L28 NA N42 Engine (Block and head)

[Blitzed]

Need some data. Purchased  73 240Z with a transplanted L28 N42 NA engine, from the mid 80's (high mileage engine).

Have zero history of the parts used in the engine. What PSI compression ratio's should I expect on an old rebuild not knowing dome or flat piston. How can I verify the piston config?

All seam to be in the 155 psi range # 4 cylinder is the highest at 162 psi . Interesting that # 4 with highest compression pulls the lowest vacuum number on the airflow meter at the intake? 

Thanks.

[Captain Obvious]

You should be able to pull the plugs and get a view of the piston tops. Won't tell you what your compression numbers should be, but at least you will know if the pistons are dished or flat topped.

[Zed Head]

There were only two compression ratios for the NA L28's.  8.3 and 8.8.  A stock N42 block should have either an N42 or N47 head.  That would give the 8.3 CR.  

Your pressure numbers seem low but the adapters or length of the hose to the gauge can affect the ratio.  More adapter or hose looks like more combustion chamber volume.

What do you mean by "air flow meter at the intake"?  Are you doing your measurements with the intake manifold removed?  

15 hours ago, Blitzed said:

Interesting that # 4 with highest compression pulls the lowest vacuum number on the airflow meter at the intake? 

 

[Blitzed]

Thanks Zed,

N42 head, the pistons look dished from what I can see from the spark plug hole. Will borrow a engine video scope to confirm. It's a high mileage engine so the compression number should be low.

With dished piston what should the PSI numbers look like? I've seen 155-170 range range 8.3.1. First test was a warm engine. Was going to run through a cool engine comp test. 

The compression tester is a OEM with a 15" hose, rental from Autozone. So not sure how accurate? 

ITB's so when I use a airflow synchrometer (in the horn), cylinder # 4 highest comp, lowest airflow reading.  

 

[Zed Head]

You said old rebuild in your first post.  Were you expecting better?  Sounds like an old used 280Z engine.

These engines will continue to run well even as they burn lots of oil.  What are you trying to determine?

I'd pull the valve cover and check the timing chain for wear (notch and groove), the oil spray bar for damage (if it has one), and the rocker arms and cam lobes for wear.  While you're there check and adjust valve lash.

[Blitzed]

This started with loss of radiator fluid. Not running a closed system or overflow tank so not sure if it's just evaporating or steaming out the cap tube. Not a lot but I'm topping off more than usual. Nothing visible on the block, water pump, hoses or floor. So I'm checking for a blown head gasket. Engine runs fine. 

Next question is it steaming or white smoke out the exhaust, yes, cold starts only for a few minutes. once up to temp gone and restarts at temp no white smoke. California climate, it's not very cold, 50's. 

I've run every test on the radiator, pressure test, holds 15lbs for 24 hours. Chemical carbon dioxide test at the radiator cap, negative, stayed blue.  Put the pressure tester back on and put 10lbs in the system, rev'd the engine to 6K rpm, no change in fluid pressure. Even checked the oil with the sizzle test. Heated a piece of tin and dropped engine oil from the dip stick on the tin, just smoked like oil should, if it's sizzles and crackles there is coolant present. All negative. No milky substance in the radiator fluid or oil, all is clean. 

So moved to compression testing, could be chasing my tail. 

The valve cover will come off to inspect and change the valve stem seals. I know, not a fun job. Will inspect all other components while preforming the service and retorque the head bolts. 

 

[grannyknot]

5 hours ago, Blitzed said:

This started with loss of radiator fluid. Not running a closed system or overflow tank so not sure if it's just evaporating or steaming out the cap tube. Not a lot but I'm topping off more than usual.

 

Back in the 70's a water hose and air hose were built into the gas pump island, adding water to your rad was done when you checked the oil level and was considered routine maintenance.  There may be no leaks in your system, a lot of Z owners just add a catch bottle for regular driving and remove it for car shows.

[Namerow]

1 hour ago, grannyknot said:

Back in the 70's a water hose and air hose were built into the gas pump island

An the compressed air was free! 

[Patcon]

Radiator cap could also be weak. Not holding proper system pressure

[Zed Head]

The original high miles engine in my car used to leak coolant from the right rear area of the head/block interface.  It only leaked when cold and sealed up after the engine warmed up.  Took me a while to figure out since there was no steam or odor from hot coolant boiling away.  I noticed some staining from where it flowed after escaping.

It was a bad headgasket but the bad part was only a coolant sealing area.  The port to the heater core is back there also.

[Blitzed]

Thanks all,

These engines can send you down a trail. Will report back on the head bolt retorque and the new valve stem seal maintenance.

Will check the seal on the radiator cap but I've put to bed any external coolant leaks with the cooling system with all previous testing preformed.

Another internal coolant leak test preform. Parked the car on my driveway incline (engine elevated) with a large coolant bleeder sealed in the cap neck and the bleeder tank full let the engine idle for 20 mins, opening the heater core valve. After the thermostat opened (140) rev'd the engine and idled for another 10 mins (160) closed the heater valve and rev'd again, zero bubbles or air escaping into the bleeder tank. 

 

[Zed Head]

I wrote a bunch of stuff but will just say instead that idle and pressure tests in the driveway might not be replicating the loss conditions of driving.  Loads and temperatures are different.