Intermittent misfire only when hot (motorway cruising)

[Pootle]

The van has started to misfire when very hot, i.e. cruising at 70+ on the motorway. It will happily pootle around town all day. When it misfires, it doesn't backfire, just stumbles and slows. On that really hot October day two weeks ago it got so bad we had to be recovered back to Bristol, it was dangerous.

The inline fuel filter is brand new, the fuel filter in the carb is clear, the air filter isn't blocked, the pipes aren't kinked. The vacuum advance "clunks" properly when I suck the tube (the van accelerates strongly, even up hills). The van was recently serviced.

The plugs look fine, one slightly darker than the others but is still OK i.e. light grey/brown tip.

The temp gauge usually stays just below the LED, at 70+ it moves just on the LED or very slightly right.

I replaced all the HT with Brickwerks stuff back in June this year, I've driven a thousand miles or so since then:
* Spark plugs - Bosch W7DTC
* Distributor cap - Shielded type (black cover)
* Rotor arm - limited type
* Ignition coil - pin type [grey label] ZS123
* HT lead set

The ignition control unit, vacuum advance and Hall sender in the dizzy are 5 years old. The dizzy itself is original. The fuel tank was replaced last year. The fuel pump is about 7 years old.

Where should I start?

[kevtherev]

Where should I start?

By slowing down?
Sustained 70 plus is not a good speed for these engines.
My temp gauge does the same at 70 and if you had an oil pressure gauge you would never sustain 70+ again, the pressure drops alarmingly as the cooling system struggles to cope with the heat removal


However Scotty says your fuel pump may be struggling to deliver the fuel needed to sustain warp speed captain
Was it greased up properly before it was fitted?

Or he suggests the radiator need a de-sludge to be more efficient at warp 7

[Pootle]

My temp gauge does the same at 70

Many thanks Kev, but you've got the wrong end of the stick a bit. I mentioned the temperature gauge to show that the engine isn't getting too hot. The temperature is what I'd expect, and how it's always been for the last 15 years. The intermittent misfire is something new.

I was wondering about the fuel pump, but it only happens when the engine is hot? If it was the pump, I would expect it to happen when cold too.

[kevtherev]

at 70+ it moves just on the LED or very slightly right.

Nope I have the right end of the stick..

Like I said my van did the same and the engine feels rough... never done it since
try opening the hot valve on the heater matrix and see if it still does it

[kevtherev]

I was wondering about the fuel pump, but it only happens when the engine is hot? If it was the pump, I would expect it to happen when cold too.

do you do 70 with a cold engine then?
just to clarify this power loss is only at 70?
or at lower speeds?

[Pootle]

do you do 70 with a cold engine then?

When I first get onto the motorway it's fine at 70+, but after about 15 minutes - i.e. the engine is properly hot - the misfire starts.

just to clarify this power loss is only at 70? or at lower speeds?

Yes only at motorway speeds.

The engine is a 1.9DG with a single Weber 34DAT carb. It's been fine for years, this is very recent behaviour. My normal driving speed is about 60mph, with my foot to the floor the van will (eventually) do about 80. The misfire first started on a very hot day at about 65mph.

I'm not totally discounting the fuel pump. Metals expand at different rates, and it could be that when it gets hot the pump is just failing to deliver.

many thanks