Prospect of warmer weather coming in Spring brings back my nightmare of having to get rid my 940 TD IC Wagon; but this tough and hard-working companion can only possibly be replaced by another one of the same, hard to find in good condition these days ! Besides, I'm not dishonest enough to sell it without saying what the problem is, so nobody in their right mind will buy it !

Trouble started last summer when it was so hot 6000 people died here during the heat-wave. Engine will only start when totally cooled down, ie means leaving it overnight. Otherwise starts from cold, runs and idles perfectly, although some of a smokescreen behind when accelerated hard. 245000 kms on the clock as yet.
Since the weather cooled down last fall, it's only happened once or twice on warmer days.

Examined by several specialists including the largest Volvo dealer in Paris who spent (and charged for !) 8 hours of labour without being able to fix it or even find anything wrong. So I said, what do you propose ? They said, well just change the whole engine. I said, OK but are you sure it will fix the problem, they said, er, no.
I could go through a list of everything that was checked, but that would be boring to any proficient mechanic !
Anybody come across this ? Volvo or other turbo diesel ?

Bonjour Mon Capitaine,

May this find you well. When you say that your car won't start, do you mean that:
(a) it cranks, but will not catch at all (no sign of combustion)?
(b) it catches, coughs and then dies?
(c) it catches, but runs briefly/roughly then dies?

I know little about diesels, never having owned one. I offer some thoughts because you seem to be in a real bind. I am sure that others, more diesel-smart, will follow.

Two temperature-driven changes, typically affect car operation:
(a) water in the fuel (usually the result of condensation as warm moist air is cooled [this tends to improve as ambient temps rise])
(b) a fuel injection system temperature sensor, that fails.

In your case, I'd bet there's a temperature sensor that shorts-out (stops working) or mal-functions (sends an erroneous signal), when it gets hot.
Without that sensor's output - or with an incorrect output - the engine won't catch.

The sensor could have been damaged by the terribly high temps of last summer. Engine coolant temp could have soared, as the radiator could not shed heat fast enough, owing to high ambient air temps.

Is there an air intake temperature sensor or a coolant temperature sensor, that is connected with the fuel injection system?

If there are no such sensors, stop reading here: what follows is irrelevant.

If your car has one or both such sensors, you may want to continue.

Check these sensors. Before ambient temps rise, you might do the following:

(a) On a chilly day, when you'd expect the car to start fine, test an air intake temp sensor by using a hair dryer to blow warm air over the sensor, to simulate the warmth on a hot Paris July day. Use a thermometer to measure the air temp in front of the sensor. I'd not exceed 40 deg Centigrade (104 deg F). If the car won't start, that implicates that sensor.

(b) if there's an engine coolant sensor, replace it, and park the car in a heated garage over-night, so the ambient temp is, say, 20 deg C (68 deg F) (the dealer likely will accommodate you). If the car starts, then that sensor was the problem.

It could be that when the engine coolant reaches normal operating temperature, the damaged sensor "has a nervous breakdown". The engine will run. However, once you shut it off, the damaged sensor won't work, until it cools and re-sets itself.

Hope this helps.

Yours faithfully,

spook

I have no experience of Volvo diesels but I had this problem a few years ago on a Perkins engine and it turned out to be the electric fuel shut off (engine stop) valve sticking closed when very hot (these valves are usually fitted on the injection pump).

Colin.

1990 740SE B200E/M47, remote C/Locking.

Hi Colin

That was one of the first things I looked at, so did the Volvo dealers, and it seems OK but as this has been mentioned several times by others, I think I'll change it anyway (for a new one from VW, less rip-off than Volvo...)

Cheers
Cap'tan Pete

Hi Spook

Wow that was a quick reply, thanks !

What I mean by not start is, it'll crank until the battery and starter motor are blue in the face, but not the tiniest cough. Won't even push-start, I don't mean someone actually managed to push the beast, but we did try in 2nd gear down a steep local hill !

I'll now go and check up on some of your suggestions, in case the local Volvo people didn't.

