I have a 2001 Jeep Cherokee Laredo. It has several issues no-one can figure out. First, it does not always want to start, it just doesn't catch you can hear it trying. Second, sometimes it just dies while I am driving or most of the time when I am applying the brake for a red light. Third, sometimes when I finally get it to start it doesn't idol right. I have to press on the gas constantly or it dies, finally after a min of me making it idol it will start to idol properly. They have replaced idol control valve, alternator, the battery and have checked plugs and wires. The mechanics are baffled and don't know what the problem is, the most recent diagnosis was that I need to replace the crank? Just wondering if maybe somebody can help me
This is one of the hardest types of problems to find. There are a number of tests that have to be performed.
All "crank, no start" conditions are approached in the same way. Every engine requires certain functions to be able to run. Some of these functions rely on specific components to work and some components are part of more than one function so it is important to see the whole picture to be able to conclude anything about what may have failed. Also, these functions can ONLY be tested during the failure. Any other time and they will simply test good because the problem isn't present at the moment.
If you approach this in any other way, you are merely guessing and that only serves to replace unnecessary parts and wastes money.
Every engine requires spark, fuel and compression to run. That's what we have to look for.
These are the basics that need to be tested and will give us the info required to isolate a cause.
1) Test for spark at the plug end of the wire using a spark tester. If none found, check for power supply on the + terminal of the coil with the key on.
2) Test for injector pulse using a small bulb called a noid light. If none found, check for power supply at one side of the injector with the key on.
3) Use a fuel pressure gauge to test for correct fuel pressure, also noticing if the pressure holds when key is shut off.
Once you have determined which of these functions has dropped out,
you will know which system is having the problem.