Can anyone help me with this problem?

First of all ,this is not an oldsmobile engine.

The engine is running on 7 cylinders,at least on idle.
Did a compression test,the results were around 135 psi on all cylinders.
Did it over again on the dead cylinder,and this time it showed only 50 psi.

I did about 10 tests and they showed pressure between 50 and 135 psi.

Something is wrong but what?

 

Any tapping or unusual noises? If I understand you correctly you checked the compression in that cyl 10 times and it sometimes read 50 psi and sometimes 135. If this is the case I would suspect a valve that is not seating sometimes. Could be bent or carbon under that valve. Also are you performing the compression test with the spark plugs out of the cylinder adjacent to the one that gives the low reading. If a head gasket blows between 2 cylinders you can have erratic readings. Usually if this is the case both cylinders will be low.

 

Can you have a leak down test performed? This would confirm a valve sealing problem.

 

Hook up a vacuum gauge and run the engine to see what the guage does at idle if it bounces up and down motor has a valve problem.

 

This could be a weak valve spring, it is not uncommon on engines that sit for extended periods. The springs that are fully compressed during storage due to rocker arm/cam position lose tension.

 

Thanks.

No unusual noises.
This cyl. is dead on idle,but seems to be working on higher rpm.
The highest reading was 135 psi(2-3times),the lowest 50 psi,but there was readings inbetween these also.
I performed the test with all sparkplugs remowed.

 

UPDATE

Pulled the valve cover,removed the rockers and spun the valves with an electric drill.Now I get constant high compression readings.

Unfortunately I think the leaking valves are a symptom,and not the problem.
When I cranked the engine without valvecover I could see, with my bare eyes ,that the intake valve on the deadline cylinder was opening a lot less than the three others.

Guess I am dealing with a worn camshaftlobe.

 

Or possibly a bent pushrod?......

 

No Sir,the pushrod is straight as an arrow.

 

It could be a collapsed lifter, but generally it would tap.

 

So would a bent pushrod - I'm thinking bent [or badly carboned] valve, too!

 

Interesting.
But wouldn`t this lead to a large axial play in the pushrod when the valve is closed?

Is there any way to check this without removing the intake?

 

You should be able to spin the push rod with the valve closed. You'll know right off the bat if it's bent. It should run true from top to bottom.

 

Pushrod is straight.
There is no clearance pushrod/rockerarm when the valve is closed.
Can rotate the pushrod.
The intake valve in question is opening less than the other valves.
Can a lifter get stuck this way?
I am considering running some internal engine cleaner in the oil,but I am kind of bewildered,some says it is good medicine,others call it poison.

The car is cruising good,seems like it is fireing on all cylinders then,but it is this irritating vibration at idle.

I want to be pretty sure what the problem is before ordering parts.

 

I think the only way to be absolutely, 100% positive is to take the intake off, examine the lifter, and measure the lobe lift directly. That being said, I'm betting on cam lobe.

Out of curiosity, what kind of engine is it? You did say it was not an Olds engine, but that leaves it pretty wide open...

 

Ford 460,on my -74 lincoln mark 4

 

The higher engine speeds will tend to hide the miss of the dead cylinder. The cleaner will probably be useless in resolving the problem. If you have reduced lift of the valve, and the rocker and pushrod are normal, then surely something is wrong with the cam or lifter, or both. The cam will not "fix" itself. The lifter could collapse and then recover somewhat with added oil, but it is unlikely. As you have normal compression on the dead cylinder at times, it makes me wonder if that cylinder may have more than one problem. I would start by removing the manifold and checking the cam and lifter. It's possible that you may also have to remove the cylinder head if there is more than one problem. Beste ønsker.

 

Keep us posted on what you find. Curiosity is killing me now. I tend to agree with others that it points to cam/lifter but I still would expect some noise or play between pushrod and rocker. Look for metal in your oil. A failing lifter or cam should be leaving some metal behind. Stop running the engine if there is metal present until you find the source. You don't want that stuff going through your bearings.

 

I am living in the outskirts of the civilized world,so it is not like a 5 min drive to the local autopartstore to pick up parts for a 40 year old car.

It will take me about 2 weeks to get parts,and I cant block my workshop with an undriveable car that long.At least not a car this size.

So I am ordering a camshaft and lifter set,a new timing set and a gasket set
and leave it as it is until the parts arrive.

I will keep you updated.

 

It sounds like you are proceeding correctly with your plans. The dead miss on that cylinder is most likely caused by the loss of compression (whatever causes that). However, dead misses can also be caused by loss of ignition and significant vacuum leaks which affect one cylinder. While you are waiting on the parts, you could check these other possibilities to eliminate them as possible problems. Also, after it is repaired, be careful about the engine oil which you use. The older engines with flat lifters (non-roller) needed sufficient ZDDP additive in the engine oil to avoid cam and lifter failure. Many current oils have reduced levels of the ZDDP additive to aid the exhaust converter vehicles. Also, many mechanics will coat the cam lobes and lifters with a special lubricant to avoid "dry" starts. I hope that you will let us know how things progress.

 

Just fixed a similar issue on a friends Olds 324. Had two cylinders that were dead at idle, good compression and spark. Finely traced it down to the carburetor gasket by spraying WD40 around the base of the carb at idle when the rpm's picked up I knew this was the problem huge leak. Installed new carb gasket. runs like a champ. Have also had this happen with bad intake gaskets

 

A leaking waterpump forced me to take action.It s been on hold for 3 years now.It was as expected,one lobe was worn down about 1/8".The others were fine. I just wonder what causes this,material failure on the cam or are ther underlaying causes.Have changed the camshaft, the lifters,and put in a straight up timing gear.Not finished assemblying it yet.This was a big job,a lot of accessories on these cars.

 

It could be material failure, but:
Use a quality assembly lubricant meant to be used between the cam and flat tappet lifters.
Use appropriate viscosity oil for the temperature environment.
Use an oil which will be correct for the flat tappet engines.

 

Could it be that if the intake lobe is warn in just the right way that it causes the valve to open late which inturn causes the down stroke to suck the exhaust valve open just for a split sec. ???????
Railguy

 

Only if the valve spring was shot.
Poor compression and a valve on the offending cylinder not opening properly, and a vibration at idle. If it walks like a duck, quacks like a duck and swims like a duck...........

Roger.

 

What brand of zinc additive do you recommend.Dont think I can find it here.Wife is leaving for US tomorrow so maybe I can get her to buy me a small stock.I have some, but don't recall the brand and don't know how good it is.

 

There are many available. Here's one I've used with success (so far).

http://www.cam-shield.com/

 

Wonder if this oil is suitable for Engines with flat tappet lifters.API SL.

 

Here is another one.API SF. About the cheapest oil you can get.For diesel and gas engines. They call it tractor oil.

 

In 1976 Ford was recommending oil meeting Ford specification ESE-M2C101-C or classification SE of the appropriate viscosity for the temperature environment (both multi-viscosity & single viscosity). I don't have the information for 1974.