no sparks involved this way.

Hi RJP,

My mechanic tells me the diagnostics report says my ORVR has failed. Well, at 110,000 miles it's about the first thing that ever broke so no big deal.

However, he says it's 7 hours of labor because he has to remove the gas tank and a lot of other huge and ugly stuff. Then I saw your post on this forum and thought maybe there's a better (and cheaper) way.

Could you provide some more details on how you accessed this through the trunk? As in, where in the trunk did you cut the hole, how big a hole, how did you start the hole to get the metal shears in w/o drilling, etc.

Maybe even a photo?

Thanks

Hi RJP,

My mechanic tells me the diagnostics report says my ORVR has failed. Well, at 110,000 miles it's about the first thing that ever broke so no big deal.

However, he says it's 7 hours of labor because he has to remove the gas tank and a lot of other huge and ugly stuff. Then I saw your post on this forum and thought maybe there's a better (and cheaper) way.

Could you provide some more details on how you accessed this through the trunk? As in, where in the trunk did you cut the hole, how big a hole, how did you start the hole to get the metal shears in w/o drilling, etc.

Maybe even a photo?

Thanks

I can believe the 7 hours labor quote. The entire tank has to come out and there is a ton of stuff under the tank that has to be removed first. Still, you may want to consider paying it, as the cutting approach involves, well, cutting, and your car will never be as strong again. Does it matter? Don't know.

To get access in my case I simply enlarged the existing hole for the fuel pump. Are you aware that there is a fuel pump access hole in the trunk? Take a look. It is about 8 inches in diameter. I cut from this hole up towards the back seat until the valve was exposed. There is a folded metal section that is a support that I had to cut through too. It is pretty thin sheet metal but does require a good pair of shears and some muscle. I am sure the structural integrity is reduced somewhat now, but it seems ok to me. Because I can not say with 100% certainty that this cutting is not a problem, I hesitate to recommend this approach, but it is one alternative, and it worked for me. I do have some pics if I can find them.

I repaired the hole with sheet metal and screws and caulked any cracks.

Going out on a limb here, but my guess at this point is that the 2 way valve is breaking FT pressure too early and only allowing my tank to reach 2.55V rather than 3V. Think I'll buy a new one.

I don't know the precise value range, but I suspect your seeing .2 to .6 inches of Mercury is a tad on the low side. Being located where it is, vulnerable to corrosive (salt) climates, I believe your suspicions about the valve are correct. It's opening too early, releasing tank pressure, hence indicating DTC P1456. Me? I would replace the valve.

I don't know the precise value range, but I suspect your seeing .2 to .6 inches of Mercury is a tad on the low side. Being located where it is, vulnerable to corrosive (salt) climates, I believe your suspicions about the valve are correct. It's opening too early, releasing tank pressure, hence indicating DTC P1456. Me? I would replace the valve.

Wardenr,

This is an old thread. I already fixed the problem. The 2 way valve was fine. 2.5V is correct. 3V was incorrectly stated in an older manual I believe.

Anyway, the problem was a leaking ORVR valve in the fuel tank.
No p1456 for months now!

Wardenr,

This is an old thread. I already fixed the problem. The 2 way valve was fine. 2.5V is correct. 3V was incorrectly stated in an older manual I believe.

Anyway, the problem was a leaking ORVR valve in the fuel tank.
No p1456 for months now!

Mea Culpa! :notworthy

While the 2-way valve is somewhat failure prone, so is the ORVR (tank) valve. I suppose I (mistakenly) started with what I considered the obvious, most probable defect. :headbash: