Chevrolet Astro 4.3l Crank No Start

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.

Chevy4.3LCamshaftPositionSignal

Chevy Astro 4.3L camshaft position sensor signal, bad signal from loose timing chain

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.

This post was written by: Martin Hand

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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

2 Comments

  • Jon Reeves says:

    Hello Martin
    I have not worked on one of these old astros for a while. I assume it is a vortec by the fact that you scoped the crank sensor. If I remember correctly they take at least 55psi fuel pressure to overcome the pop-it valve in the injectors. It seams that you eliminated a fuel problem with the starting fluid in the intake.

    I was disappointed to read that you could “tell by the sound” that it has good compression. It literally takes less than 5 minutes to put an amp clamp around the starter cable and do a dynamic compression test with your scope.

    I would have like to have seen the cam and crank signals captured together, that would have cleared up any suspicion of valve timing issues. Spark timing comes from the crank signal, the distributor and cam signal has nothing to do with spark timing, only distribution. With that said, if the cam was “bouncing around” the delta time would vary, it looks fairly fixed to me. The only anomaly I see on the cam signal waveform is at the top of each square wave, and that I suspect is caused by the battery voltage being pulled down by the starter motor (you can see it fluctuate between about 8-10 volts).

    Anyway I would be surprised if a t-chain fixed it. Please don’t take this as negative criticism but as an honest analysis of the post. Also if a t-chain did fix it shoot me an e-mail and let me know, if you feel like it, and if you read this far thanks for your time.

    • Martin Hand Martin Hand says:

      Thanks for the reply. Yes the timing chain fixed the problem. Not three months after this on we had the same issue except this one was setting cam crank sensor correlation codes which made the diagnosis simple (engine ran poorly). You can just grab the crank pulley and feel the loose chain slack turning it back and fourth. Keep it simple don’t overthink it! There’s always a logical explanation to the problems we find in these vehicles.

      Not that it matters but you wouldn’t use an amp clamp for a crank no start diagnosis. I’m not looking for a dead hole just need to know if there is compression so the engine will fire.

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