Does anyone know how the system operates? Do the glow plugs switch on for preheating every time the ignition is switched on, or only when the coolant temperature is below a certain level such as less than 5°C? If that is the case do the glow plugs pulse when the engine is started or not, or do they not work at all when the engine is started warmer than this? How long does the engine have to run for before they switch off, assuming they do pulse to aid cold running?

Will the ECM recognise a fault if one or more glow plugs were open circuit? Where is the glow plug relay located?

The ECU controls the operation of the glowplugs. There is a light on the dash that come on whilst the glowplugs are preheating. When the light goes out you can start the engine. After the engine starts, the glowplugs remain on for a futher period determined by the ECU.

NB. If you don't start the engine the ECU will switch off the glowplugs after a timeout occurs. This prevents the battery draining.

I don't think the ECU would detect an open circuit to one of the plugs. They can be tested with a multimeter. The resistance should be about 1.5 ohm.

Cant fault any of that, spot on advice.

I can expand a bit on that after checking my own car. The glow plug relay has four terminals. 30 is battery live, 87 goes to the glow plugs, 86 is ignition switched live and 85 is ECU controlled earth. When you switch the ignition on terminal 86 is live, but the relay will only switch on if terminal 85 is earthed via the ECU. This is of course dependant on what the ECU says the glow plug requirement is.

I still don't know though at what temperature the ECU decides the glow plugs are required for preheating. I guess it's probably somewhere not much above freezing, but it would be nice to know for sure.....

IIRC is 8 degrees c.

I'm pretty sure that the manual says 4 degrees C

9c. And the initial 'on/off' of the glow plug light that takes about 1 second is NOT the glow plugs. It's an indicator that the engine ECU is getting power. It's one of the clues when relay 109 is dead, that the glow plug light doesn't flash. Under 9c it will stay on longer than the flash, indicating that the glow plugs are having preheat. They can also be coded to come on longer and at higher temperatures, we do this on single tank vegoil conversions (should really be twin tank, but the VE tdi takes to it pretty well) which like more glow time than diesel.

If you have starting issues, and it's not COLD - start by checking the static timing via diagnostic computer (vag-com or similar) in the injection pump and then look at air ingress. These cars do not rely on the plugs, like old IDI pugs, think of them as an assist for extreme cold.

Greg.

These cars do not rely on the plugs, like old IDI pugs, think of them as an assist for extreme cold.

Greg.

That can only be partly true though, as we all know a diesel engine is a compression ignition engine. Only gases burn. Liquids and solids do not. Unlike petrol, diesel fuel does not vapourise at low temperatures. That is why the compression ratio on a diesel is twice as high as a petrol. The intake air needs to reach a temperature of around 500°C for the injected fuel to vapourise and ignite. This is much more difficult when the engine is cold, as so the need for some sort of preheating.

Direct injection diesels often don't need glow plugs, but quieter, smoother indirect injections do........

Like I said, they don't rely on them like old idi's did. I didn't say they don't use them, but I could case several DI's which have no glow plugs not option for them and are sold for the UK market. The Ford York 2.5di was sold with glow plugs optional and no other form of cold start assist (fluid/grid heater) for example.

As for quieter and smoother, I'll take a 130pd for NVH over a 1.9td pug any day. The idi sounds like a bag of nails in comparison. 1st gen York and Perkins Prima (Rover Montego et al) single stage di's were naily though

Greg.

Direct injection diesels often don't need glow plugs, but quieter, smoother indirect injections do........

That can only be partly true though, as we all know a diesel engine is a compression ignition engine. Only gases burn. Liquids and solids do not. Unlike petrol, diesel fuel does not vapourise at low temperatures. That is why the compression ratio on a diesel is twice as high as a petrol. The intake air needs to reach a temperature of around 500°C for the injected fuel to vapourise and ignite. This is much more difficult when the engine is cold, as so the need for some sort of preheating.

Direct injection diesels often don't need glow plugs, but quieter, smoother indirect injections do........

SO how come fuel vapourisation is a known problem with hot starting in a petrol engine then? I was always taught that you need a finely atomised emulsion (fuel droplets in air) for combustion, not fuel vapour, and this was why all injection engines run better (power and/or economy) than an otherwise identical carburetor unit (the injection gives a finer emulsion).

Because if the injector is timed for a certain milisecond to give the correct amount of liquid to make a 14.7:1 mix with air (or whatever it's trying to do) and only vapour comes out, the wrong weight of petrol will come out and the mix will be way too lean. It won't start.

Vapour by it's self isn't an issue, if the system is set up to deliver and meter it. For example LPG gas conversions, which on the multi systems inject, with injectors, propane vapour. There are also other systems, with injectors, which inject liquid propane. They have very different injectors and inject a much smaller amount by volume. By weight, both systems have to inject the same as both systems are trying to get the same amount (without getting too complicated) of fuel in.

HTH,

Greg.

SO how come fuel vapourisation is a known problem with hot starting in a petrol engine then? I was always taught that you need a finely atomised emulsion (fuel droplets in air) for combustion, not fuel vapour, and this was why all injection engines run better (power and/or economy) than an otherwise identical carburetor unit (the injection gives a finer emulsion).