Checking for Boost Leaks

by: Vernon Cradier
Member SCCoA BBS
Just so everyone is on the same page, when you do this, the entire intake is pressurized, from the back off the throttle-body to cylinders with an open intake valve. Both sides of the bypass valve will see the same pressure so even though the bypass valve will be closed as Jeff described it really is of no consequence for this test. The thing is, the IAC solenoid is spring-loaded and shuts without current. This makes the system somewhat airtight and allows pressure to form without too much airflow escaping.

I used inline pressure regulators like used for air gun painting with an air compressor. I set the first on 50 then the second on 20. Excessive pressure could blow out seals and gaskets so be prudently careful. One regulator would work but two are better for pressure safety and stability. I ran an air hose from the shop lines to the regulator set which was hung under the hood with a wire hook. I must have found a hose barb fitting that screwed into the regulator outlet port that I could clamp a section of clear vinyl onto. I ran the tubing into the intake at the boost gauge nipple, as that seemed the most unlikely place for a leak to form or go unnoticed. Once the airline valve was opened the intake was pressurized to 20 PSI. A slight hissing could be heard at the throttle-body. Using a plastic spray bottle filled with soapy water I saturated all possible leak points. It helps to use a thick soap like laundry detergent. This is easier on a cool engine. The large hose nipples at the back of the intake and the PCV line joints were leaking a shocking amount. And maybe another small one but I can’t remember for sure. Be sure to check every possible point, fuel injector o-rings, intake gaskets, all tubing joints including the spanner nut, the vacuum tree on the firewall, etc. I used small hose clamps and tie wraps to completely seal off all hoses and lines. The difference was phenomenal and very surprising, part throttle torque and throttle response improved about 20%. The engine felt like it got another cylinder. Throttle became very crisp and more precise. It was much smoother and got better mileage. Less carbon in the WOT exhaust. Boost, while unchanged at peak, peaked sooner. WOT power improved as well and the car sped up but not as much as the part throttles 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.

The following is a post I saved describing this to someone who found his or her SC made more power on lower octane fuel. This was obviously confusing. Lower octane fuel burns faster and generates more heat in the same amount of time.

Yes it’s possible, but it may mean your HP and mileage are lower than optimum. Dmitriy, I have a fix that has been proven time after time but I want you to check some things to make sure that this is it. My theory is this, that you have boost leaks. This is very common but usually not noticed on our cars, especially at their age and mileage levels nowadays. Most of the time it’s from old vacuum hoses that have gotten soft and swollen. Check the big ones going across the intakes in the back, if it’s easily pulled off by hand it positively leaks at 15 PSI. Under vacuum they pull together and seal up so it runs fine. Under boost the supercharger can still maintain 15 PSI because your engine and some small leaks are still within it’s flow capabilities, but 15 PSI is its pressure limit with your setup. Look at the tailpipes and the area around their exits; is there a heavy soot coating? Here’s the problem and reason it can use 89 octanes and actually run better with it than 93. All the air that the SC is pumping in whether it gets used by the engine or leaks back into the atmosphere is being metered by the mass air meter and therefore the EEC computer is adding fuel in excess to the engine because it doesn’t account for 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 a extremely rich full throttle mixture that not only cools the cylinder more than necessary but also generates lower than optimum power levels and therefore builds less heat than normal at peak power outputs. The limiting factor in gas octane is air charge heat in the combustion chamber. Octane is not a power rating but a volatility rating, actually lower octane fuel burns quicker and hotter, it is a measure of the fuels resistance to burning, the higher the number the more stable it is and therefore more resistant to self ignition. So in this theory, with a super rich air/fuel charge, the lower octane actually burns faster and hotter therefore perceptibly better than with the high octane which isn’t necessary with lower chamber temps. With normal fuel ratios you wouldn’t be able to tell the difference.

My advice to you if you see excessive black soot in the rearview mirror at WOT (especially at night with someone’s headlights right behind you), soot build up around the tailpipe exits, and loose fitting vacuum lines is to replace the large hoses and use small hose clamps at EVERY fitting to seal them for good. Get a handful of small zip-ties and use them on the small lines that see boost pressure to seal them up. Then change the plugs as they will already be fouled out and not giving you acceptable performance. A new Motorcraft PCV valve is good practice also. You will notice a big difference (the difference in light acceleration is phenomenal) and better mileage and power. This higher HP output and more realistic fuel ratio should generate the need for higher octane again, but the increase in mileage and power will even out the cost. You will be pleasantly surprised and thrilled with the new character your engine develops.

But with a properly tuned engine and fuel system lower octane usually doesn’t work with 15 PSI at WOT, at least in the summer. Give it a try as not only have I done this on quite a few SCs, but others I have told about this have done it and came back here giving me much thanks and having a blast with the new power for less than $15 and a half hours work.

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