Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB GT V8 Factory Originals Technical - dizzy cap condensation

Any advice please on preventing damp/condensation problems in the V8 distributor cap. Tried WD40 with good results for 2-3 days, then get misfires when heavy damp rainy/foggy conditions return. Currently running with the standard Lucas opus system, contemplating converting to Lumenition to perhaps improve spark performance. What do the Range Rover boys do when playing in the sand pits?
Richard Evans

Richard - not a general prob with the rover engine - could you have a water leak around the timing cover area ?

You have my sympathy as I get similar symptoms when driving in heavy spray. In fact the slight misfire starts soon after the rain does and gets progressively worse as the spray gets heavier. In really heavy spray the car is well nigh undrivable and I have to change down to keep going at all. This is no fun! and the car stays in on wet days. I have not bottomed it yet so anything you learn Richard could help me too. BTW I already have a Lumenition system fitted.

Ian Thomson
ian thomson

Never had any problems over 70k driving in all weathers, with conventional points and condenser, and neither have tens of thousands of other V8 engine owners with many different kinds of ignition systems. Except for one thing. Every now and again, before the plugs were due for routine replacement, it would just fail to start. As this was usually when I had to be somewhere else I only had time to change the plugs (always kept a new set in the car ready gapped) and not fully diagnose it. Until one day when I *did* have time and proved that it was the cap that was causing the problem. It seems that as the plugs went through their normal ageing process the HT voltage would gradually rise, until at some point the cap would start breaking down. Since I fitted a new cap it hasn't been a problem. The Opus is also a known problematic system, being used on 4-cylinder cars in North America, many being replaced under warranty. However they were usually total failure, I understand. A change of ignition system is unlikely to 'correct' a problem of condensation causing HT breakdown (if indeed that is the cause of the problem), unless it is a *low* voltage system.
Paul Hunt

to amplify my last:

you should not be getting water inside your cap - it is either coming from inside the timing cover or if the vacuum advance is no longer working, from the inlet manifold.

Like Paul - I have not had a problem in the 20 years I have used my GTV8 as a daily driver. It has luminition throughout and I did have one amplifyer fail (and always carry a spare)

My recipe has been to change the cap, coil and leads every 50K miles - sparks every 6K. It is essential whether or not you change leads, that they follow the precise route recommended in the handbook otherwise they will short and cause misfire.

Ian - I would start with the leads !


In my previous off-road experience there were two methods of keeping water out of the distributor: either silicon the cap to the distributor, or wrap a scrap of rubber innertube around the distributor. This was only for the worst conditions and usually wasn't necessary. There's more likely than not a great resaon to replace your cap.
Jeff Schlemmer

The water inside the cap is from condinsation,
and is usually more previalent in areaa with
high humidity. Theoretically, if one can hermetically
seal the cap (in a non-humid) environment the problem
would be solved.
I had this problem with my jeep cj during Oklahoma Summers (real humid). I found a vented distributor cap which solved my probelems, until that time I carried a role of paper towels and WD40. Now I run an HEI dist. and don't have to worry about it.
Not sure who made this cap, but they are out there.
Hope this helps,

Thanks for feed back. New plugs in, gave car run after no use for two days, clean dry cap before start, 4 miles down a coast road, air temp approx 5degC starts to misfire!! stop and remove cap, spray with WD40, runs just fine for 10 miles run then home. Not changed to Luminition yet still on std. Opus.
Paul thanks for H20 sensor info. What was problems with Opus?
Roger, did you convert std dizzy? Dizzy all works mechanically OK.
Richard Evans


Does your distributor have a missing rubber plug in the distributor body? Does the wire pigtail fit tightly through the hole in the distributor? Is there a puddle of water in the bottom of the distributor? Have you changed the cap and rotor to eliminate the problem?

You could put a bead of dielectric grease around where the cap sits on the distributor or put a band of duct tape around the cap to body joint (at least to see if the moisture is getting in there).

Wayne Pearson

Wayne, I was about to ask the same question as you - is there water in the bottom of the distributor? Condensation in that volume doesn't just come from nowhere. The volume of air in the cap will not create the volume of condensation that will cause a misfire, at least in "normal" weather conditions. You need to remove the distributor and take a better look at what you are working with.
Jeff Schlemmer

Richard - yes - standard dizzy to luminition. Works fine with a 12V coil. I have had several coils over the years and that could also be one of your problems with the condensation a parallel isue - same goes for the capacitor.

The water which condenses in the distributor cap most likely comes from your cooling system - it could be anywhere but the likely place will be the timing cover where either the gasket is weeping or sealant has broken down on one of the bolt threads which penetrate into the waterway of the block. Its not a difficult job to remove and reinstal with new gaskets but it does take time.

For the record; Paul's point about the reliability of the motor is entirely fair - problems are few and far between irrespective of the climate and how hard you push them. However they need frequent oil changes and the water must have a corrosion inhibitor/anti-freeze/water-wetter.

Safety fast.


Richard - Opus problems were simply one of poor design/construction leading to early failure, as I say usually total and within warranty. I've heard of a lot of the alternative 45DM4 systems still in use, and the electronic module is still available, but only one other Opus system that I can recall.

When it starts misfiring and you remove the cap can you see visible condensation? If so then like others have said it must be coming from somewhere, it is not simply atmospheric. I've had coolant dripping on the distributor of my roadster from a faulty heater valve, heavy beading inside, and still no misfiring. If no visible condensation and you are simply spraying WD40 as a straw-clutcher then have you tried just removing and replacing the cap? Perhaps it is a problem with the centre contact, or lead connections.

When I replaced my cap the old one had no visble tracking paths that I could see, but replacing it solved my problem. I diagnosed it but clipping a timing light onto the coil HT lead and noticing that the flashers were as they should be, but clipping it onto plug leads there was no or very intermittent flashing. Also diagnosed a cap/rotor problem on my daughters Metro the same way.
Paul Hunt


This summer my 4 cilinder roadster stopped and after checking the engine it became clear that i had no sparks. Checked the wiring and could,nt find anything so i had to be towed home. At home i checked everything and it was the rotor which caused the break down. It had an unvisible split. This was on a sunny day. Maybe your rotor is also fractured a little and with some condensation, the current will run from the rotor to the rotor shaft.
Good luck

This thread was discussed between 24/11/2004 and 30/11/2004

MG MGB GT V8 Factory Originals Technical index

This thread is from the archive. The Live MG MGB GT V8 Factory Originals Technical BBS is active now.