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When I try to start my ford ranger 2009 it won't start. I already change the fuel filter. But when I sprayed wd40 into the air intake manifold it started and the engin runs well. But a few hours later it won't start again. Any ideas??

Compression, Fuel, fire(ignition), and air
Since air is never the problem look at the systems that produce fire and provide fuel. I'm ruling out compression as you said the vehicle runs after you essentially use an accelerant

Fuel
Make sure everything is tightened properly and there are no leaks in the fuel system. Try priming the diesel system before starting.

Fire
Try letting the glow plugs heat up more than once (Turn the ignition to the on position more than once) before attempting to start.

I don't have this vehicle, I'm just giving the basics based on info provided.

Info about when the problem started, or how long you have been experiencing this problem would be helpful.

Thanks for the reply. The problem started a few days ago after replacing the glow plugs.

Maeel wrote:Thanks for the reply. The problem started a few days ago after replacing the glow plugs.

https://www.trinituner.com/v4/forums/viewtopic.php?t=759805

Service the starter, those engines like a fast turnover otherwise problems.
If you serviced the starter, try another battery.
Check battery cables and grounds.