I've searched the forum, but this one doesn't seem to be here:

'83 F150 5.0L high mileage - sudden onset of rough idle and poor performance under load. Discovered engine was 3 quarts low on oil, due to a constant rear seal leak (known for some time, but not topped up often enough)... 6 weeks ago, compression in all cylinders was normal and even @ 135-145 psi. Now Cyl #1 has 40 psi, while all others are normal @140psi.

When applying compressed air to #1 spark plug hole at TDC, NO air can be heard at carb, tailpipe, dipstick tube, or radiator (bubbles) and after 30 minutes, leakdown pressure gauge shows no significant loss of air pressure (<10 psi).

When turning over (without starting), with valve cover off, both #1 valve rockers do move and seem to be in sequence, both at roughly the same height.

Engine oil was topped up, but no improvement in roughness and no improvement in compression.

Conclusions: Valve not stuck open (no leakdown); rings not badly worn and no hole in piston (no leakdown); not broken rod, because compression is not zero.

Hypothesis: Valves not opening enough, or at all?

Question: could this be a collapsed lifter (moving rocker but not lifting valve enough?)? Anyone ever heard of that?

Question: Does anyone have any other advice?

 

Check to see how far pushrod is lifting the valves. If the cam lobes are worn out. The valves will not open fully.
zanny

 

Zanny, Thanks for the suggestion. Not sure how that could happen suddenly, but I'll check out the lift height just the same; could be the determining factor, even if it's from some other cause than cam lobe wear.

 

Broken valve spring? Sometimes these things can go and the keepers will remain in place giving the impression that the camshaft lobes are worn. Look real close and see if the springs are broken, cracked or are collapsed.