Troubleshooting Cold Hard Starting in Diesels
Perhaps the greatest mystery with my VW Jetta TDI was cold hard starting. That means, the engine is stone cold, usually after sitting overnight. In the morning, the car would just crank over starved for something. After the third attempt, it would stumble into starting and I quickly applied the fuel to higher RPM. However, sometimes, the engine would die before I could even do this and then the process would be repeated. The mystery was once it started, the car ran great. It accelerated fine and if parked for periods of up to 8 hrs., it would start instantly like it was supposed to, even if the engine temp indicated it was cold. It was just starting after sitting overnight.
This went on for a few months and a few trips to VW. There was not an engine code indicating there was a problem. So, we changed the coolant temp sensor, the fuel temp sensor, replaced the tandem pump (I have an 2006) that is a mechanical fuel pump, the fuel filter and high pressure fuel pump. Because there were engine codes triggered it was an expensive process of elimination that did not eliminate. It was obvious to the pros, the issue was fuel related. The baffling issue why would the car start immediately throughout the day and run great? Yet, overnight, the car refused to start? There seemed to be a correlation to temperature because that was the coldest time for extended duration. Even though the coolant and fuel temp sensors did not throw a code (codes are not shown all the time. Sometimes, a code will not show until it happens more than once) they were changed. The tandem pump was suspect since the car has 200K on it. This creates additional fuel pressure to boost the High Pressure Fuel Pump. Many with this issue, installed a new one and solved the problem. Of course, doing a compression check and a leak down compression check would be worth the effort for the health of the engine. A compression check revealed it was normal. Checking that the Fuel Pressure Regulator Valve is still good is important. As a bad one, causes faults with injectors. Checking the Fuel Return Line Pressure Valve is also important because it maintains pressure at the injector. For my Jetta, this is 14.5 psi. The pre-injection stage for this car should have a fuel pressure of 2611 psi when starting.
As the mechanic conducted pressure checks in the fuel system, it all was according to spec. This had the mechanic shaking his head in bewilderment. When the mechanic hooked the car to the VW diagnostic computer, a code appeared, PO302- misfire on cyl. 2. A misfire can be caused by many things but it did give direction.
Cutting to the chase, the cause of this mysterious problem was a bad injector. The car was starting on 3 of the 4 cylinders. Once the car even ran for a mile, the bad injector worked fine the rest of the day. This still baffled the mechanic. It would seem the problem would be continuous. It was found that while the electronic part of the injector was to spec (regulating fuel), the mechanical part of the injector (special to 2004-07 models) failed to maintain the fuel pressure over long periods of time (i.e., overnight). The fuel pressure is in a closed system and any vacuum leak or pressure leak by an injector will cause problems. The pressure must be maintained by all the injectors and throughout the fuel system.
VW injectors are not cheap-between $500-1000. Rebuilt ones should be as good as a new one since they are taken apart and new parts are used. You can replace just one injector, you do not have to replace all at the same time, despite what a dealer may state!
The lesson is that a cold hard start probably is related to fuel pressure, or lack of, somewhere in the system.
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