cranks but no start ran great shut it off wont start now

hey everyone, i picked up a 1991 xj 4.0 ho,has new cps, plugs wires cap rotor coil, a lot of stuff ran great shut it now it won't start again have 12.4 volts to coil, crank position sensor replaced, where do i need to start now, im trying to have it ready for the rubicon by next friday thanks in advance for the help:gonnablow

You need spark, fuel and air. Find out what is missing and troubleshoot from there. I usually start out by checking spark and listening for the fuel pump for a 2-3 second prime when you first turn the key to the run position. Look at your check engine light, is it on?

Air sounds funny, but if your IAC is stuck closed you have to give it a little pedal to get the motor running. Or maybe a Rat built a nest in your air box.

First guess, knee jerk, is your CPS wires are cooking on your exhaust manifold.

test the CPS, they have been known to be bad right out of the box. The usual recommendation around here is to only buy the OEM ones from the dealer.

i have no spark, fuel pump is priming i have supply voltage to the coil, has new cap rotor plugs wires cps iac the kid i got it from took it swimming i got a new comp for it had it programmed ran great just parked the other day and now no start how can i test the cps , what results should i get from a good cps

I haven't tried this test myself on a 91, sooooo?

http://www.naxja.org/forum/showthread.php?t=1062027

If you turn the key to the run position and get no CEL (check engine light) it is a good possibility the 91 is the same as many later years and you have a short in the 5 volt (in some early models it is 7 volts on some 8 volts) sensor supply circuit. If you get no CEL a simple test is to unplug the CPS and see if the light comes on, then unplug the Cam Sync Sensor, etc.

I usually volt test the orange wire (on many XJ's) going into the distributor and see what I get, if it reads 2-3 volts or so you likely have a short someplace in a sensor.

New parts don't necessarily mean good parts, changing out parts is a step in the troubleshooting process, but you never really know if the connectors are good (clean and making good contact) or there is something in the wiring.

It is always a good idea to grab a flashlight and check the normal culprits before you do much of anything else, The wire going to the rear O2 sensor, the wiring for the front O2 sensor (by the manifold down pipe and in the front where the harness goes up to the injector harness). The wires running from the CPS to the connector near the injector harness.

Coil to distributor cap high voltage wire and the wires running into the distributor for the cam sync sensor.

Coils are know to fail, especially the ones mounted to the side of the motor (heat). Check for in voltage to the coil with the key in the run position, Check coil continuity.

The most common failure is the CPS (like I said new doesn't mean good) secondly burnt wiring on the manifold or exhaust.

Looking at the CPS to see if it is covered in oil, mud or other crap is also a good idea. No CPS pilse during cranking, weak CPS pulse during cranking, interrupted CPS pulse during cranking ( dirty, corroded, full of water or oil connector or damaged wiring) and the PCM will shut down fuel (except for the initial priming) and shut down spark.


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Twice I have had the wire from the coil to the distributor go bad. It would work fine then not start. When I was trying things and unplugged the wire it crumbled.

i have a cel, comes on with the ignition then goes off, orange wire going to distributor has 8.8 also showing 8.8 at cps i have 12v going into coil but no spark leaving the coil cleaned all my sensors i could find this morning replaced all the wiring that waas bad and still no spark also installed another cps

Try the key test and see what codes are stored in your PCM.

http://jeep-xj.info/HowtoECUcodes.htm

They say a bad cam sync sensor can shut down the PCM making spark (triggering the coil), as well as closing the fuel pump relay, just like a bad CPS. Never seen it myself.

Have you löooked inside the distributor for water. Water can hide under the cam sync sensor mounting plate. Check out the coil to distributor cap wire in the dark for high voltage taking an odd path to ground? Do an ohm test on the coil to distributor cap wire, if it is broken the high voltage often grounds before it can get through the distributor cap, through the plug wires to the plugs. Also turn the distributor cap over and check the center electrode and any lightning looking flash burn trails inside the top of the distributor cap.

