The situation when the engine ZMZ 402 refuses to start, familiar to many owners of Volga, Gazelle and UAZ cars. This time-tested engine is famous for its simplicity and maintainability, but its carburetor system and contact or contactless ignition require regular attention. If you are faced with the fact that the starter turns, but the engine is silent, there is no need to panic - most often the problem lies in a simple lack of spark or fuel.
Before opening the hood and grabbing the tools, it is necessary to conduct an initial analysis of the symptoms. The behavior of the engine when trying to start can tell an experienced technician about 80% of possible malfunctions. The key point is the nature of the rotation of the crankshaft: does it try to grab and stall, or does it just spin limply. In the first case, we are looking for a problem in the fuel supply system or ignition settings, in the second - in compression or mechanical damage.
For successful diagnosis, you will need a minimum set of tools: a spark plug wrench, screwdrivers, a tester (multimeter) and, possibly, spare spark plugs. Remember to follow safety precautions when working with flammable liquids and high voltage. The correct sequence of actions will save you time and nerves, allowing you to quickly return your car to service without unnecessary trips to a car service center.
Diagnosis of the ignition system and lack of spark
The most common reason why ZMZ 402 It won't start, it's in the ignition system. If the spark is weak or absent altogether, ignition of the fuel-air mixture in the cylinders becomes impossible. First of all, you need to check the ignition coil and the center wire. Remove the wire from the distributor cap, bring it to ground at a distance of 5-7 mm and turn it with the starter. If the spark is bright, blue and breaks through the gap with a characteristic crack, everything is in order with the coil and commutator.
If there is no spark or it is barely noticeable, the problem may be a breakdown of the high-voltage wire, distributor cap or slider. In damp weather, microcracks often form on old covers, through which current flows to the body. It is also worth checking the gap in the contacts of the breaker (if you have a contact system) or the serviceability of the Hall sensor (in a contactless system BSZ). An incorrect contact gap leads to a violation of the ignition timing, which is critical for starting.
- 🔌 Check the tightness of all high-voltage wires in the sockets of the distributor cover - oxidation of contacts is a frequent enemy.
- ⚡ Inspect the coal in the center of the distributor slider: if it is worn out or burned out, contact with the central electrode will be unstable.
- 🧹 Make sure the breaker contacts are clean: carbon deposits or oil on them drastically reduce the spark energy.
⚠️ Attention: When checking the spark, never hold the wire with a rubber gloved hand or in a suspended position without a reliable grounding connection. This may damage the commutator or coil due to lack of discharge resistance.
Particular attention should be paid to the order of operation of the cylinders. On the engine ZMZ-402 it is 1-2-4-3. If you have recently removed the distributor cover or changed the wires, an error in their connection is guaranteed to prevent the engine from starting. Visually check the alignment of the marks on the crankshaft pulley and the runner when the piston of the first cylinder is at top dead center (TDC).
Problems with fuel supply and carburetor
The second most important factor affecting starting is the presence of gasoline in the cylinders. Carburetor versions ZMZ 402 (usually K-151 or Solex) are sensitive to fuel quality and filter condition. If the starter turns vigorously, there is a spark, but the engine does not “catch”, most likely it is “flooded” or, conversely, “dry”. Unscrew the spark plug: if it is wet and smells of gasoline, it means that fuel is supplied, but is not ignited (there is no spark or the phases are out of order). If the spark plug is dry, gasoline does not flow.
Start checking with the fuel pump. On a mechanical pump, when the engine is running or cranked by the starter, a stream should flow from the outlet fitting. Often the problem lies in the pump diaphragm or valves. Also, do not ignore the mesh sediment filter in front of the carburetor and the fuel tank itself, where water or dirt can accumulate. In winter, a common cause is freezing of condensate in the fuel lines.
| Symptom | Possible reason | Elimination method |
|---|---|---|
| The candle is dry, the smell of gasoline is weak | Nozzle or filter clogged | Blow out the system, replace filters |
| The candle is wet, black soot | Carburetor overflow, needle does not hold | Adjusting the level in the float chamber |
| The engine "sneezes" into the carburetor | Lean mixture or early ignition | Check the inlet tightness, angle |
| Unstable idle after starting | Air leak or dirt in XX | Adjusting quality and quantity screws |
How to check the tightness of the K-151 carburetor?
To check, remove the top of the carburetor and turn it over. If gasoline continues to drip from the needle hole after filling the chamber, then the shut-off needle is not holding pressure. This can often be resolved by replacing the needle or carefully grinding the seat, but sometimes the entire assembly needs to be replaced.
It is important to note the condition solenoid valve (EPHH) if it is installed on your carburetor. When the ignition is turned on, it should make a clear click. If the valve does not work, fuel will not flow through the idle system and it will be impossible to start a warm engine. In emergency mode, many motorists remove the valve needle, but this leads to increased fuel consumption and the inability to turn off the engine in the normal way.
Starter and electrical faults
When you turn the ignition key and there is silence or only a single click in response, the problem lies in the starting circuit. On vehicles with an engine ZMZ 402 The starter is often subjected to stress, and the contacts oxidize over time. If the starter turns sluggishly, first check the battery terminals and engine ground. Poor ground contact is the scourge of old cars, which simulates battery discharge.
