Q: my yukon will not start unless i prime it

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I have replaced the throttle position sensor the mass air flow sensor. It runs good unless i let it set for a couple hours then i have to prime it again. Any ideas

My car has 157000 miles.
My car has an automatic transmission.

The hard to start condition is going to be a fuel pump pressure related issue most likely. You will need to have the fuel pressure checked at the fuel pressure fitting on the fuel rail. The fuel pressure should be 55 to 62 psi on the gauge when you turn on the ignition key. You may have to cycle the key a few times to build up the pressure on the test gauge. Once maximum pressure is reached check to see if it is between 55 and 62 psi. Also the pressure should stay pressurized when key is turned off. If the pressures do not hold or are lower than specified then replace the fuel pump.

If you have good fuel pressure and it does not leak down then you should have the engine compression checked when the engine is cold to see if the compression is very low on all the cylinders. If compression is low then you may have excessive carbon build up on the valves and this would cause the hard or no start cold due to the carbon acts like a sponge and soaks up the fuel before it can get into the engine and cause the valves to stick or not seal lowering compression. If you have the fuel injectors and induction system cleaned it may improve the condition without the cylinder heads having to be removed to completely clean out all the carbon at a machine shop. The cleanings will lower the carbon amounts and help free up the valves. If after these services the problem remains then the head removal will be needed. Try to only use fuels that have cleaners in the fuel and also add cleaners in the fuel to keep the carbon at a minimum.

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