Electronic Fuel Injection: Motorcycle EFI Diagnostics and how to troubleshoot your bike.
By Mike Grage, Customer and Technical Support at Modern British Iron
“Understanding how the EFI sensor loop works is the foundation for finding a fault in the rainbow-colored spaghetti we call a wiring harness.”
The most common question I get when discussing Electronic Fuel Injection EFI diagnostics is: How do you know this stuff? For most of my adult life my job title was ‘multi-line motorcycle service technician’. Motorcycle Mechanic’s Institute only gave EFI an honorable mention when I went there in 1990-1991, and that training was for the early EFI systems used by Japanese motorcycle manufacturers. Those EFI systems were not the most dependable, they were not very well understood by dealer service technicians, and they lacked on-board diagnostics. Additional fuel injection training over the years came from Triumph, Kawasaki, BMW, Suzuki, and Harley-Davidson. All that training was a great starting point, but when it comes to EFI training means very little without shop experience. My experience with EFI diagnosis was a regular thing for more than a decade.
“Then there are the dreaded intermittent MIL trouble codes.”
On-board diagnostics help, but you cannot depend on them to solve every problem. There are times when replacing a sensor will not solve the problem that is generating a trouble code. Other times the on-board diagnostic system continues to produce multiple trouble codes after replacing all the sensors that produced the trouble codes. Then there are the dreaded intermittent MIL trouble codes. The scary truth is that most EFI problems are electrical problems and hunting them down can be frustrating. Experience has taught me that sensor faults are usually caused by wiring problems, not the sensor itself. This Electronic Fuel Injection diagnostics guide will give you a couple places to start looking.
The best piece of advice I can give is to use an analog multi-meter when testing components. The reason for this is because analog meters do not have a polling rate. The best example of why analog is better is the TPS tests. The throttle position sensor output for Keihin EFI should have 0V with throttle closed, 2.5V at half throttle, and 5V at full throttle. The needle on the analog meter should move smoothly as you open and close the throttle. A sudden drop to 0 off idle indicates a burnt spot in that potentiometer, something like that can be easily missed with a digital meter and trigger intermittent MIL codes.
This blog cannot solve every EFI problem out there. The idea is to give the readers some experience-based advice on where to start. The following are the three types of faults technicians deal with on a regular basis, and the methods I used start dealing with them. The three titles are the long form of what I would like to see a service writer put on a service ticket when a motorcycle comes into a dealership.
1. Troubleshooting No Start/Poor Running when MIL Codes are NOT present:
No start, poor running, poor engine acceleration, with no malfunction indicator light (MIL) codes after the engine starts tends to be a fuel pressure problem. Low fuel pressure is the most common EFI problem, and it will not trigger a malfunction indicator light code (MIL, check engine, FI, engine symbol light, etc.) in most cases. If the MIL light goes out after the engine is cranking, a fuel pressure test is needed. Low fuel pressure is usually caused by a weak fuel pump, bad pressure regulator, or bad fuel pump relay. A voltage check at the fuel pump connector is always a good idea before ordering a new pump, low voltage at that connector is usually caused by a bad fuel pump relay. Although it is a rare occurrence, a gremlin in the harness can cause low fuel pump voltage and pressure.
2. Troubleshooting Malfunction Indicator Light Codes:
If you have one sensor producing codes replacing that sensor should solve the problem. I suggest checking the connector and terminals before replacing that sensor. One habit I have is pushing wires into connectors after I get the connector to click to make sure the terminals have a solid connection. Missing that a tiny metal tab broke off a terminal is an intermittent MIL code nightmare. It takes 3 cycles to trigger a code and 40 cycles for the code to go away, so sometimes pushing a wire in and then starting the bike will clear the code and solve the problem. Terminal connections can come loose from vibrations. Just because you don’t feel it in your seat doesn’t mean your bike doesn’t have screw and connector loosening vibrations. Checking to make sure all fasteners are tight is part of a regular service on all motorcycles due to the high frequency vibrations.
