1990 4.9 300 dead cylinder? - Ford Truck Enthusiasts Forums



1990 4.9 300 dead cylinder?

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Old 07-02-2006, 12:27 PM
galaxiedan galaxiedan is offline
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1990 4.9 300 dead cylinder?

I have an F-150 with the 4.9 six. Truck has 85,000 miles on it. I have just (less than 500 miles) put new plugs, wires, cap, and rotor on it. The first cylinder (from front of truck) does not fire. A local mechanic told me he could listen to it crank over and tell if it had weak compression in a cylinder. He heard it and said yes weak compression. The shop wants $100 to test compression. Is there an easy way to test to see if the injector on that cylinder is working? If the injector is working is there an easy way to tell if the possible weak compression is a head problem or a piston/ring problem? I really don't want to invest the amount of money it would cost for a complete rebuild on the motor but would consider head work if needed. Thanks
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Old 07-02-2006, 12:39 PM
holland501 holland501 is offline
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A good mechanic can indeed hear if a cylinder has seriously low compression, just by cranking the engine over. If it is that low, it almost has to be a problem with a valve. Even bad rings would create enough compression for the cylinder to fire and run. Pull the plug, chances are you will smell raw fuel or it will be very black and sooty. Good luck.
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Old 07-02-2006, 01:06 PM
galaxiedan galaxiedan is offline
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Would it be more common for a valve to be sticking or wore out? The plug seems wet but it is not black or sooty at all. I can't really tell if it is fuel or oil on the plug. Engine does not have blowby from pcv and does not smoke out the tailpipe. any advice?
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Old 07-02-2006, 01:19 PM
galaxiedan galaxiedan is offline
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I had an old 250 straight six in a chevy truck that had a habit of losing the nuts holding the rocker arms on. Does this motor have that problem? Is it possible to pull the valve cover on this motor (1990 4.9 FI) without removing the upper FI manifold? Would I be able to tell if it is a valve problem without taking the head off?
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Old 07-02-2006, 08:03 PM
300I6VAN85 300I6VAN85 is offline
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You might have a stuck open valve. Bum a compression tester. I think you want at least 120 lbs. You might have luck with some snake oil. Marvel Mystery Oil. My friends Dad had luck with his sons 2 stroke dirt bike gas. Some people pour the snake oil right on the valve spring assy. 85,000 miles is a young 4.9. My 4.9, she's so fine. My 4.9.... Second thought, if you can't get a compression tester. NAPA #901, Air Holding Device, about 10 bucks. Get a hose fitting that will fit the part and buy some hose about 3 feet. Screw this into your bad cylinder spark plug hole at Top Dead Center when the valve should be closed. Blow into it. You are looking for that 120 lbs.
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Old 07-02-2006, 10:42 PM
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ive never heard of anyone blowing into a compression tester, I always just diconnect the coil and crank the engine over by bumping the starter.
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Old 07-03-2006, 10:50 AM
300I6VAN85 300I6VAN85 is offline
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Pud. You are missing the point. Dan is on the cheap and does not want to buy a compression tester. The tool I was refering to is used to hold the valve inplace when you want to replace the valve steam seal when the head is still on the motor. You get the valves closed, connect the fitting to the air compressor and that holds the valve inplace while you remove the rocker arm, valve keepers, spring and old worn valve steam seal. That is what the tool is made for. I was just trying to give Dan another cheap option to see if his valve was stuck open if he could not find a compression tester.
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Old 07-03-2006, 11:12 AM
pud pud is offline
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oh, my bad.
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Old 07-03-2006, 12:41 PM
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No problem is fixed for free. A compression tester and the proceedure are cheap and easy. Anybody can do it. You have to know that number before you know what to do next. This saves fixing things that aren't broken.
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Old 07-03-2006, 03:41 PM
galaxiedan galaxiedan is offline
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Thanks for the help guys! I'm going to buy a compression tester tommorrow (the 4th). I will let you know what I find. If I did have a stuck open valve wouldn't I get a backfire into the intake or exhaust? I guess if it was backfiring into the exhaust it wouldn't be a backfire and I probably wouldn't notice it! Would that be considered front fire? I bought the truck for $200 as a second work truck/ loaner (eliminate people asking me to help them move, etc) and I have no idea of its background. Does anyone recommend an engine oil cleaner that might free up a sticking valve without taking the valve cover off? The fuel injection manifold along with a bunch of other stuff will have to be taken off first. I have no problem with changing an engine or head but just trying to slide by easy on this one. It runs and I can still drive it but I feel like a cross between Jud Clampett and chitty chitty bang bang.
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Old 07-03-2006, 04:02 PM
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Front fire, I like it.

Have you pulled codes from this thing yet?

Again, run compression first. No point pouring in snake oil till you know what to fix. It could be anything from an injector wire to a flat cam lobe.

If you have low compression, put it on TDC, stick it in low, set the parking brake and blow air in the spark plug hole and listen to where is leaks out. If it's a valve you will hear it in the intake or in the exhaust.
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