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Old 12-06-2010, 13:46   #1 (permalink)
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4.6 low compression on 3 cylinders

hello guys, I'm new to this forum and I am currently helping someone repair their 1996 crown victoria 4.6l

The car was running bad, like it had a bad head gasket or something major. I looked under the hood and saw that the intake manifold was cracked so i replaced it with a new one. I still have the same problem.

So I proceeded to do a compression test. I found good compression on all the cylinders except numbers 6, 7, and 8. 6 had about 50 psi 7 had about 35-40 psi and i barely got any pressure on 8.

I pulled the driver side head off. nothing looks suspicous to me yet. I'm wondering what to do now to figure out what went wrong.

Your help is appreciated.
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Old 12-06-2010, 13:57   #2 (permalink)
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Re: 4.6 low compression on 3 cylinders

Did you do a leakdown test? That would let you know where the leak is.
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Old 12-06-2010, 14:06   #3 (permalink)
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Re: 4.6 low compression on 3 cylinders

unfortunately i did not. head is off now. the head gasket didnt look bad to me when i took the head off. valves look good. head looks good. I just dont know where to go from here.

Edit: also the block seems fine too.
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Old 12-06-2010, 15:11   #4 (permalink)
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Re: 4.6 low compression on 3 cylinders

I don't know what to tell you. Those engines are usually good for a lot of miles.
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Old 12-15-2010, 16:26   #5 (permalink)
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Re: 4.6 low compression on 3 cylinders

I've never had to replace any myself, but I've been told that the valve springs go weak in the 4.6's on occasion.
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Old 12-15-2010, 16:36   #6 (permalink)
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Re: 4.6 low compression on 3 cylinders

We never had any get weak but we had some break and they were noisey. One had it happen and the officer revved it up to see if he could clear it and the valve dropped into the cylinder and broke a piston
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Old 12-15-2010, 20:29   #7 (permalink)
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Re: 4.6 low compression on 3 cylinders

On my old motorcycles I would check the sealing of the valves by pouring kerosene or alcohol into the combustion chamber and see how long it took for it to leak past the valves. Time all of them on that head to get a comparison. Should have a couple of leakers unless you have ring/cylinder problems. How high is the mileage?
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