About to do my first compression test - Tips?
Thread Starter
|
Senior User
Joined: Sep 2001
Posts: 350
Likes: 1
From: Friendswood TX
About to do my first compression test - Tips?
I am about to do my first compression test ever on an engine. I have a 390 and think there is a cylinder that isn't playing ball with the others 
. Any suggestions? I have taken out all the plugs, they all look good (alike) - plugs have a bit more than 15k on them (what is the change interval, please?).
Stuart
. Any suggestions? I have taken out all the plugs, they all look good (alike) - plugs have a bit more than 15k on them (what is the change interval, please?).Stuart
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
Ok. Disconnect the coil. Keep the battery charged. When you find the offending cylinder, squirt a little oil down it to see if it's the rings or the valves. If the compression number comes up a lot, it's rings, otherwise, valves.
The spark plugs are probably due for replacement. I usually pull a couple at each oil change and take a look. If they're starting to round off, it's time for new ones.
The spark plugs are probably due for replacement. I usually pull a couple at each oil change and take a look. If they're starting to round off, it's time for new ones.
Thread Starter
|
Senior User
Joined: Sep 2001
Posts: 350
Likes: 1
From: Friendswood TX
OK - seven cylinders range from 145 to 170 psi (five compression strokes, carb wide open, good battery) but the cylinder at the forward end of the bank on the passenger side is only showing 68 (sixty eight) psi. I pulled off the valve cover and as best I could tell the springs look ok. About 15K miles ago these heads were purchased used, overhauled and bolted on to the block. All seems well except this corner cylinder. I did add oil to the spark plug hole and did not get a bump up in compression. Any more tests I can run?
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
You could run a leakdown test. That would help pinpoint the problem. But at this point, I think it's fair to assume it's in the heads.
And yes, I meant the center electrode. When it starts to round off it's probably time to change the plugs.
And yes, I meant the center electrode. When it starts to round off it's probably time to change the plugs.
Elder User
Joined: Oct 2005
Posts: 907
Likes: 0
From: Citrus Heights, CA
Just out of curiosity.
My son's 360 (that I just finished completly rebuilding) also had low compression at the number two cylinder. Is this common? This was not a truck for the track. It was just a street/work vehicle. Perhaps this is just a random event, but it is an interesting coincidence.
My son's 360 (that I just finished completly rebuilding) also had low compression at the number two cylinder. Is this common? This was not a truck for the track. It was just a street/work vehicle. Perhaps this is just a random event, but it is an interesting coincidence.
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
How low was it? As long as they're within 10% of each other, it's probably fine for a street driven motor.
Trending Topics
Elder User
Joined: Oct 2005
Posts: 907
Likes: 0
From: Citrus Heights, CA
Before we rebuilt the engine, compression was all over the map from 120 for 5-8 to 65 for cylinder #2. I noted that boltgunner also had low compression on cylinder #2 and was wondering if this was a typical pattern of just a random coincidence.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
10 Best Ford Truck Engines We Miss the Most!
Joe Kucinski
2026 Shelby F-150 Off-Road: Better Than a Raptor R?
Brett Foote
2027 Super Duty Carhartt Package First Look: 12 Things You NEED to Know!
Michael S. Palmer
10 Most Surprising 2026 Ford Truck Features!
Joe Kucinski
Top 10 Ford Trucks Coming to Mecum Indy 2026
Brett Foote
5 Best / 5 Worst Ford Truck Wheels of All Time
Joe Kucinski
Ford Super Duty: 5 Things Owners LOVE, 5 Things They LOATHE!
Joe Kucinski
Every 2026 Ford Truck Engine RANKED from WORST to FIRST!
Michael S. Palmer
The Best F-150 Deal of Every Trim Level (XL through Raptor)
Joe Kucinski
Thread Starter
|
Senior User
Joined: Sep 2001
Posts: 350
Likes: 1
From: Friendswood TX
My Ford factory shop manual stated that having a low pressure that was higher than 75% of the highest pressure was good to go. That was for any truck engine in '76. My highest was 170, so according to the chart in the book I was OK at 127 or higher. Still, I got a bleeder with the front RH corner banger.
Posting Guru
Joined: Jul 2003
Posts: 2,232
Likes: 1
From: Colville, Washington
Originally Posted by AnOldDog
Before we rebuilt the engine, compression was all over the map from 120 for 5-8 to 65 for cylinder #2. I noted that boltgunner also had low compression on cylinder #2 and was wondering if this was a typical pattern of just a random coincidence.
Thread Starter
|
Senior User
Joined: Sep 2001
Posts: 350
Likes: 1
From: Friendswood TX
OK, I have slept since I did the compression check, so now I am wondering what it would take to drop that pressure to 68 psi. Anyone have some ideas why this amount of drop in pressure? This head has not been on the truck long. She runs sweet above idle, smooths out a bit at the stop light when I drop her in neutral. Just annoying little vibration that tells me that I have some knuckle-bustin' ahead. Arrgh.
Posting Guru
Joined: Jul 2003
Posts: 2,232
Likes: 1
From: Colville, Washington
I would also expect a valve issue, a hung open valve(caught in guide) or a burnt exhaust are the most likely. I once had a valve hang up on a 289 with all new valvetrain with only 1000 miles on the motor. Never did figure out why it seized(exhaust valve).