1990 1.8 auto cabrio engine cutting out

Apologies in advance for a bit of a long post!

I have a 1990 (H reg) 1.8 automatic cabriolet, which has developed a bit of a hiccup.

The symptoms began on the way back from a trip to discuss an engine swap (ironic, I know) and after about 100 miles of motorway cruising, the engine began to misfire as though it was starved of fuel, and the car ground to a halt on the hard shoulder of the M25.  Temperature was normal, and the tank was half full.

Half an hour later the car started again and I made it home with no further issues.

Searching these forums, I picked up some spares for the likely causes (fuel filter, coil, ignition module and pump) and the car went on a 200 mile trip to recreate the problem. Things were much worse this time – barely 20 miles between each failure.  Replacing the coil and ignition module didn’t make much difference BUT keeping the tank full did.  When the fuel level dropped below about 2/3 the problem would start (and the engine would stop.)

So I suspected the filler neck or maybe some rubbish floating around in the tank.  I couldn’t find any corrosion in the neck, and the tank was equally clean (I drained it having gone in via the fuel level sensor).  The sensor and pick-up also looked sparklingly clean.

The next thing I tried (I’m trying to avoid any expensive parts, since the engine is going in August) was the vapour separator.  The old one looked ok, but I put on a spare and tried running with just 5 litres of fuel in the tank.  No problems whatsoever this time.

So, before I try another long drive, could the vapour separator really have been the problem?  Is the fuel level a red herring?  It occurred to me that by stopping every 40 miles to refill I was allowing stuff to cool down/settle.

Any thoughts or suggestions would be much appreciated!

What carb do you have fitted?

Have you had a look at the rubber flange which the carb sits on as they can split.

Is there petrol in the fuel filter?
Try removing the fuel hose going into the fuel filter and blow back down the fuel line into the fuel tank, you should hear bubbles in the tank, this is remove only blockages in the fuel line.

Blocked fuel lines causing vacuum in tank,I've just had same problem 

An update on this - after changing the vapour seperator it has done 100 miles without problems on a near-empty tank.  I gave the fuel lines a blast and changed the fuel filter again just in case.

So it looks like the problem has gone away.  Hopefully, since its about to go on a 10 day tour around Europe (France, Germany, Czech, Poland) as a last hurrah before the engine swap.

I'll keep the spare fuel pump with me.  Just in case.

Many thanks for the suggestions.

The half full  is a clue.
I would suspect a fuel pump internal ones can hole the intank interconnect, I have seen them fall off as I was removing them as that hose fell apart. Good first sign of inter-connect hose issues is over pressurizing the tank as half the fuel goes out the hole at 45psi and half the fuel goes to the sump/filter/swirlpot.

If you hear a Whine from the right hand rear wheel arch when this is happening, then your issue is a Fuel Pump, as the Main pump likes to whine about over working… until it overheats and stops.  

When it is hot like this it can lacquer the fuel on the vanes, over time and actually become unusable as the clearance between the vanes and the housing are nil…..
Rust can also stop a pump or severely impede the flow.

The sump/filter/swirl pot that is between the main pump and the in-tank pump is where the main pump sucks fuel from.  If the in-tank pump is failing from use, rust or what not, the main pump is forced to draw fuel from the Tank via the sump, so it has to work harder.

The in-tank pump uses the fuel in the tank to keep it lubed, and cool.  So when it stops it becomes an impediment to the main…

The 20 minute cool down to restart is also time for the in-tank pump to cool off .  

On a Digi- you can remove the in-tank, and external pump, the sump, and old crusty hoses and replace with a noisier in-tank pump that does the job, the worst is that less than a quarter of a tank, you might have a fuel starvation issue on a hard turn.

The best part is that the new in-tank pump is about the same as the current in-tank pump.  The flow rate and the pressure is within spec as well.

On a digi the working pressures and rates of the in-tank and external pumps are identical.

Now CIS is different for the external, and this can't be done as easily.

I have also had the fuel pressure regulator do some wonky things as in if you drove the car on a full or empty tank after shutting it down, it wouldn't pressure rise the fuel rail….for yep about 20 minutes so no start.

I have had them fail and bypass all fuel back to the tank.
The Fuel pressure tests and residual pressure tests from the Bentley nailed that as bad both times.

I have had the vacuum diaphragm fail and pass fuel into the TB.

my two pence.

its a carbed car, no in-tank pump

I'd check that your tank doesn't have particles floating around in it.  There is an inspection hatch under the rear seat.  It may be due to a rusty fuel filler neck which is very common.