HOW TO: Vacuum leak detection

Vacuum leak detection.

In this article I'll cover the general things that apply to all Fiero versions, then later I'll break out the version specific and detail items.

Tools:

Spray can of "throttle body" and Fuel Injection cleaner. (This is safe for all EFI. Carburetor cleaners can sometimes do damage to EFI parts. Don't use it.) ((I do NOT recommend most people use the gas search method. It is far to easy to start a fire with this method, especially if done with improvised equipment.))

Assorted line plugs and port caps.

General:

Eliminate the nonessential items first. Start the car after each of the following and see if it has solved your problem.

Disconnect the 2 lines to the Fuel Vapor Recovery canister. Plug the ports on the throttle body and/or manifold.

Disconnect the Cruise Control line and plug its manifold port. ((Note. Cruise control may be a branch off either the brake booster line or the Vapor canister. If it is tapped on the brake booster line then disconnect and cap it's port.))

Disconnect the EGR line from the throttle body and cap its port. (I'll get more into the EGR later)

In the front compartment disconnect and plug the line on the brake booster. The brake booster can leak internally. Carefully inspect the booster vacuum line and all it's clamps. All joints in this line should be clamped and the rubber hose sections free of cracks or other damage. Make sure there are no rusted sections of steel line. (The section inside the cabin is likely OK.) If any section of the line looks questionable disconnect the line from the manifold and cap the port there. (That port is clamped. I try to avoid removing the clamps unless I have to.)

Check the line to the MAP sensor carefully. Hard line doesn't like being bent, especially when cold. (DO NOT spray the MAP sensor with solvent when searching for a vacuum leak!)

If the above have not solved the problem then carefully begin searching with the spray cleaner. Start the car and spray the throttle body mountings and work your way around whatever isn't disconnected. DO NOT spray the distributor, coil(s), sensors or IAC with cleaners or solvents! You will likely damage them or start a fire. Use as little spray as possible to find the leak. Don't soak things. Test an area, let it dry then test another. The RPMs should change, usually up, when you spray the leak.

Fuel Vapor Recovery Canister:

This device catches vapor from the fuel tank. It uses 2 vacuum lines. The large line is connected to manifold vacuum. This line is used to ventilate the canister. The smaller line is the control line. This line is connected to the main bore of the throttle body and only opens the purge control valve beyond a certain amount of throttle.

Problems with the canister can include failure of the purge valve to close and cracks in the purge valve body or canister itself.

There is also a fiber filter in the bottom of the canister that should be replaced periodically.

Another problem that this device can develop is flooding. Flooding is caused when liquid fuel is forced up the vent line into the canister. A flooded canister can cause rich mix problems with the engine. A canister that has been flooded should be replaced. To prevent flooding avoid “topping off” the fuel tank when it’s full. If the nozzle of a retail fuel pump is defective, and they often are, then you can pressurize the fuel tank and force fuel up the vent line into the vapor canister. It doesn’t take allot of liquid fuel to screw up the canister.

Cruise Control:

Leaks in the cruise control servo cannot be fixed. The servo should be replaced.

There is also a vacuum reserve canister in the cruise system. Older style canisters are often metal and are prone to rusting out. Replace these with new style plastic canisters.

There are 2 styles of canister. The ones with 2 fittings usually have integral check valves. The ones with single ports are used with a check valve in the manifold line.

You can replace a single port and inline check valve with a 2 port canister, but you must also insure that the plumbing is redone to work correctly. The larger port on the 2 port units is usually the manifold side but always test it to make sure.

The size of the canister is not very critical. Any canister of approximately the same volume or greater will work. You can get good used plastic canisters for about nothing at any salvage yard. The plastic ones will last nearly forever. Just make sure it's check valve is working on the 2 port ones before you leave the yard. (Blowing into one port is very easy. The other is not going to let you if the valve works.)

Brake booster:

The brake booster unit should not be disassembled. If you believe it leaks then it will need to be replaced. You generally can't get parts for the booster and there is a very heavy spring inside it that likes to send thing flying.

Hard lines:

Hard vacuum line can be a pain to service. You can get the line but finding the correct fittings can be difficult at best. (The rubber fittings are reusable if they are in good condition and sealing tightly to both the line and the port.) If you replace hard line with soft line make sure the new lines are not pinched or collapsed from bending too tightly. The soft line should be sized to properly slide onto the fittings and stay tight. Don't stretch undersized line to fit. It will likely split in short order.

New Hard Line can be shaped or straightened by dipping the line in hot water. Be careful not to pinch or stretch the softened line. Don't try to form old lines. They usually break even when warmed.

Note: If you replace the MAP sensor line with soft line then keep the line as short as possible. The soft line will have a larger internal volume per inch of line than the hard line. This increased volume can affect the reaction time of the MAP sensor. A slow MAP sensor will likely hurt performance. Even when using hard line it is best to keep the MAP feed short as possible.

Make sure the steel PCV valve line is not rusted through anyplace and that both elbows are sealed. GM recommends clamps on the elbows. Be careful with the clamps that you don't cut into the soft elbows.

4 Cylinder specific:

The 4 cylinder EGR is bolted directly to the intake manifold. Make sure it's tight. The bolts have a nasty habit of coming loose. Replace the gasket if the bolts have come loose. The EGR valve is hard to test for leakage. Make sure the valve isn't binding. One way to test EGR problems is to make a block off plate and seal the manifold openings. Then run without it for a little bit. Note: Permanently running the car without the EGR is a crime in the United States. Running the car without the EGR can cause detonation, which can damage the engine. This is a testing procedure only.

87-88 4 cylinder: Be careful not to spray the MAT sensor's plastic parts with cleaner or any other solvent. You can damage the sensor. The MAT sensor is located to the right of the TBI unit and is screwed into the intake manifold. Avoid intake backfires in these engines. If the car backfires into the intake, the MAT sensor should be inspected for damage. If the MAT sensor has been cooked it should be replaced. (The MAT sensor is used in calculating fuel/air ratio.) Don't over tighten the MAT sensor or you may split the intake manifold.

87-88 TBI: There is a specific sequence to tighten the 700 TBI to the manifold. First tighten the long bolts that anchor the TBI to the manifold. Then check torque of the T30 headed bolts that connect the 2 halves of the TBI. Don't over tighten the TBI bolts.

V6 specific:

You'll need to check the EGR plumbing very carefully. Use extreme caution if you decide to use spray or gas searching on this. The V6 EGR plumbing gets hot enough to light off many solvents on contact.

The V6 EGR control valve must also work properly. If it's acting up, try electronics parts cleaner. Don't use the cleaners with lubricant in them. The lubricants will hold dirt and quickly clog the valve up again. (Most "Tuner wash" and contact cleaners have lubricants in them.)

To test this valve, with the engine off, blow into the line connected to the EGR valve. The air should exit the vent hole. Cap the vent hole and blow again into the EGR port. No air should come out the manifold port.

To test during run connect a vacuum gauge to a T in the EGR valve line. You should be able to see the vacuum rise and fall only when the ECM has activated the EGR valve and the engine RPM is in a certain range.

You must also carefully inspect the entire plenum. If bolts have come loose you may be able to simply tighten them but likely you will need to replace the gaskets.