Diesel CAV Troubles

CAV fuel pumps give an endless amount of trouble. Problems with idle speed, speed oscillation, stalling are among these typical failures. In other cases, the engine might idle normally but it gets stuck at higher or lower idle revolutions when the accelerator is first pressed and released. Or it can get stuck at a certain speed over idle regardless of accelerator position.

The most usual causes are:

• Incorrect idle speed adjustment
  The engine revs up normally on accelerator command but takes long to rev down, or even gets temporairly stuck on a higher idle, then drops to normal. Idle oscillates slowly. Engine accelerates normally when accelerator is pressed, decelerates rapidly it is released and appears to stall shortly before picking up idle speed again, or even stalls completely, but restarts normally after cranking. Engine seems to run normally (possibly with slightly high idle) but cannot be turned off by rotating the manual shut-off lever on the pump.
• Air leaking into fuel uptake
  Engine starts but stalls after a few seconds and cannot be started easily (needs priming, low resistance to pressing the priming button indicates that there is air in the uptake).
• Plugged up filter
  Priming button on the filter is sucked in.
• Water in the fuel
  Occasional (but not constant) white smoke puffs from the exhaust when engine seems to stall temporarily or run roughly.
• Fuel leaking out of certain parts of the pump
  A leak from seal on the pump shaft can be difficult to check for, this is one of the things that should be checked when you replace the cambelt. A leak from the turbo pressure compensator assembly causes sluggish turbo or no turbo response, low turbo pressure until higher revolutions are reached. A leak from idle setting / cold idle / manual shut-off lever assembly shaft makes it difficult to set the (cold) idle speed. A leak from the electric run/stop valve can cause a failure to start, uneven idling or sudden stalls. A leak from the cold idle timing advance solenoid will result in rough idle, slightly higher noise and higher fuel consumption at normal operating temperature.
• Stuck or worn auxilliary parts
  The engine starts but stops after a short while, priming shows that there is fuel in the pump and under pressure (the button is out, it barely moves or doesn't move at all when it's pressed). The pump return one-way ball valve is stuck, mostly because of corrosion. It is ball and spring assembly located inside the hollow screw that holds the return pipe on top of the pump case. If there is no difference between cold and warm idle speeds: check the paraffine element on the engine. Check the electric contacts of the cold idle timing advance solenoid as well.
• Worn out pump
  If you have eliminated all other problems and the idle still gets stuck or the engine runs away, the pump produces subharmonic noise or clicking sounds, you might have a worn pump. The engine could run hot (even with the cooling system functioning perfectly), its power could be low even in spite of the injectors, cylinder compression and turbo working fine.

A combination of the mentioned symptoms is also possible, making diagnosing difficult. The complete troubleshooting procedure is as follows:

Give the pump an independent supply of clean fuel. The best way is to use a small canister filled up using the priming pump to pass the fuel through the filter. Connect the canister to the uptake and return outlets of the pump with transparent hoses. Position the canister higher than the pump in order to be able to syphon some fuel. Start the engine without pressing on the accelerator pedal (this might prove difficult until the air passes through, watch out for bubbles in the hoses). If the engine runs evenly without you touching the various control levers or even better, the idle lowers by itself as the engine temperature goes up (provided the paraffine element on the engine is working), start looking for an air leak into the uptake line.

It is best to get rid of any air leaks before proceeding any further as this will disturbe or even hinder the accurate setting of the idle speed. You can replace other parts of the tubing with transparent hoses so that you can look for bubbles in the fuel supply.

Loosen or disconnect the accelerator cable. If the engine stalls immediately, the idle adjust is misaligned (see below). Unscrew the screw that dictates the minimum position of the speed lever (this is the one the accelerator cable attaches to). If the idle speed drops or the engine stalls, the idle is misadjusted. Keep unscrewing the screw until it no longer touches the speed lever. Loosen the cold idling speed adjust cable going from the paraffine element on the engine to the pump.

The idle speed can be adjusted by the relative position of the manual stop and cold idle levers: these two small levers are mounted on a coaxial shaft just to the left of the main speed lever. There is a screw adjusting the maximum distance between these two. The larger the distance, the lower the idle speed. Thus, decrease the distance until you can start the engine normally and the idle speed is within the specification. If the car has air-conditioning fitted, remember to adjust the idle speed with the A/C on as mechanically controlled pumps (like the CAV ones) will always have the idle higher when the A/C is off. As the idle speed needs to be adjusted at normal engine temperature, you might need to readjust as the engine heats up.

Remove the speed lever damper if there is one. Move the speed lever slowly, the speed will not increase immediately. Keep the lever at the point where the engine speed just starts to increase. Then turn back the screw that determines the default position of the speed lever to obtain a 5 to 8 mm gap between this default and the position where the speed started to rise.

You should never increase the idle speed by adjusting the speed lever default position. The CAV pump has two different mechanisms for speed regulation, and without this initial free play, the two actions will overlap causing speed oscillations. Increasing this gap makes the engine respond more readily to the releasing of the accelerator pedal but slower to the pedal being pressed because of the added movement needed to go through the increased free play. Decreasing it has the opposite effect. Besides, increasing it past a certain limit will actually make the engine stall temporarily before the separate idling mechanism might take over, finally making it impossible to start the engine without pressing the pedal down. If the gap is too slow, the speed regulation mechanisms will contend resulting in the idle speed sticking, or oscillating. It may oscillate so largely that the engine dies on the bottom of the oscillation, unless you rev it up with the throttle.

Reattach the accelerator cable making sure that the adjustment clip allows the speed lever on the pump to return to the adjusted position. It may be necessary to check the attachments and the holders of the cable to reduce any unnecessary free play and unwanted oscillations on acceleration-deceleration as described above.

Reattach the cold idle speed cable but do not put any tension to it. Wait for the engine to cool down completely, preferably leaving it overnight, making the final adjustment early in the morning or on a generally cold day. Pulling on the cable decreases the angle between the manual turn off and idle speed levers on the pump, thereby rising the idle speed on a cold engine. The mechanism also has an electrical contact attached to it controlling the cold idle timing advance solenoid. Ideally, this should disengage before the idle speed drops to normal. The only way to adjust it is to extend or compress the spring under the contact. Make sure that the timing advance does not remain engaged unnecessarily.

Pull the two levers together until the desired idle speed is reached, then position the adjustable stop on the cable as close as you can to the levers. It will be difficult to get it right so you'll probably need several tries. If you find that the engine idles too high or for too long, you can carefully loosen the cable a bit. Idle speed and length of time involves a compromise, trial and error is the key.

Željko NASTASIĆ