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1986 Nissan 720 King Cab with 2.4 Naps Z engine (Carb based) running rough (suddenly)


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Hi,

 

I have a 1986 Nissan 720 King Cab Pickup with a 2.4 Litre Naps - Z carburetor based engine. Just yesterday it made a real weird loud panging noise (Kind of metal and thumping (light) together that was noticeable on deceleration.

 

Please help me figure out what is wrong. Seems to have spark to all plugs, no apparent vacuum leaks (by spraying carburetor cleaner on carburetor, intake manifold and all apparent vacuum lines there is no change in rpm.)

 

The engine is extremely rough running and vibrates heavily at idle. It improves a bit around 1800 RPM but something is wrong. The load tapping noise on deceleration scares me. I have made no changes to timing for the past year or two.

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720s are notorious for blowing head gaskets every 100k miles. A compression test will likely show one cylinder or two adjacent cylinders with low compression. Do the compression test on the exhaust side plugs.

 

You may notice the rad water keeps going down, or there is white exhaust 'smoke'. The coolant recovery container may also over fill and leak out.

Edited by datzenmike
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Do a simple compression test and post the results. If compression is fine then it could be something else.

 

If you find a low cylinder squirt in some oil. Re test. If compression jumps up then the oil is sealing badly worn rings and this is the cause of your lost compression. I doubt this is it as they wear out slowly and yours made a sudden sound and started shaking at idle. I would expect a head gasket... but just in case...........

 

 

Be sure the valve(s) are closing by checking the valve clearances. If no clearance the rocker arm may be riding on the cam and holding the valve open letting any compression out past it.

 

If clearances are ok, the valve(s) may be burnt, bent, worn or otherwise damaged and not sealing. A leak down test can be preformed where compressed air is forced into the combustion chamber through the spark plug hole while the piston is at TDC (top dead center) with both valves closed. If you listen at the carb or the open exhaust manifold, air can be heard leaking past the bad valve. A damaged ring or piston will be heard through the vent hose on the top of the valve cover. It's not perfect but can often narrow down or eliminate some causes. Again valve damage happens slowly over some time and not like your problem. Suspect the head gasket.

 

If you are having this done by a mechanic don't let him talk you into an expensive valve job when all it is, is a simple head gasket problem.

Edited by datzenmike
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  • 3 weeks later...

most likely the headgasket blew between 3-4 cylinders... My 720 has blew 2 head gaskets in that exact same place.... I had my head sent out the first time and they machined it, then i got it allback together and it blew again in like 15k or something like that.. and the third time i just slapped a hg on and drove it..

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I did a compression test (Did not open the throttle at carburetor though)

and found cylinders one and two were 155 PSI, while cylinders three and four read only 50 to 55. Suggestions?

 

:confused:

 

sterlingstone@gmail.com

 

720s are notorious for blowing head gaskets every 100k miles. A compression test will likely show one cylinder or two adjacent cylinders with low compression.

 

Z24 h/g blown between 3 and 4.

http://i138.photobucket.com/albums/q251/datzenmike/L%20Z%20Heads%20and%20Motors/Z24hgblowout.jpg[/img]"]Z24hgblowout.jpg

 

Three and four are adjacent and both are low. Fits the description of a blown gasket between them. This is a cheap fix and can be done in an afternoon for the cost of a new gasket and some work.

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from what i have discovered about the NAPS-Z headgasket issue is that having the head resurfaced is pointless since the block decks distort in the z24 so badly. if you resurface the head usually the gasket will blow within the first 10k. If you dont surface the head it will usually form better to the bad block deck giving you as much as another 100k !!!!!!!! before it blows again.

Another thing i found is reusing the head bolts over again is good since they allready stretched, new head bolts will relax in time allowing it to blow again unless you retorque the bolts... but sometimes this will fatigue threads and upset the gaskeet too. Also the z24 HATES head gasket coating. Dont do it. Trust me....... the bottom line is with a z24 is keep it cheap and as simple as possible and you will be better off. Anyone able to confirm this?:P

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Can't fault any of that. NEVER NEVER use a coating on your head/block/gasket. This isn't 1950!!!

 

Get the head and block cleaner than clean and smooth as possible. Can't be too clean or smooth. Not necessarily flat but smooth. Because the block and head are dissimilar metals that expand and contract at different rates, being as smooth as possible reduces the 'grip' and lessens the side shear that slowly causes the gasket to fail. Nissan OEM gaskets are graphite laminar composite which also reduces internal failure. At $70 are a bit steep but arguably the best. The threads should be clean as the threaded holes. This will prevent a 'tight' bolt giving a proper torque value yet not clamping the head properly. Use the proper tightening sequence generally from the middle bolts working alternately outwards to the front and back. Torque to 1/3 the final value in sequence, then 2/3 final torque value and then to the final torque of about 60 ft lb for the Z24. Check after several warm ups and cool downs when engine is cold.

