Recently we had occasion to diagnose and repair a 2000 Chevrolet Blazer with a fuel injected 4.3l V6 that would not start, and was not throwing any error codes. The owner informed us that it ran fine, but then one day would not start. They had a mobile mechanic come out to look at it, and after many hours of diagnoses and random parts replacement he gave up in frustration.
This seems to be a very common problem on these engines, and it isn't particularly through any direct fault in the design. It is just a really hard problem to diagnose, as it was in this case. The previous mechanic tested fuel pressure, cleaned the distributor cap and rotor, checked compression, and many other things. But there were some minor signs that were overlooked.
When we got the vehicle, we gave it a preliminary look-over to check the basics. Spark plugs were good, fuel was getting to the pressure port on the rail, the fuel pump was running when it should. Spark was getting to the plugs. That left things like timing, final fuel delivery, or airflow. To eliminate potential airflow issues we removed the air filter temporarily.
After cranking the engine, we pulled the plugs and noted absolutely no fuel was reaching the cylinders, even though it showed pressure in the rail. We attempted to start the engine with ether (not recommended except for testing purposes) and it fired up and ran smoothly. However it did exhibit some driveablilty issues, notably lack of acceleration and ability to re-start on its own.
We put a fuel pressure gauge back on the rail test port and noted readings under various conditions. When just sitting and revving the engine, the fuel pressure would increase from its stable (and acceptable) 50 psi to about 60 psi. When running, it stayed stable at 50 psi. Which while on the low end, is acceptable. When starting the pressure would begin at 50 psi and rapidly drop down below 40. And this seemed to indicate our issue.
In original form, the GM 4.3l came equiped with a central 'spider' fuel injector, with a tube running down to each cylinder and a poppet valve on the end of each tube. While the pressure we were getting was fine for the injectors, it was not enough to initially open these poppet valves during starting conditions. So the problem was narrowed down to fuel filter or pump.
We decided to eliminate the fuel filter as a problem candidate primarily because when coming up from zero, the fuel pressure snapped up to 50 psi almost instantly. A clogged filter would provide more restriction and a slower increase. This left the fuel pump as our primary suspect.
After replacing the pump, the engine fired up instantly and ran without issue.
It is interesting to note that the previous mechanic did not focus on fuel issues because he noted the initial pressures were within acceptable range. Instead he focused on spark and cranking like the starter (why we don't really know). Obviously in hindsight the problem is obvious, but at the time it could have been a number of issues. However most of the potential issues can be quickly eliminated with proper troubleshooting.Although the 'spider' fuel injection system is not the best design, it is often suspected as a candidate for non-start issues. There is even a replacement unit that uses individual injectors on the tube ends rather than poppet valves. While a better design, it is often not strictly necessary to replace this system for the newer one. As long as a good quality fuel pump and filters are installed the system should function normally as long as the pump remains strong. Obviously a downside with this system is that the fuel pressure must be in a very tight range in order for proper starting. With a direct injection system fuel will still reach the cylinders (albeit less of it), while with this system and a weak pump no fuel at all can initially open the poppet valves due to reduced fuel volume in the lines. The pressure may be very close to specification, but there is no good way to measure the volume. Reduced volume may be very hard to detect and will prevent initial startup. We recommend watching the fuel pressure carefully in all conditions before condemning any specific part of this system. While spark and timing can be issues with this engine (as with any engine), fuel problems are more common. primarily for a quick diagnosis, if the engine will start with ether or starting fluid or gasoline poured in the intake, and run mostly ok afterward, then suspect the fuel pump.