1999 Chevrolet Astro 4.3L engine cranks but will not start. This would normally be fairly easy problem to diagnosis. The problem is consistent so we just begin the process of elimination. We need fuel, ignition and good compression to fire. Listening to the engine crank I can tell there is compression so I will start with fuel. Checking the fuel pressure is easy, there is a valve test port you just have to remove the dog house. The fuel pressure checks good about 38 psi while cranking. To eliminate any other fuel problem possibility’s we tried spraying starting fluid into the intake to see if it would fire and no change.
Next easy check would be spark. Checking spark at the coil shows good strong spark. A common problem is spark jumping inside the distributor cap, it had tracing marks so I replaced the distributor cap and rotor. It was a typical problem we have seen before, causing the engine to try and start and randomly kick back. Breaking the nose cone of the starter and even sometimes grinding the teeth of the flex plate gear. I thought we had fixed the problem but no, it continued to have the same symptoms with no change. So now I’m starting to think about what I could have missed and broke out the lab scope. That crank signal was good but we had something funny with the camshaft position sensor signal, take a look.
You can see that the sensor is doing it’s job switching on and off creating a square wave but the off time signal is bouncing up and down. It took me a while to figure out what this means. The timing chain was completely stretched out and flopping around. The mechanical timing was never correct, when you rotate the engine to check the distributor to be timed correctly the slack is taken up in the chain. While trying to start the timing would jump around as you can see in the cam sensor waveform. This is a rare problem in my experience though. The chain is very short and I never expected it to develop enough slack to cause a crank but will not start condition. Never say never, stay humble and always keep an open mind. Thanks for reading.