I tend to check out the basics before I start swapping sensors. I've had the center electrode in a distributor cap fall out before. No kidding, I super glued it back in lol. I'm sure glad I looked in there before I started replacing sensors, doing involved time consuming tests and even worse, started ohm testing every wire to every sensor.

i put my coil in a friends xj and low and behold it didnt start so im going to buy a coil tommrrow also fixed some more wiring today i guess this is what i get for $750 but its a 5.5re short arm kit on 33s will update once new coils in thanks

so coil didnt fix it at this point it has had 3 cps, new cap rotor plugs wires coil and still no spark im now lost i have signal voltage where i should source voltage where i should so could it be the distributor being a little dirty it hasnt been in the mud or around very much moister in like 8 months

8Mud posted:

They say a bad cam sync sensor can shut down the PCM making spark (triggering the coil), as well as closing the fuel pump relay, just like a bad CPS. Never seen it myself.

Click to expand...

I've had 2 bad cam sensors within the last 3K miles.
The magnet apparently falls out of it's holder.

The 91 has a different CMP than my 96, but I'd
suspect it as the cause of the no-spark. There'll
be a trouble code present if you can read it...

You can do a quick test with the coil circuit. Stick a volt meter plus lead on the coil out wire (I forget the color maybe black and purple, not sure), but the other wire, not the orange in wire and the other meter lead to battery ground. It should show near battery voltage with the key in the run position. It should pulse when you crank the motor over, easier to test with an old analog meter with a needle pointer.

If you get no voltage you have an open wire in the coil (most likely). If you get no pulse it is most likely you have a bad sensor (or open wire) and the PCM is shutting down the spark. The coil triggers on the ground side through the PCM.

Hopefully you didn't get a burn through inside your coil and pulse the PCM with 35,000 volts. I don't know what the outcome would be, but it is likely not to be good.

Typical resistance in a coil is around 12-14 ohms, but they really vary, I've seen some as low as 4 ohms. Zero ohms or infinite ohms and you definitely have a problem.

I remember reading long ago about one XJ with a wire broken on the out (ground) side of the coil, between the coil and the PCM. Only heard of it once, but problems like that tend to repeat eventually. Not likely your issue, but something to keep in mind. Vibration can break the wire under the insulation, you can't see it. I've found a few broken wires under the insulation, it will drive you nuts trying to find the break.

If you're not getting spark, and your vehicle has a CPS this is the first place to check. However before getting under the car and taking the CPS off, get out your multi-meter and verify connectivity/conductivity/resistance through the wiring loom. The #1 issue I've had with CPS' has always been the wires going to it, I've never had a bad CPS.

so ive tested everything and still no no spark i believe the sencor in my distributor is bad it is showing ol on my dmm when i try to resistnce test it, i pull the dist apart and it was filthy inside and had oil in it so im going to try that

Im having the same problem on my '90. replaced distributor, cap, roter, cps, checked all the coils. At a complete loss here:gonnablow

radioman925 said:

Im having the same problem on my '90. replaced distributor, cap, roter, cps, checked all the coils. At a complete loss here:gonnablow

Click to expand...

The 90 is a renix, which is a vastly different animal.

The first thing to check if your sensors test good is the wiring harness that goes to the CPS. I know a lot of people blame the CPS, but I usually find the wires are the issue.

My suggestion, get one of those "spark plug testers" vatozone has them for <$10, it's a little light that you can plug inline with either the sparkplug or coil wire. At this point if you try starting it, if there's no spark you will get no light, if you get light, that means you're getting spark and the problem is somewhere else.

If you're getting no spark that means either CPS, coil, or the wires leading to each.

ok i got spark the pickup coil in the dist was bad now im having timing problems i think partially because my exhaust has a leak and is so shor so today im redoing the exhaust from the header back, im going to do full length of the vehicle any other suggestions

Did you replace your distributor, and then have timing troubles? Timing is all distributor rotation.

How'd the exhaust job go?