Engine compression test, 351M
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Elder User
Joined: Dec 2007
Posts: 813
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From: Up Nort Minnesota
Engine compression test, 351M
Hi guys, I started a thread complaining about some turbo mufflers that i thought were giving me trouble.
Long story short, it now seems that uncorking my exhaust and going to duals has uncovered or brought to light an obvious engine problem.
I did a compression test, warmed the engine a bit, tested on dry cylinders then with a bit of oil on each piston.
Cylinder / Dry psi / Wet psi
one / 137 / 146
two / 137 / 145
three / 117 / 120
four / 147 / 152
five / 142 / 149
six / 148 / 149
seven / 150 / 150
eight / 140 / 140
My whole RH side is a little weak.
It could be something I could live with if it weren't for cylinder number 3, I must have a bad valve or something there.
I pulled the valve cover on the RH side, no obvious bent pushrods or cracked/broken springs.
But, I did notice that especially on exhaust valves the springs seem loose when the valve is closed, like no preload.
I havea spare 351M on a stand in the shop from a 79 bronco. I'm going to go and feel the springs on that one and see if they seem any different.
Any input/suggestions much appreciated.
Thanks.
Dave
Long story short, it now seems that uncorking my exhaust and going to duals has uncovered or brought to light an obvious engine problem.
I did a compression test, warmed the engine a bit, tested on dry cylinders then with a bit of oil on each piston.
Cylinder / Dry psi / Wet psi
one / 137 / 146
two / 137 / 145
three / 117 / 120
four / 147 / 152
five / 142 / 149
six / 148 / 149
seven / 150 / 150
eight / 140 / 140
My whole RH side is a little weak.
It could be something I could live with if it weren't for cylinder number 3, I must have a bad valve or something there.
I pulled the valve cover on the RH side, no obvious bent pushrods or cracked/broken springs.
But, I did notice that especially on exhaust valves the springs seem loose when the valve is closed, like no preload.
I havea spare 351M on a stand in the shop from a 79 bronco. I'm going to go and feel the springs on that one and see if they seem any different.
Any input/suggestions much appreciated.
Thanks.
Dave
Thread Starter
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Elder User
Joined: Dec 2007
Posts: 813
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From: Up Nort Minnesota
****...
lol!
yeah it does, dammit it forgot to do that.
The valve springs on my "spare 351M" are fairly tight with closed valves. Like they have a bit of preload.
I'm thinking about a low buck head swap, pull a head off my spare engine and slap it on the trucks engine.
I'd be out a little time and a few bucks for gaskets.
1, 2 and 3 are weak, 3 is especially bad and really points to a valve issue. Probably exhaust valve since I'm not backfiring through the intake.
I'm thinking 3 stands to gain the most from a head swap.
1 and 2 probably have ring issues.
This isn't a restore effort at this point, I just want it running better than it is right now.
lol!
yeah it does, dammit it forgot to do that.
The valve springs on my "spare 351M" are fairly tight with closed valves. Like they have a bit of preload.
I'm thinking about a low buck head swap, pull a head off my spare engine and slap it on the trucks engine.
I'd be out a little time and a few bucks for gaskets.
1, 2 and 3 are weak, 3 is especially bad and really points to a valve issue. Probably exhaust valve since I'm not backfiring through the intake.
I'm thinking 3 stands to gain the most from a head swap.
1 and 2 probably have ring issues.
This isn't a restore effort at this point, I just want it running better than it is right now.
Elder User
Joined: Aug 2011
Posts: 685
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From: Cape Coral, Florida
The diff between wet & dry is good, I have a van with one low reading and it still runs great. If you don't feel it don't fuss. How many miles on it? Get some manifold vacuum readings, somebody was passing around a very good vacuum diagnostic chart maybe Montana.
Fleet Mechanic
Joined: Jan 2012
Posts: 1,668
Likes: 13
From: Saskatchewan
If I were you I would swapp the heads out. Like you say it's only a little time and some gaskets. What's the point of having sweet duel exhaust if you don't like the way it sounds. Good luck and keep us posted.
Thread Starter
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Elder User
Joined: Dec 2007
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From: Up Nort Minnesota
The manifold vac readings lead me to believe restricted exhaust, that's why I stuck the dual exhaust on the truck from the manifolds back.
The dual exhaust really made the issue come to light. (sounds like hell)
besides the #3 cylinder i'm within 8.5% on pressure.
I really think it's an exhaust valve on #3.
