Compression Check.
Laughing Gas
Joined: Apr 2002
Posts: 846
Likes: 2
From: Bakersfield Ca.
Compression Check.
[updated:LAST EDITED ON 08-May-02 AT 08:00 PM (EST)]Greetings. I finally ran a compression check on my truck. Engine warm, plugs out, WOT. I have a 1975 460 with at least 124,000 miles on it. All cylinders read between 150 and 155 except no.4 which reads 120 
I squirted some oil in the hole and it came up to 130. Do you think this a signifacant incress to suspect the rings? All opinions welcome. I should also mention that the vaccum gauge reads 17 to 18 with an intermitent drop along with some hesitation. My trouble shooting guide says that it may a valve sticking, which would help explain the low reading in no. 4 would it not? But then again there is the question of the slightly higher reading after adding oil into the cylinder. Again all opinions welcome. The hesitation could also be a ignition miss which brings me to another problem which I'll post in the electrical forum. Thanks.
I squirted some oil in the hole and it came up to 130. Do you think this a signifacant incress to suspect the rings? All opinions welcome. I should also mention that the vaccum gauge reads 17 to 18 with an intermitent drop along with some hesitation. My trouble shooting guide says that it may a valve sticking, which would help explain the low reading in no. 4 would it not? But then again there is the question of the slightly higher reading after adding oil into the cylinder. Again all opinions welcome. The hesitation could also be a ignition miss which brings me to another problem which I'll post in the electrical forum. Thanks.
Postmaster
Joined: Mar 2001
Posts: 4,968
Likes: 3
From: So Cal
Compression Check.
The 30psi difference is a bit out of bounds for a normal spread, especially if all the others are close, 10% is a rule of thumb which means you should see maybe 15psi. I would suspect a combination of 3 things causing what you saw.
You have slightly leaky piston rings, thus the oil helped a little.
You probably have worn valve guides/seals which would not be fixed by the oil
The worn valve guides may also be causing the sticking which would cause lower compression.
With 124K miles on it, especially an early 70s, when they were switching voer to unleaded and started to have wear problems... It would not be unusual for you to have one or two valves with worn guides and or sticking. Sounds like it may be time for a valve job and hardened guides and seats or maybe just knurling the guides will do it.
I am not a Ford engine expert so I cannot be sure when the hardened guides and seats started showing up on the engines. I would guess after 74 since I don't think trucks had to go to unleaded until 1976?
Good Luck,
Jim Henderson
You have slightly leaky piston rings, thus the oil helped a little.
You probably have worn valve guides/seals which would not be fixed by the oil
The worn valve guides may also be causing the sticking which would cause lower compression.
With 124K miles on it, especially an early 70s, when they were switching voer to unleaded and started to have wear problems... It would not be unusual for you to have one or two valves with worn guides and or sticking. Sounds like it may be time for a valve job and hardened guides and seats or maybe just knurling the guides will do it.
I am not a Ford engine expert so I cannot be sure when the hardened guides and seats started showing up on the engines. I would guess after 74 since I don't think trucks had to go to unleaded until 1976?
Good Luck,
Jim Henderson
Cargo Master
Joined: Apr 2001
Posts: 2,364
Likes: 3
From: Eustis FL
Compression Check.
My truck used leaded in 1978 (F150 Ranger Explorer) according to the manual. It said only use unleaded if it said so behind fuel door or on dash.)
I haven't used leaded in years, yet I have 500,000 miles and no problems with the motor!
Ford started it; Ford will finish it!
I haven't used leaded in years, yet I have 500,000 miles and no problems with the motor!
Ford started it; Ford will finish it!
Cargo Master
Joined: Apr 2001
Posts: 2,364
Likes: 3
From: Eustis FL
Compression Check.
If you have a hesitation, check to see you don't have a Motorcraft 4350 carb. They are known for that problem. I have the same carb, same problem. Ask the old Jeep guys, as the same carb went on their 360 and 401 engines.
They are also known to stumple on inclines (climbing hills) and even dying for no apparent reason. Float problems, I guess...
I have a 4 cylinder engine in a Nissan car with 155 psi all cylinders, but she smokes like crazy when warm... Me thinks valve stem seals, OR head gasket break near bleed down hole, OR piston rings, OR ...etc...
Who really knows? I have a new head gasket on standby. I also have another used engine on standby (low miles, but low compression: No smoke)
These things are tough to figure!!! Even mechanics won't guess: They just recommend trying one thing at a time... If it don't work, go i and 'fix' something else...
I would rather do the fixing myself and save the $60/hr shop rate!
Good luck.
Mark
>Greetings. I finally ran a compression check on my truck.
>Engine warm, plugs out, WOT. I have a 1975 460 with at least
>124,000 miles on it. All cylinders read between 150 and 155
>except no.4 which reads 120 I squirted some oil in the
>hole and it came up to 130. Do you think this a signifacant
>incress to suspect the rings? All opinions welcome. I should
>also mention that the vaccum gauge reads 17 to 18 with an
>intermitent drop along with some hesitation. My trouble
>shooting guide says that it may a valve sticking, which
>would help explain the low reading in no. 4 would it not?
>But then again there is the question of the slightly higher
>reading after adding oil into the cylinder. Again all
>opinions welcome. The hesitation could also be a ignition
>miss which brings me to another problem which I'll post in
>the electrical forum. Thanks.
Ford started it; Ford will finish it!
They are also known to stumple on inclines (climbing hills) and even dying for no apparent reason. Float problems, I guess...
I have a 4 cylinder engine in a Nissan car with 155 psi all cylinders, but she smokes like crazy when warm... Me thinks valve stem seals, OR head gasket break near bleed down hole, OR piston rings, OR ...etc...
Who really knows? I have a new head gasket on standby. I also have another used engine on standby (low miles, but low compression: No smoke)
These things are tough to figure!!! Even mechanics won't guess: They just recommend trying one thing at a time... If it don't work, go i and 'fix' something else...
I would rather do the fixing myself and save the $60/hr shop rate!
Good luck.
Mark
>Greetings. I finally ran a compression check on my truck.
>Engine warm, plugs out, WOT. I have a 1975 460 with at least
>124,000 miles on it. All cylinders read between 150 and 155
>except no.4 which reads 120
I squirted some oil in the >hole and it came up to 130. Do you think this a signifacant
>incress to suspect the rings? All opinions welcome. I should
>also mention that the vaccum gauge reads 17 to 18 with an
>intermitent drop along with some hesitation. My trouble
>shooting guide says that it may a valve sticking, which
>would help explain the low reading in no. 4 would it not?
>But then again there is the question of the slightly higher
>reading after adding oil into the cylinder. Again all
>opinions welcome. The hesitation could also be a ignition
>miss which brings me to another problem which I'll post in
>the electrical forum. Thanks.
Ford started it; Ford will finish it!
Senior User
Joined: Mar 2000
Posts: 360
Likes: 0
Compression Check.
A low of 120 with a high of 155 puts the low cyl. about at the 75% point. In the edge area for the 75% rule. For a cyl. with 120 lbs. to come up only 10 lbs. with oil makes me think that cyls. problem may be valve related. A leakdown test would show it.
