compression issues?
Thread Starter
|
New User
Joined: Feb 2018
Posts: 12
Likes: 2
compression issues?
Hi all,
I have a 79 f100 with a 302 cid. I doesn't seem to have power like it should (what ever that means. I know it is 40 yrs old). So, i removed a valve cover and made sure the valves were closed on the cylinder i was working on. Removed the spark plug and attached a fitting (compression hose type that screws into the spark plug hole). Next, I attached a bicycle pump that has a pressure guage to the hose and stated pumping. I hand one of the kids listen at the exhaust. No sound out the exhaust. But, hardly any pressure reading at the guage. I could here air flowing but not sure where. I'm guessing going past the rings? I did not feel any air rushing out the carb.
I did this on cylinders 1,2 and 3. Same results.
Any ideas? Thoughts?
Thanks
I have a 79 f100 with a 302 cid. I doesn't seem to have power like it should (what ever that means. I know it is 40 yrs old). So, i removed a valve cover and made sure the valves were closed on the cylinder i was working on. Removed the spark plug and attached a fitting (compression hose type that screws into the spark plug hole). Next, I attached a bicycle pump that has a pressure guage to the hose and stated pumping. I hand one of the kids listen at the exhaust. No sound out the exhaust. But, hardly any pressure reading at the guage. I could here air flowing but not sure where. I'm guessing going past the rings? I did not feel any air rushing out the carb.
I did this on cylinders 1,2 and 3. Same results.
Any ideas? Thoughts?
Thanks
Grumpy goat
Joined: May 2017
Posts: 3,058
Likes: 594
I wouldn't trust a bike tire pump to check compression. A parts store compression test kit is cheap. Also, get a vacuum gauge. They're cheap also and can tell you a lot about what's going on with your engine. But if you really do have no compression in one, or four cylinders as in your case, it could be several issues. None are good. And it would be enough for me to want to pull the engine and at very least remove the cylinder heads to inspect them, cylinders, and piston tops.
That said, it could be stuck valves, worn/burned valve seats, worn valve guides, holes in piston tops from pre-detonation, worn/oversize cylinder walls, and worn rings. And of course when you have any of these problems in an engine with lots of miles, there will be plenty of other badly worn components. But again, before declaring it dead, I'd get a proper compression test gauge, vacuum gauge, and retest.
That said, it could be stuck valves, worn/burned valve seats, worn valve guides, holes in piston tops from pre-detonation, worn/oversize cylinder walls, and worn rings. And of course when you have any of these problems in an engine with lots of miles, there will be plenty of other badly worn components. But again, before declaring it dead, I'd get a proper compression test gauge, vacuum gauge, and retest.
Fleet Owner
Joined: Jan 2004
Posts: 26,007
Likes: 575
From: Brentwood,CA
Not sure where the bicycle pump idea came from, that's a new one to me. However to do a leak down test requires the leak down seat of gauges and a air compressor. Compression testing requires a compression tester and a fully charged battery with a battery charger attached during testing. I would do the test over with proper tools.
FTE Legend
Joined: Nov 2015
Posts: 30,860
Likes: 4,106
From: Angier, NC
Not sure where the bicycle pump idea came from, that's a new one to me. However to do a leak down test requires the leak down seat of gauges and a air compressor. Compression testing requires a compression tester and a fully charged battery with a battery charger attached during testing. I would do the test over with proper tools.
It sounded like you were trying to do 2 different test at the same time and not using the right tools to do either.
As compression test is a gauge in a spark plug hole and crank the motor and see where the needle stops.
A leak down test you set the cly to check on TDC and using air from a compressor, not a bike hand pump, and see what the gauges read on the leak down set reads and listen where the air is coming out of.
Do a compression test and give us the readings and we can let you know if good or bad.
Also how many miles on the motor, last time a full tune up was done.
Is this a 4x2 or 4x4 and what trany and rear gear does it have.
Dave ----
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
A brand new compression gauge can be picked up for under $20 delivered to your door. Quality is "good enough", I suppose. Pick up a remote starter switch while yer at it. About $5
Remove all the spark plugs. Engine is supposed to be warmed up, a lot of people ignore this one. I know I do. If I get good consistent numbers on every cylinder cold, I don't worry about it anyway. Need a good strong fully charged battery. Block open throttle and choke.
Count the # of "puffs" it takes to reach the highest reading. Use the same # of puffs for recording compression reading for every cylinder, say 5, and write it down. It's encouraging to see the needle rise quickly to the highest #, and it's also a good sign that all the cylinders are close to each other in compression within say +/- 10%
Remove all the spark plugs. Engine is supposed to be warmed up, a lot of people ignore this one. I know I do. If I get good consistent numbers on every cylinder cold, I don't worry about it anyway. Need a good strong fully charged battery. Block open throttle and choke.
Count the # of "puffs" it takes to reach the highest reading. Use the same # of puffs for recording compression reading for every cylinder, say 5, and write it down. It's encouraging to see the needle rise quickly to the highest #, and it's also a good sign that all the cylinders are close to each other in compression within say +/- 10%
Trending Topics
Thread Starter
|
New User
Joined: Feb 2018
Posts: 12
Likes: 2
Thanks for your suggestions. I will do a compression test on all cylinders with the engine warm and check vacuum. I will get back in about a week or so.
