Lack of Vapor from Gas cap + P1457 + P0171: Likely suspects?


  • When I open the gas cap, not much vapor is trying to escape anymore (lack of the usual sound)
  • P1457 → EVAP Control Sys Leak (Canister side)
  • High LTFT could throw P0171

From the shop’s last diagnosis, purge solenoid is not the culprit. I tried different gas caps but no luck. Smoke test seems difficult as it might be under the car.
This video explains it well - but my Vent valve is different than the one in this video:

Link to my Vent Valve - it seems mostly plastic except the base hence little change for leaks:

Any thoughts, pointers?

The video also has a diagram with Green line dividing P1456 vs 1457. This coupled with different Vent shut valve makes troubleshooting tricky (besides, I do not have the expensive scanner that is being used).

What could cause lack of vapor in the gas tank? How is it escaping and making engine to think more air or lack of gas?

1999 Year Acura

you did not say what year your acura is. it might still be under warranty for the evap system. if not, take it to a more reliable mechanic that will actually fix the problem or the dealer.

Its 1999 Acura - no warranty at all

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have you checked all the hoses for cracks or dry rot?
also this is for your p0171 code
P0171 ACURA: Code Meaning, Causes, Symptoms, & Tech Notes (

If the evap system is working properly, there shouldn’t be a significant vapor pressure when you remove the gas cap.

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The diagnosis should include finding the leak, telling the customer what isn’t leaking is no help.

Getting under the vehicle to inspect for leaks is necessary most of the time.


But I have P1457! IM monitor set EVAP to ready around the same time.
It was the last IM monitor to become ready!

The vent valve is normally open. Why would PCM closes it?
In my car, Vent valve connects to the Purge Valve. Outside air connection is likely from the Canister itself (not from the Vent Shut valve).

Would a failing Vent valve brings more air into the system? Perhaps it sends more air into the manifold via Purge Valve?

Use an OBD2 scanner to erase the codes.If they come back,you need to take care of it.Most of these codes are thrown because of a major gas vapor leak escaping into the atmosphere. A bad gas cap or using an aftermarket clone are usually the culprit. For this reason I would buy the OEM and see how it goes.

It seems like it is time to replace this vehicle or have a shop fix it properly . How many threads does it take before something is done to solve this .

Replace a car for a solenoid?

Thanks those been the sounding board - the intersection of P1457 and P0171 is Purge Valve assuming no issues with the hoses and wiring. I replaced it and waiting. Denso doesn’t make the part anymore!

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