3. Troubleshooting Multiple MIL codes for Electronic Fuel Injection EFI Diagnostics:
I wrote a companion piece to this blog on the breakdown of OBD-II codes. Go read it first if you have codes caused by multiple sensors first. The sensor loop codes always start with P01xx or P11xx, multiple sensors producing codes with those designations are generally a single bad connection or a single bad sensor. Coolant and MAP sensors are notorious for causing multiple MIL codes. However, multiple system codes (ex. P01xx and P03xx or P03xx and C16xx) are usually wiring related. The wiring on motorcycles is not very protected from corrosion, and wires corrode. EFI is more prone to corrosion related malfunctions because it works with 5V DC circuitry. The part of the circuit that is most prone to corrosion are the case and frame grounding points. A bad ground for the sensor loop is the type of nightmare that will make wrenches fly across the shop and has the potential to cause alcoholism in motorcycle technicians. I remember the first time I dealt with this problem like it was yesterday. In 2001 a Triumph Daytona 955i rolled onto my lift for a warranty repair, the problem it had was so baffling that I was the third technician to get stuck with that demonic motorcycle. The idle speed control motor was bringing the idle up and down. The ECU produced multiple MIL codes, and then the bike would shut down when it reached operating temperature and would not restart until it cooled down completely. All the EFI sensors were replaced by the two previous techs and the Hinckley factory continued to throw parts at it to the point that they sent a new ECU for me to install, none of which solved the problem and all of that made me reconsider my career choices. Instead of giving up like the other two techs I decided to go old school. Got out the analog Micronta meter and started checking the individual EFI component readings at the ECU connector and tugging on wires and the wiring harness. A funny thing happened when I checked one of the black wire’s continuity to ground, the needle twitched every time I tugged on the harness. There was a loose engine ground wire, replacing the eye connector on that wire fixed the bike. It only took 6 months, 3 technicians, and 1 smashed computer monitor to find that loose (not even broken) wire. That bike taught me a lot about EFI diagnostics, and it taught me that throwing a wrench in the general direction of a computer is a bad idea.
Here’s the explanation of what was happening there. The EFI sensors operate on a 5V DC reference voltage which makes them more prone to failures from loose/bad connections. Understanding how the EFI sensor loop works is the foundation for finding a fault in the rainbow-colored spaghetti we call a wiring harness. The ECU has two modes, open loop and closed loop. The ECU takes readings from a group of sensors in a specific order in closed loop mode to determine fuel mixture/trim and it ignores the sensors in open loop mode. The specific order is why faulty readings from the coolant temperature or oxygen (lamda) sensor can trigger other MIL codes. The loose wire on the 955i was the ground for the sensor loop and ISC stepper motor circuits.
Those are the most common conditions and starting points for Electronic Fuel Injection EFI diagnostics and faults. There are a lot of things that can go wrong in the rainbow-colored spaghetti called a wiring harness. There are over 5,000 OBD-II codes and I am not going write a blog on the causes and cures for each one of them anytime soon. Honestly, tugging on wiring harnesses and checking connectors is a good first step towards preventing heart attacks and strokes associated with EFI problems. Hope you enjoyed this, and I hope it helps.
Also, check out our blog on Explanation of EFI Diagnostic Trouble Codes for OBD-II for more information.
Frequently Asked Questions
What is the most common motorcycle fuel injection problem?
Low fuel pressure is the most common EFI problem, and in most cases it won’t set a malfunction indicator light (MIL) code. If your bike won’t start, runs poorly or accelerates poorly with no MIL code once the engine is running, suspect fuel pressure first. Low pressure usually comes from a weak fuel pump, a bad pressure regulator or a bad fuel pump relay. Find fuel system parts in Carburetor and Fuel Delivery.
Why won’t my fuel-injected motorcycle start when there’s no check engine light?
If the MIL goes out while the engine is cranking and no code stays on, you need a fuel pressure test. Before buying a new fuel pump, check the voltage at the pump connector. Low voltage there usually points to a bad fuel pump relay, not the pump. In rare cases, a fault in the wiring harness can also cause low pump voltage and pressure.
Are most motorcycle EFI problems electrical?
