Car occasionally cranks very slowly, but then cranks normally

Question

Got a strange starting problem. 19 times out of 20, my '07 Honda Fit starts instantly. But about 1 time in 20, it will crank very slowly, as if the battery is nearly flat.

If I turn off the ignition, then try again after a couple of seconds, it will (usually) start instantly, as if nothing was wrong.

Also, if I keep it cranking for a couple of seconds, it will suddenly speed up and start. So I don't think it can be a flat battery.

I haven't been able to link this behavior to using any accessories (like the A/C or demister), engine temperature, weather, or anything else. It really has no clear pattern.

However, it's slowly getting worse. A few years ago it was 1 time in 100. Now it's about 1 time in 20. It's also failing to start on the 2nd (or 3rd) attempt more often now. So I'm worried about getting stranded somewhere now.

Thanks for any suggestions what to check.

Answer 1

I can think of a few possible faults -

1. Faulty/dirty contacts in the starter solenoid. It may be possible to dismantle the solenoid and clean the contacts. If not you would need to replace it.

2. Worn brushes in the starter motor, or a dirty commutator.

3. Oxidised connections on the high current cables that run from the battery to the solenoid and to the starter motor. Included here would be the earth straps that connect the battery negative to the body and the engine.

I had a similar problem with my 1971 Triumph Spitfire. I unbolted all of the high current wires and cleaned them with wire wool. The difference was quite amazing afterward. With such high currents, you only need a very small resistance in a connector to cause a large voltage drop, which will cause the starter to run slowly.

The large voltage drop on the connector can then cause heating of the connector and therefore causing a small amount of expansion movement that then improves the connection briefly. The improved connection will then make the starter run better.

It is also possibly to get an oxidised connection where the terminals are crimped onto the ends of the wires. This is a bit more complicated to fix and may be simpler to replace the cables.

I would start by cleaning all the connections you can see, including the battery terminals.

Make sure you disconnect the battery before doing any work.

Answer 2

It is worth checking the earth connection (earth strap) from the engine to the battery or car body. These are a common failure item as they are exposed to water from the road, and when they fail it will prevent the starter motor from pulling enough current. This can be an intermittent issue and will often appear to "fix itself" after a few tries.

A good way to rule out this particular issue is to connect a jump lead from the battery negative terminal to the engine block. If the engine then cranks normally, you can be fairly sure that it's an earth connection fault.