Knock knock….

Who’s There?

Ping Who?
Ping I just knocked a hole in your piston….

Engine Knock, Fuel Octane and other myths

Thing One: Never use higher Octane fuel than the engine needs to prevent knock.

I’m going to try to greatly simplify a very complex topic here. They write encyclopedias about this stuff. I’m going to try to keep it to a few pages.

What is Knock/Ping?

When the engine is running normally, the spark plug fires and the flame spreads evenly thru the cylinder. The mix burns evenly until consumed. When the engine Knocks or Pings it means the fuel was set off at the wrong time and/or the fuel was set off from multiple sources. Both conditions are bad for the motor and rob you of performance and fuel economy.

What causes it?

There are many things that can cause it. Most of them you can and should fix.
Here’s a list of common items. (In no particular order.)

  • Over advanced ignition timing. (Very easy to do with mechanical distributors but still possible with electronic ignition.)
  • Vacuum leaks or EGR problems. (These are way more common than people think.)
  • Exhaust leaks. (Even small upstream leaks can screw up O2 sensor accuracy.)
  • Wrong spark plugs or other bad ignition parts.
  • Carbon buildup in cylinders. This alters engine compression and can create hot spots.
  • Engine sensors dirty/bad or damaged sensor wiring. If a sensor is lying to the ECM the car will never run properly.
  • Compression increased as a result of engine or head rebuild. (This can be intentional or a side effect.)

What is Fuel Octane Rating?

That number you see on the pump comes from a bunch of tests that determine how much the fuel helps resist engine knock. The higher the number the more it resists knock and it usually means the fuel is also less volatile. This last bit is the big rub… Less volatile fuel will often perform worse, not better as many people think. How much you may notice this depends on the specific engine in question. In any case, using more Octane than needed will not increase engine performance. Higher-octane fuel sells for more than regular fuel. Compared to most 87 Octane fuels, you pay 10-20 cents more per gallon for a product that costs them almost nothing more to make. That extra dollar or two per tank can add up really fast. Why spend it if you don’t have to… The carmakers all list the minimum Octane rating for fuels in the owner’s manuals. For most Fiero I’m pretty sure it’s 87 Octane. (I know the 87 owners manual lists 87 for both the 4 and 6 cylinder.) Oh, and the factory 2.8 has lower compression than the 4 cyl, so don’t go thinking bigger engine means more Octane. (2.8 is 8. something:1 while L4 is 9:1) Even motors with 100,000 miles or more can run the specified Octane if they are in good shape. Mine has over 150,000 miles and is running wonderfully on 87 Octane fuel. (Even WaWa fuel that is usually one of the lowest priced fuels around the region.) Note: Someone in the forums pointed out the European Octane ratings are higher than the U.S. numbers. Keep in mind other regions of the world use different methods and rules for grading fuel. While the European Octane number might be higher, that doesn’t mean they are automatically better than U.S. fuel at preventing knock.

Bending the rule, additives in Pump Fuel

All U.S. fuel is required by the EPA to have cleaning additives in it. Fuel meeting the minimum requirements is perfectly fine for most cars. Some fuel vendors promote their fuel based on the additive package. These additives won’t add power so much as restore or maintain it by removing carbon and dirt. These additives have contributed to confusion over octane ratings. These cleaning additives, which are unrelated to Octane rating, may help performance in some vehicles. This can be a function of brand, grade, or both. Some brands use the same additive package in all grades of fuel, while others put the “good stuff” only in higher grades. As an example, Texaco puts “Cleansystem3®” in all their fuel. Other brands may put their best additive package only in premium grades. What results you’ll get from any additive packages will depend greatly on the condition of your engine. None of them are likely to clean up a really dirty motor but they may help an engine expel minor carbon buildup and other dirt. Some no doubt do this better than others. Other than cost, it won’t hurt anything to shop fuels based on their additive package. If one product works better for you than something else that’s great. Otherwise don’t buy more than you need. To use Texaco again… Since they put Cleansystem3® in all grades, you’d still run the lowest Octane fuel the engine will take. If Texaco 87 Octane fuel works, their 89 and higher isn’t going to do anything but cost more. (Texaco is just a handy national brand for an example… I use a regional brand most of the time.)

