Compression Low- 1cyl

[jdeignan]

Hey guys- 74 bronco. Engine was rebuilt by PO
155
152
155
162
118- 130 w/ a little oil added
158
158
162

Test was done on a cold engine, didnt count revs but they were all similar.
Any thoughts on the low cylinder? From reading it looks like rings, but only 10psi increase with oil?

 

[mike77]

possibly a bad valve

 

[cldonley]

Usually if you add oil and the compression comes up it indicates leaking past the rings. In my opinion you get the most accurate readings with the engine warm. Truth is that 118 probably isn’t anything to be concerned about, especially if it isn’t burning oil. A trick I use on aircraft reciprocating engines is apply pressure to the cylinder with it at too dead center. If Air is getting by the rings you can hear it at the oil filler, by the intake valve you hear it at the carb, exhaust valve at the tailpipe. But if you squirted oil in the cylinder and it came up it is because the oil sealed the compression ring to the cylinder wall. I think the Ford shop manual talks about compression check limits.

 

[mike77]

Try a shot of tranny fluid in that cyl. It worked on mine.

 

[cldonley]

If you have carbon build up on a valve causing it to leak, you can take a mallet and a drift and smack the valve stem allowing th valve to smack back into it’s seat which may knock the carbon off and let the valve seal. This is a real live procedure you can find in aviation maintenance manuals called “staking”. Worth a try.

 

[cldonley]

If you do the tranny fluid thing, don’t over do it. Fluids don’t compress.

 

[jdeignan]

If you do the tranny fluid thing, don’t over do it. Fluids don’t compress.

This will probably be my first move. How much ATF are we talking?
Let it sit a while? Burn it off via combustion? Or blow it out before the plug goes back in?

 

[mike77]

All of my cylinders were low, less than a 100.I just added a squirt of tranny fluid to each cylinder & let it sit for 10 to 15 minutes then rechecked. All checked above 100 the highest was 185.i think I had stuck rings due to sitting up since 1993.Hope this helps.

 

[DirtDonk]

What made you do the compression check in the first place? Was it not running right? Smoking or using oil?

Is this an older and seldom driven engine by any chance? This is common under those conditions. The key is to just get out and drive it more.
But like the transmission fluid trick, it's recovery (to the extent it can recover) can be helped along by using gas and oil additives (the dreaded snake oils!%)) that are designed to help with just these symptoms.
Usually some type of solvent-based setup for cleaning internal components. they really can work. Do they always? Of course not. But if your engine is borderline or just suffering from a buildup of crud, they can help greatly.
It always has helped in the engines I've used them in.

Good luck.

Paul

 

[B RON CO]

Hi, if you squirt tranny fluid in the cylinder and let it sit overnight you can crank the engine with the spark plugs out (remove the coil wire) and then you don't have to worry about the left over fluid.
We used to slowly dribble tranny fluid in a running engine with the throttle held open a little. We hoped it would be carbonize it, but it also made a great smoke show.
I would try Marvel Mystery Oil in the gas tank and crankcase. I know guys who swear by Rislone oil treatment.
Good luck

 

[Apache Bronco]

We used to slowly dribble tranny fluid in a running engine with the throttle held open a little. We hoped it would be carbonize it, but it also made a great smoke show.
Good luck

I remember this from the late 70's. In auto shop the instructors mentioned a spray of distilled water in the carb with the engine running, 1 time only, to steam clean combustion chamber.

 

[Rustytruck]

send a picture of the plug that came out of that hole compared to the other plugs. a long term misfire in the plug wire or a bad plug could be the initial cause of this problem. if your going to soak the rings with something do it with the piston at the top of the cylinder. The biggest wear in the cylinder is at the top so your more likely to get fluid down into the rings.

I would still do a pressure leak test to find out where the leak actually is.

 

[blubuckaroo]

I remember this from the late 70's. In auto shop the instructors mentioned a spray of distilled water in the carb with the engine running, 1 time only, to steam clean combustion chamber.

