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Mechanic baffled

Tried to start my 2005 Sienna the other morning and all it did was whir as if the timing belt was broken (replaced 15,000 ago) the crankshaft turns but no compression tried it several times over three days, had it towed to my mechanic and when he tried to start it – it started right up he has tried it and drove it several times , no CEL and no codes stored. Anybody have an answer?

I don’t know for sure without hearing the sound it made, and neither can your mechanic until it fails again when he can hear it, but I suspect that you only heard the starter spinning and not the motor turning over. That would indicate a bendix spring in the starter is getting weak. Normally you just replace the starter, but at this point, that would be just an expensive guess.


I was scratching my head on this one until you made that suggestion. Could be that or a bad solenoid or weak electrical supply to the starter assembly…

How do you know there was no compression?

And you saw the crankshaft turning? If so, this IS baffling. Next time it happens, take a video with you phone and post it.

And assuming you didn’t see first-hand evidence that the crankshaft was turning, if this happens again, get a rubber hammer and whack the starter and see if it goes away.

Was the truck parked outside and if so how cold was it and what weight oil is in the engine? At nears Zero*F 10W-40 oil can thicken enough that the lifters will hold valves open resulting in loss of compression. If hauled to a shop where the car is in the heat for a few hours the oil can get warm enough that the engine can start normally. I’ve BTDT more than once.

The Bendix drive in the starter has a sprag clutch inside it. It’s a one way clutch which basically functions like a ratchet. This is what is probably failing.

Unlikely the crankshaft was turning. More likely the starter motor was spinning but was not engaged with the flywheel teeth, due a problem with the “Bendix” mechanism. If you take the starter motor out, an auto electric shop can fix it easily. A good time for new copper contacts inside the solenoid, too, and a cleanup of the plunger disc.