How to tell which Z20 engine in my 85 720?

[NissanEgg]

I have a base model 85 720 truck that is supposed to have the Z24S.  A few weeks ago I replaced the cracked cast iron exhaust manifold with a Pacesetter header.  While doing that job I cleaned off that side of the engine and realized the engine has been swapped out for a Z20.  Current engine mods are new Weber 32/36DGEV, new Pacesetter header, full EGR delete, ignition timing set at 8* btdc, full tune-up using Nissan spark plugs/both fuel filters/cap and rotor/NGK wires and valves adjusted as per the FSM.  Carb is adjusted as per Weber instructions and idles at 750 rpm.  I have a nasty stumble if you slam the throttle open when the secondary opens, but clears right up and pulls hard to redline.  Now if you gently roll into the throttle it does not stumble when the secondary opens and still pulls hard.  I think my carb is over-jetted for the Z20?  I bought it new thinking I had a Z24S engine.  Is the carb jetted wrong for the Z20S?  I bought it new from Redline Weber.

 

Also recently at hot idle I have a light random noise coming from some where in the lower part of the engine.  But I cannot determine if it is chain or crank/rod/piston related noise.  Raising the rpms above 900 will make the noise go away.  Recently at idle the engine has developed a roughness, and seems to get worse with mileage.  Again, raising it above 900 and the roughness goes away.  No vacuum leaks found.  No oil/coolant loss or smoke, and mpgs are still pretty good (about mid 20s).

 

My concern is that the engine is in need of a rebuild in the near future.  I have been reading that there are differences between the Z20E and Z20S, mainly piston/rod combo and heads. 

 

Without tearing the engine apart, how do I determine if someone swapped in a Z20E or Z20S?

Will the casting numbers on the head/block tell if it is a efi or carbed engine?

I would like to know so I can figure out what I have and what compression ratio I have.  I would prefer the higher CR for power.

 

I need to know so I can plan ahead for a complete rebuild.  I also plan on throwing in a reground cam from Comp or others in the future too.  Also plan on installing a wideband o2 sensor to keep things in check.

 

Any help will be greatly appreciated.  Also I am a certified senior Nissan tech and have covered all the basics with this truck.

 

 

Or scrap all this and install a fresh rebuilt Z24S like it is supposed to have?

[datzenmike]

First of all get your timing back where it belongs about 5 degrees on the Z20. Running without EGR increases the chance of pinging. There's no gain and everything to loose running it that advanced. If by some chance you have the 720 Z20 motor the compression is 9 to one!!!! Even more reason to run the correct advance.

 

As for the random noise, change your oil and always the filter. Run 15W40 it's slightly thicker and will up the oil pressure without having to raise the idle to 900.

 

 

 

Check the engine tag under the pass side hood hinge on the inner fender. See if it says Z24 or Z20 on it. If it says Z20 the differential ratio will also be 3.364 and you have a Mileage Option 720

[NissanEgg]

Truck is a Z24S standard model, regular cab short bed with bench seat and rubber floor mat and no center console.  I have the original window for this truck as well and it says Z24S with 3.70 rear end.  The US built trucks do not have the chassis tag you speak of, only the Japanese built trucks with VIN starting with JN6.  Mine starts with 1N6.

 

1985 MPG model was only offered in regular cab long bed and has a knock sensor on the block.  This according to the FSM and Nissan FAST software.

 

I currently run 10W40 with genuine Nissan oil filter.

 

EGR doesn't reduce chance of pinging, it's sole purpose is to lower NOx emissions.  I had to remove it as the BPT and valve were screwed beyond repair and was leaking at idle causing massive stumbling and stalling.  Removing that made the engine run better.

 

I haven't heard any pinging yet and run 91 octane.  I am well aware of what pinging is as I have a 86 300ZX Turbo completely built that pings from time to time under long boost pulls in 4th/5th gear.   I still may back off the timing alittle. 

 

Also verified the vacuum advance works properly with and without vacuum applied to actuator.

 

Also this noise has been there since I got the truck when it still had the out of whack Hitachi carb and broken exhaust manifold.  Also when I got this truck the timing was set at -1* of timing and was a slug.

[datzenmike]

Truck is a Z24S standard model, regular cab short bed with bench seat and rubber floor mat and no center console.  I have the original window for this truck as well and it says Z24S with 3.70 rear end.  The US built trucks do not have the chassis tag you speak of, only the Japanese built trucks with VIN starting with JN6.  Mine starts with 1N6.

All N American 720 have a credit card size tag on the right inner fender below the hood hinge that lists (among other things) the engine/trans and diff ratios.

 

Looks like JN6 to me...

 

 

 

1985 MPG model was only offered in regular cab long bed and has a knock sensor on the block.  This according to the FSM and Nissan FAST software.

My FSM and Nissan info shows the Z20 as available from November '82 through '86.5 on Non California short wheelbase and King Cabs.

 

 

EGR doesn't reduce chance of pinging, it's sole purpose is to lower NOx emissions.  I had to remove it as the BPT and valve were screwed beyond repair and was leaking at idle causing massive stumbling and stalling.  Removing that made the engine run better.

It reduces NOx by adding inert exhaust to lower peak cylinder temperatures. Since heat is the cause of pre ignition it will lower the chance of pinging although this is not it's intended use.

 

 

 

I haven't heard any pinging yet and run 91 octane.  I am well aware of what pinging is as I have a 86 300ZX Turbo completely built that pings from time to time under long boost pulls in 4th/5th gear.   I still may back off the timing alittle.

 

That's a problem with pinging ... is can be light enough that you can't hear it. The damage takes longer to accumulate.

 

 

 

Also this noise has been there since I got the truck when it still had the out of whack Hitachi carb and broken exhaust manifold.  Also when I got this truck the timing was set at -1* of timing and was a slug.

 

The factory suggests 5 degrees + - 2 degrees for altitude and variances in fuel octane.

 

 

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