I recently bought an EZ-EFI throttle body fuel injection system from FAST for my ’70 Buick Skylark. I installed it according to the instructions and it started and ran pretty good for the first few days. I’ve noticed now that the engine runs very rich and it’s starting to load up the spark plugs. I’ve not changed any of the settings yet it runs a lot richer than before. My idle air-fuel ratio is 13.0, the cruise setting is 13.5, and wide open throttle is 12.5. I don’t understand why the engine seems to run much richer than these settings. You have any ideas?



This is an EZ-EFI throttle body on a big-block Chevy we did a couple of years ago that worked out very well. Attending to the details will make a big difference in how the system works.

Jeff Smith: We’ve installed several of these new-generation, self-learning fuel injection systems and they seem to work very well. But there are a couple of areas where it is essential that the initial installation be performed properly and then attention to details can make all the difference. Let’s see if we can focus in on your problem.

The first thing to emphasize is that the main power and ground wires are in fact connected directly to the battery. All EFI systems operate on electrical signals from sensors that normally operate on a 0-5-volt scale. This leaves very little room for error with regard to proper grounds. So it is essential that the main power and ground leads go directly to the battery. This allows the battery also to operate like a giant voltage spike suppressor which allows the electronic side of the system to operate normally. Don’t be afraid to lengthen either or both power and ground leads if that what it takes to plumb them directly to the battery.

The next item to try is to run back through the initial calibration process again. This essentially purges the computer’s memory of the previous tuning changes. This means starting with engine displacement, idle rpm, rev limit, fuel pressure, and resetting the throttle position sensor (TPS) calibration. Then set your idle, cruise, and WOT settings the same as before. Once all this is complete, the engine should run as it did when you first installed the system. Drive the car for a couple of days and make sure to shut the engine off and restart it several times during the test. You may notice that the engine begins to run rich again after several cycles of running and then shutting the engine off. If the over-rich condition returns, I think what you may be experiencing is a very common problem where there is a leak in the exhaust system that is causing the over-rich air-fuel ratio problem.

If you’ve ever driven a car with an exhaust leak, you can often hear a very audible tick-tick-tick from the exhaust. The tick you hear is the result of an exhaust pressure spike that travels down the exhaust pipe, finally exiting after the muffler. When that pressure spike hits the atmosphere, it sends a reverse or low-pressure pulse back up the pipe and back to the engine. This low-pressure spike will pull in outside air as it passes through the leak area. Think of it like a siphon pulling fresh air in from the outside. Unfortunately, this can cause problems with a self-learning EFI system.

All of the self-adapting EFI systems use a wide-band oxygen sensor placed roughly at the header collector, downstream in the exhaust system. The sensor reads the exhaust gas and the engine control unit – the computer (ECU) uses the information as feedback to the tune. As their name implies, these wide-band sensors (WBOS) operate over a much wider area of air-fuel ratio from very rich at 10:1 to very lean at 20:1. All WBOS operate the same way. They do not actually measure the ratio of air to fuel. Instead, they are calibrated to sense the amount of free oxygen in the exhaust system. The ideally-balanced chemical air and fuel ratio for any fuel is called the stoichiometric ratio. For gasoline, this ratio is 14.7:1. With lean air-fuel ratios such as 15.8:1, there is more free oxygen in the exhaust because there is minimal fuel to complete combustion. This is called an excess-air ratio. An air-fuel ratio of 12:1 for example would be considered an excess-fuel ratio and contain very little free oxygen because there is more fuel available to use more air to combust the fuel.

The reason it’s important to know how a WBOS works is because there are several conditions that can fool an oxygen sensor into thinking that the engine is running lean. The first situation is actual misfire. Often, an engine might create a misfire that you may not necessarily feel as a hesitation. This happens quite often under high load at WOT. When this misfire occurs, that unburned fuel and free oxygen travel out the exhaust where the WBOS reads the free oxygen. This is then interpreted that the engine is running too lean and needs more fuel.

If, as may be the case with your car, you either have an engine misfire or – more likely – a leak in the exhaust system, this higher oxygen content in the exhaust will quickly fool the WBOS into reporting to the ECU that the engine is running lean. This adds fuel to the engine. What makes matters worse is that most (if not all) self-learning EFI systems store the new air-fuel ratio tune to the ECU when the engine is shut down. Then the process starts all over again because the leak in the exhaust system is still fooling the WBOS, each time the engine is operated and then shut down, the new, richer self-learning table is added to the existing tune. This is the downward spiral that makes the engine run richer every time it is started and run.

So it could be the solution to your problem is to check the exhaust system very carefully. Two logical places for air to enter the exhaust system is the gasket between the header and the head and the other at the collector flange where there is often a gasket that can easily allow fresh air into the exhaust. This can occur even if there is no audible tick or leak in the system. If the header gasket shows black exhaust tracks past the gasket, it’s likely that it is also allowing outside air into the exhaust. Another possible leak path is the WBOS mounting bung itself. Make sure that this fitting is fully welded into place with no pinholes that could allow air very near the WBOS itself. Another possible leak path could be any place where two exhaust pipes are welded together. This is a very common place for leaks to occur. Look for these holes at the top of the pipe where it is most difficult to weld when the pipes are in the car.

A great way to check for exhaust leaks is to pump smoke into the system. There are several commercial smoke generators on the market designed for this kind of testing, but we have also seen some ingenious home-built smoke leak detectors built with liter soda bottles that can do the job. Eliminating even a very minor leak or crack in an exhaust system will make a huge difference in improving the performance of your fuel injection system.

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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.