- Contaminated O2 Sensor: If you run certain fuels, such as race gas or leaded fuels, the O2 sensor may become contaminated and read incorrectly. Result? Improperly mapped fuel table, possible catastrophic engine damage from running too lean. The fix: Calibrate the O2 sensor often and replace the O2 sensor if it cannot be calibrated within specification. If you're unsure or in doubt of O2 operation, replace it! Better to replace an O2 sensor than a blown engine.
- Engine Misfires: If one or more cylinders are misfiring for any reason, chances are pretty good that the O2 sensor will signal a lean condition and the ECU will compensate by increasing fuel flow. If the ECU is allowed to go too far, in other words, if it's given enough control authority, it can over-fuel the engine and wash down the cylinders. The engine won't blow, but it'll suck! A lack of power or a lack of compression is usually the result of cylinders being washed down with excess fuel that prevents proper piston/cylinder lubrication. Keep sharp when tuning and learn to realize when the engine is running richer than it needs to. Symptoms include rough-running, lack of power, black exhaust smoke, fouled spark plugs and chugging under acceleration. You may also notice that the engine sounds or feels "loaded up", as if it's struggling to pull the vehicle. Keep the preceding in mind, and do yourself a favor and use two O2 sensors when tuning so that you can cross-compare and verify readings.
To address the control authority issue mentioned above, take a look at the following screen-shot:
The table above is the Closed Loop Compensation Limits table. It allows you to set the maximum authority for fuel trim in terms of percent (%). On the y-axis, you'll find manifold pressure in kPa while the x-axis is engine speed. At low engine speeds (up to 875 RPM) and below 1-bar (105 kPa) the ECU is only given 10% authority. Everywhere else, at all other engine speeds and loads, the ECU has been programmed with +/- 40% control authority. This is way too much for racing purposes.
Stock, OEM vehicles have 25% Closed Loop control authority for "Short-term" fuel trim and 25% for "Long-Term" fuel trim. Long-term trim is meant to address wear issues or changing environmental factors, such as driving from sea-level to high-altitude. Short-term fuel trim is meant to address instantaneous issues, such as changing barometric pressure or fuel composition variation. But again, these are stock, OEM vehicles that have a blanket, mass-production ECU calibration when they leave the factory and may be driven who-knows-where under who-know-what conditions. Your race car will probably be driven in relatively consistent conditions with high quality fuel, and be tuned properly with care and effort. There's no need for more than 10 or 15% authority to be granted to the ECU if you're tuning correctly. The beautiful thing about the Dominator EFI is that the fuel map is presented in a format that allows you to use logic to determine if the fuel map looks proper. Here's the rule of thumb:
Most widely available fuel used in most automotive engines results in a brake-specific-fuel-consumption (BSFC) of roughly 0.5 lbs/hp/hr. Therefore, if your engine produces 100 hp, you'll need roughly 50 lbs of fuel per hour to satisfy the fuel demand. A 400 hp engine needs roughly 200 lbs of fuel per hour. See where we're going with this? Take a peek at the fuel map below:
The fuel map here is for a Chevy 350 with a mild cam and it is presented in lbs/hr of fuel delivery. On the left-hand-most circle we've highlighted, you can see the value at 60 kPa at 800 RPM is 6.18. Using the math we showed you above, you can determine that the engine requires roughly 12 to 13 hp at that load while idling at 800 RPM (6.18 x 2 = 12.36). The next highlighted section, corresponding to 49 kPa and 2,500 RPM, a typical condition at cruise speed, you see the value of 23.2, or roughly 46 hp worth of fuel. About right for that engine speed and load, isn't it? Looking up top at the WOT condition of 105 kPa and 6,500 RPM you'll see the value of 246. This would correspond to an engine output of roughly 490 hp. The point here is that the layout of the fuel map lends itself well to logical thinking. If you consider your engine, you can calculate the approximate fuel flow your engine will require and enter the appropriate values in your fuel map to get yourself in the ballpark even if the EFI system doesn't have a base map close enough to your build or engine specification. This, then, allows you to use a Closed-Loop control authority that is much tighter and thus prevents the probability of over- or under-fueling your precious engine. To make your life easy, you can always start with a slightly wider range of authority while tuning, such as 30%, but then once you've nailed the fuel map down and the engine runs great, cut the authority down to 15 or 20% to prevent the ECU from commanding harmful fuel trims due to something as simple as a contaminated or defective O2 sensor. Now, having covered these 4 common mistakes, the last one is simple but often overlooked...
If you've made any of the mistakes listed above, I suggest a new set of spark plugs because by the time you've sorted out any one of the above, let alone all of them if you've gone that far astray, chances are the spark plugs are fouled and useless. Get a new set of plugs with the proper heat range for your engine and set them to the proper gap before installing them. If I had a nickel for every time I heard "The plugs are new, Dan!", well, I'd be somewhere else...probably doing something evil as the dictator of my own private island. The point is, don't make this amateur mistake. Not even once. Certainly not to save $40 on spark pugs.
"Why $40?" you ask?
Because you should buy two sets when tuning your race engine. If you think you need to run a medium heat range spark-plug, then buy one medium set of plugs and then get a set that's one or two heat ranges hotter. Start with the hot set and get your engine idling properly and running well during part-throttle cruise conditions. Then do a couple of sessions mapping the WOT portion of the fuel and ignition maps. When you're closer with the fuel trims and ignition timing, then swap in the colder plugs and go to town. The hotter plugs you used earlier on will have prevented, or at least reduced the probability of fouling while you worked your magic and then the colder plugs allow you to dial the engine in completely without worry of misfires or lacking performance due to over-heated plugs when doing long, hard pulls on the dyno or at the track. That's the easy way to do it. The $35 or $40 you spent (use regular ol', conventional copper plugs) will be well worth the time you saved and avoiding any spark-plug related headaches.
So there you have it: the Top 5 mistakes.
Index the CKP, get the ignition system straight and operating properly, choose the right fuel injector and set the fuel system up properly, enter the proper fuel parameters into the ECU, map the fuel tables carefully and don't rely solely on Closed-Loop or Auto-Tuning (use your head...it works better than trusting sensors or fuel quality) and use spark plugs that'll allow you to get the job done with a clean-running, misfire-free engine. If you do this, you'll avoid making the 5 most common mistakes made when tuning EFI and you'll have a nice, easy time of making gobs and gobs of power. MET