The vehicle had been off the road for over 4 years undergoing a rebuild, at the beginning of March it was given full service and taken for an MOT, it was running well and passed the mot and emission tests.
Last week it was pressed back into service, and after two days of commuting it developed a high idle when hot, despite the high idle it was still running fine. The CEL was not illuminated.
Investigation:
Initially, a visual inspection was made. Oil and coolant levels, temperature check with an IR thermometer, hot compression test, timing (valve and ignition), throttle cable, coolant and vacuum hoses - all were ok.
Time to check with RhinoView. There were no stored fault codes, and all readings appeared normal, but the idle speed was still around 1100rpm, and the ISC was reading 10%. lowering the target idle speed had no effect, although when raised the ECM was able to maintain a higher stable idle.
Armed with the knowledge that the sensors and actuators were operating correctly, the problem must be too much air. I started by removing each vacuum line in turn and blocking the intake, no change was noticed. Next I removed the ISC valve air pipe and blocked the end, the idle fell to 950rpm, this confirmed an air leak and the correct operation of the ISC.
My next approach was to spray brake cleaner around all the manifold and throttle body gasket joints, a leak would show itself as a dip in engine speed as the flammable cleaner was drawn in, but there was no change. It must therefore be an internal leak.
The only remaining device was the cold idle air valve, I removed the valve to clean and check it, and on removing the valve found the water passage full of pink gunge, blocking the flow of coolant to operate the valve.
It turns out that a P.O. had filled the cooling system with OAT coolant, when i drained and refilled the system with the correct coolant, it had reacted with the remaining OAT causing it to gel and block the small waterways around the manifold. As a result the throttle body was not getting hot enough to fully close the cold idle valve.
After performing a good full back flush of the cooling system, followed by a chemical clean, flush and refill, everything is working normally again.
