With this particular cold start issue, a possibility could be related to the diesel fuel waxing up during cold temperatures caused by the fuel not containing winter mix / additives, or its does and its even more colder than the winter mix is designed/produced for.
If your symptoms is not occurring frequently and only during very cold conditions, then this could be a possible cause. The remedy is to ensure that when you're entering cooler / cold conditions such as ski fields / alpine regions that you have filled your fuel tank up with winter mix.
Another scenario is outback Australia were its get fairly hot during the day, but bloody freezing at night, in this case none of the fuel outlets will have winter mix, and so to rememdy this you go to a local auto store or fuel station and pick up fuel additive ensure that you add to the tank when you expect these conditions are about to occur.
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A subject to discuss and expand on is when you don't have access to winter mix / fuel additive is the use of a small amount unleaded petrol to help prevent waxing. I've read a few articles and they state pros and cons of doing this. Please until I've fully understood it myself do your own research and if anything lock it away as an option if absoloutely needed when you are unable to obtain winter mix / additive and there is a change you're about to encounter cold condition and you fell you diesel will wax.
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2. Low Compression
As expected with diesel engines the fuel is ignited under the correct compression ratio and heat. If one or all of your cylinders are starting to experience low compression, then this could make it harder to start a cold engine.
I found the following article - "Compression Test" useful for detailing Compression Tests on both Petrol and Diesel engines.
3. Air in Fuel Line / System
There are a few causes that may cause air to enter the fuel system.
a. Fuel Hoses - There are a few to check / replace
- Input and Output fuel hoses of the Fuel Filter / Primer (8mm I/D)
- Return Fuel Hose (6mm I/D) from Injector Pump to pipe back to the tank.
- Sender (8mm I/D) and Return Fuel (6mm I/D) Hoses on the Tank End.
- Spill Over / Fuel Leakage Pipe (small pipe from middle injectors back to injector pump).
These hoses can perish over time. This could lead to air entering the system and you may not even be able to find the crack. It only cost about few bob per meter of fuel hose worth the investment and piece of mind nothing that you've done them.
The other part to this one is the fuel hose clamps, make sure they are secure / tight proper hose clamps (smooth inner bore) not standard jubilee clips that will tear / pierce into the host
b. Clogged Fuel Filters
Obvouiosly this can affect performance of a diesel engine, and eventually hinder even stop a diesel engine. Make sure you replace fuel at the specified km's / interval.
c. Water in the Fuel / Fuel Filter Water catchment full
Water in the fuel system can affect the performance of a diesel engine, cause it to stall and if large enough quantities kill the engine (ie water in via the air intake).
The fuel filter water catchment chamber that collects water from the fuel system may be full. Normally fuel filters have a water sensor that screws into the buttom of ther filter which will alert the drive when the chamber is full or nearing full. But this should normal not occur between regular fuel filter replacements unless you have encounter a very dirty source of diesel or water has contaminated the fuel tank some how.
d. Fuel Water Sensor leak
The water sensor that screws into the buttom of the fuel filter should always have the o-ring / replace before being screwed into the filter. This ensure that leak are prevented. However leaks can still occur if the plug is not total and securely screwed into the fuel or worse and shamelishly the water drain plug is not tightly shut off /screwed on.
e. Fuel Filter / Primer leak
The fuel filter seal onto the primer may not be air tight. Each time you replace your fuel filter you should make sure you add a little oil / fuel on the new seal of the fuel filter before screwing it back on to ensure a good air tight seal.
The Primer pump itself can be a source of air leak.
f. Fuel Pickup pipe corroded within the tank can allow air leak
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g. Fuel Pickup Filter and by pass can be blocked / dirty.
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4. Engine Block / Sump oil Heating Unit Failure
Depending on were you live and the weather condition you experience, vehicle come with or have aftermarket block heaters fits which plug into mains to heat the engine sub oil and block to enable diesel engines to start easier / possible. I've have no experience with these units but can appreciate that they can and do break down. And when the breakdown your engine maybe next to impossible to start without the vehicle being kept in a warm area.
