EFI: Vacuum leaks

VACUUM LEAKS

[photo] A huge source of problems with late model cars is vacuum leaks. Use the under-hood emissions diagram and check every single vacuum circuit with a vacuum tester such as the MityVac. In the case of my car, I found a split hose going to the MAP sensor and two small holes burned through the vacuum line going to the cruise control vacuum tank. When I tried to remove the vacuum line from the tank I broke the nipple so it may have been cracked and also leaking.

I saw a neat (but probably expensive) tool for locating vacuum leaks on TNN's Shadetree Mechanic show. It's called VACUTEC and it uses nonflammable, nontoxic smoke to find problems and it works with the engine off. Basically, you hook it up to the vacuum system and turn it on. It pressurizes the vacuum circuits with smoke and makes it pretty obvious when you have a leak.

I've had a lot of trouble with the fuel purge canister solenoid on my car. When energized, it's supposed to hold vacuum for at least 15 seconds. The first time I replaced it, you could hear it click when power was applied but it would not hold any vacuum. 18 months later, I found the replacement (which tested fine when it was new) it would barely hold vacuum for 1 second. It turns out the '89 is special and you must buy the whole frigging canister assembly. $65 at the local AC/Delco parts house.

The symptoms of a defective canister solenoid are interesting. "Good" values for the integrator and block learn multipliers (BLM) are defined to be 128 ± 6. Starting in 1993, GM refers to the integrator as the short term fuel trim (STFT). STFT values indicate the percentage of fuel that must be added (or subtracted) in order to achieve the desired fuel mixture. An integrator of 138 means that %10 extra fuel must be supplied; 118 means %10 less fuel is used. So larger numbers mean the engine is running lean and extra fuel must be added and smaller numbers mean the engine is running rich and fuel must be reduced.

My scan tool showed the integrator and BLM to be 100 at idle and 140 above idle. This means the engine was running rich at idle and slightly lean at other times. I believe what was happening is that fuel vapor was getting sucked in at idle; this extra source of fuel was causing the rich condition. At engine speeds above idle, the extra oxygen became the dominant problem.

Here's a sub-hint, don't buy AC/Delco parts from the dealer, find a local AC/Delco distributor and save some money! I recently bought a defogger timer relay that was $38 from the dealer but only $20 from the distributor.

Test, or since they're so inexpensive, replace the PCV valve. I prefer to use the genuine part (about $3).

One last thing you should do is clean the throttle body. A buildup of dirt can cause the minimum air flow to be too low; this can degrade the quality of the idle. But be careful about what you use; Brake-clean or carburetor cleaner can damage gaskets, hoses and wiring insulation. Also, since you are supposed to replace any throttle body gaskets you remove, you should buy the gasket kit (available from your AC/Delco distributor) BEFORE you take the car apart. Leaks in the intake system are just as bad as leaks in vacuum systems.

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