Peugeot 405 Manual

Spark plug renewal (12000 Mile (20 000 km) / 12 Month Service)
Routine maintenance and servicing / Spark plug renewal (12000 Mile (20 000 km) / 12 Month Service)


Note: On models from 1994, the makerТs specified interval for this procedure is 18 000 miles (30 000 km).

1 The correct functioning of the spark plugs is vital for the correct running and efficiency of the engine. It is essential that the plugs fitted are appropriate for the engine (the suitable type is specified at the beginning of this Chapter). If this type is used, and the engine is in good condition, the spark plugs should not need attention between scheduled replacement intervals. Spark plug cleaning is rarely necessary, and should not be attempted unless specialised equipment is available, as damage can easily be caused to the firing ends.

2 On 16-valve models, to gain access to the spark plugs, the access cover fitted over the centre of the cylinder head must first be removed. Undo the eight bolts, noting the position of the wiring retaining clip, and remove the cover (see illustration).

11.2 On 16-valve models undo the eight bolts (arrowed) and remove the access
11.2 On 16-valve models undo the eight bolts (arrowed) and remove the access cover to reach the spark plugs

3 On other models, to improve access to some of the plugs, it may be necessary to remove the air inlet duct (refer to Chapter 4 for further information).

4 On 1998 cc 16-valve models, pull the HT coils off the spark plugs. If necessary, to remove the possibility of the HT coils being connected to the wrong spark plugs on refitting, mark the coils 1 to 4 (No 1 cylinder is at the transmission end of the engine).

5 On all other models, if the marks on the original-equipment spark plug (HT) leads cannot be seen, mark the leads 1 to 4, corresponding to the cylinder the lead serves (No 1 cylinder is at the transmission end of the engine). Pull the leads from the plugs by gripping the end fitting, not the lead, otherwise the lead connection may be fractured (see illustration).

11.5 Pulling the HT leads from the spark plugs
11.5 Pulling the HT leads from the spark plugs

6 It is advisable to remove the dirt from the spark plug recesses, using a clean brush, vacuum cleaner or compressed air before removing the plugs, to prevent dirt dropping into the cylinders.

7 Unscrew the plugs using a spark plug spanner, suitable box spanner, or a deep socket and extension bar (see illustration).

Keep the socket aligned with the spark plug - if it is forcibly moved to one side, the ceramic insulator may be broken off. As each plug is removed, examine it as follows.

11.7 Tools required for spark plug removal, gap adjustment and refitting
11.7 Tools required for spark plug removal, gap adjustment and refitting

8 Examination of the spark plugs will give a good indication of the condition of the engine.

If the insulator nose of the spark plug is clean and white, with no deposits, this is indicative of a weak mixture or too hot a plug (a hot plug transfers heat away from the electrode slowly, a cold plug transfers heat away quickly).

9 If the tip and insulator nose are covered with hard black-looking deposits, then this is indicative that the mixture is too rich. Should the plug be black and oily, then it is likely that the engine is fairly worn, as well as the mixture being too rich.

10 If the insulator nose is covered with light tan to greyish-brown deposits, then the mixture is correct, and it is likely that the engine is in good condition.

11 The spark plug electrode gap is of considerable importance as, if it is too large or too small, the size of the spark and its efficiency will be seriously impaired. The gap should be set to the value given in the Specifications at the beginning of this Chapter.

12 To set the gap, measure it with a feeler blade, then bend the outer plug electrode until the correct gap is achieved (see illustration).

11.12 Measuring the spark plug gap with a feeler blade
11.12 Measuring the spark plug gap with a feeler blade

The centre electrode should never be bent, as this may crack the insulator and cause plug failure, if nothing worse. If using feeler blades, the gap is correct when the appropriate-size blade is a firm, sliding fit.

13 Special spark plug electrode gap adjusting tools are available from most motor accessory shops, or from some spark plug manufacturers.

14 Before fitting the spark plugs, check that the threaded connector sleeves (on top of the plug) are tight, and that the plug exterior surfaces and threads are clean. It is very often difficult to insert spark plugs into their holes without cross-threading them. To avoid this possibility, fit a short length of hose over the end of the spark plug (see Haynes Hint).

15 Remove the rubber hose (if used), and tighten the plug to the specified torque (see УSpecificationsФ) using the spark plug socket and a torque wrench. Refit the remaining plugs in the same way.

16 Connect the HT leads in the correct order, and refit any components removed for access. On 1998 cc 16-valve models, connect the HT coils in their correct order.


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