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Which sensor is going out??

J

jephs422

NAXJA Forum User
Location
Mission viejo CA
Okay I did a search and I am still a little unclear on what is going on with my truck, so I'll describe it the best I can.

My 88 XJ has been running fairly decent since I reserected it a few weeks ago. I had a few starting issues, then I switched out my ignition and now that's fine. I have spark issues, and changed out my coil, now that's fine.

Now the truck will only start every couple times. I'll drive it to work, then when I go out to start it, it won't start. It turns over just fine, and will ocasionally catch and start, then die immediately. I thought it might be because of all the rain, but I pulled my coil out and my distributor cap, and they are both fine. I don't know if this has anything to do with it, but the past few days, it has had a fairly high cold idle for about three seconds, until returning to normal. but when it does start, she runs like a champ. Any idea? I'm stranded at work!!
 
Sounds like the CPS. They are notorious for getting flakey. Many times it is a corroded connector and plugging and unplugging can get it working for a while. Do a search on CPS and you will find tons.
 
well here's another twist I guess. One of the things I had to replace to get the thing running, was the CPS sensor. Sot it is les than a month old, and only has about 500 miles on it. But, after I changed it, I pulled the oil pan off and replaced the gasket because it was blowing oil so bad. It is not leaking at all anymore, but I wonder if the few miles of leaky oil may have bene enough to kill it..... I'd hate to spend another $50.00 on it if the one that is in there is fine.
 
There tends to be only three sensors that can keep a 4.0L from starting. The CPS, the cam position sensor (in the distributor) and sometimes the MAP sensor. Since you had a lot of oil, pull the connector on the CPS and spray some cleaner in there and then put it back together. Maybe you just have a contamination problem. Otherwise get on your local chapter forum and see if anybody has a spare CPS you can borrow for a week to troubleshoot.
 
possibly the IAC motor-
you probably need to clean the throttlebody anyway-all of us do.

the IAC is the one sitting to the left of the TPS on the throttlebody. Its a pintle motor- go`s in and out covering a small hole- controls throttle at idle and just off idle by supplying air when starting--- pull it when you clean the TB and check her over.

guesses.
 
When I first got mine it would start then die and I changed the O2 sensor and it was fine. Unplug it and see how it works. Probably a long shot but something to try.
 
old_man said:
Sounds like the CPS. They are notorious for getting flakey. Many times it is a corroded connector and plugging and unplugging can get it working for a while. Do a search on CPS and you will find tons.
Click to expand...

Ditto...

I had one last me 2 months last year...
Lesson?
Buy 2 carry a spare

side note: I have been told that from the factory the CPS came with some type of "cover" to guard it from the elements...
Of all the CPS I've bought over the years (5-7), I've yet to find this elusive "cover"
 
okay, now maybe I am way off. By CPS sensor, I thought you meant crank positioning sensor, which is on the bell housing. Does my 88 have a Cam sensor? I know I haven't changed that.
 
Cam position sensor is in the distributor. The test is with the connector connected and using a needle (analoge) type volt meter, measure the voltage (by pricking the wire or through the back of the connector), between the blue (+) wire and the grey/with strip (-) wire. Should be 5 volts.
Pull the high voltage wire from the distributor and lay it to the side, well away from any ground and crank the engine, the needle in the analoge meter should pulse.
i started mine up and ran it for awhile with the cam position sensor disconnected once. Don´t really know, if it was a fluke. Seemed to run just fine.
CPS wire goes through multiple connectors, before it makes it to the computer. I´ve checked the wiring for my CPS sensor (between the sensor and the computer) and have found resistance in the connectors. Eagle has said there is a kit to bypass many of the connectors and shorten the wire run, between the CPS and the ECU. Designed to give the ECU a stronger pulse from the CPS.
 
There were so many problems with the connector C101 that DC issued a TB about it and recommended a fix. Cut the connector out and wire the wires together directly. I did it on mine. There are like 30 wires.
 
I had a similar problem and just fixed it today, My 1991 cherokee would start then die. After much research I found that their is a ceramic resistor on the drivers side on the front fender. This resister is connected to the fuel pump and can be disconnected and bypassed to see if that is the problem

It worked for me.
 
xjacker said:
I had a similar problem and just fixed it today, My 1991 cherokee would start then die. After much research I found that their is a ceramic resistor on the drivers side on the front fender. This resister is connected to the fuel pump and can be disconnected and bypassed to see if that is the problem

It worked for me.
Click to expand...

I might be wrong, but that ballast resistor on the fender is for the fuel pump, if I remember correctly it controls the voltage to the fuel pump and if you bypas it (I did it about 3 years ago) it slightly increases (?) the voltage to it... I think the only side effect was that it made the pumpa tiny bit louder (not that I notice anymore)
 
Gil BullyKatz said:
I might be wrong, but that ballast resistor on the fender is for the fuel pump, if I remember correctly it controls the voltage to the fuel pump and if you bypas it (I did it about 3 years ago) it slightly increases (?) the voltage to it... I think the only side effect was that it made the pumpa tiny bit louder (not that I notice anymore)
Click to expand...

I think that you should definitely check that ballast resistor out. I'm not sure that I would permanently bypass it, I figure that it's there for a reason, but certainly test that out because a failed ballast resistor will do exactly that, with the Jeep starting and then dying out a few seconds later.
 
The reason it works by bypassing it is because usually a resistor will open up when bad therefore not letting voltage to the pump. It could have also changed value, you may check that but I doubt it.
 
Well it's day four and I'm not sure if I'm any further along now, but we'll see. I checked the cam sensor in the distributor and it seems to be fine. I did however, seem to get a feeling that it could be a coil issue, so I started checking that. This is what I came up with. At the battery posts, I read 12.46 volts. At the terminal where the coils drops in, I read 11.76 volts. Is that normal, or is that a lot of resistance. when I checked my connections for the coil, they seemed a little gummy, so I cleaned them up. I fear that I may have had an intermitant coil connection (hence the reason it would sometims start, and sometimes not) So I checked the coil, and there is no resistance between the two posts. I guess I have a short. But I just put this coil in less than 500 miles ago. I will replace it and hopefully it will run fine, but what could have caused it to fry? Maybe water from the rain? Or an intermitant connection?
 
okay, I pulled out the CPS just to see if I could visually see something wrong. And low and behold, the wire was eaten all the way through from the manifold. I soldered the wires back together, but still nothing. Could I have burnt it out from the wires touching?
 
The test for the CPS is to disconnect the plug and check for ohms between the A and B pin, should be aroud 200 ohms.
Not in the book but I check mine by using the low voltage scale on my analoge meter and checking for a pulse when I crank the motor over.
 
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