# Why would a PCM / ECU retard timing at idle?

I'd like to understand better the reasons that a PCM / ECU would decide to retard timing at idle.

I'm aware of the following:

knock
noise similar to knocking picked up by the sensor
a defective knock sensor
engine overheating
missing knock sensor on some cars ( some Hondas I think )


are all possible reasons; are there any other reasons why the PCM would retard timing at idle?

Would a code always be set when the timing is retarded?

My intention in this question is retarding beyond what is normal, due to some kind of fault condition or out of spec operation.

• rough idling (independent of the root cause)? crankshaft position sensor malfunction/noise? – Martin Nov 6 '17 at 15:52
• @Myself rough idle as a result of retarded timing, but as a cause? crank or camshaft sensor noise / failure that wouldn't set a code? – Robert S. Barnes Nov 6 '17 at 15:57
• On an Renault I once had a CPS, contaminated with metal shavings, resulting in starting problems. For unknown reasons there was no code present. The tendency for ignition timing is "early as possible, but not too early", so if there are problems/unclear situations the ECU might try to retard the ignition. – Martin Nov 6 '17 at 16:58
• Also leaner mixtures are burning faster that rich mixtures, resulting in a need to delay the ignition – Martin Nov 6 '17 at 16:59
• I've seen several instances of an engine with mechanical problems like a loose timing chain or rod knock that could fool the knock sensor into retarding the timing to near undrivable states. – vini_i Dec 8 '17 at 18:53

I assume you're unconciously talking about the natural behaviour of retarding at idle, i.e. not additional retarding because of a fault. Retarding at idle is an old technique with several reasons;

1. Retarding makes for a less efficient combustion, so that more energy gets converted into heat instead of work. This causes the engine to run hotter, which creates a more complete combustion. It's meant to reduce HC and CO emissions while idling.

2. Retarding creates less peak pressure, resulting in less NOx emissions.

3. Retarding makes it possible to make the engine run slow and smooth, making for more comfort, less noise and less fuel consumption and emissions in overall while sitting in traffic.

4. With the old carbureted engines, it also made for better driveability. As soon as the accelerator would be pressed while idling, the ignition would instantly be advanced. This caused the engine to suddenly run more efficient and create more power. This makes driving away at the traffic light considerably more easy and comfortable.

Addendum: OP mentioned that he was talking about an abnormally retarded ignition. There could be two reasons for that:

1. Either the engine is supplied with faulty information by misbehaving or broken sensors, causing it to think that it runs more advanced that it actually does, or;
2. The sensors are fine, and it is a protecting strategy to prevent eg. knocking. If an engine is equipped with a knock sensor and active ignition control, it will retard the ignition in order to reduce the knocking. There could be other reasons to protect the engine by retarding the ignition, but I can't think of additional reasons.
• Sorry my question wasn't clear enough, I mean excess retarding beyond what is normal spec. – Robert S. Barnes Nov 7 '17 at 15:31
• @RobertS.Barnes Note taken, answer revised. – Bart Nov 7 '17 at 15:51

It's a simple matter of obtaining a fuel-efficient and smooth idle.

You don't want the piston attempting to compress an already burning charge. There's no kneed to generate power, so an idle ignition timing of even after TDC works.

Chase the piston down with just enough energy to keep the engine spinning smoothly.

• My intention is a situation where the PCM is retarding timing beyond what the normal range is for that engine. For example, I've got a car that calls for between 6*-18* of advance at idle, but the PCM is retarding it to between 0.5* - 2.75*. – Robert S. Barnes Nov 7 '17 at 7:34
• That is not the same question you asked.... – Solar Mike Nov 7 '17 at 9:12
• @SolarMike OK, I thought asking about the setting of a code in the question made it clear I meant it was about retarding due to a fault condition. Apparently I need to make it more explicit. – Robert S. Barnes Nov 7 '17 at 15:28
• It is often said that to get the right answer, one needs to ask the right question... – Solar Mike Nov 7 '17 at 15:33
• @SolarMike Word. – SteveRacer Nov 8 '17 at 2:55

One of the reasons an engine computer would retard the timing at idle when there are no fault codes present is in order to meet the computers RPM Target. For example if the engine is idling slightly high for some reason like a small vacuum leak, and the idle air control valve is already fully closed then as a last resort the computer will retard the timing to reduce power and slow the idle down. Such a situation could also occur if the idle air screw is opened too much.