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Erratic rpm

Question : I have a problem with the throttle response of my Wira Aeroback automatic. When idling, with aircon on, usually after the car has run for some time, the rpm meter reading goes up and down, between 500 to 3,000rpm. At running speed, the response is OK.

Before this happened, I adjusted the tuning near the throttle body with a screw driver until the timing was higher. I did this because the engine suddenly switched off during low speed (during standstill), which I thought was due to low timing.

Prior to this, I did a top overhaul, changing piston ring, valve seal, and skimming, and other things normally done during the exercise.

Does this self revving (up and down) of the engine mean something is wrong?

A mechanic suggested a cleaning of the injection part. Another guy assumed that the place where I've done the top overhaul has stolen the throttle body and replaced it with bad one.

Answer : The adjustment that you did was the idle bleed that adjusts the amount of air bypassing the throttle butterfly to vary the idle speed. However, looking at your problem I do not think it was caused by that adjustment unless you have unscrewed it too far but rather I think you may be having an air leak in any of the hoses in the intake system. One thing to remember about EFI systems is that the air mass sensor must measure all the engine air requirements in order that the fuel is properly metered through the injectors. So, if there is any leak, i.e. if there is any air getting into the engine that has not passed through the air mass sensor, this would cause the air-fuel mixture to get too lean and the engine will start to "hunt", i.e. go up and down.

You need to check all the hose clips connecting the air-mass sensor to the throttle valve and the intake manifold and also all the hoses connected to the intake manifold for crankcase emissions, vacuum, etc, and make sure that they are tight, and the clamps are tight also. I think when the engine was put back together, some work was not done properly, and this is the result. 


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