We would do this back when compressions were higher and the carbon build up actually caused interference between the piston and head. We just used plain tap water.
We would use a Pepsi bottle full of water in one hand and trickle it down the carb while blipping the throttle with the other. If you kill the engine, STOP POURING THE WATER or you could end up damaging a piston or rod.
It's best to do this out where the exhaust doesn't spray black crap all over your driveway.
Also, it's best to follow up these type of procedures with an oil change.

 

[needabronco]

I'd pour a half quart of atf in the oil and run it for 30-45 minutes with the radiator partially blocked off to get the engine hot, then change your oil. The solvents in the atf will clean out whatever build up or sludge you have. Don't be surprised if chunks come out when you drain the oil...

Also with the engine idling, slowly pour atf in a vacuum port on the intake. Yes it will smoke like a freight train and knock and clack like crazy but it will burn all of the built up carbon out of the combustion chamber and free sticking rings. We regularly do this on oil field engines, it's like a 60 second tuneup if you do in fact have a carbon buildup or sticking rings.

 

[73azbronco]

Crank the motor with the plug out a few times to remove oil. Then Start it warm it up and try some seafoam in the carb. Then if it's not smoking, drive it and worry about it when it smokes.

 

[jdeignan]

What made you do the compression check in the first place? Was it not running right? Smoking or using oil?
Is this an older and seldom driven engine?

No real reason for compression test. I was getting erratic vacuum and decided to check compression while I was resetting idle fuel mix (it was a nice day to work on the truck). Ended up getting it to a steady 15in vacuum and idles at 550 in drive a whole hell of a lot better now.
It doesnt smoke, and I dont think it consumes a lot of oil. It does, however, dump a ton of moisture from the tail pipe. I try and drive it once a week at least.

 

[jdeignan]

send a picture of the plug .

I will next time I pull them out, but Ill be honest they were all surprisingly identical.. tan/brown as expected without any of the classic indicators that they were having issues

 

[DirtDonk]

No real reason for compression test. I was getting erratic vacuum and decided to check compression while I was resetting idle fuel mix (it was a nice day to work on the truck).

Ahh, the mark of a true tinkerer at heart. Good for you!

Ended up getting it to a steady 15in vacuum and idles at 550 in drive a whole hell of a lot better now.

Ok, steady is great. But 15hg, not so much.
You want closer to 20 with a stock engine. Is this one relatively stock or do you have a cam?
If stock, check your gauge against a known good modern engine. If it still reads 15 you probably have a bad gauge (but at least you know what it's reading now).
If you get 19-22 on the newer engine though, go back to yours and try some more tweaks.
For instance, bump the timing up three or four degrees to about a max of 14 BTDC or so. If you're at stock or below now, you probably only need to go up to about 10° BTDC.

It does, however, dump a ton of moisture from the tail pipe. I try and drive it once a week at least.

Normal for this time of year with older vehicles. Not anything great, but nothing to worry about. Yet at least!

Paul

 

[jdeignan]

Ahh, the mark of a true tinkerer at heart. Good for you!

Ok, steady is great. But 15hg, not so much.
You want closer to 20 with a stock engine. Is this one relatively stock or do you have a cam?

Hell yeah man- it’s half the reason I got this thing!
Ok so cam wise— I have no idea what is in there... it isnt stock according to the PO but I didn’t ask what kind. Regretfully. I suppose I cant find out without tearing it down.

I will try and bump the timing tomorrow and see what it does to the vac reading. I’d do it now if i didnt have ATF sitting in the Cyl

 

[pcf_mark]

I've seen some modern cars trail a stream behind them I would not chase that as a symptom of anything.

When I my stock engine was carbureted i was pulling 19" steady in neutral. I had 16 degrees of timing but that made the exhaust smell nasty backing it down to 12-14 is better.