1997 Chevrolet 1500 5.7L - Cranks, No Start, Weak Spark
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General Information:
Customer’s truck was towed in because it would not start. The battery was also dead and looks like it had been sitting for a while.
First thoughts:
On any vehicle, you need the essentials: air, fuel, spark. Of course you can go into more detail with compression, timing, and all the other stuff, but those three are what you can always use to start the diagnostic process for a no start.
These trucks are known for have some common issues that result in a no start.
Fuel pump
Distributor Cap and rotor
Coil and Ignition Module
So those are my first go-to starting point.
Diagnostics:
I put a fuel pressure gauge on the truck to see if it was getting correct fuel pressure. I smelt fuel when cranking on it earlier but that’s not s good enough test to know if the pressure is correct. The pressure gauge showed around 55 psi, which is fine for this truck. So after that point, I wasn’t worried about fuel.
I wasn’t too worried about the air coming into the engine, but I did notice that some of the clamps were loose on the tubing coming from the air box, I tighten them up just in case. This truck does have a Mass Airflow sensor (MAF) so the engine computer does have to read the correct amount of air coming into the engine using the MAF. Any vacuum leaks past the MAF can result in an engine not running To its full potential.
Seeing that there were clamps loose, I also took note that there were new spark plug wires on the vehicle and decided to just check if the wires were going to the correct spark plug from the distributor cap. It’s a possibility that whoever worked on this before I did, did not correctly put things back together, just clicked the clamps from the air box. The wiring though, was correct.
My next steps was to check spark. I took a known good spark plug and connected it to number one cylinder spark plug wire. As someone cranked on the engine, I noticed that it did have spark, but it looked a little weak.
I decided to check the coil and to see how far the spark jumped out of it.*
* This should only be done by a trained professional. Spark from a coil can seriously hurt. Don’t ask me how I know.
The spark from the coil jumped about an inch or more, which is fine for this truck.
So what is wrong with this truck? Good spark at the coil, but bad spark at the spark plug. With, what looks like, new spark plug wires.
I decided to look under the distributor cap.
Just as I suspected, there is a lot of corrosion under the distributor cap and on the rotor.
I put a new cap and rotor on, and now it runs like a champ.
Why wouldn’t it start even though it had spark?
It had spark in the atmospheric pressure, but once the spark plug is in the cylinder with compression, the spark can not jump the gap. The resistance from the corrosion caused a voltage drop that would not all the spark to jump the gap at the spark plug during the compression stroke.
During your diagnostic process, it’s important to eliminate the simple factors that could cause your problem. For example, once I saw the weak spark on a known good spark plug, I could’ve just called it a cap and rotor, but what if it was the coil that was producing a weak spark? It’s easy to guess and hope that you got the problem fixed, but it’s better to take a few extra minutes to check the other factors.
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