The car knocks/pings…

We’re talking generally stock engines here. Even a regular rebuild should run on the OE octane or maybe one step higher. Obviously if you’ve done a big build up or super/turbo setup, you could need more octane. If a car that was built to run 87 Octane only runs on 90+, the engine may just be dirty or you could have mechanical problems. If the car knocks even on the highest Octane pump fuel you almost certainly have engine problems that need repair. These are some things I look for when an engine knocks. Triple check for vacuum leaks. Even a tiny leak can lean out a cylinder enough to cause knocking without tripping ECM errors. (You’d be surprised how easy it is to have a vacuum leak that only messes with one cylinder.) Check the cooling system very carefully. Knock/Ping can also be an early sign of overheating. Don’t assume the temperature gauge is correct. (Even if it is correct, don’t assume there are no cooling problems. A common problem here is a head gasket installed wrong. This can make some cylinders hot even if coolant temp reads ok on the gauge.) Check timing on distributor motors very carefully. The difference between knocking and not can be just a couple degrees of advance. (Defective or improperly configured Mechanical Distributors are great for this.) Use OE EGR valves (GM, AC Delco, Motorcraft, etc.) whenever possible. If you must use a “universal” style valve, make sure it’s a good brand. A crappy EGR valve can really screw up an engine. (I have seen aftermarket valves die in as little as a year.) Is the engine suffering carbon buildup? You might want to try one of the carbon removal processes. Carbon buildup can be the result of past fuel system problems and other things. It usually won’t go away without help. Is the engine sucking oil? This can both alter the octane of the mix inside the engine and cause carbon buildup. It’s probably time to rebuild some or all this engine or find another. Higher Octane fuel or carbon removal is at best a temporary fix. 87-88 Fiero L4 motors with DIS may need a different PROM in the ECM. GM had an update to tweak ignition timing a bit. (The new PROM numbers are in DIS notes. This kind of stuff is common with ECM/PCM controlled motors.) This should be considered ONLY AFTER all other mechanical items are checked out.

An example

Awhile back, my car started knocking under certain load conditions. (87SC L4 w/ DIS) I’d also lost some fuel economy. This went on for weeks and since the problem wasn’t consistent, it was really a pain to find. It did it even on 90+ Octane fuel. (Since it hadn’t done this previously, I didn’t think the PROM update would help.) In searching for a vacuum leak, I took off the EGR to see if maybe the middle of the gasket had partly blown out. (A common problem with this type of EGR setup.) The EGR wasn’t that old, and like many aftermarket units not easy to test. Since it wasn’t very old I didn’t really expect it to be bad but just out of habit I checked to see if the pintle was moving ok while it was off. It wasn’t moving at all… It turned out that the 2-year-old aftermarket EGR valve had failed totally. It had jammed in such a way as to not open and still leak slightly. (It never set any kind of code in the ECM during all this.) The spring in the top had actually broken and you could hear it rattling around when shaken. Replacing the EGR with a good used GM valve solved both the knock problem and cranked fuel economy back up to 26 mpg even in really crappy traffic. (The EGR gasket cost about a dollar.) The lesson, yet again, is never to assume recently installed parts are good.

Fuel additives

Octane Boosters and Cleaners

Octane boosting additives don’t help much except maybe for when you have a modified/race engine and can’t find suitable fuel at the pump. A stock vehicle that won’t run on pump gas has something wrong. Octane booster products, like higher Octane fuels, usually will not increase performance. How much, if anything, a cleaning product does will depend greatly on how messed up the engine is. Don’t expect any miracles from most of them. Even professional cleaning products may not remove heavy buildup. Some add in Octane boosters and cleaning products may actually damage the older styles of port fuel injectors used by GM and some others. In the older injectors, used thru the late 90′s, the solenoid coil is exposed to the fuel. (This helps cool the coil.) Most products are probably ok but make sure they are used as directed on the container. Mix them stronger than directed and they might eat your injectors. (The newer GM injectors are redesigned and do not expose the coil to the fuel just because of this problem.)

I’ve heard fuel changes by season….

Yes fuel does change by season and it also changes by city and region. This double whammy can make buying fuel quite confusing. If you suddenly drop or gain 2-4 MPG, you might be a victim of Reformulated or Oxygenated fuel. In some vehicles these fuels may also induce knock/ping that forces you to a higher, more expensive, grade of fuel. What many people call Winter Gas is defined as Oxygenated or Reformulated Gas (RFG) depending on where you live. They are supposed to reduce air pollution. These fuels contain MTBE and other chemicals to increase the oxygen content of the fuel. They cost more, often cause odd problems, bite performance, and you get less MPG. There is little you can do about the problems caused by RFG and Oxygenated fuels. On HEI engines, try retarding the timing very slightly if you have ping/knock.

Acceptable Alcohol Fuel Content

Methyl Tertiary-Butyl Ether
Fuel containing Methyl Tertiary-Butyl Ether (MTBE) may be used, providing there is no more than 15% alcohol by volume. MTBE is under investigation in many places. Some states are likely to ban it in future. MTBE is intended to reduce air pollution but has been shown to be a serious problem when it gets in the water. Small amounts of MTBE can pollute large amounts of water and the stuff is hard to filter from drinking water.

Fuel containing ethanol (ethyl) or grain alcohol may be used, providing there is no more than 10% ethanol alcohol by volume. Ethanol plays a roll in Octane rating and pollution control. Its presence in motor fuels is likely to increase greatly as MTBE (Methyl Tertiary Butyl Ether) is phased out in many regions. Higher concentrations of Ethanol will alter the fuel beyond what the ECM/PCM can deal with, and may be too strong for some fuel system parts to handle. Pre-computer engines may need considerable carburetor and distributor adjustments to run right.

GM recommends against any methanol use. It can damage the fuel system parts. (Nearly all carmakers say stay away from methanol even now.) “Dry” gas typically contains Methanol but not enough to hurt anything. Fuel containing methanol (methyl) or wood alcohol may be used, providing there is no more than 5% methanol by volume. Use of fuel (gasohol) that contains more than 5% methanol can corrode metal fuel system components and damage plastic and rubber parts.


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