Ford f-250 4.9 in-line 6cyl., has 140-150 compression in the first 4 cylinders and only 100 in 5th and 6th cylinders
Can this be my crank but no-start issue?
Added (1). I know low compression in any cylinder is a sign of bad Rings or valves or worn cylinder wall, What I want to know is if that is what is causing my crank but no-start problem!
Could be valves or rings, but if it is running Ok don't worry about it
I would pull that head off and have a valve job done on it, low compression WILL make it VERY hard to start, thats 1/3 of your cylinders your talking about.
You didn't say how many miles it has on it
Were you getting a lot of blow by before it quit running? If so i would plan on doing a overhaul on it. At least then youd know what youve got.
That will keep it from running smoothly but it should run with compression numbers like that.
Like Kayleen said - valves OR rings - For a cylinder below 100 psi, pour 1 teaspoon of engine oil into the plug hole and retest. If the reading jumps, the piston rings are worn. If not, think valve problems.
It should start and run with those numbers. If one third of your cylinders were down to around twenty you would have problems. Even with the low fuel pressure it should at least start. May not run well or have any power but it should start.
Should start, as said, but may have head gasket blown between the last 2 cyls.
Stop guessing. Check for good spark on all the plugs. If there's spark, spray brake parts cleaner on the intake and try to start. No start, the low compression is the cause. If the truck starts, check fuel pressure with a gauge when cranking and check if the injectors are getting command to fire with a noid light.
Engines need compression (yours low but OK), vacuum, fuel, spark, and timing. Check spark At all plugs when cranking, set timing by hand and see, make sure no obvious vacuum leaks (stuck open EGR valve?), and for fuel. Pump throttle, Look--any gas in carb? (if carbureted). Or remove plug from fuel rail and see. Try squirting a bit of starting fluid, if it pops, its fuel problem. Try some fresh gas.
Pull the spark plugs and see if they have gasoline on them. If they are dry and you have spark take the air filter tube off of the throttle body and shoot a blast of starting fluid into the throttle body and see if it starts. If it starts for a few seconds put a new fuel pump in the gas tank.
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