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  • October 20, 2021, 09:17:30 AM

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Author Topic: Low Compression. Missing on cold start  (Read 94 times)

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Offline MIPS

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Low Compression. Missing on cold start
« on: October 07, 2021, 09:54:47 PM »
As the mornings are getting colder I'm starting to notice at least one cylinder has really lousy compression at odd times.
A few times I've cranked the engine during the summer and you could clearly hear the engine cranked faster on one cylinder but as soon as you started the engine it idled fine.
Since the beginning of the week however I've started the car in the morning and it does it usual thing in cold mornings:
-Engine cranks fine with no sounds of a low cylinder
-Starts with a tap of the  pedal to set the choke. Doesn't need a squirt of gas.
-Revs up to 1200 or so, drops down to about 600 with a bit of cold engine missing and then settles down to idle as the choke starts to open after 45 seconds or less and the carb heater warms up.

But now it's just stuck missing on at least one cylinder until you tap and rev the engine once or twice. You can drive away with it still being unhappy like that but OH MAN it doesn't like it. No black smoke or backfiring. You step on it and there's just no power and a lot of missing but once you are on the highway for a few clicks it's hot and happy. No missing, no power loss and the choke's full-open.
The valves, lifters and guides were serviced last summer so I'm doubtful one is sticking up in the cold but the block is due for a rebuild in the spring do to some bottom-end knocking. Does this sound like a ring that's on the way out and sticking? Oil consumption has been okay but I can't check the compression as the threads on my tool are not deep enough for the plug holes on the 258's head.....
« Last Edit: October 07, 2021, 09:57:22 PM by MIPS »

Offline TheBirdman

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Re: Low Compression. Missing on cold start
« Reply #1 on: October 08, 2021, 11:37:17 AM »
Theres not much I could speculate on without knowing compression numbers. You might want to get a compression tester that fits.
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Offline AMC of Houston

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Re: Low Compression. Missing on cold start
« Reply #2 on: October 08, 2021, 12:04:46 PM »
Don't get too focused on compression and ignore other possible causes; such as coil breaking down, choke pull-off not working, moisture in the dist cap, etc.   Good luck on the diagnosis.
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Offline MIPS

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Re: Low Compression. Missing on cold start
« Reply #3 on: October 08, 2021, 10:14:36 PM »
I checked this morning to verify the pulloff and the choke were indeed functioning fine and it was good. On the way home I dropped into Lordco and bought a deep plug thread adapter to fit the head and here's your mostly hot engine compression numbers:

1 - 142
2 - 150
3 - 150
4 - 145
5 - 152
6 - 160

Eh, not terrible but my issues were happening on a cold engine, so I'll try it again first thing tomorrow morning.
« Last Edit: October 08, 2021, 10:15:41 PM by MIPS »

Offline MIPS

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Re: Low Compression. Missing on cold start
« Reply #4 on: October 09, 2021, 02:02:03 PM »
And now the cold engine tests:

1 - 140
2 - 148
3 - 151
4 - 150
5 - 148
6 - 157

So the numbers seems pretty healthy over and hot/cold, except for cylinder #6 has exceptionally good compression all the time.

One thing I've noticed was that it has a lifter that's the loudest when you let the engine sit for a while so it's leaking down faster than the rest. During compression testing by the time I was at the 5th pot I could hear excessive pressure popping into one of the manifolds. Had to run the engine to pump up the lifters and test again.
My theory is that the #6 has a bad lifter and one of the valves are not opening fully.
« Last Edit: October 09, 2021, 02:02:35 PM by MIPS »

Offline AMC of Houston

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Re: Low Compression. Missing on cold start
« Reply #5 on: October 09, 2021, 04:33:30 PM »
Fairly common on these engines with high miles to have the rocker arm pivots worn down to where the valves aren't opening fully.   I tend to replace all of the pivots at 100K miles or so just for grins.   My 200K-mile waggie is almost due for them again!
George G.
'81 Eagle Sundancer
'85 Eagle Waggie
1960 1902 Rambler Replica
'64 American
'70 AMX (Big Bad Blue), '70 AMX (White)
'77 Gremlin
'78 Pacer Coupe, '78 Pacer Wagon
'79 Pacer Wagon
'73 Jensen Interceptor
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'09 Nissan Titan
'10 Nissan Maxima

Offline Illeagle1984

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Re: Low Compression. Missing on cold start
« Reply #6 on: October 10, 2021, 10:12:17 AM »
I think if it was worn valvetrain parts, it would likely be more consistent and with accompanying clatter.  How's your oil pressure?  It could very well be a dirty lifter.  The tiny oil hole gets partially plugged, and while the engine is warm the hot, thin oil will make its way in there to pump up the lifter and lube the top end.  Sometimes it may sit and be fine.  Other times it may sit and run like a 5-cylinder for while.  It can seem random because the problem may only show itself when that particular valve is held open when you shut it off.  The cam & valvespring squeeze the oil from the lifter, and when you restart it takes time for oil pressure to pump that lifter back up.  There is a check valve inside the lifter to prevent this, but apparently it doesn't work when its dirty.  This could explain why revving the engine makes it go away (increased oil pressure overcomes the dirt).

Just my theory though, it can be tough to be 100% sure on something like that.  You can try repeated cold compression tests, or possibly shut it down once it's doing it and then test.  I had good luck fixing that exact problem on 80's Caravans back when they were still around.  I had one that all the lifters pumped down so hard it wouldn't even start!  The solution was an oil change and a bottle of Valve Medic.  You can try a motor flush too but sometimes those solvents can accelerate oil leaks on real old gaskets.

Also if you are able to get the lifters out, they're super simple to take apart, clean, and reinstall.  And you could inspect the entire valvetrain for wear/damage while you're there.  Easier if you can pinpoint which one it is.  If it's popping through the intake, it's probably an exhaust valve not opening.  Cylinder 1 looks sus...
It's getting crowded down here:
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Offline MIPS

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Re: Low Compression. Missing on cold start
« Reply #7 on: October 10, 2021, 11:14:24 AM »
Last summer the head was also rebuilt and decked. The shop pointed out two worn pivots and one worn rocker. I didn't have new replacements on hand so I replaced them with considerably better pivots and rockers from an '84 head.
The rebuild included new lifters in the quotation so even though these ones are basically new we can change them out again when it goes into the shop.

 

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