Would explain the crappy sound on that RH exhaust pipe too.
The dual exhaust really made the issue come to light. (sounds like hell)
besides the #3 cylinder i'm within 8.5% on pressure.
I really think it's an exhaust valve on #3.
Would explain the crappy sound on that RH exhaust pipe too.
Thread Starter
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Elder User
Joined: Dec 2007
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From: Up Nort Minnesota
Actually no, I just looked at the tester again, it just has the relief buton for resetting the gauge.
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Senior User
Joined: Sep 2011
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Head swap sounds like an easy fix if it is valve springs, I think if it was a ring issue your wet and dry would be DRASTICALLY different, pluss if you pop that head off you can atleast inspec cylinders 1-4, also do you burn any oil, like are you down a quart every oil change? If not i really think this points to the springs and not a ring issue.
Good luck, keep us posted
Mason
Good luck, keep us posted
Mason
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Cargo Master
Joined: Oct 2011
Posts: 2,251
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From: Kenner,La.
weak cylinder
northern dave, Do you have a balance tube between the exhaust pipes? If not add one after the heads are swapped , this will smooth out the exhaust sound even more or an "X" pipe. Kenny
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Elder User
Joined: Dec 2007
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From: Up Nort Minnesota
A little update here. I inspected one of my spare 351 heads (assembled) and I think I have a winner.
Valves aren't sucked up into the seats, springs look good, I poured a little brake fluid into the exhaust ports with the head face down then checked on the head a little later. I got some minor seeping (a little wet areound the exhaust valve inside the combustion chamber) but the fluid definately didn't run through.
I repeated with the intake side and it was the same or better.
So, I'm thinking I'll lap these valves and stick this head on the truck on my 1,2,3,4 side of the engine.
Valves aren't sucked up into the seats, springs look good, I poured a little brake fluid into the exhaust ports with the head face down then checked on the head a little later. I got some minor seeping (a little wet areound the exhaust valve inside the combustion chamber) but the fluid definately didn't run through.
I repeated with the intake side and it was the same or better.
So, I'm thinking I'll lap these valves and stick this head on the truck on my 1,2,3,4 side of the engine.
Thread Starter
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Elder User
Joined: Dec 2007
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From: Up Nort Minnesota
I tore down one of my "spare heads" tonight, cleaned and lapped valves/seats, measured my springs, cleaned all my gasket surfaces.
Look at this nasty intake valve! The stem seal was shot, drinking oil down the guide. Ick.
unlapped seats.
Lapped seats
Nasty valve on the left, cleaned/lapped on the right
Lapping tool on an intake valve.
They all look like this now, ready to rock.
I measured my free height on my springs, my exhaust springs look to be .037", .030", .024" and .045" short.
I'd shim them if new springs from rock auto weren't so damn cheap. $1.78 each :grin:
So, I'll order some gaskets, some springs and stem seals and I think I'm going to go through my other spare head and swap them both out as long as I'm doing this.
Look at this nasty intake valve! The stem seal was shot, drinking oil down the guide. Ick.
unlapped seats.
Lapped seats
Nasty valve on the left, cleaned/lapped on the right
Lapping tool on an intake valve.
They all look like this now, ready to rock.
I measured my free height on my springs, my exhaust springs look to be .037", .030", .024" and .045" short.
I'd shim them if new springs from rock auto weren't so damn cheap. $1.78 each :grin:
So, I'll order some gaskets, some springs and stem seals and I think I'm going to go through my other spare head and swap them both out as long as I'm doing this.
Thread Starter
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Elder User
Joined: Dec 2007
Posts: 813
Likes: 6
From: Up Nort Minnesota
I've also learned that the heads i'm going to toss on my 77 have a little different springs/keepers. My "spare heads" are off a 79 bronco and they went to a different spring retainer in 78, well, shorter spring, a rotating spring retainer and different keepers actually.
Actually, i don't think enough people are aware of this.
The exhaust valve springs on 78 and later M engines will coil bind at .490" valve lift.
So, if you install an aftermarket cam that aproaches .500" gross lift on the exhaust valves, You're going to bend pushrods. (if You have the later style exhaust valve spring system)
Good stuff to know.
Actually, i don't think enough people are aware of this.
The exhaust valve springs on 78 and later M engines will coil bind at .490" valve lift.
So, if you install an aftermarket cam that aproaches .500" gross lift on the exhaust valves, You're going to bend pushrods. (if You have the later style exhaust valve spring system)
Good stuff to know.