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Recently bought a 1985 720 ST Z24, has a coolant leak and smokes like a chimney. I got to pulling the head off today to see whats up. No doubt the head gasket is blown, the oil has mixed with coolant so much its become thick like molasses. What has me scratching my head is it seems cylinder 3 is the only one affected by coolant making its way into the combustion chamber. The intake valve on #3 is covered in coolant, the chamber clearly has been burning coolant by looking at the head, the spark plugs are fouled. But this chamber seems to be the only one compromised. I can see coolant passages running through the intake directly under the runners, though I'm not too sure on how its piped. I'm guessing Nissan designed the intake this way in an attempt to raise the intake air temp to promote a more complete combustion in an effort to reduce emissions? I see 2 pipes coming off the radiator, lower one to the water pump which is attached to the block, upper one to the thermostat which is attached to the intake, and then a coolant pipe coming strait from the block to the center of the intake. This has got to be the strangest design I've seen so far. Any experienced veterans care to fill me in on what Nissan was thinking I appreciate it 🙂




Cylinder 3 intake runner, the others were dry. The hose directly in between 2&3 is attached to the coolant pipe coming off the block.







Ive got big plans for this truck, its got a bright future 😎

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Water comes out of the back right side of the block and into the cab for the heater. Back out of the cab and forward to under the intake below the carb wher all the water from the intake runners merge with it and then piped down and forward to the lower return rad hose and back into the engine block. The coolant in the intake just warms the air to a constant temperature. The engine runs more predictably/efficiently if the air is held at a more constant temperature summer or winter.

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Got it put back together. Tried starting it without the ignition coil fuse to get the new oil filter pumped up, and to my surprise it started up and ran for a few seconds before dying. I gave it another shot and it started no problem, no more coolant out the exhaust. Although it does idle quite high and rich on cold, smokes white out the exhaust but it just smells like fuel. It starts up at about 2Krpm and quickly goes down to about 1Krpm then immediately goes back up to 2K and stays there. It even idled at 3K one of the times. I remember starting it before I bought it and it also had a fast idle about 2-3K, so i doubt I introduced any issues when I took the head off. Besides I didn't mess with the carb, I left the entire intake inside the engine bay supported by a plank of wood. I will try adjusting the fast idle screw tomorrow, any ideas what else I should try?

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Well, I just read up on what the fast idle rpm should be and it’s 2600-3000rpm for the z24 according to the FSM. So this seems normal, although it does seem excessive. I would imagine the necessary cold start rpm being 1200~. I will try bringing the fast idle screw down to the 3rd cam. Anyone have any info on what the hot idle rpm should be on the z24? I could do some digging and find it tho I’m about to pass out

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The fast idle setting is a measurement of the opening gap on the primary throttle plate when the stop is on the second step on the cam. For the Z24 this is 0.028" to 0.033". No idle speed is shown in my FSM from Nissan. 2,600-3,000 is too high and could damage an automatic placed in gear. Earlier trucks like the 620 specified 1,800 to 2,200 RPM which is much more sensible.


Set the fast idle cam stop on the second highest step #2. The set screw can be unlocked and adjusted. Unless someone messed with it it should be the same as set by the factory. With it on the #2 step measure the throttle plate opening as being between 0.028' and 0.033".


Here's the set screw with the lock nut, but like I said it shouldn't need adjusting...




Warm idle speed of 750-850 is fine.


Idle won't lower?


Make sure throttle cable is loose enough to not hold the throttle open.

Make sure idle speed screw is turned out enough to lower idle speed.

Make sure the choke is off and the fast idle cam not engaged.

Make sure the secondary throttle plate is fully closed and not hanging up.

Make sure the primary throttle plate is not catching on the bore and not returning fully.

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I checked out the choke and it seemed stuck closed. When I would budge the throttle it would open. I greased up the fast idle/choke link and it seems to work now. Adjusted the idle screw and idling at 1K. The smoke blowing out the exhaust was coolant left over from the blown head gasket, has been running for an hour and cleaned up quite well. Still smokes a bit, not sure if its coolant burning off or a rich mixture. Engine sounds healthy at idle. Will drive it soon to heat up the exhaust burning all coolant off

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I drove it for a while and the exhaust cleaned up. I adjusted the idle to be about 1K and its running great.




I threw on a set of 17” 245/65. Had to bang out the front wheel wells for clearance, they really do fill up the empty space. A few rubs here and there but nothing a ball peen hammer cant fix. Ordered 25mm spacers for the fronts to clear the steering rack and bring the poke out to match the rears.



Edited by TommyBsolid
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