Fix Zetec Idle

Now the Zetec is a pretty reliable engine and can last well in excess of 150,000 miles. But if there is one thing that crops up time and time again it is a poor idle, that can be so bad as to stall the car. Now obviously it didn''t do this when new so its not something you should put up with. What makes it harder to diagnose is that I have found a large number of things that can affect the idle. Some are common, some not so common. Here I will try to list all that I know off, in order of the commonest to least common, symptoms, tests to do and costs to replace.

Bare in mind that all 16 valve engines tend to have a rougher idle than 8 valve equivalents. Also the EEC unit on the 16v''s values low emissions over good running so even on a perfect car, there will be some unevenness of idle as the computer hunts for  the lowest emissions and occasionally goes too far and has to increase the revs to compensate.

Also this is a large list. Don''t for one minute think that the fiesta has a load of faults, quite the opposite in fact. But with so many millions sold there will always be some that have problems and of course if you play with yours as much as I have you put extra stresses on the lump so things do occasionally go wrong. this list tries to list everything that could be an issue rather than say you will have these problems if  you have a Fiesta

HT Leads

These commonly break down. Often showing as a slight stutter when Idling. or a misfire at high load. Best diagnostic is to swap for a known good set.

The Zetec has a very powerful (and reliable) wasted spark, capacitor discharge coil. Due to the large plug gap Ford specify this really has to crank up the current to fire the plugs. This puts a lot of stress  on the HT leads and they often go. I have eaten several cheap sets in less than 10,000 miles. Setting the plug gap smaller than the 1.3mm standard can help and I personally use a 1.1mm gap now, Down to 1mm seems to work fine and can give a smoother idle.


Not normally a problem if you use good plugs. I have used NGK, Boschand Motorcraft (Fords) with no problems. If however you have used Champion plugs that''s your problem right away. You can also try gaping them a little less than recommended. I gap mine at 1.1mm rather than the recommended 1.3. I find the HT leads last longer and the engine seems smoother at idle. Always change plugs and leads as a set as a problem in one can damage the other.

Rubber L-Piece

Hiding underneath your coil is the a vulnerable part of the Zetex and many people miss it. It is a 5cm Rubber L piece that connects the engine block breather to the inlet air system. Due to the movement of these pieces it often splits and lets out vacuum, resulting in a bad idle and or stalling.r. To remove this stick your hand under the coil (towards the rear about 5cm below the coil base), feel for the rubber bit and pull firmly. It takes about 20 seconds to swap this

Crankcase breather valve

Designed to allow excess crankcase pressure to escape it causes a bad idle when partially blocked and will stall the engine when completely blocked. Located on the front right hand side of the breather.

Ford say this should be changed at 30,000 mile intervals but don''t list it in the service guides (nor do Haynes), mine failed at 110k. It is a pig to change as it is hidden behind the exhaust manifold. Symptoms of this blocking are a good idle when cold, getting worse as you warm up leading to complete stalling at working temps. The engine seemed to pulse and hunt with a one second frequency between 1500 and 700 rpm

PAS pressure switch

Designed to increase the idle speed when you turn the wheel (hence increasing the load on the engine)

Its located on the top of the engine on the power steering pipe that runs across the rocker cover and the wires for it often rub on the bonnet and break. Then of course when you turn the wheel, the load increases but not the revs to compensate and you may stall. To test the wires just plug and unplug the switch while the engine is running and the steering is turned. The engine should change idle speed and may stall.

Vacuum Leak
If any are leaking or damaged, the vacuum will drop and the idle will be terrible. Theres a few vacuum lines on the back of the manifold., best bet is too replace these with silicone items or the like.

Idle Speed Contributing Valve

This allows small variations in idle speed to be "smoothed out". It works by varying the amount of air induced into the engine via an auxiliary air passage in the throttle body housing. It is controlled by a signal from the EEC module.

The Idle speed control valve is open to the Inlet manifold and so can get contaminated and gummed up. Taking it off and giving it a good soak in Throttle Body cleaner is a good idea and cant hurt. If this doesn''t solve it, buy a new one!

Throttle Position Sensor
This provides the EEC IV module with data regarding the position of the throttle plate (i.e. how far open or closed). It consists of a potentiometer mounted on the throttle shaft.

To test, use a multimeter and while watching the resistance, gently rotate the inner spindle (don''t force it), you should get a nice gentle sweep of resistance. Any spike or anomaly''s and its dead. Unfortunately the place its most likely to wear is the idle position which is hard to test. You can wash the inside out with WD-40 as a precaution. When refitting make sure the rubber seal on the TB is intact and flat. There has been problems with this seal distorting and sitting the potentiometer at an angle and forcing it to give duff reading


Can i point out here the need for a good quality air filter. Not to mention make sure its not blocked of got holes in it. All it takes is a bit of crud too hit the wires to bugger it up.

This measures the quantity of air passing into the engine via the air filter. The information is then fed back to the EEC IV module. The system can take into account the changes made by varying ambient conditions.

The Mass Air Flow meter is a clever little bugger. It consists of a hot wire that is kept at 200C . When air blows over it the wire cools and the unit increases the power to keep the wire at 200C. So the air flow is proportional to the current. Also when you stop it heats the wire up very hot (300C) for a second or so to burn off any crud

Unfortunately that crud can still stick and it insulates the wire and hence it gives false readings. You can prolong the life of your MAF by gentle cleaning with Throttle Body cleaner and a thin paint brush. Don''t spray hard on the wires, they are very delicate. Symptoms of a duff MAF are a slightly rough idle and overall down on power

Lambda/Hego, O2 sensor (all the same thing)

This provides the EEC IV module with data relating to the quantity of oxygen remaining in the exhaust gas. This is necessary for the correct operation of the catalytic converter. The sensor does not operate until it has reached its working temperature.

Lambda sensors as they are often called seem to last about 80-100k miles before they die. A dead one generally gives all sorts of running problems and lack of power including a bad idle when warm

Air Intake Temperature sensor

This provides the EEC IV module with a signal proportional to the temperature of the incoming air charge. Make sure the connections are OK and that the sensor is in position. Its located on the back of the inlet manifold, bit of pain to get too sometimes as space is limited on the fiesta.

Throttle Body Gasket/Inlet Manifold Gaskets

If its leaking then your getting un-metered air into the engine which will hurt the idle. Check both inlet gaskets, before and after the injector sandwhich plate and the throttle body itself.

Intake Leaks

Chech all pipes are properly connected from the air box to the throttle body. check all rubber pipes have no splits as unmetered air entering the engine will cause bad idle.

Wrong oil

If you have used too thick a grade of oil (Such as 15W/40) then this could effect the hydraulic valves, causing them to be sluggish until warm which could give a rough cold idle. This is what people think of as the "Sticking Valves Problem" a quick oil change to the correct viscosity oil will fix this. I''ve always used Mobil1 and would recommend it to anyone. Symptoms are valve rattle noises on start up which go when warm. If you don''t cure this the the valves will wear and you''ll have a permanent rattle.

Best prevention is to use the correct grade oil. Ford say that you should use a min of 5W winter rated oil (so 10W is bad). If you have put in the wrong oil or don''t know what is in there then do an oil change using Castrol or Valvoline synthetic 5W-30 oil, don''t forget to change the oil filters whenever you change the oil. Use a good quality oil filter too, id recommend Bosch.

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