With the information you haver posted here are my best answers. I work on mostly gas engines as well but info is out there for those of us that are looking.
1. Glow Plugs
If slow cranking isn't the problem, perhaps there's something wrong with the glow plug system. Most passenger car and light truck diesels have glow plugs to assist cold starts. The glow plugs are powered by a relay and timer that routes voltage to the plugs for the prescribed number of seconds. When the timer runs out, the relay is supposed to turn off the voltage. But relays sometime stick and continue to feed voltage to the glow plugs causing them to burn out. One or two bad glow plugs on a V8 engine might not cause a noticeable starting problem during warm weather, but it can when temperatures drop.
Glow plugs can be checked by measuring their resistance or continuity. Excessive resistance or a lack of continuity would tell you the plug is bad.
If one or more glow plugs have burned out, are heavily coated with carbon or are not receiving their usual dose of start-up voltage, the engine will become progressively harder to start as temperatures drop, and will idle roughly and produce white smoke in the exhaust for several minutes once it finally starts. If all the glow plugs are burned on the end, you'd better check the injection timing because it is probably over-advanced.
To see if the glow plug module is providing power to the glow plugs, use a voltmeter to check each plug for the specified voltage when the ignition key is turned on. No voltage? Check the glow plug control module connections, ground and wiring harness. The glow plugs themselves can be checked by measuring their resistance. Replace any plugs that read out of specifications.
Hard starting can sometimes be caused by a glow plug module that fails to turn the glow plugs on or doesn't keep the plugs on long enough when the weather is cold. On GM 6.2/6.5L diesels, there have been reports of heat from a still-warm engine causing the 125-degree inhibit switch inside the controller to shut off making the engine hard to restart. The cure here is to relocate the control module away from the engine. On Ford 7.3L diesels, the control module can cut off early if there are two or more bad glow plugs. We have also heard of control modules that do not keep the glow plugs on long enough for easy cold weather starting. The on-time is sufficient for warm weather, but not cold weather.
2. Diesel engines are real misers when it comes to sipping fuel. They're also known for their pulling power and rugged durability. That's why diesels continue to be a popular option in many pickup trucks today. But diesels are also known for their idle clatter, black smoke and cold-weather starting woes. So let's start with the ill-effects of cold weather.
When temperatures drop, several things happen that can make a diesel hard to start. First, the oil in the crankcase thickens. At the same time, battery output drops, reducing the number of amps available to crank the engine. The 15W-40 multi-viscosity motor oil, a popular warm weather choice with many diesel owners these days, may become too thick when temperatures go below freezing or plunge to zero or below. Straight 30- or 40-weight oils would definitely be too thick. The increased drag created by the cold oil can reduce cranking speed to the point where the engine may not generate enough cranking compression and/or fuel pressure to light the fire.
One of the first things you should check when diagnosing a "hard to start" symptom, therefore, is the dipstick. If the oil is thick and globby, it may not be the correct viscosity for winter driving. What kind of oil is in the engine? When was it last changed? Switching to a lighter oil such as a 10W-30 (never anything lighter in a conventional oil!) may be all that's needed to improve cold cranking. For really cold weather, you might switch to a CG-4 rated synthetic motor oil.
The next thing that needs to be checked is minimum cranking speed. The rpm needed to light the fire will vary according to the application, but General Motors says its 6.2L and 6.5L diesels with Stanadyne rotary injection pumps need at least 100 rpm when cold, and 180 rpm when hot.
If the engine isn't cranking fast enough, check battery charge and condition, as well as the cable connections and the starter's amp draw. Problems in any of these areas can make any engine hard to start. If the battery is low, recharge it and check the output of the charging system, too.
3. Unlike gasoline, diesel oil is adversely affected by cold temperatures. Diesel is made of heavier hydrocarbons that turn to wax when temperatures drop. The "cloud point" or point at which wax starts to form for ordinary summer-grade No. 2 diesel fuel can range from 10 to 40 degrees. If the fuel tank contains summer grade fuel and temperatures drop, wax crystals can form in the water/fuel separator, causing a blockage.
The cure here is to pull the vehicle into a warm garage so it can thaw out, replace the water/fuel separator as needed, then add an approved "fuel conditioner" additive to the tank (some manufacturers do not approve any additives or prohibit the use of specific ingredients such as alcohol that are found in some additives), or drain the tank and refill it with No. 1 diesel fuel. To prevent the same thing from happening again, you might install an aftermarket fuel heater.
Water in the fuel is another problem that can cause starting and performance problems. Condensation that forms during cold weather is the primary source of contamination. Water that gets into the fuel tank usually settles to the bottom because water and oil don't mix. The water is sucked into the fuel line and goes to the filter or water/fuel separator (if the vehicle has one). Here it can freeze, causing a blockage that stops the flow of fuel to the engine. So if the filter or separator is iced up, the fuel tank needs to be drained to get rid of the water.
4. A diesel engine that cranks normally but won't start regardless of the outside temperature either has low compression or a fuel delivery problem. If compression is okay, check the fuel gauge (out of fuel?). Then check the fuel filters and lines for obstructions.
If the injection pump isn't pushing fuel through the lines to the injectors, it may have a faulty solenoid. Listen for a "click" inside the pump when the ignition switch is turned on. No click means the solenoid and/or pump need to be replaced. If it clicks but there's no fuel coming through the injector lines (and the filter and lines are not obstructed), the pump is probably bad and needs to be replaced.
ECM could possibly be the issue but there are alot of others possible. My best bet is glow plugs i've seen and have helped friends with you're problem glow plugs salved it.
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