Lucas Stop solenoid help

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ohms
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Lucas Stop solenoid help

Post by ohms » 28 Aug 2014, 18:57

My ax diesel was idling away quite happily when all of a sudden there was a huge bang the engine stopped and a big cloud of smoke from under the bonnet. I opened the bonnet to find that the stop solenoid had blown it's innards out. The cable connecting the 12v feed was still attached to the nuts but hanging underneath the car and everything inside the solenoid has been expelled including a load of diesel.

I'm thinking it must have internally shorted out somehow, has anybody ever seen anything like this before or got any ideas what may have caused such a catastrophic failure?

BX
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Re: Lucas Stop solenoid help

Post by BX » 28 Aug 2014, 23:55

I am replying because no one else has. I haven't come across this before. If the problem was caused by an electrical short then it would most likely blow the fuse. Check with a test lamp to see if you still get 12V to the solenoid with the ignition on. If you do then probably either the solenoid failed mechanically due to poor construction or was subjected to excessive fuel pressure which blew it apart.

Peter.N.
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Re: Lucas Stop solenoid help

Post by Peter.N. » 29 Aug 2014, 12:04

No I haven't either. I would imagine that the winding had developed a short and heat generated had produced more pressure then the casing could cope with. I think you will find that everything will be OK when you have replaced the solenoid. It may have blown the fuse but not necessarily, a low resistance can generate quite a lot of heat without drawing enough current to blow the fuse.

Peter

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CitroJim
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Re: Lucas Stop solenoid help

Post by CitroJim » 29 Aug 2014, 13:01

I believe it was more than likely poor construction. They do fail through leakage from the top plastic bit that acts as the insulation around the electrical connection. It's only crimped in and if this is a bit marginal coupled with the pressure of fuel behind it the prospect of it completely popping out is quite likely.

Filberton
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Re: Lucas Stop solenoid help

Post by Filberton » 29 Aug 2014, 20:52

You may want to check your timing belt in case it has slipped a notch or two. I don't know the AX system so may be way off beam (so apologies if I'm talking through my hat :-( but other diesels I have seen or worked with with a shut off valve they work by being shut when engine running but spring open when ignition is turned off to let excess air in to the inlet manifold to over weaken the mix (is that the same on the AX?). If the timing is out so the inlet valve is open on the mid compression you can effectively get a backfire pressurising the manifold and valve which could blow out the valve and spew exhaust fumes out under the bonnet. Problem is it can also blow back through the Mass air flow.

Big question is, if you close off the valve, will she re-start and run? [if you've not tried yet, might be worth turning over by hand in case you've dropped a valve.]

Peter.N.
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Re: Lucas Stop solenoid help

Post by Peter.N. » 29 Aug 2014, 22:18

The fuel solenoid is purely a cut off valve that closes when you switch the ignition off, its only controls the flow of fuel into the pump and doesn't have any connection with anything else so I would have thought it was an internal fault that caused it to self destruct.

Peter

BX
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Re: Lucas Stop solenoid help

Post by BX » 29 Aug 2014, 23:10

I am not by any means an injection pump expert. However I have done several minor repairs over the years such as replacing o rings and seals and on the CAV (Lucas) pumps having to free stuck metering valves which are sometimes a cause of no start or lack of power particularly on engines that may be parked up for ling periods. Also cleaning out the gauze in the fuel inlet. On the old manually operated pumps pulling the stop lever closed the metering valve and cut off the fuel to the high pressure pump. My understanding of the stop solenoid valve is that when it is released the plunger blocks the flow of fuel from the transfer pump to the metering valve. All the while the engine is running the valve is subject to transfer pressure. I do not know if a fault in the transfer pressure regulator could cause a huge rise in pressure at idle speed. As i say I don't really know and I am throwing it out there. Someone is bound to have the answers but are they are reading this thread. More than likely replacing the valve will solve the problem Whilst these pumps are getting scarce on cars there is still a large number of tractors and industrial engines with similar pumps that are likely to be in service for another 20 years.

ohms
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Re: Lucas Stop solenoid help

Post by ohms » 01 Sep 2014, 16:37

I tested the 12v+ feed to the solenoid and it was still giving out 12volts with the ignition switched on. So fitted a new solenoid and it's been as good as gold ever since.

I can only conclude that it must have somehow shorted out internally and then blew itself to bits. Most strange and a little disconcerting to say the least.

I have also just finished re-shimming the valve clearances to brng them back into spec. The car now seems to be slower than it was before and unfortunately just as tappy on the top end. Atleast they're correct now though I suppose. It no longer smokes out of the exhaust either. Next thing I want to try is locking the engine to see if the cambelt is a tooth out somewhere. I know the car had a cambelt replacement just before I purchased and there is a suspect red paint mark on the new belt which suggests to me whoever did the job may not have gone to the trouble of locating the locking bolts. We will see.

I'm sure i'll love this car eventually! It does have that characteristic citroen soft suspension and seat combination. Reminds me of my BX's! :cry:

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CitroJim
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Re: Lucas Stop solenoid help

Post by CitroJim » 01 Sep 2014, 17:05

Transfer pressure is 7 or 8 bars so well enough to pop a marginal seal...
BX wrote:However I have done several minor repairs over the years such as replacing o rings and seals and on the CAV (Lucas) pumps having to free stuck metering valves which are sometimes a cause of no start or lack of power particularly on engines that may be parked up for ling periods.
Apart from leaks, that's about the only fault these excellent pumps ever suffer from unless you foolishly try to run veg through them.
BX wrote:there is still a large number of tractors and industrial engines with similar pumps that are likely to be in service for another 20 years.
And the rest :-D

The DPC has been around since the 50s and virtually unchanged in all that time... They'll still be going, albeit weeping diesel all over the place, well into the next century I expect...

A classic design.

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Re: Lucas Stop solenoid help

Post by Peter.N. » 01 Sep 2014, 19:38

The Perkins 4/99 and 4/108 engines I used for diesel conversions used a DPA pump, same principle except for the fuel solenoid which hadn't been invented then, they had a stop lever with a choke type cable attached.

Peter