Engine Trouble Diagnosis

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Trouble shooting is only a process of elimination and provided the procedure is carried out correctly and systematically an accurate diagnosis of the trouble can be made in the minimum amount of time. These problem can be caused by many things. So there is no one simple answer to fix it. It will take some patience, and methodical approach to finding the cause. The fix will follow once the cause is understood. Don't get too frustrated -- if you find it overwhelming, take your 1200 to an experienced mechanic, or feel free to ask for specific advice in the Datsun1200.com main forum.

Overview

For an internal combustion engine to run there are three basis requirements, these are ignition, fuel and compression. There are other factors of course, but as a rule an engines failure to start can be attributed to a fault in one or more of these three systems.

The most likely cause of an engine wont start issue is an ignition or electrical failure first followed by fuel, with mechanical or compression failure the least common.

Before getting into ignition troubleshooting it is worth checking, is there fuel in the fuel tank? If yes begin the ignition/electrical diagnosis.

To see other Diagnoses pages (Electrical, fuel, etc) search for Diagnoses.

Engine Will Not Start

but cranks normally

possible cause -remedy

(a)dirty or corroded distributor points. -clean or renew and adjust points

(b)carburettor flooding -check needle valve and float, clean out fuel system

(c)moisture on high tension wire and/or inside distributer cap. -dry out high tension wires and cap

(d)dirt or water in carburettor or fuel system. -clean out carburettor and fuel system

(e)incorrectly set spark plug gaps. -reset spark plug gaps to specification

Verify spark is strong enough to jump in the cylinder. Rare failures will produce a spark when the plug is grounded outside the engine, but won't jump under higher cylinder pressures. A fat blue spark is a good indicator. A weak yellow spark is suspiscious.

(f)faulty coil or capicitor. -test and renew faulty components.

(g)faulty high or low tension wires. -test and renew faulty wires.

(h)fuel vapor lock. -check source of vapor lock and insulate against heat.

(i)faulty fuel pump. -test and overhaul fuel pump.

(j)incorrectly set ignition timing. -check and retime ignition.

(k)broken or short-cicuited low tension lead to distributer points. -test and renew lead.

weak or erratic cranking

(a)weak or faulty battery. -recharge or renew battery.

(b)fault in starter lead or solenoid. -test and renew faulty components.

(c)faulty starter. -test and overhaul starter.

Engine Stalls

• idling speed set to low: adjust idle speed stop screw
• idling mixture to rich or lean: adjust idling mixture screw and idling speed screw

The above are quick fixes. However, they do affect other parts of the engine, so if the problem is really somewhere else, you simply trade one problem (stalling) for another.

For stalling problems, it is best to do a complete tuneup, starting with (as usual for a yearly tuneup) a headbolt re-torque, valve adjustment, compression check, etc. etc.

Part of the tuneup should be a close inspection 
of the vacuum hoses and vacuum motors on the 
carburetor. Leaks here are common causes of
stalling

For details on Tune-up, see your Nissan Factory Service Manual, your Gregory's or other repair book, or for an overview see here: Tune-up

Engine Won't Fully Rev

A stock A12 revs easily to 6,000 RPM in first and second gear. And obviously will also rev in neutral to the same RPM. You may not have enough road to get to 6000 RPM in fourth as that is 140km (90mph).

Problem symptoms:

• revs OK in neutral
• 1st gear revs out to full rpm reasonably smooth
• 2nd gear revs to about 4000 rpm dies for a couple o seconds then revs to about 5500-6000 rpm. 3rd is about the same
• 4th at higher RPMs starts to surge a bit and when you put the foot down a little further and it start jerking

CAUSE: Fuel pressure out of spec (too low)

TROUBLESHOOTING to verify cause: Fuel pump pressure test

RESOLUTION: Replace or rebuild fuel pump. With electric fuel pumps it may be the wrong type. With turbos it generally needs to be ^POST 5psi above the boost pressure.