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V8 Rough running under hard throttle


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Hi all,

Since the weekend and changing the oil, my disco 1 V8 has become rough( struggeling to pick up, sounds like its on about 3 or 5) when squeezing the throttle hard in park and seems ok with a slow squeeze. However under load it is apparent at low revs particullarly after coasting in traffic and then accelerating again.

It seems to be apparent on both LPG and Petrol but much worse on LPG. This is steering me towards an electrical issue, as i know how the ignition system need to be in tip top condition for LPG. I have magnecor leads and NGK spark plugs which were changed about 4000 miles ago. I have cleaned the contacts up on the dizzy and the rotor arm but its still apparent. I have checked the vacum between the dizzy and plenum and it is working fine?

Can any one suggest anything i should be looking at or what besides the rotor arm and dizzy cap i should change? Any ideas why its started to occur after an oil change?

Cheers, Foss

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new rotor arm? leads to sprak and coils renew magnicor...... check the tops of each spark plug if the tops are loose.

the coil, is your alternator / battery giving out enough, when my alternator started to fail i had this and struggled to rev in the end. maf?

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Based on my experience - I wouldn't limit myself to ignition. You might have knocked off (or split) a breather pipe, moved the dizzy, over-filled the engine, kinked the fuel pipe, left a rag where it got sucked into the intake, knocked off a sensor wire (don't rule out the ECU just because the problem also happens on LPG - my ECU still controls the air-bypass valve) - and any one of a dozen other things that I have done or seen over the years*.

Basically, since the problem started after you have been working on the engine, your first thought should be 'what have I done?' and you should have a really good look around. Only after you have excluded 'you' being the cause should you start to suspect that a general ignition (or any other) fault just happened to occur at just the right time.


*The rag in the intake and the over-filling the engine were both done by an very experienced home-mechanic I used to know, the rag took a while to find because for some reason we suspected the carburettors, and the over filling was due to him filling it twice - how can you do that? :o:)

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hi fozzy

I had the same problem with my v8, I tried the dizzy first and had no joy it turned out to be 1 of the ht leads was arking out to the engine and a cheap rotor arm, I put a Lucas rotor arm on and insulated the ht leads and it runs like a dream now,not saying will be your resolve but start with the cheapest things first.hope this is of help good luck.


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Thanks all, I will try the simplest and cheapest items first and report back how i get on. Its single point lpg btw. I think the 'gunson's flashtest' looks like a very hand tool to have in the V8 tool kit, may have to invest in one of those.

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I have checked and cleaned all wiring to the coil, checked the connection to the afm, take all ht leads of the plugs and had a good look at the ceramic, everything looks ok, tightened all of the screw ends, put all HT leads back on checked the firing order and made sure none of the leads are touching each other or the engine, checked all breathers, checked all sensors, taken air box off to see if a new filter is needed, checked the oil level, everything seems fine. And its still running rough, probably getting worse. I have noticed it is a lot better first thing in the morning when every thing is cold but as it warms up it turns back into the bag of spanners. How would i go about checking the ecu?

I dont know wether it could have anything to do with it but i think the torque convertor or gearbox is on its way out? ive been off roading in the past and it was coming to the end of the day trying to climb a gentle slope there was very little drive for the given engine revs, changing the oil and filter in the box helped but its on my list of things to do.

Thinking now about changing the coil, dizzy cap and rotor arm to see if they help.

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