Missfire on overun and light throttle

While driving home (380miles:rolleyes:) I noticed a slight miss when on light throttle, I didn't think much of it but after a little play with a merc I noticed the missing coming in during overun. I got home and didn't think much of it as it went away after 100miles and the temperature gauge never moved.

This morning I went to start the car and it was hard to start. When i did get it started it conked out while reversing out the drive. It would keep dropping revs untill i saved it from stalling, like it was firing off a cylinder. I checked the coolant and it was really low, topped it up and it sucked it all in. The rad cap was screwed and fell to bits so I have that coming this afternoon at 2.30 from mr.t and the engine now runs as sweet as ever now that its warmed up and full of coolant.

My question is wouldn't this indicate HG failure? Missing and coolant loss with dodgy startup and dropping a cylinder from cold start? Any advice before i start spending copious amounts of money would be greatly appreciated.

What's the spec of the car?

If the rad cap was leaking the system wouldn't be holding pressure and the coolant has probably boiled over. Is there any evidence of escaped coolant in the engine bay around the rad cap and over flow bottle? The leaking coolant could be the reason why it was missing if it got into the wiring somewhere.

What's the spec of the car?

 

If the rad cap was leaking the system wouldn't be holding pressure and the coolant has probably boiled over. Is there any evidence of escaped coolant in the engine bay around the rad cap and over flow bottle?

 

Misfire could be something as simple as the plugs, coil pack or connectors.

The car is bpu, 3" blitz with no cats running 1 bar with standard everything else. The plugs are NGK's the one step colder/hotter (cant remember) that is recommended. Coil pack clips were changed about 8 months ago and the plugs were changed 3 months ago. I can't see any eveidence that coolant has pissed out of the expansion tank either. When the car is sat running and upto temperature I can't see any leaks under the car either. The temperature gauge has never indicated any problem with overheating either but whether its that reliable or not is another story I suppose. The car isn't missing now as I just drove it for a couple of miles and back again and its not missing, however I didn't give it any beans as I dont know whats wrong with it yet.

How low was the coolant? There is an ECU water temp sensor in the system, maybe airlocks affected it?

And the car cant possibly missfire on the over run, theres no fuel injected

How low was the coolant? There is an ECU water temp sensor in the system, maybe airlocks affected it?

And the car cant possibly missfire on the over run, theres no fuel injected

What could cause a similar feeling to missfire on overun then? Thats what it feels like. Like someone is dabbing the brakes just as im coming right of the throttle and on overun.

Coolant was pretty low, overflow had practically nothing in it and I could see the core of the rad when i took the cap off. The cap fell to pieces when i removed it too. I have a new cap here so i'll fit that and see. Just the missfire (or feeling thereoff) that is worrying me now.

You may have been getting hot spots due to lack of coolant, and this subsequently caused high resistance in the ignition system. Just a theory as it now seems to work when full of coolant

Get the system pressure tested. Make sure you take the plugs out just in case as you don't want to hydraulic it should the HG be a bit the worse for where.

You may have been getting hot spots due to lack of coolant, and this subsequently caused high resistance in the ignition system. Just a theory as it now seems to work when full of coolant

Yeah I think you may be right. I drove it this morning and there's not even a hint of missing now. No problem with cold start and runs sweet as if nothing ever happened. Must of just been the dodgy rad cap, new one is on and everything is now fine:search: Could of been worse if I'd overheated it but all is well it seems, rad cap wasn't cheap right enough, £17.50. Cheaper than a HG though;)

Get the system pressure tested. Make sure you take the plugs out just in case as you don't want to hydraulic it should the HG be a bit the worse for where.

I'm going offshore tomorrow but when i get back it'll be getting new filters, plugs and a compression test just for piece of mind. Dont think there's a problem with the oil but i'll change that too. Who can pressure test the rad system? Any garage?

OK a little update with this. I got home and ran the car, all was well so i drove down to Northamptonshire with it with no problems. Now that i'm down here the car is lossing coolant from somewhere and it runs perfectly on one journey into town then it will die as i pull up to a junction. Its fine aslong as its on boost but when it goes on the overun and stays in vacuum it starts to miss untill i get to walking pace when it can even die (which its done 3 times now).

I'm thinking this is a headgasket between the cylinder and the water jacket. I think in vacuum its suckin coolant in and running like shit and in boost everything is fine but it over-pressuries the coolant system as the overflow is pissing fluid out occasionally. Anyone have any other idea's or am I right?