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1987 - 1996 F150 & Larger F-Series Trucks 1987 - 1996 Ford F-150, F-250, F-350 and larger pickups - including the 1997 heavy-duty F250/F350+ trucks

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Old 03-09-2010, 03:12 PM
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Low Compression All Cylinders... Help.

I had originally posted in this forum on a no start issue but I think it has turned into another issue all together so thats why the new post.

Background: 89 f350 efi 460. I have swapped a 84 Short block under a set of fresh 89 efi 460 heads. The shortblock came as a complete carbed running motor out of a 84 van with 89K on it. After finding out that the cyl heads have about the same CC combustion chamber size, I figured I would be OK swapping the heads onto the short block even with the larger dish in the pistons. Figured I would be a little lower than stock 89 but that would be OK.

I put the motor in and no start. I have fuel, I have spark, I checked compression. Cold and dry cranking compression was rock solid 85 - 90 psi on all 5 cyl I tested. Decided to try a different compression tester and added a little oil. Same numbers. (I stopped testing after the trend). The guage held good for a while on each cylinder. Timing should be good, I popped the valve cover and watched the valves to make sure I was compression stroke and not 180 out.

Could it be the head and piston combo? I didn't think it would come out that low.

Could it be bad Cam timing? I get a lot of popping in the engine while it is trying to turn over, sounds like intake backfire almost.

Any help would be appreciated.
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Old 03-09-2010, 03:28 PM
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It could be a bad timing chain , turn the crank buy hand & whatch the rotor , then turn it back the other way & see how far you have to turn the crank to get the rotor to turn , this will tell you how much slack you have in the chain ...Lew
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Old 03-09-2010, 03:32 PM
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Your compression appears to be hella low.
My 500cc kawi twin wouldn't fire at 90psi.
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Old 03-09-2010, 06:33 PM
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OK, so I ripped the timing cover off and checked the timing marks. They seem to line up for the most part. I popped both sprockets off to see if it had jumped a tooth, doesnt look like it.

The chain is way shot though. I get 1.6 inches of deflection when the manual calls for .5 max. Would that be enough slack to cause bad cam timing and lead to low compression? Or is it something else? I don't want to throw parts at the thing if I still am going to have 90 psi in each cylinder.

Can anyone answer my dilema about the efi heads on the 84 short block; did it make that big of a difference as to be my problem?

One other thing I noticed was that when the timing marks were lined up, my distributor was out by about 30 degrees from the #1 mark. Is it supposed to be TDC when the timing marks are lined up?

Thanks
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Old 03-09-2010, 07:27 PM
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Yeah you need a new chain & gears , your timing is way off ...Lew
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Old 03-09-2010, 08:24 PM
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Yeah, but the question is... Is it really off enough to cause that low of compression. It hadn't jumped a tooth. Or, is it my head piston combo?
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Old 03-10-2010, 05:41 AM
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I don't know , but it needs a timing chain for sure ...
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Old 03-10-2010, 08:23 AM
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Ill bet its the timing as well,UNLESS, depending on how the 460 is made maybe the pushrods are different between the years.
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Old 03-10-2010, 08:43 AM
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Quote:
Originally Posted by deeznuts822 View Post
depending on how the 460 is made maybe the pushrods are different between the years.
Didn't even think about that. I'll measure them today.
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Old 03-10-2010, 11:13 AM
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Pushrods are the same part # 84 -89 so I think I will leave the top end together. They should be the same.

I have a Cloyes double roller gear set coming in this afternoon so that should take care of the chain slop.

Can anyone tell me if it is supposed to be at TDC when the timing gear marks are lined up?
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Old 03-10-2010, 11:30 AM
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Yeah it should be at TDC with the timing marks aligned. Your probably a tooth off on your distributor also. Get the timing chain on and aligned and then make sure your rotor is pointing towards the #1 mark on the cap and see how it improves
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Old 03-11-2010, 10:18 AM
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Well the cloyes didn't fit because it was a 88-92 year specific, lame. So a FRPP set is on its way. One other thing I noticed, and this is probably why I couldn't get it started as well, and would explain why the dizzy was off a tooth or two; I compared the damper with my one off the old engine and it looks like the outer ring spun about 160 deg out of whack on this one. Boy what a project. At least the rings seem good on it as it holds the pressure #s for quite a while on the compression guage.
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Old 03-11-2010, 11:22 AM
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Yeah thats no good , the new chain & damper should do it , good luck ...Lew
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Old 03-17-2010, 05:19 PM
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After the install of the timing set at 2 deg. advanced, the cold PSI went up 15-20 to 105. Just rechecked compression warm and they are all 125 to 130 across the board at 5000 ft above sea level.
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Old 03-17-2010, 06:13 PM
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Good , with that much slack in the chain , that will do it on the 460s
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Old 03-17-2010, 06:13 PM
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