Help confirming charcoal canister diagnosis
11-04-2017, 06:50 PM
#1
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Location: Oregon
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Car: 1991 Z28
Engine: L31-R 350 w/ EBL P4
Transmission: 700R4
Axle/Gears: 2.73
Help confirming charcoal canister diagnosis
Background: In all eight years I've owned this car, the gas tank will sometimes make a burp noise when idling. For years I didn't think much of the issue until a small crack formed in the bottom of the tank earlier this year. After fixing the crack, I wanted to know what caused the crack to occur so I could prevent a repeat issue. The only possible causes I could think of had to either do with too much pressure in the tank or a vacuum being created. My first attempt to fix these possible causes was to clean the white valve back by the tank. While it was pretty clogged up, this did not stop the burping.
On a somewhat related note, since I did my engine swap last year, the car has trouble with warm starts. 75% of the time if I let the car sit for 30-40 minutes after driving it, the engine will stall out when attempting to start it. After the engine dies, sloshing sounds come from the gas tank as the fuel pump winds down. When this occurs, there is a lot of pressure built up in the gas tank, observed when pulling the gas cap off. This leads me to believe that my exhaust is causing the gas to boil in the tank when it sits, creating a lot of pressure in the tank. Since the charcoal canister is supposed to vent excess tank pressure, I took a look at it today.
Tests Performed
First, I checked the tank pressure relief valve by the canister. Applying a 10" Hg vacuum to the control port of the valve produced air flow through the valve from the tank. Removing the vacuum stopped the air flow. From this I concluded that the pressure relief valve was working correctly.
The canister in my car is a two line system with the vent port capped from the factory. The bottom port of the canister solenoid connects to the pressure relief valve and receives the vapors from the tank. I was able to blow air through the port with ease, so I concluded that the intake port was not clogged.
The top port of the canister runs a line to the throttle body and acts as the purge line. According my Helms manual, if the purge solenoid is good, the port should be able to hold 15" Hg of vacuum for 15 seconds. No vacuum is held when attaching a vacuum pump. Attempting to blow through the port was difficult but some air did flow through.
As a last check, I looked at the voltage across the solenoid connector at key on. With the ignition on but the engine off, there is about 11.6V across the connector. The solenoid acts the same both with ground and 11.6V applied to the connector: no vacuum is held and it is difficult but possible to blow a small amount of air through the purge port.
All of this information seems to point to a bad purge solenoid on the canister. When the solenoid was open, I was expecting to be able to blow air through the purge port easily, but in both ground and 11.6V applied to the solenoid, it was difficult to blow air into the canister. Am I wrong with this diagnosis? And if I am correct, how do I make forward progress on this? From what I can tell the canister and the solenoid is a discontinued part. Threads on the site show how to replace the charcoal in the canister, but nothing talks about how to fix a broken solenoid.
On a somewhat related note, since I did my engine swap last year, the car has trouble with warm starts. 75% of the time if I let the car sit for 30-40 minutes after driving it, the engine will stall out when attempting to start it. After the engine dies, sloshing sounds come from the gas tank as the fuel pump winds down. When this occurs, there is a lot of pressure built up in the gas tank, observed when pulling the gas cap off. This leads me to believe that my exhaust is causing the gas to boil in the tank when it sits, creating a lot of pressure in the tank. Since the charcoal canister is supposed to vent excess tank pressure, I took a look at it today.
Tests Performed
First, I checked the tank pressure relief valve by the canister. Applying a 10" Hg vacuum to the control port of the valve produced air flow through the valve from the tank. Removing the vacuum stopped the air flow. From this I concluded that the pressure relief valve was working correctly.
The canister in my car is a two line system with the vent port capped from the factory. The bottom port of the canister solenoid connects to the pressure relief valve and receives the vapors from the tank. I was able to blow air through the port with ease, so I concluded that the intake port was not clogged.
The top port of the canister runs a line to the throttle body and acts as the purge line. According my Helms manual, if the purge solenoid is good, the port should be able to hold 15" Hg of vacuum for 15 seconds. No vacuum is held when attaching a vacuum pump. Attempting to blow through the port was difficult but some air did flow through.
As a last check, I looked at the voltage across the solenoid connector at key on. With the ignition on but the engine off, there is about 11.6V across the connector. The solenoid acts the same both with ground and 11.6V applied to the connector: no vacuum is held and it is difficult but possible to blow a small amount of air through the purge port.
All of this information seems to point to a bad purge solenoid on the canister. When the solenoid was open, I was expecting to be able to blow air through the purge port easily, but in both ground and 11.6V applied to the solenoid, it was difficult to blow air into the canister. Am I wrong with this diagnosis? And if I am correct, how do I make forward progress on this? From what I can tell the canister and the solenoid is a discontinued part. Threads on the site show how to replace the charcoal in the canister, but nothing talks about how to fix a broken solenoid.
11-04-2017, 08:25 PM
#2
Supreme Member
Re: Help confirming charcoal canister diagnosis
The TBI trucks 88-95 use a solenoid separate from the canister, it also 12 volts and same connector type. The canister is also the exact same diameter, but its almost twice the height. But the solenoid is not built in. The truck canisters are still being built and you can buy them new off of amazon.
So if you didnt want to mess around with used junk, you could just get all new stuff. Since the solenoid is separate from the canister, you will need an extra hose that runs from the solenoid to the canister. Also a place to mount the solenoid. since the diameter of the canister is the same, it should sit in the mount. And i would think it should clear the hood also.
So if you didnt want to mess around with used junk, you could just get all new stuff. Since the solenoid is separate from the canister, you will need an extra hose that runs from the solenoid to the canister. Also a place to mount the solenoid. since the diameter of the canister is the same, it should sit in the mount. And i would think it should clear the hood also.
