Ford Focus Misfire

A misfire caused by corroded core plugs?

This sounds strange so I will explain; in this case the vehicle is a 56 reg Ford Focus 1.6 ti vct.

Upon inspection it was found that coolant had passed through the core plugs in the cylinder head and was shorting out No.1 Spark Plug causing a misfire.

Solution, replace the core plugs? Not that simple, engine coolant should not corrode the core plugs. After further investigation the problem was

mobile care mechanic kent

core plug location

found to be water leaking from the bonnet where the washer jets attach and laying in the trough in the cylinder head which corroded the core plugs from the outside in.

Ford sell modified washer jets (they have a rubber seal at the base) to address this problem.
If you are going to tackle this job yourself please be aware that the spark plug or plugs may have corroded threads and are liable to snap.

If you require any further advice please contact RP Automotive Services.

20 comments. Leave a Reply

  1. Neil

    this is a really useful thread as I broke down yesterday on cylinder Number 2 misfire & found water in the trough. My plugs were quite badly corroded & I put it down to heavy rain etc as there was no loss of coolant. I dried it out & replaced plugs & ht leads and its now running well. I will check my washers to see if they are causing it. May change core plugs as they look a little rusty from the outside there was a tiny little bit of water in centre of core plug this morning. My cars an 05 focus. Many thanks for posting this topic

    • Hello Neil.
      I am glad you found the blog useful and it has given you some insight into the possible cause of the misfire.
      Thank you for your comment.

  2. V12AML

    Brilliant bit of diagnosis. I couldn’t work out why I was getting water in the trough and initially assumed it was coolant from the core plugs but it looked like fresh water and didn’t have the sweet smell of antifreeze. The misfire also usually occurred after heavy rain and I started to look at areas where the water could come in externally eg bonnet seal at the rear, also wondered if it was a leaking washer jet pipe but know this makes perfect sense as it would be landing on top of the HT lead and tracking down. Was suprised at the quantity of water though! Thanks for the tip.

    • Thank you for taking time to comment, I am glad you found the article of use.


  3. Aqeel Ahmed

    Your issue is why they are corroding, which I can only guess would be related to your coolant. Do you have a slow coolant leak and regularly top up with tap water, or potentially a different coolant to what is in there?

    • The corrosion is generally not due to the coolant mix/type, it is due to water finding it’s way past the washer nozzles on the bonnet.
      There is a modification available from Ford for this.

      R Pearce

  4. It’s nearly impossible to find knowledgeable people about this subject, but you sound like you know what you’re talking about!

    • Terrance,
      Thanks for taking the time to reply, experience is one of the most used tools in my trade.

  5. Pingback: Runny

    • Thank you.


  6. Tony

    Thanks for helpful recommendation. There are lot of people using Ford cars and misfire problem can be deadly if within time it’s not treated well. So I hope by reading this post Ford car owners will be aware to double check before running into cars.

    • There are numerous reasons for a misfire, this is just one which is often not spotted.

      Thank you for the comment


  7. driving course online

    Is this a common problem for the ford focus? If it is a common problem then this could be the downfall of the ford focus.

    • I have seen quite a lot on this engine.

  8. Huh…. I find it very interesting to see how other people diagnose problems. How long did it take to figure out it was the coolant that caused it to short out?

    • only a few minutes, when you pull the plug leads off you will be able to see coolant or evidence of coolant.

      Thank you


  9. Hello! I just would like to give a huge thumbs up for the great info you have here on I will be coming back to for more soon.

    • Your welcome,
      Thanks for the comment


  10. Why did this misfire happens?What are the other causes?


    • The misfire happens because of a build up of water in the trough between the camshafts, this is where the spark plugs are situated, therefore the ht lead to the spark plug shorts and creates a misfire.There can be many other causes of a misfire,such as coils or coil packs, spark plugs, ht leads bad earths and more. A competent mechanic with experience and the correct equipment will be able to diagnose the cause.

      Thank you for your comment


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