Cheers
Cap'tan Pete

If it cranks at a decent speed, and there is no smoke out the pipe, I would suspect a sticking fuel shutoff valve.In the following diagram it is called a 'magnet valve' http://www.dieselkorea.co.kr/images/photo_bosch%20type%20VE%20injection%20pump_002.gif
you might be able to pull the valve and clean it. you should be able to substitute one from any VW 4cyl diesel. Possibly a lubricity enhancing addative in the fuel might help? Also you should have battery voltage at the valve with the key in the crank or run positions. If a jumper wire solves the problem, you might have ignition switch/radio relay issues.
hope this helps.

Hi there TurboGreasel

The valve, which I suspected straight away, seems to work. When the problem happens in hot weather, I disconnected it and you can hear the solenoid clicking up and down if you touch the connector onto the terminal.
All the same I'm going to change it.
Interesting URL link you sent me ! Nice to have a diagram of that pump. Thanks.
Cap'tan Pete

Dear Cap'tan Pete,

Good p.m. I have a further thought, which you might want to try first. Your car might have an RPM/Crank sensor: all gasoline/petrol-fired 940s have them. I think that 740s and 240s also have them. This sensor is located atop the bell housing, which connects the engine and transmission.

The crank sensor reads the rotation of the flywheel (gear-shift) or flex-plate (automatic). When the starter operates, the sensor signals the fuel pump to keep running. Without this signal the fuel pump does not run, so no fuel reaches the injectors. The engine will not start,for love or money.

At least, that how this device works on a gasoline/petrol-powered car.

If there's a crank sensor on diesel-fired models, you'll find it as follows. On the passenger-side of the engine, go to the back corner. Look down towards the ground. If there's an RPM sensor, it will sit right on top of the bell housing, just on the passenger side of top dead center. The wires run straight up and hook into the harness, on the firewall.

If there's a crank sensor, and it's the original, it may have been cooked by last summer's heat. It works when its cool. When it gets hot, it shorts out.

The total failure to crank is consistent with crank sensor failure. It is not an expensive part (about $40-50 here, probably twice that in France).

Hope this helps.

Yours faithfully,

spook

G'day Spook
"La nuit porte conseil" as they say around these parts ; Indeed any trouble of this kind from various sensors should only happen with those who cut out the engine some way. For example, the coolant temperature sensor on the block only activates the warning lamp on the dashboard. An overspeed device does cut out the fuel supply on a diesel engine so it's a good lead which I'll try ASAP ! Unless it's a mechanical device.
To tell the truth I know there is one, but have never dared to try it by over-revving, this 6 cyl VW turbo motor already revs too high for my liking - redlined at 5500 RPM which is as far as I went (when the poor old lady had less mileage)
Will keep you posted
Cap'tan Pete

I have tested the VW rev limiter.
It is all mechanical, and the only way to defeat it is if you VW chooses to give you first when you are reaching for 5th. Their shift linkage is good for that. I think the motor will hang together arond 7000 in a no load situation like that, untill rod stretch lets the piston hit a valve, or maybe even the head surface itself. My non IC only gets 5200 ; (

You're a brave guy to test that limiter ! I couldn't find an electronic limiter described by Spook so that's it : Mechnical. Don't think it could prevent hot start ?

Dear Cap'tan Pete,

Good a.m. and may this find you well. I beg leave to point out that the device I described is not a speed limiter.

Rather, it is a device - present on gasoline-fired cars (possibly not on diesel cars) - that prevents fuel being sent to the engine, if the engine is not turning. It reads the rotation of the fly-wheel or flex-plate and signals the engine computer to order the fuel pump to send fuel to the engine.

If an accident damages and so stops the engine, this device will cut power to the fuel pump. That prevents fuel from being moved towards an engine, which may be on fire.

If this device - a crank or RPM sensor (Volvo Part No. 3547847-8) NOT a speed limiter - fails, the engine dies. If the wires to this sensor deteriorate with time, it will work intermittently, until it stops working entirely. A failing crank sensor definitely could work when cold, but not work when hot.

Ask your Volvo dealer if your car has this device. It is located on top of the transmission bell housing, just to the passenger side of the center line. It is visible from the passenger-side, rear corner of the engine. It is held on by a single 10mm bolt.

I'd be surprised if there isn't one.

Am I right to think that coolant or air temp sensors proved not be present?

Keep digging: you'll find the cause.

Yours faithfully,

spook