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81 ford 4.9 6 valve problems


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heinz57
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Aug 2, 2012, 3:10 PM

Post #1 of 18 (2944 views)
81 ford 4.9 6 valve problems Sign In

One year ago I replaced the head on this truck with a reman head. Had a little problem getting the manifold to seal but fianlly got it. Recently I started getting popping when accelerating and then backfiring with hard acceleration. Long story short found #6 cylinder not firing. Compression check 5 cylinders at 120 and #6 at 60. Have pulled and checked rockers, push rods and springs. Turning over engine valve movement is same on all cylinders. Any alternatives to pulling the head. Appreciate any help, this head weighs about 90 lbs. Almost neeed engine hoist.


Hammer Time
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Aug 2, 2012, 3:14 PM

Post #2 of 18 (2936 views)
Re: 81 ford 4.9 6 valve problems Sign In

If you haven't stripped it down yet i would do a wet compression test and a cylinder leakdown test.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



heinz57
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Aug 2, 2012, 3:50 PM

Post #3 of 18 (2918 views)
Re: 81 ford 4.9 6 valve problems Sign In

Thanks for quick reply. Did wet test went from 58 to 60. Have leak down testor but need aplug adaptor for this engine. Will post results then. Thanks


Hammer Time
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Aug 2, 2012, 3:53 PM

Post #4 of 18 (2914 views)
Re: 81 ford 4.9 6 valve problems Sign In

The wet test pretty much eliminates rings so now you are down to valves, head or head gaskets.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Discretesignals
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Aug 2, 2012, 5:30 PM

Post #5 of 18 (2894 views)
Re: 81 ford 4.9 6 valve problems Sign In

Popping and backfiring under engine load could be a valve not sealing since you have low compression on that cylinder.





Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.


heinz57
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Aug 3, 2012, 7:02 AM

Post #6 of 18 (2873 views)
Re: 81 ford 4.9 6 valve problems Sign In

No coolant in oil so don't believe it's gasket. Did paper on tail pipe test and get rapid push pull which from other posts indicates an exhaust valve. Still plan to do leak down test to verify. My ? is is there anything, other then pulling the head, that I can try.


Hammer Time
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Aug 3, 2012, 8:18 AM

Post #7 of 18 (2870 views)
Re: 81 ford 4.9 6 valve problems Sign In


Quote
No coolant in oil so don't believe it's gasket.


That would be a wrong assumption.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
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Aug 3, 2012, 9:32 AM

Post #8 of 18 (2864 views)
Re: 81 ford 4.9 6 valve problems Sign In

Guess of the day..... Valve, valve spring can be elusive and sudden, head gasket/head/block somehow still not right gasket torque didn't hold properly?

Elusive to a quick inspection is a cracked valve spring. I know you said you looked but could be on a blind side but be like this if it shows...........



Not common but has happened to me and elusive to find. Why - not sure why on some without provocation?

T



heinz57
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Aug 4, 2012, 10:04 AM

Post #9 of 18 (2837 views)
Re: 81 ford 4.9 6 valve problems Sign In

Using a mirror went all around spring and can see no obivous problems. When I replaced the head I did the 3 step torque sequence outlined in my Chilton manuel. Triple checked it before putting it back together. Drove maybe 200 miles and rechecked torque. Could still be gasket but I don't know how. Head & block ? Checked all around block & head and again can see no obivous signs of leakage. Coolant in radiator near top. When I pulled the spark plug it was wet with unburned fuel but no signs of coolant. Put #6 cylinder piston at TDC and checked with light, top of piston was black with no evidence of coolant. No evidence of collant coming thru exhaust, no white smoke or coolant at tail pipe. Don't know if this will help but this came on gradually beginning with slight intermittent hesitations when accelerating and then advancing to popping and backfires over a couple of weeks.


Hammer Time
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Aug 4, 2012, 10:10 AM

Post #10 of 18 (2835 views)
Re: 81 ford 4.9 6 valve problems Sign In

You still haven't done the leakdown test?



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



heinz57
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Aug 4, 2012, 4:10 PM

Post #11 of 18 (2820 views)
Re: 81 ford 4.9 6 valve problems Sign In

Results of leak down test. Air coming thru carb.


Hammer Time
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Aug 4, 2012, 4:11 PM

Post #12 of 18 (2816 views)
Re: 81 ford 4.9 6 valve problems Sign In

OK, you have an intake valve problem on that cylinder.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



heinz57
User

Aug 5, 2012, 4:45 PM

Post #13 of 18 (2795 views)
Re: 81 ford 4.9 6 valve problems Sign In

Any options other than a tear down?


Hammer Time
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Aug 5, 2012, 4:49 PM

Post #14 of 18 (2792 views)
Re: 81 ford 4.9 6 valve problems Sign In

Not if you don't see any kind of external failure or anything holding the valve open.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



heinz57
User

Aug 14, 2012, 11:07 AM

Post #15 of 18 (2745 views)
Re: 81 ford 4.9 6 valve problems Sign In

Finally bit the bullet and pulled the head. Cylinder (#6) valve has damage. Piston seems ok, seat seems ok and guide feels tight. It appears as though the valve was damaged during rebuild leaving a very thin edge that eventually burned thru. Plan to just replace it and put it back togeather unless there is some other explanation or advice you might have. I appreciate your help.


Discretesignals
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Aug 14, 2012, 7:01 PM

Post #16 of 18 (2737 views)
Re: 81 ford 4.9 6 valve problems Sign In

Burnt valve is a symptom. You need to figure out the cause. Valves burn when they can't transfer heat to the seat or the combustion temperatures are too high from things like overheating, lean air fuel mixtures, etc.

You can't just slap a valve in there an expect it to work. You have to make sure the seat is cut to match the valve face, the seat isn't worn out, and the valve actually seals. Best thing is to send it to the machine shop so they can make sure things are right.





Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on Aug 14, 2012, 7:06 PM)


Tom Greenleaf
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Aug 14, 2012, 7:50 PM

Post #17 of 18 (2731 views)
Re: 81 ford 4.9 6 valve problems Sign In

Agree with DS - send it out or you'll just be back there again. Valve interface needs to be right or it can burn out. Couldn't verify it but make sure the head bolts are reusable. Some are one time "turn to torque" idea and don't do that twice. Couldn't verify that for this year so easily..........

What valve needs to seat properly and cool itself too....... (sample pic)



T



heinz57
User

Aug 20, 2012, 3:01 PM

Post #18 of 18 (2667 views)
Re: 81 ford 4.9 6 valve problems Sign In

Valve had obivious physical that had to be done during rebuild work or they used a damaged valve when they asembled the head. Rebuilder had sent two heads, both wrong before this one was sent. I think he purposely screwed this one up just out of spite. None of the valves were seated correctly. I filled the valve ports with brake parts cleaner and they all leaked. Spent a number of hours hand lapping all the valves and got all to seal. Cranked it up today and it runs- sounds great. Need only to get another year out of this engine before retiring it. By the why used all new head bolts.
Thanks for all your help.






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