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Old May 14th, 2005, 14:35
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old_man old_man is offline
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Join Date: May 2003
Location: Loveland, Colorado
Posts: 15,717
How to Troubleshoot a No Start XJ

Troubleshooting a NO Start XJ / MJ.
By Old_Man

When an XJ doesnít start, the first thing people tend to do is throw parts at the problem. While this works in most cases, it can actually cause more problems than it solves. The purpose of this write up is to give a logical series of steps and decisions that should solve virtually all starting problems.

Before we get into troubleshooting the causes, there is a common problem that can easily mask the original problem. If you crank an engine too long when it isnít starting, you could have flooded the engine during your attempts to start it. Flooding is nothing more than getting too much fuel in the combustion chamber, leading to a situation where there is simply not enough oxygen available to support ignition. Too many times there is enough gas in the cylinder to wet the plugs, and after fixing the actual problem, it still doesnít start due to being flooded. The problem is that you donít realize you fixed the problem and then keep throwing parts and $$$ at a non-existent problem.

I will address this flooding issue after determining the circumstances that preceded the failure to start.

The first step it to determine the circumstances under which the starting problem occurred.
1.Have you done any work on the engine, and then it wonít start?

2. Were you driving and the engine quit and wonít start?

3. Does it start only when cold/hot?
4. Does it try to start? In other words do you feel it try to fire?

If #1 is true, you have most likely caused the problem in your work.

1.1 Did you mess with the distributor/plugs/wires?

1.2 Did you mess with the fuel system?

1.3 Did you wash the engine?

We will hit a few of the most common things that can go wrong for the #1 situation.

If #1.1 is true, and you removed the distributor, most likely you got the distributor inserted incorrectly. The most common error in this event is to get the distributor inserted with the rotor 180 degrees off. The rotor turns at half the rate of the crank and it is easy to set the rotor in place when the crank is showing 0 degrees, but the engine isnít on the compression stroke. Do a search on indexing the 4.0L distributor.

If you didnít pull the distributor but replaced the spark plugs or wires, you could have gotten the wires out of sequence. The firing sequence is cast into the top of the intake manifold. The rule on doing plugs is to remove one at a time.

Make sure you didnít pull a no-brainer and forget to install the rotor back in the distributor or that you have the coil wire disconnected.

If #1.2 is true and you were messing with the fuel system, you need to make sure that you still have fuel pressure at the fuel rail. When you turn on the ignition switch, before you actually start to crank it, you should be able to hear that the fuel pump runs for a few seconds and stops.

If the fuel pump keeps running you have one of two problems. Either the fuel pressure regulator is stuck open, or you have a defective fuel pump. A defective fuel pump that continues to run, normally wonít cause a no start condition, since it is simply going to generate a higher fuel pressure, so the fuel pressure regulator is most likely the problem. A fuel pressure tester will tell you a lot. With the vacuum hose disconnected from the regulator, you should have about 39 lbs of pressure.

If you donít hear the fuel pump, the problem can be simply a very quiet pump or a defective pump, or a bad power source for the pump. Turn the key to the on position, and remove the protective cap from the Schrader valve on the fuel rail. A Schrader valve is the type of valve you have on a tireís valve stem. If you have fuel pressure, when you depress the center of the valve, you should get a healthy squirt of fuel. If you have either measured the fuel pressure or done the quick test and there seems to be fuel pressure, you can pretty much rule out the fuel system, other than not having the injectors plugged in or the computer not firing the injectors, but for now donít go down that path. At this point, you need to troubleshoot the power to the pump. That will be covered later.

If #1.3 is the case, you most likely got the distributor wet. There is a problem with getting the throttle position sensor wet, but that very rarely will cause a no start condition. Unscrew the distributor cap and get some paper towels and dry the inside of the rotor cap and the rotor. Let all condensation evaporate and then put the cap and rotor back on. Wet wires will rarely cause a no start condition. Make sure the engine didn't get flooded while trying to start.

If you werenít working on the engine or even had the hood open, you will need to go down a different path. The first task is to determine that you have both fuel and spark. First, turn on the key and test the fuel pressure, either with a gauge or the shade tree mechanic technique of depressing the center of the Schrader valve. If you donít have fuel pressure, then you will go to the troubleshooting the fuel system section.

To test the spark, pull a spark plug. Make sure the plug isnít wet from gas or very corroded. Push the sparkplug back on the wire and lay the plug metal body on a metal part of the engine. Have a buddy crank the engine and watch for a spark from the plug. If you are getting a spark, you most likely donít have a problem with the ignition. It is possible to have a defective cam position sensor in the distributor that is causing the spark to happen at the wrong time, but this is not real common. Probably the most common source of no spark is the crankshaft position sensor. They have several failure scenarios. The wire to the cps can flop over against the exhaust header and be melted, the connector can be corroded, or the sensor is just bad. Visually inspect for a melted wire, and then plug and unplug the connector a few times for good measure. Then check to see if you have spark. If still no spark, a new cps will fix the problem in the majority of cases. A bad camshaft position sensor will normally not cause a lack of spark, only an erratic spark. If you still have no spark, in the earlier models there is a connector C101 above the vacuum brake booster that gets intermittent. Plug and unplug it a couple of times and look for spark. If there is still none, perform the same task on the firewall connector that goes from the engine compartment into the back of the fuse block.

