Low compression!!!

[zeiss150]

Hello!  So I got a little bored today and decided to pull all of my plugs and check my cylinder compression.  I’m going to be doing a turbo build so I just though it would be a good idea to give the old L28 a once over.    

I pulled all the plugs and got a consistent 

100 PSI on all cylinders. 😡

al little history on my L28 ...

i rebuilt it about 15 years ago and it has about 20 thousand miles on it.  It’s an F54 block with flat top pistons with an HKS steel head gasket (2mm I think) with a freashly re-built P90a head. 

So what’s up??? Why 100 PSI??? 

I mean it’s 100 exactly on all 6 cylinders

HELP!

[seattlejester]

Assuming off the bat that your gauge is good....

Any tell tale signs? White smoke out the back, hydro carbons in the coolant, blue smoke out the tail pipe? 

You can isolate it to see if it is a problem with the head or rings. 

Run it again and drop a little bit of oil in the spark plug hole, a small amount like a cap full. If the compression shoots up, then you know it is a problem with the rings. Did you gap them correctly would be the question that comes up.

If the compression stays the same, then the problem is most likely above the piston so either a leaking head gasket (some MLS head gaskets are notoriously hard to get a good seal) or the head it self (valves not closing all the way from carbon buildup, incorrect valve lash, incorrect valve stem height). 

 

[grannyknot]

Was your engine cold and the throttle butterflys closed? Engine must be  up to temp, butterflys must be open.

[Chickenman]

^ That makes a huge difference. Engine must be to full operating temp and Throttle Blade held wide open when cranking. 

I'll add to it. Battery should be fully charged ( and on a Charger ) , all plugs removed, Injection disabled and starter must be in good condition. Cranking RPM makes a huge difference.  The old direct drive starters did not spin the engines over very fast, even when new. The gear reduction starters are much stronger.

Camshaft duration also makes a big difference. More duration effectively lowers cranking  CR. 

The fact that all the cylinders are the same is a good sign. Specs are  within 15 psi of each other. 

If it's not burning oil... I wouldn't worry about it too much. 

 

[Chickenman]

The HKS 2mm head gasket is alo going to lower compression. Stock Nissan gasket is 1.25 mm ( installed thickness ) if memory serves me correct ( OZ dat shows 1.25mm for calculation ) 

With the HKS 2.0mm head gasket ( installed height I'm assuming ) Oz dat calculates 7.9 CR with Flat Top Pistons and Zero deck height. I calculate with pistons .020" ( .450mm ) above deck height ( figure shown in calculation is -.020" or -0.450mm to show piston above deck height ) , as that I believe that OZ Dat has the piston pin height wrong for the NA Flat Tops. Using the latter figure, that gives a CR of 8.3 to 1 with FT and P90 head. Not a lot of CR.... but good for Turbo charging 

Stock Turbo CR with dished pistons sitting .020" below deck is a rather anemic 7.35 to 1. 

 

[zeiss150]

Ok in answer to some of the questions...

1.  Brand new compression tester ( bought it yesterday new)

2.  Engine was stone cold. Oops. I didn’t think about that.  

3. I’m adjusting the rocker arms now. .012 exhaust and .010 intake on a cold engine. 

4.  I did not have the (throttle) butterfly valve open AT ALL forgot about that one too. 

5. Yes the rings were gapped and oriented properly 👍🏻

6. I do have a reduced gear starter   

7. All spark plugs were out when cranking  

8. I’m running SU’s so I can’t disconnect the fuel. 

I didn’t think that compression ratios had any effect on cylinder psi? 🤷🏻‍♂️

My F54 block was bored .20 over and I used new flat top pistons. 

Do you have a link to that engine calculator?  

Im going to re torque the head bolts while I have the valve cover off. 

Once I have all that stuff done I’ll warm up the engine, pull all the plugs, have WOT and try it again.  

Thanks for all the tips guys.  It’s been a long time since I worked on my L28 and it’s clear I forgot a bunch of stuff. 

[zeiss150]

I took a few photos. 

I checked the timing and it looked fine except... the bright link was on the 3. Isn’t  it supposed to be on the 1?  

I know the 1,2,3 are for timing chain stretch but my timing chain has maybe 20 thousand miles on it and I’m pretty sure when I put the head on I had the shinny link on the # 1 position. 🤔

then I started thinking about it... doesn’t the bright link move around with every rotation of the engine🤷🏻‍♂️

Man i suck at this.  

I finished my valve adjustments. They were almost all about .001 to tight. 

I retorqued the head bolts to 60 ft/lbs. I think they were torqued to 55 last time I put the head on because I was able to turn it a little tighter until I got a click at 60lbs

i ran out of time to run a compression test so it’ll have to wait till Tuesday.  

