Mercedes sedans and wagons 1965-1985
1965-1985 Mercedes troubleshooting guide
These cars are very sensitive to air, even the smallest of air leaks will cause rough running and/or poor performance. Make sure all fuel system connections are perfectly sealed.
Problem: Lacks power, runs rough, stalls on vegetable oil and/or diesel.
*Before attempting to troubleshoot this or any other fuel system problem, verify vegetable oil filter and diesel filter are not clogged
*Verify all plumbing connections are correct and match manual exactly.
*Verify wiring is correct: Use a test light to observe that both A and B valves energize when on veg, Only B valve energizes on purge and no valves energize when on diesel. Also confirm that valves are grounded securely.
You can also test operation by listening/feeling the valves, they should produce an audible click when turned ON or OFF.
Air leaks: To find an airleak, a clear section of fuel line can be installed in various locations throughout the fuel system to determine the source of the leak. Run the engine to observe any air bubbles in the clear line, if necessary, move the section of clear line to other locations to determine the exact location of the leak. A vacuum/pressure gun (Mityvac or cylindrical type suction gun) can be handy to apply pressure or vacuum to fuel system to help find the source of an air leak.
1) Check all hose connections, especially connection at inlet to lift pump
2) Check primer pump. These pumps are prone to leakage and rely on the seal under the knob to seal out air, these seals get worn from repeated screwing down of the knob. Replace primer pump if it is old or leaking.
3) Replace all copper seal rings at banjo fittings, they are often damaged from repeated use, do not overlook the seal ring and O rings found on the hold down bolt for diesel filter, this area of the filter has vacuum applied while on diesel and is a common airleak..
4) Verify solonoid valve stems are tight. These are the parts in the center of valve which screw into the body. If the stems requires tightening, a spanner wrench is the appropriate tool. If no spanner wrench is available, you can lock two nuts together at the top of the stem and then tighten with wrench. Do not use pliars to tighten stem as it is easily dented and will cause valve to jam.
5) Inspect injector return lines for leaks, replace as necessary. These are often in poor condition and will cause air in system when on vegetable oil due to the presence of vacuum on these lines while on veg with the looped fuel return.
6) Verify pex line in tank is submerged and is secured to bottom of tank.
7) Verify drain screw on filter is secure.
8) Verify filter is tightened securely and seal is intact.
9) Verify pressure valve at return banjo on IP is not stuck or spring damaged.
10) Leaking fuel hoses that connect to diesel tank. These lines are usually in poor condition and tend to leak, it is possible that air drawn through these hoses can enter the veg system as well. Air leaks on diesel are not as noticeable since return to tank purges air, air in the veg side will be more noticeable since air cannot escape to tank.
Restrictions/excessive vacuum: To check for restriction, use a vacuum gauge to monitor vacuum while engine is running, preferably under load. Install the vacuum gauge at the inlet of lift pump, if vacuum exceeds 14”, a high vacuum condition exists and can cause poor performance, air leaks and decreased filter life. A vacuum gun (mityvac) or suction gun can be used to apply vacuum or pressure to help diagnose restrictions and to clear any obstructions.
1) Clogged filter, change filter and inspect contents. Poor filter life can usually be attributed to inadequete filtration, poor oil quality or the presence of algea contamination in fuel system.
2) Tank not venting, verify vent in fuel cap is not plugged. To test for this leave cap loose or remove a screw from hatch plate. Also verify diesel cap is venting, the diesel cap must vent well since when purging, the volume of fuel displaced from tank is increased due to return fuel being sent to veg loop. This can cause increased vacuum in diesel tank at shutdown and may cause fuel to be drawn back to tank (loss of prime)
3) Debris in tank obstructing pex line or covering inlet of pex line, inspect tank and pex line for debris.
4) Kinked fuel hose or pex line, excessively long lengths of fuel hose or pex line.
5) Obstruction in valve, verify that valve flows freely and seals in both positions.
Problem: Vegetable oil tank filling with diesel:
1) Left on purge cycle
2) B (return valve) stuck in ON position when on diesel, sending returned diesel to veg tank
3) A (supply valve) not energizing when switched to veg or has poor ground, causing diesel to return to veg tank
Problem: Diesel tank filling with vegetable oil
1) Not purging long enough or frequently switching between fuels.
2) B (return) valve not energizing or has poor ground, leaving it in N.O. Position
3) A (supply) valve stuck in ON position
Problem: Filter and tank not getting warm, coolant not flowing
1) Coolant hoses air locked, try filling hoses with coolant and then driving vehicle for approximately 20 minutes. Monitor coolant level and add as necessary, as air is displaced, coolant level will drop.
2) Coolant hoses kinked or routed excessively high.
3) Coolant hoses not plumbed correctly. Supply coolant must be plumbed into heater supply hose from Cyl head, return coolant hose must return to water pump.
Problem: Low vegetable oil temps (MBZ's tend not to heat the vegetable oil as much as other cars. These cars are generally fine with veg temps in the 120F range, the important thing is that the car is started on diesel or they can be very rough and smoky when started, even in warmer weather).
1) Verify meter is accurate, double check with thermometer or another meter. Verify that meter is wired correctly and is grounded well.
2) Incorrect vegetable oil filter installed and does not fit in coil securely, causing poor heat transfer. If a crossover filter is used, use a strip of aluminum flashing or foil tape around the filter so that it fits in the coil securely.
3) Low coolant temps. Verify coolant is approximately 190F, if running to cool, replace thermostat. In cold weather, MBZ's may require that the radiator is blocked enough to maintain heat.
4) Filter or hoses exposed to direct wind, insulate as necessary and avoid excessive hose length.
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