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3.5 V8 Missfire - one dirty plug?


Davewillb

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Hi,

Apologies in advance as I guess there was a collective groan at the subject line but I think I've done the usual first things to sort this problem.

The engine is a 3.5V8 in a 1992 Discovery, 97K basic LPG system fitted, single point with no stepper motor at the inlet. It is currently running on 7 cylinders only on both gas and petrol but runs a lot better on petrol than gas (not suprising in view of the setup I'm told).

I've changed the plugs, leads, distributor cap, rotor arm all for genuine items and this got me to 7 cylinders firing from 6 but the first plug on the righthand bank (from car front) is sooting up badly and not firing but there is a spark to it.

A compression test on this cylinder gave me 140psi a quick test on one other cylinder gave 150psi with a borrowed tester so I'm thinking that should be enough for the cylinder to fire ?...... but pulling the plug lead makes no difference to the engine at tick-over.

Could this be a faulty injector that is leaking and staying open.... even when switched to gas and flooding the cylinder..... or is this more likely a cam fault, any suggestions would be much appreciated.

I should have said that the problem with this cylinder was there right from the start and none of the ignition stuff had any effect, the rest of the improvement was due to the carbon brush in the distributor cap beinG detached from the spring and generally very loose.

Cheers for any help.

Dave

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Thanks for that and you fixed it for me!!! .... by getting me to look again at the injectors to see how hard it would be to do a swap I noticed something I should have seen before but didn't - I only bought the her last week so she is completely new to me is my excuse!.

Who ever did the gas conversion had connected the injector emulator for the cylinder with the black sooty plug to the temp sensor which is nearby and has the same connector. I guess this had the double benefit of giving a double fuel dose when on LPG and an incorrect temp level to the ECU when on both LPG and Petrol so it ran rough on both and missed particularly on LPG.

Now she runs really nicely and I'm more than a bit chuffed to say the least.

Grateful thanks.

Dave

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Its the best possible time!! and I've made a donation, not massive but V8 disco is eating a hole in my pocket at the moment, now its running nicely it just needs prop shaft UJs, front springs and shocks, exhaust mounts and front wheel bearings for the MOT .... note you are the honourable treasurer :D - nicely done.

Thanks for forum - it really is excellent.

Dave

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Oh right - thought it mentioned your forum name when I went to the donate section ..... prob just your Swiss bank account details where you are saving for tax free crayons then..

Cheers anyway (still on a high at this end).

Dave

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