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Spark plug blown out!


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A worrying moment this morning. Started car, bit lumpy, sounded like firing on three, took it for a drive as it sometimes does that then catches up. Still not right, sounded rough and noisy after 5 minutes. Pulled up opened bonnet, stuck head in and bang! Nr 4 plug shot out attached to lead. Fortunately I was on the other side. After checking and cleaning put plug back and all running normally for a drive of 10 miles or so. All the more worrying as we are on holiday in France at the moment, but all seems well now.  Apart from me not putting plug in properly, which is possible but I am usually pretty careful, I don't see how a plug can just blow out. Any thoughts? 

 

Cheers 

Dave 

 

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Unless the thread is stripped (unlikely) its more likely not screwed in properly Im afraid. I have seen one that wasnt fitted properly get blown through a bonnet on an early XJ many years ago.

Stuart.

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Hi Stuart 

 

Yes I suspect I am the cause, but grasping at straws! 

 

Dave

 

Edited by dpb
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Hello Dave

When plugs are new, when you torque them up there is a compressible washer that is compressed. If you remove and refit a plug then this washer may not hold the plug as well as it did new because it has already been compressed. I suspect a senior moment unless the threads are stripped as Stuart says. Just check all plugs are tight before you set off for a drive and hope it was a one-off.

Keith

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In order to avoid cross-threading, I insert each plug by hand and twist through 180 degrees to get started, then apply the wrench gently (should be easy if properly engaged in the thread of the head) before tightening fully.

Ian Cornish

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Thanks all. Crossthreading and thus a supposed tightness sounds like the cause. More caution in future. A flying sparkplug can be dangerous... 

Dave

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It is perfectly possible for an insufficiently tightened plug to blow out of a cylinder head. This happened to me many years ago on a Vauxhall Cavalier 1.8 GSi (incidentally one of the best and most enjoyable company cars I ever had). I was on my way home one day and had no warning of impending trouble wth the car running perfectly when all of a sudden there was a loud bang and the car started running on three cylinders. I stopped to investigate and found a plug sitting on the engine still attached to its lead. Fortunately, and to my surprise the cylinder head thread had not stripped, nor had the ejected plug damaged the underside of the bonnet and I was able to replace the plug with no ill effects. I found the other three plugs to be secure. The car had been serviced a few days previously  by Oggelsbys, ertswhile Vauxhall main dealers of Harpenden, and I could only conclude that a mechanic had failed correctly to torque the one plug.

Tim 

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On 6/29/2022 at 5:26 PM, tim hunt said:

It is perfectly possible for an insufficiently tightened plug to blow out of a cylinder head. This happened to me many years ago on a Vauxhall Cavalier 1.8 GSi (incidentally one of the best and most enjoyable company cars I ever had). I was on my way home one day and had no warning of impending trouble wth the car running perfectly when all of a sudden there was a loud bang and the car started running on three cylinders. I stopped to investigate and found a plug sitting on the engine still attached to its lead. Fortunately, and to my surprise the cylinder head thread had not stripped, nor had the ejected plug damaged the underside of the bonnet and I was able to replace the plug with no ill effects. I found the other three plugs to be secure. The car had been serviced a few days previously  by Oggelsbys, ertswhile Vauxhall main dealers of Harpenden, and I could only conclude that a mechanic had failed correctly to torque the one plug.

Tim 

I think I am right in saying that Vauxhall used to use plugs with a taper seat rather than a washer, so needed to be well tightened so they didn`t rattle loose.

I was taught that a new spark plug was screwed in until the washer contacted the head, then a further 1/4 turn was right. With a used plug where the washer is already compressed,  1/8th turn was needed after the washer contacted the head.

Ralph

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