Lost compression on cylinder 5

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OpossumWrangler

OpossumWrangler

Joined
Aug 3, 2024
Threads
4
Messages
32
Location
South-East United States
So my 80 died the other day and when I started it up it had a terrible miss. After a compression test and some investigation I found cylinder 5 is leaking and the compression is escaping out one of the intake valves. Are there any other investigative steps to take before pulling the head? I can see both lifters touching the cam lobes at top dead center, and I can push down on them and move the valves. My bore scope won’t look doesn’t look up so I can’t see the valve face. I did run it down the intake manifold to look at the top side of the valves and didn’t see anything alarming. I tried shining a bright light into the cylinder to see if any light leaked past the valve but that didn’t yield any results. Leak down kit won’t build past 10 psi and the draft inside the intake manifold will clearly blow a grill lighter. Any advice would be appreciated
 
How many miles are on the engine/head? Has the head been rebuilt or the valves adjusted in the past or recently? Any overheating or hard running events of note lately?

If the leak down kit will build any pressure, even a few psi, then it may be hard to see the issue with a bore scope. Larger damage can be visible but the hole will also prevent any build-up of air pressure. The outcome in either case is typically similar and results in needing to pull the head for repairs.

One possible failure scenario is that the intake valve(s) on that cylinder were running too tight and were held open too much, allowing the valves to overheat and burn/erode until they stopped sealing well. Valves have to close securely and for sufficient duration to transfer some heat to the seats and avoid the valves overheating.

Another possible explanation for what's going on is that debris, like a chunk of carbon, is stuck between an intake valve and seat causing it to seal poorly or debris ran through the valve(s) and damaged the sealing surface(s) and/or bending the valve stem so that the valve doesn't sit flush in the seat.

In a situation where you couldn't, or didn't want to, remove the head for a full valve job/etc. you might next check the adjustment on the intake valves for this cylinder to possibly learn some relevant details. For instance, if one of the intake valves is loose then it may be being held open by debris/damage, causing both the lost compression and the larger gap. Conversely if one of the intake valves is too tight then there may be relevant valve face or seat erosion keeping it from closing fully and throwing the adjustment off. Trying to clean the valve/seat in the first case or re-adjusting in the second case could possibly buy you some time and return some compression but that's a long shot and more of a bush repair.

Depending on the engine history, condition otherwise and your goals for it I'd be seriously considering pulling the head for a thorough inspection and then replacement or full repair. Unless you find a clear explanation for what's going on that is reliably repairable it's now time for a full valve job at least.
 
jpoole said:
How many miles are on the engine/head? Has the head been rebuilt or the valves adjusted in the past or recently? Any overheating or hard running events of note lately?

If the leak down kit will build any pressure, even a few psi, then it may be hard to see the issue with a bore scope. Larger damage can be visible but the hole will also prevent any build-up of air pressure. The outcome in either case is typically similar and results in needing to pull the head for repairs.

One possible failure scenario is that the intake valve(s) on that cylinder were running too tight and were held open too much, allowing the valves to overheat and burn/erode until they stopped sealing well. Valves have to close securely and for sufficient duration to transfer some heat to the seats and avoid the valves overheating.

Another possible explanation for what's going on is that debris, like a chunk of carbon, is stuck between an intake valve and seat causing it to seal poorly or debris ran through the valve(s) and damaged the sealing surface(s) and/or bending the valve stem so that the valve doesn't sit flush in the seat.

In a situation where you couldn't, or didn't want to, remove the head for a full valve job/etc. you might next check the adjustment on the intake valves for this cylinder to possibly learn some relevant details. For instance, if one of the intake valves is loose then it may be being held open by debris/damage, causing both the lost compression and the larger gap. Conversely if one of the intake valves is too tight then there may be relevant valve face or seat erosion keeping it from closing fully and throwing the adjustment off. Trying to clean the valve/seat in the first case or re-adjusting in the second case could possibly buy you some time and return some compression but that's a long shot and more of a bush repair.

Depending on the engine history, condition otherwise and your goals for it I'd be seriously considering pulling the head for a thorough inspection and then replacement or full repair. Unless you find a clear explanation for what's going on that is reliably repairable it's now time for a full valve job at least.
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Valve clearance looks good. Looks like the head will be coming off. The truck has 290,000 on it so it’s probably ready for some valve work. It has also burned oil quite a bit of oil since I’ve owned it. I started removing hardware this evening. I’ll post some photos and keep the thread up with how I’m moving along. I’m just finishing up an overhaul on an old continental flat head engine in my garage, other than that I don’t have much experience going this deep in a motor so I should learn a lot along the way.

IMG_2896.webp
 
Good luck!

I'm guessing you've seen this here but in many cases valve stem seals account for most oil consumption in these engines. I replaced valve stem seals (w a head job) at 225k ish and haven't been close to needing to add oil between oil changes in the 100k+ miles since then.

How deep will you go into the 1FZ? Keep us posted on how it goes.
 
Follow the procedure in the manual and you'll be fine.
 
jpoole said:
Good luck!

I'm guessing you've seen this here but in many cases valve stem seals account for most oil consumption in these engines. I replaced valve stem seals (w a head job) at 225k ish and haven't been close to needing to add oil between oil changes in the 100k+ miles since then.

How deep will you go into the 1FZ? Keep us posted on how it goes.
Click to expand...
I believe my goals with this project will be to solve the compression issue, address oil consumption, and a new timing chain. Is there specific gaskets/studs typically recommended or just OEM?
 
You won’t be doing a Timing chain unless you are removing the upper oil pan and timing cover. I wouldn’t be worried about a chain as much as a slipper those tend to have the plastic bits break off.
Looks like you are well on your way, good work.
 
Fj80oregon said:
You won’t be doing a Timing chain unless you are removing the upper oil pan and timing cover. I wouldn’t be worried about a chain as much as a slipper those tend to have the plastic bits break off.
Looks like you are well on your way, good work.
Click to expand...
Yes, I’ve done some reading on that. Figured I will go ahead and replace the chain tensioner slipper, vibration damper, and chain its self. Also all of the gaskets and seals
 
OpossumWrangler said:
Anyone know if a puller is normally required to remove the crank pulley? Service manual shows the use of a SST puller with the language “If necessary”
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Sometimes they come off easy sometimes they need a puller. I usually use a brass hammer and replace the pulley it’s a wear item in my book.
 
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