Erratic idle diesel

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rob-81
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Erratic idle diesel

Post by rob-81 »

Hi. I have a 157k mile, 1991, 1905 Bx. Recently, I have noticed the when idling it sometimes seems to hunt between around 750 rpm to 1000 rpm and then often settles on an idling speed of around 1000 rpm. I have checked the throttle is returning properly (it seems to be fine). Anything else to check? I'm getting married on Saturday and going on honeymoon in it on Sunday and don't fancy breaking down in deepest darkest Wales!
Thanks in advance!
Rob.
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jonathan_dyane
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Post by jonathan_dyane »

Has it just started doing this, and does the car have a Lucas pump? If so, an uneven idle can be caused by the bearings starting to break up, and clogging up the pressure relief valve and correspondingly causing the pressures to be all over the place and an uneven idle.

This is easy to check for, as the pressure release valve is within the banjo that attaches the diesel return pipe to the injector pump. I would recommend that you remove the banjo for inspection (removed you should be able to easily rattle it's ball bearing by shaking it, and there should obviously no sign of swarf within it).

Another (probably more likely) possibility, is the fuel heater on the back of the engine gone/going porous. This is a casting fitted to the rear of the water pump area, which is intended to heat the diesel in winter conditions. Sadly the heater is made out of a form of allow which is closely related to cheese, and they always go porous. I suggest that you check to see if the diesel feed still flows through the heater (access best from below IIRC) and if so bypass it with some suitable tubing. The car will not suffer any ill-effects from not having a fuel heater.
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rob-81
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Post by rob-81 »

Thanks for that Jonathan; very much appreciated.

The symptoms first showed themselves a few weeks ago. I noticed it once and didn't notice again until yesterday. Today, it's fine.

I haven't got any time this week - last minute wedding preparations but if needed, I can probably get the car to my local specialist. Are either fault likely to cause more problems or get rapidly worse if left for another 2000 miles (I am thinking I would rather not have to sort out anything car related until after my holiday in it)? I appreciate that it is a bit of a "how long is a piece of string question" but am I on borrowed time?

Thanks again,
Rob
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Post by rob-81 »

sorry, I forget to say, it is a Lucas pump
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rob-81
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Post by rob-81 »

I've just thought, I put a bottle of injector cleaner at the last tank full and put some in a while again too (maybe when the symptoms first reared their head). No chance that this is related?
thanks!
Rob
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docchevron
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Post by docchevron »

It's possible.
What make of injector cleaner was it mate?
They dont all do quite the same thing.

Equally, Lucas pumps do some eraatic stuff if any air gets in to the system, but quite sporadically sometimes..
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Post by rob-81 »

Hi Doc,
I an't remember what make it was, just the cheapest from halfrauds. I also filled up with the "super" diesel at a shell garage last tankful - I'd queued for ages in a line and realised that there was no normal diesel. Couldn't be arsed to queue again!

Thanks,
Rob
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mat_fenwick
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Post by mat_fenwick »

Whereabouts in deepest darkest Wales are you off to? As I live in Blaenau Ffestiniog, if you're up this end you're welcome for a cuppa. Or if you break down I may be able to help...
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Post by rob-81 »

Hi matt, thanks for the kind offer. I'm spending a week in a cottage near Lampeter (not sure I've spent the correctly!) then off to Llangollen for a night followed by a week on a narrowboat on the llangollen canal. We're hoping to to to Llangollen via Harlech. I love Wales, I only really discovered it a few years ago when my twin moved there (he's an outdoor instructor) but have had some great holidays over the last couple if years. Where's ffestinog?

Welsh place names scare me witless - I dread getting lost whenever I'm there!
Best wishes,
rob.
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Post by DLM »

Another possibility is that the rubber diesel filter primer diaphragm is perishing away - but I'd expect that to show up at times other than idle. Air bubbles will be seen in any subbed-in clear tubing upstream of the pump, (a giveaway that it's happened before) and random minor surges and dead spots become an everyday occurrence - if you can get the thing started in the first place..

I'd also recommend not pushing away at the primer button in an attempt to provoke the problem ... as it almost certainly will provoke this problem. A spare diaphragm is a useful part to have salted away.
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Post by Alcyone »

Interesting stuff... I have a Lucas on my A8A with a different problem. Sometimes, when driving at a steady speed, it suddenly starts to accelerate the car for a short while. Although the idling is always steady, I will suspect the pressure release valve.
The most problems I've had was with faulty wiring to the fuel cut-off valve, having a large resistance somewhere. Since I'm lazy I removed the valve and now stop the engine by popping the hood and push the stop lever.
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