Yes. Most EFI problems are electrical, and sensor faults are usually caused by wiring problems, not by the sensor itself. On-board diagnostics help, but you can’t count on them to solve every problem. Replacing a sensor doesn’t always clear its trouble code, and a bike can keep setting multiple codes even after every flagged sensor has been replaced. Start by checking connectors, terminals and grounds. Wiring and connectors are in Electrical and Ignition.
Should I use an analog or digital multimeter for EFI testing?
Use an analog multimeter when testing EFI components. An analog meter has no polling rate, so its needle shows every change in voltage as it happens. A digital meter can miss a split-second dropout that triggers intermittent MIL codes. A throttle position sensor (TPS) test is the best example of why analog is better.
How do I test a motorcycle throttle position sensor (TPS)?
On Keihin EFI systems, TPS output should be 0 V with the throttle closed, 2.5 V at half throttle and 5 V at full throttle. Using an analog meter, open and close the throttle slowly. The needle should move smoothly. A sudden drop to 0 V just off idle means a burnt spot in the sensor’s potentiometer. That fault is easy to miss with a digital meter and can trigger intermittent MIL codes.
What should I check before replacing a sensor that’s setting a code?
If one sensor is setting codes, replacing that sensor should solve the problem, but check the connector and terminals first. After the connector clicks into place, push each wire into it to make sure the terminals have a solid connection. A tiny metal tab broken off a terminal can cause intermittent MIL codes that are very hard to track down. Vibration you can’t feel from the seat can still loosen connectors and fasteners, so checking them belongs in every service. The Hinckley Triumph maintenance and restoration guide covers this.
How many cycles does it take to set or clear a motorcycle MIL code?
It takes 3 cycles to set a MIL code and 40 cycles for the code to clear on its own. That’s why sometimes pushing a loose wire back into its connector and starting the bike clears the code and fixes the problem. To understand what a specific code means, see British Iron’s explanation of EFI diagnostic trouble codes for OBD-II.
What does it mean when my motorcycle shows multiple MIL codes?
It depends on which code groups are showing. Sensor loop codes start with P01xx or P11xx. When several sensors set codes in those groups, the cause is generally a single bad connection or a single bad sensor. Coolant temperature and MAP sensors are notorious for causing multiple MIL codes. Codes from several different systems at once, like P01xx with P03xx, or P03xx with C16xx, are usually caused by wiring. In standard OBD-II codes, P01xx covers fuel and air metering and P03xx covers the ignition system or misfires (Wikipedia).
Why are fuel-injected motorcycles prone to corrosion problems?
EFI sensors run on a 5 V DC reference voltage, so even a little corrosion or a loose connection can throw off their readings. Motorcycle wiring isn’t well protected from the weather, and wires corrode. The case and frame grounding points are the parts of the circuit most prone to corrosion. A bad ground for the sensor loop is one of the hardest EFI faults to find.
How do I find a bad ground or loose wire in an EFI wiring harness?
Go old school. Use an analog meter to check each EFI component’s reading at the ECU connector while tugging on the wires and the harness. Watch for the needle to twitch. On a 2001 Triumph Daytona 955i, Mike found that the idle was surging, multiple MIL codes were setting, and the bike shut down at operating temperature. Every sensor and even the ECU had already been replaced. When he checked a black wire’s continuity to ground, the needle twitched each time he tugged the harness. The cause was a loose engine ground wire, and replacing the eye connector on it fixed the bike.
What is the difference between open loop and closed loop on a motorcycle ECU?
The ECU has two modes. In closed loop, it reads a group of sensors in a specific order to set the fuel mixture and trim. In open loop, it ignores those sensors. Because the sensors are read in order, a faulty coolant temperature or oxygen (lambda) sensor reading can trigger other MIL codes. In closed loop, the oxygen sensor measures leftover oxygen in the exhaust so the ECU can adjust the mixture, mostly at idle and cruising speeds (Motorcycle Cruiser).
Where can I find help with Triumph EFI trouble codes and parts?
Start with British Iron’s explanation of EFI diagnostic trouble codes for OBD-II, which explains what the codes mean. To make sure you order the right sensors and harness parts, decode your bike’s model year and engine with the USA vehicle identification number explanation. Fuel-injected models like the Triumph Street Cup use the same diagnostic approach. If you’re still stuck, contact British Iron.