[lowrider]

I've always been of the opinion that the health of an engine isn't determined by a certain value of compression. The more important thing is that the individual compression on all of the cylinders are within a certain percentage of each other. I typically say that if the difference between your highest and lowest cylinders is within10% The engine is healthy.

 

This is what was taught to me and makes logical sense to me, so take this with a grain of salt.

[seattlejester]

You can disconnect the fuel by just pulling the feed line off the mechanical fuel pump. 

It is alright sometimes it takes a while to reason through things. I believe the link lines up at TDC. Looking at it at another point would not be the most helpful. The crank gear I imagine spins at double the speed of the cam gear. So it will move depending on which part of the 4 stroke cycle you are on.

What lowrider says is also what I was told. Compression can be a bit lower then expected, but as long as they are even that is better then if you had high compression with one outlier. Granted when your compression gets really low you have to consider there may be something systematically wrong. In this case a cold engine with the throttle shut may be the culprit. 

[HuD 91gt]

The link will move from one engine rotation to the next.  Don't worry about it.  Turn the engine 3 or 4 more times and it should show one spot 1 as installed.

[madkaw]

2&4 are your major problems . Test will not be accurate at all with those two oops

[blu240z]

Check vacuum at idle should be between 18-21" inches of vacuum.

[softopz]

Sounds fine to me numbers do mean a thing but not much. Two factors you are running a thicker headgasket + it was cold. Did you add oil after and run a wet test? add about a table spoon of oil in each cylinder and crank it 3-5 times see if your numbers increase by much thats usually a big tell. 

[softopz]

Turbos love LOW compression you can run crazy boost on it. However I want to know why you had such a thick headgasket on NA build? Or was it always going to be turbo in mind. 

[zeiss150]

Hi guys.  So I did some more testing. I got the engine up to temp and tested with WOT and the results... 100psi in all 6 cylinders 😐.  

So let’s recap 

1.  Valves are adjusted properly 

2. Timing is correct

3.  Head gasket is good and tight. 

4. Engine was a TOTAL rebuild about 20 thousand miles ago.  

5. I put about 5ml of oil in each cylinder and got a bump of 20 psi in all 6.  So with oil on a hot engine with WOT I get 120 psi in all 6 cylinders.  

Should I do a leak down test?  

The engine runs and revs fine.  

In answer to “Softops” question... I did build it to be a turbo engine. I just never got around to it.  

I’m not sure where to go from here.  Do I chase this down or do I just say F-it and put a 2JZ in there and call it a day?  

Edited November 9, 2017 by zeiss150

[seattlejester]

I'm used to seeing higher numbers, but as many have said if they are all the same and the car is not burning oil, coolant, etc, then not much to worry about. 

If you are on the fence of going 2jz and you need a reason, this really isn't a great one, but I have seen worse (I did mine because of a bad relay and leaking rear main seal). 

[NewZed]

If you're using an extender to make it easier to reach the plugs, or a long hose, that will lower the pressure reading.  The extender tube and/or the long hose effectively become part of the compression chamber, lowering the testing compression ratio.  Nissan's 170 psi number is only valid if you're using their pressure measuring tool.  They kind of screwed up by not mentioning that.  If you look at their picture though, they're using the smallest connection volume between cylinder and gauge.  so they get a high number.

[softopz]

5. I put about 5ml of oil in each cylinder and got a bump of 20 psi in all 6.  So with oil on a hot engine with WOT I get 120 psi in all 6 cylinders.  

You should have done the dry test warm then wet warm so you could compare numbers. However, If you saw an EVEN increase of compression with oil on all cylinder one could argue that the motor is still healthy. I would have been more worried about an increase in only 1 of them. The 2mm headgasket definitely lowering your numbers and different gauges give different results.  I wouldn't worry about it Boost it have fun with your motor then if you do blow it go 2jz my 2 cents... 

[zeiss150]

Well... I think I’m gonna build this turbo L28 and if it doesn’t work ... then I’ll welcome the 2jZ swap.  

I found a new problem today... intermittent spark 😡

i put a spark tester on my #1 cylinder and I got a very  intermittent spark.  I made a video but it wouldn’t upload. 

I have an electronic distributor from an 82ZX... so no points. 

And the coil is the original 72 240z coil. 

so let me state right up front ... I don’t know anything about electronics. 

I suppose it could be the coil so I’ll check that first tomorrow.  But if it’s not the coil then it would have to be the distributor ... right???

im gonna switch to ford edis anyway with my turbo mod so I guess I should do that first.  

Any suggestions or tips? 👍🏻

[zeiss150]

ok ... here’s my $hitty spark