courtesy of Bob, The Auto Answer Man
My husbands '98 Chevy Z71 pickup has been sitting for a while and we desperately need it. For about three to six months, I would be driving along and then everything would die. I'd pull over, wait a few minutes, and then it would start up. I never knew when it might happen again.
Finally, it did it and would not start again. We had it towed home and it has been sitting for two months until we could get the money for a fuel pump. We put the new one in and all the stuff that runs off the battery works, but anything else will not work. For instance, we cannot start the truck! From sitting so long the battery was dead. I had it checked and it's still a good battery. After having it charged up, the truck still will not start. When we tried jumping it off from another vehicle, it went completely dead. Now what? I know you mentioned in a recently that it could be a relay switch, but I hate to put more money into it if it won't work. Help!
Well, we need to find out what is not allowing it to start.
In order for an engine to start, a few things need to happen. First, and most obvious, the engine needs to turn. Is the starter turning the engine?
Then, at precise timing, we need a proper mixture of air and fuel. Is the fuel pump actually working? The best way to check would be to measure the fuel pressure at the rail on the engine. There is a valve that looks similar to a bicycle tire valve (it's called a schrader valve). With a fuel pressure gauge, you can check the fuel pressure. It should be close to 40 psi or so.
If you are getting fuel, the next thing you need is air. Air is pulled into the engine by vacuum. The vacuum is created by the pistons moving up and down and the valves opening and closing at the correct time. You should be able to feel some air moving either by the air filter or by the exhaust pipe.
Now, assuming the engine is spinning fast enough and there is the correct amount of fuel and air, the next part of the equation is spark. We need to have the spark plugs fire at the correct time. But, before we worry about that, check to see if there is spark at all.
The easiest way to do this is to take the spark plug wire from the spark plug and put a spare spark plug on the wire (leave the original still in the engine). Then, find a place to ground the terminal on the spark plug (the engine block should work nicely). Now, ensuring that your hands and fingers are all in a safe place, have someone try to start the engine. You should see a blue spark at the end of the spark plug. If you don't, then we need to find out why.
If any of the above items are not present (i.e. not enough or too much fuel, not enough air, or no spark), the engine will not start. Once you have that information, it would be easier to try and narrow things down. Each individual problem can have a bunch of reasons behind it.
Bob, The Auto Answer Man
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