yet another CPS gremlin

I know to some this is like ---->!!!1 but i'm stumped. So my wife bought me my first jeep for christmas (WOOHOO). I put a new diehard battery, starter, and crank sensor in it, and low and behold it came alive!! so we drove around the block a few times (not registered yet, but wanted to check it out), came back parked it and did the usual christmas stuff. went to jump in it today to tinker and the damn thing wouldn't start. it would crank FOREVER, but no fire. i checked the cap, rotor, plugs, wires, coil, ignition module, fuel pressure, injectors, and nothing seemed to work. long story short, i ended up pull the crank sensor back out, inspected it, cleaned it off with some simple green, put it back together and it started right up. so my question is, What's causing my CPS to foul? is this common?? should i re-attach it with velcro so i can get it apart faster in the future?? thanks for your help

Since this Jeep is new to you, may I suggest what I believe every new MJ or XJ owner should do upon purchased of a used model? Remove the ground connections at the engine dipstick tube. Clean until shiny the terminals and the block. Also do the same with each end of the little ground strap fron the back of the head to the driver side firewall. We want clean, shiny and tight. Accept no substitute. If your XJ s of the 87 to 89 vintage, you will see a black box connection on the driver side firewall with a bunch of wires going in and out. Remove the center bolt, clean the black tar crap out of the connectors with brake cleaner or equivalent and reconnect. PS-the CPS wires connect through that box!! Couldn't hurt to unplug the CPS connector and spray in there with a contact cleaner just for grins.

X2 on the clean grounds.

Also, are you setting the CPS gap correctly?

You are more likly to get more responces if you include year and engine size with your post. :spin1:

Greenz said:

X2 on the clean grounds.

Also, are you setting the CPS gap correctly?

You are more likly to get more responces if you include year and engine size with your post. :spin1:

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What do you mean CPS gap? It has two bolt holes and i dont think you can adjust it.

cruiser54 said:

Couldn't hurt to unplug the CPS connector and spray in there with a contact cleaner just for grins.

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This is good advice.

IF unplugging and plugging the crank sensor connector back in restores function and allows the engine to start, the problem is likely to be one of electrical "contact" ie; connector or associated wiring issue.

Spray clean the connector with electrical contact cleaner, inspect that connector very closely for any damage inside of the connector; if okay then apply a dab of dielectric grease and reconnect.

If you are cleaning connectors, don't forget that there is a crank sensor connector inside the engine bay near the firewall #6 injector. Repeat above procedure on that one too.

Gotta make sure that the crank sensor is making it all the way to the computer or you're not going anywhere.....

You could also try wiggleing the wires in that area while it is running. If it dies, you know where your problem is. Could be a disintigrating wire or short to ground. Better to have it die at home than on the road in a storm.

to be honest i'm afraid to drive it more than around the block just because of this problem, it seems that i can average about four starts before it goes out. trying to find loose or damaged, dirty connectors, but all are bright silver and the "wiggle" doesn't seem to affect the idle. isnt there a way to bypass it??

Rafterman45 said:

to be honest i'm afraid to drive it more than around the block just because of this problem, it seems that i can average about four starts before it goes out. trying to find loose or damaged, dirty connectors, but all are bright silver and the "wiggle" doesn't seem to affect the idle. isnt there a way to bypass it??

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No way to bypass the crank sensor signal. Without that critical input to the computer, you will not have spark or fuel (to the injectors). You will have to keep working on isolating the problem.

Question; where did you purchase your crank sensor and what brand is it. It is not impossible that you have a flaky sensor right out of the box. It happens more than you think; I have personally seen it. If you continue to have problems, trying a new oem JEEP sensor might be a good idea. I have found aftermarket crank sensors to be kind of hit and miss. Especially crap from ebay.

It doesn't matter what the grounds LOOK like. You need to follow the procedure outlined above to be sure. Wiggle the CPS connector while running and see if it stumbles. Remember I mentioned the black wiring block that the CPS connectors go through? Did you clean it or just a$$ume it's OK. Don't assume anything here. These are known issues that cause the problem you are describing. Your keyboard isn't gonna fix your Jeep. You need to get out there and work on it.

When you get the CPS make sure its a dealer part!!!!! For some reason my jeep hated autozone and advance auto CPS sensors. Also when you put it in make sure the wiring is up off and away from the exhaust manifold. If it touched the manifold it probably melted through.