5. Glow Plug System Faulty
The function of the glow plug is to heat the cylinder area to help diesel ignite inconjuction with the correct compression ratio. It usally comes down to glow plugs for a diesel engine having trouble starting cold. There are a few components within the Glow Plug System that could fail causing the failure to correctly heat the cylinders during a cold start.
a. Faultly Glow Plugs - Short / Open circuit
When testing Glow plugs, I've read there are two typical faults that occur with the glow plug itself (Open / Short Circuits).
You can test glow plugs in situ, however only for Open Circuits. To test glow plugs in situ you will need to remove the interconnecting powe bus bar / rails connecting each glow plug else if at least one of them working the test would always work. Once you have the bus bars off, if you use a test pen / lamp and connect one end to the positive terminal and then touch the tip of each glow plug, if it lights up then the circuit is complete and the glow is allow power through it.
However if there is a short with in the glow plug then the previous test will work, but because there is now a shorter path for the current to travel the plug will not heat up.
The best test is take them out and visually inspect them but applying power. To save me a bit of time, the following articles are brilliant source of information about glow plugs and how to test them. Even though its targeting VW vehicles, majority of the concepts are shared amongst all diesel engines:
VW Diesel Glow Plugs
VW Diesel / Glow Plug System
VW Diesel Glow Plug Control Circuit
One important bit of advice from these article I got and that is you should always test NEW Glow Plugs, just because they are new doesn't mean they are not faulty.
Here are some You Tube videos, the first one is current proedure for testing glow plug, the second video, well lets just say I wouldn'y advise it, but its cool to look at :).
Video 1 - VW Rabbit pick-up diesel glow plug test
Video 2 - Glow plug test
b. Incorrect Glow Plugs for the engine
I've read of instances where people have incorrectly put glow plugs for the Turbo Diesel Intercooler engine into a Natural Aspirated Engine causing it to be a very hard start. The other scenario is putting 12v Glow plugs into 6v systems meaning the glow plugs don't heat correctly.
Another common fault I've read about is that of "false ecomony" were people have experienced the hassles of purchasing cheaper brand of glow plugs but only to find out they are fairly short lived and start playing up.
Please consult the catelogs and look for the glow plugs for your vehicle and engine type (ie Turbo, Intercooled). Also stick with the tried and tested brands of glow plugs.
c. Poor or no power supply to Glow Plugs
There are a couple of places along the glow plug power supply line that could cause problems.
First test is to perform is to see if you are getting power to the glow plugs. This is fairly straight forward in that using a multimeter on DC volts mode you put the negative probe to the negative terminal or a good ground source on the chasis. Then put the positive probe on the 1st glow plug. Have someone else turn on the ignition to warm the glow plugs and you should see 6v or 12v depending on your vehicle for about at least 10 sec (even after the glow plug dash board indicator has went off). Repeat this for each glow plug.
Even if you are meauring correct voltage there could still be an issue with the power supply in regards to poor connections or faulty wiring which prevent the correct amount of current to be supplied to the glow plugs. Remember that glow plugs on average draw upwards of 36/48 amps depending on number of cylinders and voltage 6v or 12v - see here for more.
Second test to perform is to trace / check the wiring to the glow plugs, look for poor connections from battern to glow plug relay and then onto the first glow plug. Make sure now of the connections have corrusion and are tightly secured and making a good connection. With the pajero and many other diesel egines, the power is wired to the first glow plug and then distributed to the other glow plugs via a power bus rail (several metal strips interconnecting each glow plug. Each of these connections need to be checked and cleaned securely bolted to the glow plugs.
If this has resolved your issue, the next test to check for faulty glow plugs see previous section.
d. Malfunctioning Glow plug Control Module / Relay
If even after checking the glow plug power supply (fuse, wiring, power bus bar/rial) and assuming you have a good battery then it starting to indicated that your glow plug control module and or relay is stuffed.
The easiest way to test this is power / jump the glow plugs manually using a length 6mm wire with appropriate connection to bolt to the first glow plug and run to the positive battery. If you clip it on to the positive and wait say 10 seconds and then attempt to start the car.
If this resolves your troubles and the engine starts, then you will need to look at replacing the glow plug control module / relay or if that looks like its going to be too costly, I've read articles on how people have installed a proper manual power circuit to power the glow plug manully from the vehicle's dashboard. Please read this article - "Pajero Glow Up"
6. Injector Pump
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7. Injectors
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