I've got a p1456 code that continually reoccurs. This code indicates a leak in the evaporative emissions system on the fuel tank side.

Man oh man this is a difficult DTC to troubleshoot! I have tried every test in the service manual and the updated modified tests described in Tech Bulletin 99-075 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 Honda 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 don't have that car or the Helm book any more, but your thinking on the 2-way valve really makes sense to me.
(no I'm not a Honda engineer...)

IIRC, the 2-way valve is controlled with a solenoid, although the valve & solenoid are separate in the parts catalog. Have you verified that solenoid working OK? If so, I'd try a new 2-way valve.

For me, the Helm book's description of reconfiguring the hoses for testing the 2-way valve were very confusing. You have any better luck interpreting that? I lucked out becuase mine was a P1457 caused by a CVS valve that was obviously corroded into one chunk of crud.

BTW, WELCOME! With your apparent technical background, I hope you stick around here.

 

Thanks Jim, and thanks for the welcome. I'm new to Honda, but have done a lot of work on cars in general. I do have the Helm factory manual, and yes, I find the diagnostic procedure very twisted. I think this is why Honda released a Tech Bulletin revising the entire test procedure.

After studying this system for a week I think I finally understand how it works, and so now, in retrospect, the diagnostic steps in the manual make sense because I can now understand what each step is testing, but I do feel it is not clearly written.

For example, the Helm procedure says to apply vacuum to the 2 way valve (canister side) while the tank side is plugged and look for 1.5 V at the FTP sensor. The TB revises this to say leave the tank side connected to the tank (i.e. don't plug it) and pull vacuum to 2.1 V instead. The reason for the reduced vacuum (I am guessing) is that the ORVR valve in the tank seems to open at about 1.8V and prohibits any further reduction. This is not documented, just observed by me.

The advantage of the updated procedure of course is that it leaves the fuel tank, the gas cap, and all its hoses still connected and therefore under test. Makes sense since that's where the suspected leak is.

I wish there were a way to test the two way valve.

Can anyone explain how this valve is designed to work? Some of the details I listed are speculation and I'd like to confirm.

 

I don't know exactly how the valve works, but I think you're right about the ORVR valve. One problem is now you gotta pull a vacuum on the WHOLE tank. Yes, that's where the suspected leak is, but in your case the suspicion rests on the valves.

How about the ORVR valve? You've watched the tank pressurize, but you said it only goes to 2.5 (not 3). Maybe the ORVR valve opens too easily?

For the 2-way valve, compare the test for the same valve, but under the P1457 procedure. Is the test any different? Maybe another way of looking at the different tests to guess at the proper action of the valve?

My knowledge of that system ended when I found my corroded valve & replaced it. I never had to deal with the ORVR & tank pressure sensor or any of that stuff. I actually got confusing results testing MY 2-way valve. Then I just bypassed it & did the subsequent tests hoping to find something (which I did).

I remember someone, once upon a time, posted a pretty good schematic drawing of the ORVR & EVAP systems. I gotta go looking for that...

 

Yes. That's what I need, a good schematic diagram showing the functionality of the EVAP system and *how* the diagnostics run would be a great bonus. If you do find it please pass it along asap! THanks.

I agree with your suspicion about the ORVR. Here is what I will do to test this. Tonight I will crimp the vacuum hose between the canister and the two way valve. If the two way valve is leaking internaly, this is the only place it can go. (NOTE: I already confirmed that the two way valve is not leaking externally. If I plug any two of the three ports the remaining one holds vacuum and pressure fine.)

Now with this passage sealed off, the only place the tank pressure can go is to the FTP (on top port of the two way valve) or out the ORVR.

I will start the engine and wait for pressure to build and watch the FTP voltage and see if it climbs to 3V now. If it does, the two way valve was opening early and limiting the pressure. If not, I will next plug the ORVR hose at the canister. Then there would be nothing left but the tank, hoses, cap, and FTP sensor, all of which hold vacuum well. To find one not holding pressure would be strange.

Btw, can you tell me why the fuel tank builds vapor pressure? What is the source of this pressure? Is it due to the fuel pump, or just evaporation of fuel?

 

It should build pressure from constant evaporation. Gasoline vapor pressure causes any lawnmower gas can to pressurize itself somewhat. Auto fuel tanks can't take that pressure, so they have always been vented from 1900 till mid 1970s. That's when US EPA decided that evaporation will be regulated as an unburned hydrocarbon - and applied against the car's allowable limit.

In general (any car) the EVAP system has to allow it to vent THRU the charcoal canister. Honda seems to do this with a releif valve that opens at a pretty low pressure. But it seems to open & close, it's not a simple open vent. People have noticed a quiet venting sound - INTERMITTENT - coming from the canister area when the car's parked.

Then when you start the engine, the purge valve allows fresh air to suck in thru the canister into the engine to be burned. This prevents the charcoal from becoming saturated over long-term. The system also seems to block off the tank vent, allowing it to pressurize a bit more.

All the rest of this nonsense is required by OBD-2 for the SOLE purpose of verifying that the EVAP system is working. The details of how to verify that is for Honda's engineers to dream about.

 

Well I performed the tests tonight that I described, and neither hose crimp enabled the pressure to rise to 3V. In fact they made no difference at all. SO next I took my vacuum pump and switched it to pressure mode and just connected it ti the tank system using a tee. Like this I was able to pull proper vacuum, but could not get the system to hold pressurize for long. I could force it up to 3V with rapid pumping, but it would drain back down in a matter of 10 secs. To make a long story short, I found a leak and fixed it. The leak was on top of the ORVR valve on the top of the gas tank (I had already cut out the trunk floor to get to it last week so access was easy). There is a little tower with a plastic lid. Soap bubbles showed it was leaking. The lid was lose and popped off inside was a steel ball and spring. Appears to be a one way check valve? Who knows. ANyway, I cleaned it up and epoxyed it back together and no more leak, Now I can pump the system to 2.996 volts using my hand pump. It can go no higher because at precisely this value the 2 way valve opens and dumps pressure to the canister. I guess this is why the manual said 3 volts was normal. This makes me think the 2 way valve is working as it should since it has enough resistance to allow around 3 V. So everything looks good so far. Except, I still don't see 3 V from the car's fuel alone, only when I pump it up manually. Maybe the gas needs some magic to evaporate properly? Maybe new gas? Maybe tomorrow.

I reset the code. Tomorrow will tell.

 

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

 

rjpjnk, I'm currently battling/investigating a P1457 code. In case I have to access the valve on top of the tank, can you give some pointers as to where/how to cut the hole in the trunk for access? Hole saw maybe? Mine is a 2000 Accord 2-door, if that helps.

I'm thinkin' I DON'T want to drop that tank. Thanks.