Low compression on all cylinders

[Yousef]

Hi all,

 

I recently picked up a 1974 Datsun 620 with an L18 with a rebuilt head.  Apparently the previous owner replaced the head gasket and did a valve job (no porting).  

 

After getting it looked at by a mechanic they told me that I should get work done on the head, based on me turning on the engine ( no tests were done).  They also showed me that the exhaust pipe was sucking in air every couple seconds.  They said that it should be taken to a machine shop and could cost upwards of 1000 dollars.

 

I decided to do some tests of my own.  

 

Besides the exhaust pipe, the biggest symptom seems to be I'm experiencing some low power.  The vehicle doesn't have much get up and go but can reach 80-90mph, it just takes some time.  

 

I decided to do a compression test and found that compression was low but consistent on all cylinders.  About 100-110psi.  This also stayed the same after squirting oil in each cylinder.  

 

I wouldn't say the engine runs rough and it has never died on me after warmed up.  

 

If compression is low on all cylinders that would normally indicate worn out rings right?  However adding oil should then increase my numbers significantly right?

 

Could this be a timing issue perhaps?

 

Any thoughts would be appreciated.  Thanks everyone!

 

-Yousef

[Draker]

Check timing, check valve lash and start from there. It's valve/head related.

 

You could buy a used head for $200 or less.. 

[geojaxon]

I'm no expert but I know that the valve lash can contribute to the loss of compression if it's setup incorrectly. Also the camshaft timing would control your valves and if set wrong could lead to loss of compression.

 

My L20B had the camshaft gear setup wrong and had no compression when tested because the valves were not closed when they should have been.  If your truck runs you are at least not as bad off as me.

 

I would start with checking the valve lash.

[sebpv]

Before you go and say your truck lacks this and that, I would strongly suggest that you share those numbers. A Datsun 620 has roughly 95hp. Its not a rocket. People will chive in driving around 130-140kmh. Even then the rpm is at 4000 and you cant listen to your radio without first cranking it to maximum. A good engine should have around 150-160psi or more. Confirm that no cylinder is 15% lower than others. And also do some more research.

[datzenmike]

90MPH from an L18 is pretty damned good. Your mechanic is in business to take your money. $1,000 to rebuild the head is 3x the maximum you would ever pay.

 

 

Start doing your own work on it. Buy a manual, preferably a Nissan factory service manual (FSM) for your '74. This will give you all the numbers and a few tools as you need them. Generally a good tune up first. Set the valve lash, set the timing. Clean set or gap the plugs and points.

[Draker]

90MPH from an L18 is pretty damned good. Your mechanic is in business to take your money. $1,000 to rebuild the head is 3x the maximum you would ever pay.

 

 

Start doing your own work on it. Buy a manual, preferably a Nissan factory service manual (FSM) for your '74. This will give you all the numbers and a few tools as you need them. Generally a good tune up first. Set the valve lash, set the timing. Clean set or gap the plugs and points.

 

 

He's in the bay area.. so a full top to bottom rebuild of a head could cost that much.

[Lockleaf]

Isn't busta in the bay area?

[Draker]

Isn't busta in the bay area?

 

Yes and happens to work as a professional mechanic.

[DatsuNissanLove]

This is a problem I have n it's because someone in the past swapped the original head with a newer head with lower compression.. so maybe make sure it's the correct head on der?

[datzenmike]

L16 needs the 210 head. Anything else drops the compression from slightly 8.2 down into the 7s.

 

L18 and L20B can both use the largest open chamber heads and they are fine.

[flatcat19]

Old Datsuns.com has the FSMs to download, for free.

 

 

A cylinder head lowering compression is not the same as have low compression pressure from testing.

 

Cylinder balance testing should be performed for diagnosis after a wet compression test yeilds no results.

[yello620]

Hi all,

 

 

I decided to do a compression test and found that compression was low but consistent on all cylinders.  About 100-110psi.  This also stayed the same after squirting oil in each cylinder.  

 

Any thoughts would be appreciated.  Thanks everyone!

 

-Yousef

 

Any chance that there was an error in your testing method? Or an inaccurate guage? Truck will do 90, just don't believe compression, tuning issue - yes.

Most common thing to miss when doing a compression test is to make sure that the throttle body(carburator) is open so that the cylinder can get air. The cyclinders can only compress the amount of air that they receive. I think that the next thing would be the number of revolutions the motor is spun for the test. I was always told ten revolutions, but it usually maxes out around five revolutions IF you have pulled all the spark for the test.

 

You could also have valves way out of adjustment causing low compression.

[datzenmike]

 Compression ratio and compression numbers are different things but are directly related. A lower compression ratio will give lower cylinder compression numbers. A rule of thumb is the PSI of a compression test should fall between 15 and 20 times the compression ratio. There is no exact value as intake valve closing is well past BDC and depending on the cam they are all different.

 

An L18 compression ratio should be about 8.43, so anywhere from 126PSI to 168PSI. With 150PSI probably closer to real world.

 

 

Again if it will go 90 MPH it can't be that bad. Borrow another tester and repeat your readings. Get the kind that screw into the head like a spark plug.

[distributorguy]

Decent top end would indicate the cam timing is retarded.  That will also result in low power and low cylinder pressures.  

With that said, low cylinder pressures can just be a bad gauge.  I recently had my trusty old gauge fail, after 20 years of use.  It read on cylinder at 110, the next at 100, then 80, then 0 on everything I tested.   

 

Mike maybe you should post your cam timing mark photo for Yousef to see while he sets valve lash?

[Roadster-ka]

Are you holding the throttle open while cranking?

[datzenmike]

It was mentioned.

 

To check chain stretch or cam timing is where it should be.  Line up the pulley 0 and the ignition timing scale or pointer. This must be done by turning the crank pulley, do not turn the cam sprocket. If you overshoot the mark, back up at least one quarter turn and try again. You must set TDC and stop while turning in a clockwise direction only or you will introduce slack in the timing chain and your reading will be wrong. If it takes you 5 tries that what it takes.

 

Now look through the cam sprocket top hole. You should see a V or U notch in the back and right above it a small horizontal etch line, like the picture below.

 

 

If the V or U notch is below or just to the right of the horizontal line you cam timing is perfect. If not say so, it can be adjusted.