View Full Version : Boost leaks, what they do and how to deal with it

03-05-2003, 04:03 AM
Checking for and removing boost leaks using compressed air.

The following is from a post I saved describing leaks to someone who found his Supercoupe produced more power on lower octane fuel. This was obviously confusing. There is a usual cause for this with forced induction, boost leakage. This is very common on our cars, especially at their age and mileage levels nowadays but usually goes unoticed. Here's the problem and reason it could use 89-octane fuel and actually run better with it than 93. All the air that the supercharger is pulling through the mass air meter is measured whether it gets combusted by the engine or escapes back into the atmosphere through a leak and therefore the EEC computer is adding excessive fuel to the engine because it doesn't account for any boost leaks. The oxygen sensors are ignored under wide-open throttle and the air fuel ratio is calculated for a rich mixture based mostly on the mass air readings. So what you end up with is an extremely rich full throttle mixture that not only cools the cylinder more than necessary but also generates much lower than optimum power levels and less combustion heat than normal ones. The largest factor in engine octane need is air charge heat inside the combustion chamber. Octane is not fuels power rating, rather itís a volatility rating, or it is a measure of the fuels resistance to combusting. Actually lower octane fuel burns quicker and hotter. The higher the octane number the more stable fuel is and therefore more resistant to self-ignition. In this theory of a super rich air/fuel charge, a lower octane fuel burns faster and hotter, therefore perceptibly producing more power than fuel with higher octane. In an engine that doesnít need higher-octane fuel you normally wouldnít be able to tell the difference between the octane ratings. But with a properly tuned high-octane need engine lower octane fuel usually doesn't work.

Most of the time boost pressure leaks from the ends of old vacuum hoses that have gotten soft and swollen. Check the 3/8Ē hose connecting the intake plenum in the back, if it's easily pulled off by hand it positively leaks at 15 PSI. Under vacuum leaking fittings can pull together and seal up so it runs fine. Donít be pacified by normal boost readings, even with some leakage the supercharger can maintain 15 PSI because the engine and some small leaks are still within the superchargers flow capabilities. Look at your tailpipes and the area around their exits; is there a heavy soot coating? My advice to you if you see excessive black soot in the rearview mirror at WOT (especially at night with someoneís headlights right behind you), soot build up around the tailpipe exits, and loose fitting vacuum lines is to take an hour to search for and eliminate any possible leaks. Give this a try as not only have I done this on quite a few Supercoupes with very impressive results, but others I have shared this method with have reported back giving thanks and having a blast with the new found power and fuel efficiency for less than $15 and an hours work.

Just so everyone understands, when you do this the entire intake will be pressurized, from the back off the throttle-body to all cylinders with an open intake valve. The idle air bypass valve will be closed with the key off so it is of no consequence for this test procedure. The IAC solenoid is spring-loaded and shuts without current. This makes the system somewhat airtight and allows pressure to build without excessive air escaping. Both sides of the bypass valve will see equal pressure so donít tie it open. Use an inline pressure regulator like those used for air gun painting with an air compressor. Set it on 20-PSI maximum as excessive pressure could blow out seals and gaskets so act in a prudently careful manner. One regulator would work but two in series are better for pressure stability and safety. Run an air hose to the regulator hung under the hood with a wire hook. Find a hose barb fitting and a reducer fitting to thread into the regulator outlet port and clamp a section of small clear vinyl tubing onto it. Run the tubing into the intake manifold at the boost gauge nipple, as this seems the most unlikely place for a leak to develop or go unnoticed. Once the airline valve is opened the intake will pressurize to 20 PSI. A slight hissing can be heard at the throttle-body as pressurized air leaks through it. Using a plastic spray bottle filled with soapy water saturate all possible leak points. It helps to use thick film soap like laundry detergent. This is easier on a cool engine. On mine the large hose nipples at the back of the intake and the PCV line joints in the back were leaking a shocking amount. Another car Iíve sealed had a large one at the PCV valve opening itself. Be sure to check every possible point such as: fuel injector o-rings, intake gaskets, all tubing joints including the spanner nut, the vacuum tree on the firewall, etc.

If you find similar leaks and chances are you will, first replace the large engine mounted vacuum hoses and use screw type hose clamps at all large fittings to seal them permanently. Get a handful of small zip-ties to use on all the small vacuum line fitting that hold boost pressure. Installing a new Motorcraft Supercoupe specific PCV valve should be considered mandatory. It is designed to seal boost and many aftermarket PCV valves arenít capable of this. The correct valves are rarely in stock under any brand; beware if they donít have to order this part. Also make sure the inlet tube clamps are very tight to avoid vacuum leaks between the throttle body and the MAF.

At this point seriously consider changing the spark plugs, as they will already be fouled out from the rich mixtures preventing full performance. Since Supercoupe engines have a low compression ratio Ford specified emissions type spark plugs that are notorious for misfiring under boost. Their extended tip design is to place the spark kernal in the center of the charge with dished pistons but it under power it absorbs too much heat being exposed like so and promotes detonation. The extra wide gap helps to positively ignite leaner air/fuel ratios under light loads thereby preventing misfires at those speeds but has the opposite under power when cylinder pressure is high. I recommend using Motorcraft AWSF-32PP plugs, these are the standard 3.8 double platinum plugs. Their smaller tip design will remain cooler reducing pre-ignition and they have a smaller gap, about 0.047Ē, which greatly reduces misfiring under boost. Both are good things for any supercharged engine, proven to improve efficiency and power under boost without a significant emissions penalty. For a manufacturer who is producing thousands of cars it mattered but for your emissions testing these plugs wonít cause a failure unless your car was already on the edge of passing, in that case you have bigger concerns than normal type plugs. The most important reason to prevent misfires under boost is so you will be able to accurately judge air/fuel ratios. Rich ratios on the dyno or excessive soot in the exhaust gasses are more often from misfiring plugs than boost leaks, but most cars have both until corrected.

After sealing a leaking intake system like this you will notice an immediate difference. The increase in part throttle torque is phenomenal along with better mileage and higher power levels across the entire engine range. This higher power output is due to a more realistic air/fuel ratio and should generate the need for higher octane again with boost, but the increase in mileage and power will even out the cost. You will surely be surprised and pleased with the new character your engine develops. In my experience the difference is phenomenal, part throttle torque and throttle response improved about 20%. The engine felt like it got another cylinder. The throttle became very responsive and more precise. The engine runs much smoother and gets better mileage. Less carbon in the WOT exhaust. Boost, while unchanged at peak, peaked sooner and held longer. The WOT power improved as well and the car sped up but not as much as the part throttle improvement, more like 10%. With a 5-speed the car will be easier to drive away from dead stop.

The reason being is two-fold. Not only do boost leaks rob power producing pressure but all air going through the mass air is metered for fuel requirement whether it gets used as charge or blows out as leaks. Therefore, with leaks the engine always runs rich causing lower power and crappy mileage when using some boost. Because the blower can outflow the leaks and engine, boost will still peg at expected levels but the blower will be pumping a lot of air through the mass air that doesn't stay in the intake. The fix traps all air and restores factory fresh fuel ratios.

Vernon C.