Question about EVAP system in gen3/3.5

[Nervous]

Hey guys,

Just studying everything regarding to emission these days due to failed emission with way too high HC number.
I have a question about EVAP system in gen3/3.5 1MZ. As far as I understand, the main idea is to burn fuel vapors from the fuel tank. Gen 3/3.5 has non-ECU controlled EVAP, meaning it is merely mechanical system: there is a TVV that opens when coolant is hot enough (60 deg C and above) and this allows vapors to escape from charcoal canister into the intake. Gen3/3.5 doesn't have any monitoring systems for EVAP. So, I have a couple of questions:
1) Can 1MZ 94-96 throw any EVAP-related MILs, and if yes - how exactly it monitors it?
2) Since EVAP is controlled mechanically by TVV, it may allow vapors leak and this might enrich the mix, right? Will ECU compensate it by negative fuel trims?
3) What if I just disconnect EVAP hoses and seal them - this should not make emission test result worse and may make it only better, am I right?

[Mister_Perkins]

Non-ECM Controlled EVAP Systems typically use the following components:

1. Fuel Tank
2. Fuel Tank Cap (W/ vacuum check valve)
3. Charcoal Canister (w/ Vacuum & Pressure Check ValveS)
4. Thermo Vacuum Valve (TVV)
5. Ported Vacuum Purge Port (Port P; on the throttle body)

If the 1mzfe system is OBDII it will have a sealed system and the EVAP will be controlled by the ECM. Pre-OBDII it will be controlled by a mechanical Evap system much like the 92-96 camry.

I'm sure you could just disconnect the EVAP system on a pre-OBDII system, but why?

1. You will fail emissions, because you NEED to have an EVAP system in place
2. You will be releasing Approximately 20% Hydrocarbons that are captured in the EVAP system into the air.
3. The EVAP system is designed to keep a low amount of pressure in the fuel tank as well, you could lead to other problems such as build up , plugged fuel lines , etc..

When the engine is running, stored fuel vapors are purged from the canister whenever the throttle has opened past the purge port (Port P) and the coolan temp. is above a certain point (Usually around 129 Degrees F). Fuel vapors flow from the high pressure area in the canister past the check valve #1 in the Canister, Through the TVV, to the low pressure area in the TB. Atmospheric Pressure is allowed into the canister through a filter located at the bottom of the canister. This allows the purge flow to be constantly maintained whenever purge vacuum is applied to the canister. It does make your fuel richer, but its only when your fuel pressure is lean. It bascially brings the fuel mixture back to where its supposed to run.

When the coolant temperature drops below (usually 95 degrees F), the TVV prevents purge from taking place by blocking the vacuum signal to the check valve #1.

In our systems, we also have a VSV that runs side by side with the TVV. The VSV is normally closed and duty cycle controlled, which means the ECM rapidly opens and closes the VSV passage to provide precise, variable control purge flow volume and inhibit operation.

Because this system uses a manifold vacuum purge port, it may provide a slight purge flow durring idle if conditions can tolerate its enrichment. The ECM uses engine speed, intake volume, coolant temp, and Oxygen sensor info to control the EVAP operation

To Avoid emissions failures, the vehicle shouldn't be tested after prolonged idle or after sitting in the sun on a hot day. All these conditions cause large amounts of fuel vapor to be stored in the charcoal canister. To put the EVAP system through its normal purge cycle, the vehicle should be driven at highway speeds for 5 minutes. This should purge any vapors from the canister which would normally accumulate during the prolonged idle or sitting

[Nervous]

94-96 1MZs are OBD-II and have Non-ECM controlled EVAP with TVV. MD state does not read OBD-II data for cars older than 96 regardless to presence of OBD-II. They use 5-gas analyzer instead. Therefore they would be unable to detect disconnected EVAP system. I was thinking to disconnect it just for the emission test on my 95 Camry.

[Mister_Perkins]

Originally Posted by Nervous

94-96 1MZs are OBD-II and have Non-ECM controlled EVAP with TVV. MD state does not read OBD-II data for cars older than 96 regardless to presence of OBD-II. They use 5-gas analyzer instead. Therefore they would be unable to detect disconnected EVAP system. I was thinking to disconnect it just for the emission test on my 95 Camry.

Its worth a shot i guess

[Nervous]

Plus, I don't see how OBD-II with not-ECM controlled EVAP can detect disconnected EVAP either.... But, anyway, first of all I need to read fuel trims to see what is happening....

[Mister_Perkins]

Originally Posted by Nervous

Plus, I don't see how OBD-II with not-ECM controlled EVAP can detect disconnected EVAP either.... But, anyway, first of all I need to read fuel trims to see what is happening....

Originally Posted by Mister_Perkins

In our systems, we also have a VSV that runs side by side with the TVV. The VSV is normally closed and duty cycle controlled, which means the ECM rapidly opens and closes the VSV passage to provide precise, variable control purge flow volume and inhibit operation.

If the TVV or VSV or Pressure sensor goes bad you could throw an EVAP code

[Nervous]

There is no VSV in 94-96 1MZ for EVAP. And TVV is purely mechanical device - no code as well.

[Mister_Perkins]

Originally Posted by Nervous

There is no VSV in 94-96 1MZ for EVAP. And TVV is purely mechanical device - no code as well.

I don't know then. Sorry i couldn't help