If none of that helps, crawl under the dash and pull the computer down where you can plug and unplug the computer (ECU) a couple of times. If there is still no spark there are only a few things that can cause it. A bad coil/ignition module, a bad ECU, or a bad ignition switch that isnít feeding power to the ignition module or the ECU are the most likely the problem. I hope to be able to post a bit more on where to probe with a voltmeter to troubleshoot this condition.

If you have both spark and fuel pressure, the problem can only be a couple of things. Most commonly, the engine is flooded and the plugs are wet, but it could be a timing problem. Crank it for a few seconds, then go smell the tailpipe. Do you smell gas? If so, the injectors are most likely working and the fuel system is fine. There are only three sensors on the Renix system that will cause a no start condition. The most common is the CPS, surprisingly the second is the MAP sensor. Make sure the hardline from the throttle body runs to the MAP and that the MAP is plugged in. Plug and unplug the MAP connector a couple of times for good measure.

Before trying to crank the engine any more, I recommend getting the cheapest set of Champion spark plugs you can find and pull the existing plugs. If you have an air compressor, spray air down the spark plug holes to evaporate any excess gas. Otherwise crank over the engine a few revolutions with the plugs removed. Hold the gas pedal all the way down when you do this. The computer will sense the throttle position as a flooded condition and will stop any new fuel flow. Once you complete this procedure, put in the new plugs and replace the wires, making sure to get them on the right plugs.

A note on starting a Cherorokee: When starting a computerized engine, do not depress the gas pedal until the engine actually fires and starts. The computer will adjust the fuel air mixture automatically. Pumping the gas pedal when starting can actually screw up the computer and confuse it. If in doubt, disconnect the positive battery cable for a couple of minutes. This will reset the computer. It may run a bit rough for the first few minutes until it relearns the operating parameters of the engine, but it should get it in a startable condition.

If you havenít been screwing with the ignition and could not have indexed the distributor incorrectly, and you have spark and fuel pressure after having made sure the plugs arenít wet, the engine should start. If it simply never fires, and you can smell at least a little raw gas in the exhaust, timing is pretty much the only thing left other than the injectors not firing. To test the lack of fuel in the cylinders scenario, have a buddy crank the engine and squirt a little starting fluid down the throttel body. If it tries to fire, the injectors are not giving you any gas. If that doesn't get an attempt to fire up it is probably back to the timing issue. If you have done the CPS routine, then you either have a bad camshaft sensor, bad computer, or bad wiring. It is possible that the coil is so weak that it will spark the plugs when they are not under compression, but doesnít have enough power to spark under compression. The ECU is probably the least common, followed by the cam position sensor, and the coil.

I you are not getting fuel pressure, there are several possible problems. If this happens when wheeling or shortly there after, you could have mashed the fuel lines running from the tank to the fuel rail. Do a good visual inspection. For good measure, change the fuel filter. They are cheap and it canít hurt. If you donít hear the fuel pump ever run, then the most common failure is the fuel pump ballast resistor. It is white and mounted on the drivers side fenderwell. It has two push on terminals. The ballast resistor is really only there to cut the noise in the fuel pump. Take a jumper wire and short the two wires going to the resistor together. Then turn on the switch and listen for the pump. If that doesnít fix it, the next test is to measure the voltage on both sides of the resistor when the key is on. You should read 12 volts on one side and between 5 and 12 volts on the other side, depending if the pump is running or not. If you donít have voltage on either side, you can possibly have a bad fuel pump relay, but the relay is normally only used to short across the resistor and does not feed the power to the resistor, so I tend not to blame the relay. You will need to follow back up the wiring to find where the break in the power is happening. Some people have claimed the key switch can be causing the problem, but I havenít looked at the schematic to verify if that can be the case. Hopefully I can look into that in the future.

If there is power on both sides of the resistor and it is 12v on both sides at all times, then you either have an open power wire to the pump, an open ground wire on the pump, or a bad pump. You can crawl under the back and measure the voltage going to the pump. If you have 12 on both the pump wires, then you have a bad ground on the pump in the tank. If you have 12v on one side and 0v on the other, the pump is most likely bad.

Replacing the fuel pump is a pain. If you are not mechanically inclined then donít bother trying it yourself. There are two ways to change the pump. You either remove the gas tank and then change the pump or change the pump with the tank in place. Either way, the tank needs to be as empty as possible. If it is not below about a quarter of a tank, then you should siphon or pump the fuel out of the tank before proceeding.

If you have fuel pressure and spark but no fire and you donít smell gas in the exhaust pipe, your injectors may not be firing. The best way to troubleshoot this condition is to use a set of noid lights to test that the injectors are getting pulses. They mount between the injector and the harness. The will flash if there is injector drive when you crank the engine. If they do not flash, you most likely have a computer problem since a wiring problem would have to hit all six injector wires to not at least get a sporadic cylinder firing. You can do the plug and unplug routine on C101 and the firewall connector as well as the computer connector. If that doesnít get the light to working, either the computer is defective or isnít getting power. Check on the boards and see if you can borrow a computer from someone and do a swap. I will see if I can get the pinouts posted for the ECU to make sure there is power getting to the ECU.

Hopefully this rambling has gotten you rig to start. If you have gone through all the steps and it still doesnít fire up, post on the OEM forum and Iím sure that we can help you get through it.
Tom Houston - Former Colorado Chapter Treasurer , Terminal Adrenaline Junkie, and Rocket Scientist
Loveland, Colorado My teardrop build
You're just jealous because the voices don't talk to you.

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