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Old 09-07-2004, 12:46 PM
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Join Date: Aug 2004
Posts: 42
collapsed gas tank

When I bought my car it has a collapsed gas tank. It is about 9 gallons now.
I am wondering if I need to do something with it?

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Old 09-07-2004, 01:22 PM
MrCjames's Avatar
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Join Date: Jun 2003
Location: SF Bay Area
Posts: 599
Collapsed Fuel Tank


Yes, you will need to replace the vent valve which is located underneath in the area of the fuel tank inlets/outlets. The 86-88 (124's primarily) would develop an issue with the vent valve from time to time. Without proper ventilation the tank collapses due to the suction power of the fuel pump!

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Old 09-08-2004, 11:16 AM
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Do they check the vent valve on the emission test? The test was Ok.
I read somewhere about some kind of a regeneration module? Could it be a problem?
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Old 09-08-2004, 12:33 PM
MrCjames's Avatar
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Location: SF Bay Area
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The fuel tank vent valve is not a device that is normally tested during a state emissions test. The check (or vent) valve has two functions, 1) allow the fuel tank to draw (breathe) outside air into the tank as the fuel is depleted 2) closes to the outside air when the tank is under pressure (at rest) in order to direct the fuel vapors to the charcoal canister.

The regeneration valve is working correctly, that is why the fuel tank is collapsed. The regeneration (Purge) valve is connected to the intake manifold on one side and to the charcoal canister on the other. When certain conditions are met the valve is operated electrically which in turn opens the valve to a vacuum source (Intake Manifold) in order to evacuate the fuel vapors form the charcoal canister. Since a vacuum is present a ventilation device needs to be incorporated in order to allow outside air to enter into the system, otherwise the result is a collapsed fuel tank.

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Old 09-09-2004, 11:34 AM
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Join Date: Aug 2004
Posts: 42
So, The Fuel Tank Vent Valve is connected to the intake manifold on one side and to the Charcoal Canister on the other:

Air Intake Manifold<--Fuel Tank Vent Valve<--Charcoal Canister

And, if Charcoal Canister is plagged, the fuel tank collapses because of vacuum force from Air Intake Manifold.

What is the part number for Charcoal Canister? How to teach it to replace?
Attached Thumbnails
collapsed gas tank-fuel-tank-vent-valve.jpeg   collapsed gas tank-charcoal-canister.gif  
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Old 09-09-2004, 12:34 PM
Wes Bender's Avatar
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Join Date: Mar 2001
Location: Alpine, AZ / Green Valley, AZ
Posts: 733
The cannister being plugged would not cause the tank to collapse. It would prevent the suction from getting to the tank. The Fuel Tank Vent Valve is (or was) probably the culprit.

Back to your original question. If you can live with 9 gallon capacity, you can leave it as is. I wouldn't myself because I do a lot of distance driving and need larger capacity. I would remove the tank, purge it and see if I can pressurize it and pop it back into it's original shape. Be sure to leak check it afterwards to make sure it didn't develop a crack somewhere. Don't do any of this until you get all the gas fumes out.

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Old 09-09-2004, 02:30 PM
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THE CHARCOAL CANISTER is a tank ventilation unit in a closed loop from the fuel tank towards the vacuum source of the engine. Due to the anti-pollution legislation, all cars must now be fitted with a single or dual stage charcoal canister. HC, or fuel fumes must not enter the atmosphere. Instead they will be sucked off into the engine's intake manifold via a purge valve on the charcoal canister. If the fuel tank slowly becomes empty, the vacuum from the fuel tank sucks air back into the fuel tank via the purge valve from the engine manifold, to replace the fuel used up. The purge valve is a pressure or vacuum equaliser, operated by a diaphragm and seals. The charcoals in the canister is used as a filtering compound for liquids like petrol, and air. Petrol may enter from time to time when the fuel tank is over filled and the vehicle is left in the sun on a hot day. Fuel will expand and follow the breather line into the charcoal canister. On rainy days the overall humidity is about 98% which passes via the filtrations of the charcoal canister back to the fuel tank. The continuous moisture filtration over the years makes this unit internally look like an engine with a severe head gasket leak, with grey slime inside the tappet cover. This brings the charcoal canister out of service for tank ventilation. Excessive vacuum build up inside the fuel tank will restrict the fuel flow and pressure. By removing the fuel cap, a big air suction or air pressure from or into the tank is the result of a blocked tank ventilation which may also be caused by a kinked vent line to the charcoal canister. Checking the charcoal canister operation is quite simple with the INTER-JECT LB-291/F or 2F. Connect the servicing instrument in series into the pressure supply line. Close the diversion valve and start the vehicle. Use a 20 litre container, connect a diversion hose to the valve outlet and insert the hose into the 20 litre container. Open the diversion valve to a point of 40 l/h on the flow meter indicator. See if the flow changes to 35 or a lower indication before the 20 litre container becomes full. Remove the tank ventilation hose from the charcoal canister and at the same time see the flow increase on the flow meter which returns to 40 l/h. The service instrument is talking to you; CHANGE THE CHARCOAL CANISTER as it is blocked.
Shortcut back K-Jetronic site plan.


DISCRIBTION HOW TO CHECK THE CHARCOAL CANISTER, on any EFI or K-JETRONIC vehicle is to ventilate the fuel tank in to the inlet manifold of the engine, or replace the tank space which become empty from the used up fuel. Any faulty charcoal canister show up by extreme vacuum build up in the fuel tank or extreme pressure which is very noticeable by removing the petrol cup. A strong fuel smell is high pressure build up. A strong vacuum suction with no fuel smell cause a fuel flow reduction due the faulty charcoal canister. If it is either way, the charcoal canister may be faulty or the line to it is crimped off. To check this problem with the INTER-JECT LB-291/F test equipment is just very easy.
1. Connect the service equipment as listed in the instruction to the vehicle. The temporally return line, insirt in a 20 litre container.
2. Make sure there is at least 3/4 full of petrol in the fuel tank and the fuel cap closed. Do not start the engine. Activate the fuel pump manually by bridging the fuel pump relay.
3. The flow meter will show up a flow somewhere above 100 l/h. By opening the fuel diversion valve, bring the flow down to 40 l/h and make sure the 20 litre container to not over filling.
4. If in some reason the set fuel flow changes to 35 l/h, remover the tank ventilation hose from the charcoal canister and see if the flow on the meter goes back to 40 l/h
5. Change the charcoal canister and investigate for vent pipe restrictions

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