GV 250 EFI, 2008 – C27 fault / ISC issue, idle speed too high

[tangio]

Bike Model = : GV 250 EFI 2008

Hi everyone, I’m new here and glad to have found this forum. I’d like to share my current troubleshooting issue and would really appreciate any advice, ideas, or similar experiences.

Bike details

• Model: Hyosung GV 250 EFI
• Year: 2008
• VIN starts with: KM4MJ53A
• ECU: Sentec 85-6816, marked GV 250
• Battery: Yuasa YT12A-BS, 10.5 Ah

Fault description

My Hyosung GV 250 EFI is showing fault code C27. According to the diagnosis, this relates to the Idle Speed Control / ISC system. The main issue is that the idle speed is far too high.

The bike already had this fault when I bought it from the previous owner. Up to now, the idle speed could only be brought back down to a normal level by manually restricting the bypass air for the ISC valve.

Parts already replaced:

• New OEM ISC stepper motor
• New OEM stator
• New OEM regulator/rectifier, as the wiring had already been burnt
• New battery: Yuasa YT12A-BS, 10.5 Ah
• New spark plugs
• New spark plug caps
• Oil and filter change carried out at the same time

Condition after replacing the parts

After replacing the parts, I carried out a test run. The bike started very well. The previous manual restriction in the bypass air line had been removed before the test.

With the bypass air not restricted, the C27 fault was initially no longer shown on the FI light. However, the idle speed then rose to around 4,500 rpm.

If I manually restrict the bypass air, the engine speed drops to around 1,500 rpm. In this condition, the C27 fault is currently shown again. No other fault codes are displayed.

If the bypass air is fully restricted or blocked, the engine stalls. For that reason I assume that there is no significant false air / air leak causing the high idle speed.

ISC stepper motor check

When the ignition is switched on, the ISC can be heard moving against its end stops. It closes the valve fully once and then opens it again. This was also checked visually by removing the bypass hose.

The ISC stepper motor plunger fully closes the valve and then opens it again. However, while the engine is running, the valve remains fully open all the time. Even after running for a longer period, there is no visible regulation.

Measured sensor values

Engine temperature sensor

The measurement was taken at the ECU connector. I measured between pin 8 and pin 5. The value was 12.86 kOhm at just under 21 °C.

Intake air temperature sensor

I measured between pin 5 and pin 22. The value was 13.2 kOhm at just under 21 °C.

IAP sensors at the throttle bodies

The IAP sensors were tested according to the diagnostic guide. With the ignition switched on, I measured a 5 V sensor supply voltage directly at both IAP sensor connectors.

The connectors were then plugged back in and the sensor output voltages were measured from the rear of the connectors using test probes. With the engine running and the idle speed reduced to around 1,500 rpm, the front sensor measured 3.48 V and the rear sensor measured 3.2 V. At higher rpm, both values were around 2.8 V.

Further troubleshooting

As I initially suspected a faulty or incorrect engine temperature sensor reading, I connected a substitute resistor instead of the installed sensor to simulate temperatures between roughly 20 °C and 100 °C.

This only caused minor changes in idle speed at a very low resistance value, meaning a simulated very high temperature. I could not get the ISC to regulate normally while the engine was running.

Important detail: no TPS fitted

One important point: this bike does not have a TPS / throttle position sensor fitted to the throttle body. The corresponding wire is also not present in the wiring harness and is not connected to ECU pin 6. Pin 6 is unused.

Questions

1. Has anyone experienced fault code C27 on a GV 250 EFI together with an ISC valve that stays fully open while the engine is running?
2. Which input signals does the Sentec ECU use for idle control on this model if no TPS is fitted?
3. Are there any known issues with the Sentec 85-6816 ECU that could cause this behaviour?
4. Any Information about the Hyonsung ECU Partnumber for my bike with Sentec ECU 85-6816?
5. What further checks or measurements would you recommend to find out why the ECU keeps the ISC fully open during engine operation?

Many Thanks for reading

[♠️ M77]

It may sound crazy, but have you removed the throttle bodies completely and cleaned them out and replaced their o-rings , because if it is a sentec 2008, it will have been nearly 20 years of those o-rings cooking on engine head, microscopic leaks are possible cause uneven metering behavior for the ECU

Then also check your lambda sensor against the service manual values , it’s something i would check next ,  being a Sentec , it will be using an MS3-250 Hyosung lambda sensor if it has the old big white connector.

The check the resistance of the wire plug from the ISCV  to the ECU plug, and see if there is a breakage in the harness , then find contact cleaner.  Clean every socket you can find on the plug (every last one) , the ECU tends to get funny if  one sensor is OK but resistance high due to oxidized or dirty contacts.  Do not spray the ECU pins.   Wait for the harness to fully dry and retest ?

The worst case scenario is replacing the ECU if the internal chip has a fault (intermittent)  – I am suspicious that a Sentec GT250 ECU will work on GV250 but I can’t promise this claim , so it is just my suspicion

There was a time years ago i saw ECU marked GV/GT250-Sentec   on sale. And it was factory made for both GT and GV  (behaves the same as they used same parts in 2008)

Things changed a lot after 2009 which Daewoo and Delphi ECU’s

If it gets too crazy, you could switch to Delphi electronics which you can find easier on ebay but it means changing too many parts and you want to keep the cost low.

I pray the current ECU is OK and it is just a matter of harness checking , cleaning & throttle body O-Rings + lambda sensor checks

Info on the Sentec is limited as information is mostly disappearing from the web ,   it’s a good ECU if there is no issues , its just Daewoo was the worst ECU for the GV series.

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