Thanks again.
Thanks again.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
AEV FXL Super Duty - the Super Duty Raptor Ford Doesn't Make
Brett Foote
Lobo Vs Lobo: Proof the F-150 Lobo Should Be Even Lower!
Michael S. Palmer
Ford's 2001 Explorer Sportsman Concept Looks For a New Home
Verdad Gallardo
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
Thread Starter
|
New User
Joined: Feb 2018
Posts: 12
Likes: 2
Compression Issues?
Thanks 77f250c for the pic. I haven't pulled the heads off yet.
Let me give more info on the truck:
79 f100, 302cid, manual trans(T18), 9" diff ratio 3.0, 2wd, dura-spark II distributor.
I measured compression cold (90 degrees F) and vaccuum: Results
Cylinder # psi
1 75
2 100
3 95
4 110
5 110
6 100
7 80
8 85
Vacuum at idle (RPM????) ~ 13" Hg in the yellow some small oscillation.
opinions??? Rebuild engine?? I'm on a budget.
Thanks.
Let me give more info on the truck:
79 f100, 302cid, manual trans(T18), 9" diff ratio 3.0, 2wd, dura-spark II distributor.
I measured compression cold (90 degrees F) and vaccuum: Results
Cylinder # psi
1 75
2 100
3 95
4 110
5 110
6 100
7 80
8 85
Vacuum at idle (RPM????) ~ 13" Hg in the yellow some small oscillation.
opinions??? Rebuild engine?? I'm on a budget.
Thanks.
Laughing Gas
Joined: Apr 2018
Posts: 1,244
Likes: 12
I'm no expert (and I am 100% sure someone that is will have something to say), but your numbers don't look terrible to me. You probably have a vacuum leak somewhere. Have you gone through all your vacuum lines?
Let's go back to your original post... you felt the engine didn't have enough power. What makes you say that?
Also, post up some pics of your engine bay from some different angles please.
Let's go back to your original post... you felt the engine didn't have enough power. What makes you say that?
Also, post up some pics of your engine bay from some different angles please.
Senior User
Joined: Dec 2015
Posts: 235
Likes: 2
From: Southeast Missouri
Thanks 77f250c for the pic. I haven't pulled the heads off yet.
Let me give more info on the truck:
79 f100, 302cid, manual trans(T18), 9" diff ratio 3.0, 2wd, dura-spark II distributor.
I measured compression cold (90 degrees F) and vaccuum: Results
Cylinder # psi
1 75
2 100
3 95
4 110
5 110
6 100
7 80
8 85
Vacuum at idle (RPM????) ~ 13" Hg in the yellow some small oscillation.
opinions??? Rebuild engine?? I'm on a budget.
Thanks.
Let me give more info on the truck:
79 f100, 302cid, manual trans(T18), 9" diff ratio 3.0, 2wd, dura-spark II distributor.
I measured compression cold (90 degrees F) and vaccuum: Results
Cylinder # psi
1 75
2 100
3 95
4 110
5 110
6 100
7 80
8 85
Vacuum at idle (RPM????) ~ 13" Hg in the yellow some small oscillation.
opinions??? Rebuild engine?? I'm on a budget.
Thanks.
1,3,7&8 seem pretty low to me. No 1 is definitely concerting. You want the want the highest and lowest readings within 20% of each other. Vacuum is also a little low. Does the engine consume much oil or smoke excessively?
Thread Starter
|
New User
Joined: Feb 2018
Posts: 12
Likes: 2
Compression Issues?
When i add oil (teaspoon) to the cylinders this is what i get:
Cylinder # psi (no oil, cold engine) psi (added teaspoon of oil, cold engine)
1 75 100
2 100 110
3 95 110
4 110 115
5 110 skipped by mistake
6 100 100
7 80 ~90
8 85 95
I guess i couldn't get enough oil in cylinders 6,7,8. Why should the compression be all over the place? And #1 cylinder goes up quit a bit when oil is added.
I will try to get pics of the bay.
Cylinder # psi (no oil, cold engine) psi (added teaspoon of oil, cold engine)
1 75 100
2 100 110
3 95 110
4 110 115
5 110 skipped by mistake
6 100 100
7 80 ~90
8 85 95
I guess i couldn't get enough oil in cylinders 6,7,8. Why should the compression be all over the place? And #1 cylinder goes up quit a bit when oil is added.
I will try to get pics of the bay.
Thread Starter
|
New User
Joined: Feb 2018
Posts: 12
Likes: 2
It doesn't seem to consume a lot of oil. The truck, when standing still engine running, has a vibration that can be felt on the steering wheel and the transmission shifter when rpm is increased and then smooths out when past that rpm. I don't know what the rpm is. i don't have a Tach.
Any guesses to what is causing this?
Any guesses to what is causing this?
Regular User
Joined: Sep 2014
Posts: 1,798
Likes: 15
From: Bathurst, New Brunswick
If the numbers go up when you add oil that is an indicator that there is a problem with your piston rings (as opposed to valves). The oil helps seal the piston. 10% difference from high to low is generally considered OK. Higher than that you are looking at a rebuild. You could probably drive it as-is for years if you can live with it.