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Asked: Jul 04, 2009 - 02:08 PM

Status: Closed

I have a 93 GMC Jimmy that won't start. Already replaced the fuel pump and fuel relays.

I recently replaced the fuel pump and fuel relays and it still won't start.

In Maintenance & Repair > Fuel System
In Makes & Models > GMC > Jimmy
In Maintenance & Repair > Engine > No start
1 answer - 124 days ago

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zaken1

Date: Jul 06, 2009
Time: 02:47 AM

First of all; the battery must be sufficiently charged to crank the engine at a normal speed. The fuel system is only one of four other factors that can prevent starting. The other three are the ignition system, the cylinder compression, and the timing chain. In order to find out the cause of your starting problem, you need to check each of these areas, in order to eliminate the areas that don't have problems. Once you have found the area where the problem is; it will be much easier to zero in on the actual problem.

You can prove or disprove that the problem is in the fuel system by buying a can of starting fluid, and spraying a two second burst into the throttle body air inlet, while the throttle is held part way open. Try to start the motor immediately after that. If the motor starts, it will stop soon afterward, when the starting fluid is consumed. If the starting fluid makes the motor start, or makes it fire a few times and almost start; then the problem is in the fuel system. In that case, I would replace the fuel filter and check/replace the fuel pump fuse. If the gas gauge reads low; it may be that the vehicle has run out of gas. It is always a puzzler, when a vehicle runs out of gas right when the engine is shut off. In the event that this is likely; it is recommended to add AT LEAST 5 GALLONS of gas to the tank. Many times; adding a gallon or two will not be enough to bring the fuel level up to the point where it can be pumped out of the tank.

Other fuel system issues which can prevent starting are a defective fuel pressure regulator, defective injector(s), clogged throttle body fuel filter, defective MAP sensor or idle air control valve, EGR valve sticking open, or a defective throttle position sensor.

If starting fluid does not make the engine start; the next thing to check is the ignition system. Disconnect the coil wire from the center terminal of the distributor cap, and insert a bolt of the same diameter as the terminals on the distributor cap into the boot on the end of the wire. The bolt should be long enough to fit into the metal connector inside the boot, while the bolt head extends out beyond the end of the boot. Hold the coil wire so that the bolt head is about 1/4 inch from the engine, and have someone crank the starter. There should be a steady stream of sparks from the bolt to the engine. The sparks should be blue-white. If the sparks are as described; take the bolt out of the coil wire, and put the wire back on the distributor cap. Then remove one of the plug wires from a spark plug; insert the bolt into the boot, and hold it 1/4" from the engine, while someone cranks the starter. There should be a stream of sparks from the wire; they will come at a slower rate than they did from the coil wire.

If there are no sparks; or if they are yellow instead of blue-white, the motor may be unable to start as a result. This could come from bad plug wires; or a bad coil, ignition module, distributor cap, or rotor. It could also come from a defective ignition switch, poor grounding of the motor or battery, or corroded or loose battery cable clamps. No spark at all could also come from a broken timing chain.

If there are no sparks from the plug wire, but there were good sparks from the coil wire; the problem is either bad plug wires, or a defective distributor cap or rotor.

If the spark and fuel system tests did not indicate a problem; check to see if the timing chain is not broken, but has jumped out of position: Turn the engine until the timing mark on the crankshaft pulley lines up with the 5 degree mark on the engine. Then remove the distributor cap, and see where the tip of the rotor is pointing. It should either point to the cap terminal for the plug wire that goes to # 1 cylinder; or to the terminal that is directly across the cap from that one. If the rotor points anywhere else; it means the timing chain has jumped out of position. And if the rotor does not turn when the starter is running, that means the timing chain has broken.

If the timing chain has slipped, it will also cause abnormally low cylinder compression.

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