Another common problem is the starter solenoid relay. If a loud click is heard when you turn the key, but the bendix does not extend and does not turn the engine, most likely the relay has “died” or the nickels inside have burned out. Sometimes gently tapping the relay body with the handle of a hammer helps, which allows you to get to the garage, but it is better to replace the assembly. It is also worth checking the condition of the flywheel teeth: if they are licked, the Bendix will slip with a characteristic crack.
- 🔋 Check the voltage at the battery terminals under load: it should not fall below 9-10 Volts.
- 🔧 Clean the ground contact between the body, engine and battery to a metallic shine.
- ⚙️ Inspect the flywheel crown through the inspection window at the gearbox or by removing the starter for chips and wear.
⚠️ Attention: If the starter hums but does not turn, immediately release the key. Prolonged operation of the starter in an inhibited state leads to melting of the commutator and a fire in the engine compartment.
Mechanical problems: compression and timing
If everything is in order with the electrics and fuel, but the ZMZ 402 engine still does not start, you will have to dig deeper. Compression is the pressure created by the piston at the end of the compression stroke. For normal starting, a gasoline engine needs at least 8-9 atmospheres in the cylinders. If the compression is low, the mixture does not warm up and does not ignite from a spark. It can be measured with a compression gauge screwed into the spark plug hole.
Low compression can be caused by worn piston rings, burnt-out valves, or a blown cylinder head gasket. On 402-series engines, warping of the “head” due to overheating is common, which leads to the breakthrough of gases into the cooling system or out. It is also worth checking the thermal clearances of the valves: if the clearances are off (especially “tight”), the valves may not close completely, which sharply reduces compression.
The timing drive deserves special attention. The ZMZ 402 uses a chain that stretches over time. If the chain jumps a tooth or two, the valve timing is disrupted and the engine stops starting or runs extremely unstable. Checking the marks on the camshaft and crankshaft gears will help eliminate this problem. In the worst case, the chain may break or the gear may break, which will require major repairs.
When checking compression, unscrew all spark plugs and turn off the fuel supply. Crank the engine with the starter until the reading on the pressure gauge stops increasing. For accurate results, the battery must be fully charged.
Influence of external factors and seasonal problems
The owner of the ZMZ 402 must take into account that this engine, like any carburetor unit, is sensitive to weather conditions. In winter, the main problem is condensation in the tank and freezing of the carburetor. Water that gets into the fuel system blocks the jets or freezes into ice, cutting off access to gasoline. Using special fuel dehydrators and keeping the tank full helps minimize this risk.
In summer, especially in the heat, a “vapor lock” effect may be observed. Gasoline boils in the float chamber, forming vapors that displace liquid fuel from the channels. The engine stalls and will not start until it cools down. In such cases, wiping the carburetor with a wet rag or installing a heat-insulating spacer between the carburetor and the intake manifold helps.
Also, do not forget about the condition of the air filter. A filter clogged with dust creates a too rich mixture (too much gasoline, not enough air), which makes starting difficult. In dusty road conditions, the filter element on Volgas and Gazelles requires frequent replacement or purging.
Algorithm of actions in case of emergency startup
If the engine does not start on the road and you need to leave urgently, you can use the “turning off unnecessary” method. If you suspect that the engine is “flooded” with gasoline, remove the central wire from the distributor cover and, holding it at a distance of 10-15 mm from ground, turn the starter for a few seconds. This will purge the cylinders of excess fuel. Then put the wire back in place and try starting the engine with the throttle fully open (pressing the gas pedal to the floor).
If you suspect a lack of spark, have a set of spark plugs and a slider in stock. Often replacing these consumables solves the problem instantly. If possible, check the ignition timing “by ear”: when driving at low speed, press the gas sharply. If a detonation ringing (“fingers knocking”) is heard, which quickly disappears, the ignition is normal. If there is no ringing - later, if it knocks loudly and for a long time - earlier.
The main cause of problems with starting the ZMZ 402 is complex wear and tear of the ignition system and contamination of the carburetor. Regular maintenance (replacing spark plugs, filters, checking clearances) prevents 90% of sudden failures.
Why does the ZMZ 402 stall immediately after starting?
Most often this is due to a malfunction of the carburetor idle system or excess air leaking through the intake manifold gaskets. It is also possible for the EPHH needle to get stuck. Check the tightness of the intake tract and the adjustment of the mixture quality screw.
Is it possible to start the ZMZ 402 from a pusher?
Yes, an engine with an overhead camshaft and a timing chain drive starts well from the pushrod, even if the battery is weak. However, on carburetor versions, this can lead to backfire in the carburetor if the timing or ignition angle is off.
How often do you need to change spark plugs on ZMZ 402?
The recommended interval for replacing spark plugs is every 15-20 thousand kilometers. However, when using high-quality fuel and a working ignition system, they can run up to 30 thousand km. A visual inspection of the color of the soot will tell you when it is time for replacement.
What to do if the cylinder head gasket is blown?
Operating a vehicle with a broken gasket is prohibited. This will lead to engine overheating, antifreeze getting into the oil (emulsion) and further deformation of the head. The gasket needs to be replaced and the head plane checked for deformation.