My 720

[living_4kicks]

Bought this truck from a kid after my 86 broke down. I am having problems with it though. Ran good for a while then changed plugs wires distributor and put on a Weber carb. Ran okay had a high idle, that I couldn't get to slow down. One day it died, I haven't been able to get it to run since. It turns over sounds like it's only hitting on one or two cylinders. When I get heat in my shop I will do a compression test. Anyone else have any ideas?

[Draker]

Check the basics... check timing, check plug order, check spark. Check both coils are functioning..

[Draker]

Make sure you are getting fuel to the carb also.

[datzenmike]

New Redline weber? or some used unknown POS? Carbs are a pain to trouble shoot if used and unknown. Rebuilt 5 times,  (or needs rebuild) parts missing, adjustments and jets all wrong....

 

There are 4 intake and 4 exhaust plug wires... easy to get them mixed up. Check them. If it ran fine after the plug and wire change just ignore this.

 

Also confirm when starting that you have spark on both sides.

 

Take fuel line off carb and direct into a pail. Turn key on.... got gas? Electric pump should supply gas and then time out and shut off until ignition turned off and then on again. No gas??? Is the pump running? check the fuse. It's the #5 over to the right from the left hand side.

 

 

 

 

The most common cause of sudden non running Z24 is a blown head gasket. The Z24 is problematic for this. It's easy to prevent, but most owners are third or forth and have never cared to look after their trucks. They usually blow at about 100K. A compression test might show a low cylinder or two adjacent low cylinders but they may be low from other than a blown gasket.

[Charlie69]

Mike on a 720 the electric fuel pump does not run until you engage the starter or you have oil pressure.

[datzenmike]

I know it's somewhat like a room lamp timer. Certain inputs keep it running. The point of the relay is to shut it off if there is no alternator charge OR no oil pressure ONLY if the engine is ON but not running..... like after an accident.

 

I have the FSN diagram but have always assumed the pump relay would shut off shortly after starting if the oil pressure was low. or loss of alternator charge... but the diagram shows this is NOT the case. 

 

 

Yes only during START and RUNNING under any combination of charge/no charge and/or oil pressure/no oil pressure the pump keeps pumping. Here I thought the loss of oil pressure would shut it off to save the engine.... guess not.

[Charlie69]

On the 85 ST I put several fuel pumps on it until I realized that I had to to the ignition switch barely into the spring loaded start area before the pump actually comes on.  On the 85 ST I can do this without engaging the starter to check the pump.  I have not tried this on my 86 720 King Cab.

[datzenmike]

There is a lot of mystery surrounding the pump relay. The '84 FSM is as I have previously described it but if I was designing it, I would have it come on with the key in ON for a timed 10 seconds to top up the carb and to let you know the pump was working. If you do not try to start the engine, it stops in 10 seconds. IF the engine was cranked the pump would keep pumping until 10 seconds after you stop cranking OR if started and you had oil pressure the pump would keep running. If for any reason there was no oil pressure (either out of oil or engine stalled, possibly an accident) the pump would shut off in 10 seconds by the timer.

 

This is too simple for Nissan.

[Charlie69]

Mike I do not know that they way mine works is correct or not.  I thought when I first purchased my truck that the starting problem I was having was the fuel pump relay.  The problem was intermittent, some would crank and start fine and other times the fuel pump relay would click but the starter would not engage (crank).  I finalyy diagnosed this problem as the ignition switch itself.  This problem went away when I replaced just the electrical ignition switch.