Help 3 Intermittent cutting out on hot idle

Technical advice Q&A

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swindler
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#1 Help 3 Intermittent cutting out on hot idle

Post by swindler » Sun Jun 10, 2018 4:11 pm

Hello all

The recommissioning of my Dad's car continues in earnest. I am very keen to have the car ready to take to Le Mans classic early next month. It's a March 1964 late 3.8 OTS with 2700 miles/23 years since full restoration which included full engine rebuild.

The main and most stubborn problem is an intermittent cut out on hot idle which it's had for many years now (possibly since rebuild as it seldom when on long trips). It is not predicable but most often occurs during period in slow traffic. Once it has cut out, the car will not restart for 15-25 minutes. During this period the starter spins the engine strongly but it does not catch at all on any cylinder. Then, upon restarting there is sometimes a puff of smoke from the intake side louvres.

• I initially assumed it was an ignition problem so I have replaced the coil, condenser, points, rotor arm and spark plugs have been replaced but the problem still occurs.
• Fuel is full tank fresh Shell optimax – problem still occurring. Glass petrol bowl by carbs appears to be clear.
• The old plugs for cylinders 1-4 were pink; 5 and 6 were sooty
• I don't think there's a vacuum in fuel tank (no suction when removing fuel tank)
• Car runs well otherwise – pick up below 1000 rpm under load slightly spluttery but above this very healthy including at full throttle. Car is fast; faster than E30 M3 BMW tested upto 110mph
• Hot starting when not cut out is very quick/easy
• Timing measured from cylinder 6 at hot idle appears to be 20 degrees BTDC (using mark on front crank pulley and a digital timing gun. I haven't adjusted it. Handbook recommends 10 degrees BTDC. Is reading wrong? There is one timing mark (that I've assumed is TDC).

I'm totally confused now. Any advice very welcome. Mark :shrug:

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Tom W
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#2 Re: Help 3 Intermittent cutting out on hot idle

Post by Tom W » Sun Jun 10, 2018 6:48 pm

20degrees BTDC sounds high if the engine is otherwise standard. Over advanced timing can make for difficult starting. The mixture ignited and burns before the piston has reached the power stroke, so it pushes back against the rotation of the starter. Having said that, this isn’t really temperature dependent.

I would start by checking where your timing mark actually is, by using a dial gauge and a probe down the spark plug hole of either cyl 1 or 6. When you know where TDC is, you can accurately set the timing to the book value and see if that improves things. I’d also check that the outer ri g of the crank damper is still secure on the nose of the crank. It’s not unknown for them to de-laminate and then the timing marks move.
Tom
1970 S2 FHC

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phil.dobson@mac.com
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#3 Re: Help 3 Intermittent cutting out on hot idle

Post by phil.dobson@mac.com » Mon Jun 11, 2018 12:08 pm

if possible you need to use a spark tester when it cuts out to determine if its ignition(which it sounds like it is). If so there is a current thread on the jag lovers site that suggests that the ignition leads are probable candidates. (especially the ignition coil to distributor one)
Just an idea.

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cactusman
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#4 Re: Help 3 Intermittent cutting out on hot idle

Post by cactusman » Mon Jun 11, 2018 12:15 pm

Couple of thoughts...
If it suddenly dies as though the ignition has been turned off I would be suspecting some sort of electrical issue.
If it splutters and coughs and dies then it may still be an electrical issue or it may be some sort of fuel vaporization issue. If it happens when sitting for a while at idle then engine bay temperatures rise. Standard unleaded seems more prone due to the ethanol content so use super. It might help.
Lastly it may be an issue with the fuel pump itself stopping and then the engine coughs and dies...
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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swindler
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#5 Re: Help 3 Intermittent cutting out on hot idle

Post by swindler » Mon Jun 11, 2018 2:56 pm

Thanks i'll test all these. It does abruptly cut out at hot idle but the problem is getting it to do it in a controlled environment - last time was on the Vauxhall gyratory, so not ideal as you can't listen to whether the fuel pump is working or whip out a spark plug tester.

I'm using Shell optimax.

Do you have the link to the thread about plug wires? Many thanks

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Tom W
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#6 Re: Help 3 Intermittent cutting out on hot idle

Post by Tom W » Mon Jun 11, 2018 6:54 pm

I’ve just re-read your first post. My comment about the over advanced ignition causing difficulty starting isn’t likely to apply if the starter spins the engine easily.

I would think it unlikely that all the plug wires fail simultaneously. Maybe the king lead has a problem?

You could check if there’s an ignition or fuel problem by spraying a little easy start or similar into the air intake next time it happens. If it fires, the problem is fuel related, if not, then it’s likely agnition problem.
Tom
1970 S2 FHC

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swindler
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#7 Re: Help 3 Intermittent cutting out on hot idle

Post by swindler » Sat Jul 21, 2018 8:43 pm

Quick question:

I have two timing marks on the crank pulley. One is a line on the face of the pulley and the other a few degrees (10?) further along across the edge of the pulley. Which is TDC?

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Tom W
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#8 Re: Help 3 Intermittent cutting out on hot idle

Post by Tom W » Sun Jul 22, 2018 7:46 am

I’m not sure about the 3.8, but on the 4.2 the TDC mark is labelled, and ‘before’ and ‘after’ are marked too, along with 2 degree increments. Maybe try cleaning the pulley to see if there’s some more lettering on there. Failing that, you’ll need to set the engine to TDC, with a probe down the spark plug hole, either with a dial gauge, or using the double stop method. See which mark the pointer lines up with, and adjust the pointer to suit. Add the rest of the marks from there, a few degrees either side of what you’re aiming for. You probably want 2 degree intervals, unless your timing light has an advance function. Don’t trust that the original marks are accurate any more, pointers are adjustable, and the dampers can also delaminates and shift.
Tom
1970 S2 FHC

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christopher storey
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#9 Re: Help 3 Intermittent cutting out on hot idle

Post by christopher storey » Sun Jul 22, 2018 9:39 am

Swindler : turn the engine so that the marks are near the top of the pulley ( just because it's easier to see ) . The crank turns clockwise viewed from the front , so that the mark nearer to the exhaust side of the engine when viewed from the top will then be the "before tdc" mark ( probably 9 degrees ) and the other one will be the tdc mark


As far as your cutting out problem is concerned , it sounds suspiciously like the conductive rotor arm defect. I suggest you get a new rotor from the Distributor Doctor, whose genuine rotors are non-conductive which usually eliminates the problem

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