 

Additionally my FSM states to loosen each head bolt and re-torque it one at a time, in no special order, every tune up when engine is cold. No one I know does this but it is factory recommended and I see no harm in this practice.

 

Two other thing are to use good quality anti freeze year round and change every 2 years, and NEVER allow your motor to overheat.

Edited by datzenmike
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  • 4 years later...

    Hi Guys!!

     I'm a newcomer here. It looks as though I' not alone with my head gasket issues.

 

    Have had a 84 720 KC 4x4, since 1989 (45,000 Miles). In 2010 at 169,000 miles, seemingly out of nowhere, the head gasket blows between #1 & #2 cyls. Had the head milled with valve and guide service as well. Very carefully reassembled using Fel-Pro gaskets and Factory Repair Manual for specs. Adjusted the valves and retorqued the head bolts at 171,000 miles, then at 175,000 miles.              

     Now at 181,000 miles, after running magnificently for 3 years, the head gasket is blown between #1 & #2 cyls, and again, seemingly, out of nowhere. Drove to the grocery store running fine. 20 minutes later, it started with a loud rattle and severe miss.

     A compression test showed 75 psi for 1 & 2, 180 psi for 3 & 4. While disassembling, the head bolt torque did seem to be loose, though I did not test them for torque . 

    My questions regard what I found after disassembling the motor today. The gasket has a 3/8" gap "burned" away between 1 & 2 with an obvious hot spot between 2 & 3. The combustion chambers and piston tops for cyls #s 2, 3 and 4 look normal (slight carbon build up with brown exhaust valves). Cyl. #1, however, is clean of any deposits whatsoever (I mean dishwasher clean!), piston top as well. The port side of the #1 intake valve has what appears to be a very fine layer of "speckled" rust or a copper colored substance of some kind sprayed on to it. I see no "obvious" signs of coolant leakage into the port, nor has the car ever experienced an overheat condition.

     Has anyone else experienced these manifestations or offer an explanation?

     I fear I may have a cracked intake manifold, however , the port shows no sign of "washing" whatsoever.

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The Z24 is well known for blowing head gaskets every 100K. All aluminum heads expand from heat both faster and more than the cast steel block. The gasket material is tightly bound to both surfaces so there is a shearing action applied to the gasket material spread over many thousands of heating and cooling cycles. The Z24 is worse because of the metal used in the block and the poor clamping force of the head bolts. I've heard that the deck surface is thinner than previous Z series motors (the Z22 and Z20) also the holes and threads don't go deep enough.

 

One way or another Z24 owners are advised to to do the following from the Nissan factory service manual....

 

Loosen each head bolt and re-torque to 60 ft. lbs. Do not loosen more than one bolt at a time. You can do this in any order you like. The motor must be completely cold. Do this at every tune up.

 

 

This is a preventative measure that keeps the head firmly tight against the gasket. Most 720 owners don't know of this and even fewer second and third owners.

 

One other thing is the repeated blowing of your gasket so soon after changing. This may be from the head being slightly warped or lifted in the middle. Have it checked, it can have a small amount shaved off to true it up.

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  • 3 years later...

Thanks for the info Mike. I was lucky enough to find this site just after gettimg the truck. I have loosened and retightened every 35k to 50k. Prob not often enough though.

 

Before reassembly, i am having trouble locating the head bolts. Can anyone confirm wether or not the head bolts for a KA are the same?

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The Z24 head bolts are totally reusable if undamaged. KA bolts are torque to yield and even if the didy fit, are not reusable, you don't want them.

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There is of course a local place i found that will make any thing i want, but ive not called em to check lead time or pricing. I cant imagine it would cost more than the dealership @79$. Lead time surely not more than a few hours??m

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Z head bolts aren't really a torque to yeild bolt. They don't stretch the way newer bolts (KA) do. Its that stretch that makes the bolts unsafe to reuse.

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I doubt the KA and the Z series are the same length. KA are thinner and will stretch when torqued. It's sup[posed to be more accurate but I see it as having to replace something at your expense and someone else's profit when regular bolts used for years will do the job.

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  • 2 years later...

I know this is a very old thread but figured I'll ask anyway. I bought an 84 720 with a blown head gasket. It cranked over like a blown head gasket would but I didn't even think that it actually blew about two years ago and the owner just parked it. Took it apart and of course there's rust in the back two holes. I can't afford to get too far into this job money wise and i think I can get most of the rust to clean. I know it's the hokey way but right now it's about all I can do. I may end up with a bit of rough spot when I'm done but I think it might fly. Any thoughts? 

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