wgebb

Two weeks ago I broke down halfway to LA from San Francisco. Parents were nice enough to pick me up, and I hitched a ride back up with a friend, who dropped me off at the mechanic's place where I left it.

When I broke down I lost combustion, like gas wasn't getting to the engine. Mechanic said this was due to crankshaft sensor which he replaced, along with my ignition coil which he said was weak. It drove fine for about 100 miles on my way back North, but twice the car lurched-did not fire and then picked it up again a second later. I thought maybe the new sensor was being broken in or something, I made it back okay. However I barely made it work the next morning, at one point the car totally died and I was pulling to the side of the highway before it started firing again. I left it at work and took the train home.

Since then, I've replaced the fuel filter and cleaned the throttle body (which was alarmingly dirty). I took it for a spin after this but the Jeep exhibited the same issues, sometimes losing ignition and causing it to jerk like someone smashed the brakes very quickly and let go. I made it back to the yard gate at my work before it totally died and would not start - had to pull it in with a fork truck. The engine kept cranking but no gas.

My service manual says to test CPS by placing an Ohmmeter across terminals B and C, and the resistance should come up 0 - any reading and the sensor is bad. The sensor the mechanic put in failed this test, so I ordered a new one. When the new sensor came I tested it in the same manner and received the same result. Oh well, I installed the new one anyway. Tested my fuel pressure and got good reading of 50psi. Drove it around the block at lunch today and still no good. Died once again before getting it into the yard. This time I left it there and came out at the end of my shift - started right up and pulled it into the yard, where it then died again. Right after it died I tested the fuel pressure - still good at 50 psi, so I know the pump is okay. I also tested for spark, which was there so I don't think its the camshaft sensor.

Moral of the story: It starts right up and will drive until it dies. Then it will crank and not fire, or sometimes fire up but die soon after a few seconds of idling. Side note- I noticed today it was idling around 1k, whereas my idle has been more around 850 since I've had the Jeep. Also my air filter is clean and just a few months old.

Sorry for the long post, but I think I've taken care of the traditional avenues for the issue of cranking and not starting, so I would really appreciate some input on this issue. My next thought is that maybe it's the IAC that's faulty? Figure I'd post here and see if anyone has had similar issues before I start throwing money at it and replacing parts that aren't the issue.

cruiser54

Thanks for your post.

We’re here to help, but you must understand that all Jeeps are not the same.
In order for us to give you more accurate advice and help, we must know the following, as applicable, along with detailed information as to the symptoms/problems etc:

Year
Model
Engine
Transmission
Transfer case

IAC won't cause those problems.

wgebb

Wow, sorry I forgot the basics! Thanks for the reply. '96 XJ, 4.0L Manual Trans. 4WD. Stock trans and T-Case.

cruiser54

I'm wondering if you have a clogged catalytic converter.

Cruiser’s Vacuum Test for Exhaust Restriction

Your vacuum gauge should come with an instruction booklet outlining this procedure.

Hook the vacuum gauge up to a source on the intake manifold.
Start the engine and note the vacuum reading.
It’s usually 17 to 21 inches of vacuum.

Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle.

Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading.

If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

jordan96xj

If it dies even when you are trying to give it gas with the pedal, you can probably rule out the IAC for now. Based on your description, it is most likely not your problem.

The IAC simply allows air into the throttle body when the pedal is not being pressed and the main throttle plate is all the way closed. IAC symptoms would be more like "it dies as soon as I let my foot off the pedal".

wgebb

Thanks for the advice, I'll check this out tomorrow!

I'm wondering, would a clogged catalytic converter come up on an OBD scanner? Because the mechanic I took it to originally said he ran the diagnostic and it came up with the code for a bad crank sensor. I was a bit skeptical of this in the first place because I was watching them work on it when I first brought it in and never saw them plug a scanner in.

wgebb

Ahh, hence the term "Idle" Air Control. Thanks for that.

jordan96xj

If I was losing combustion, I would want to test for 3 things first (they are not the only 3 things that could be wrong, but on a 96 they are the 3 things I would address first). In order:

1. Am I losing spark
2. Am I losing fuel delivery
3. Am I losing something besides spark or fuel delivery

Remember, for fuel delivery, you are going to lose it in one of two ways. Either a fuel pressure problem between the fuel pump and motor. Or a failure of the fuel injectors to be commanded to fire. (We can typically rule out a single fuel injector problem, as a sudden complete loss of combustion would be more consistent with ALL injectors failing to fire).

Since you have already tested for fuel pressure as best you can (though it could still be intermittently dropping out on you), I would next want to know if the injectors are failing to be commanded to fire.

You can do this with a test light, but I chose to buy a universal injector test lamp so that I could shut the hood and stick it to the windshield, which is useful for failures that typically happen when the vehicle is being driven.

Lisle 27800 Universal Noid Light
by Lisle
Link: https://amzn.com/B0002SR57Q

Based on your description I would want to confirm fuel and spark first, if one of these is dropping out, it can also help guide towards the correct sensor(s) that are most suspect. An sense you have already tried 2 CPS (which would kill spark), I would be especially curious if fuel at the injectors was being shut off.

jordan96xj

By the way, since you have a 96. The very next thing I would want would be the BAFX bluetooth obd2 scanner in my signature (CHEAP!) and some type of cheap android device to use with it. It can be very helpful for getting insight into what the computer is doing, even when no engine codes are being presented. (for example, long/short term fuel trim numbers and O2 sensor voltages are worth their weight in gold. Because almost nothing can happen with air or fuel that won't reflect itself in the fuel trim numbers). Since it is so cheap and useful, I cracked the scanner open, did a little soldering, and permanently wired it into the vehicle (basically it gets its power from the accessories circuit instead of from the obd2 port now).

Oops. Sorry, just realized I don't have it in my signature here. Here is the linke (its only $22 now...man these are getting so cheap).

BAFX Products 34t5 Bluetooth OBDII Scan Tool for Android Devices
by BAFX Products
Link: https://amzn.com/B005NLQAHS

wgebb

Thanks Jordan. There's some great info in there. I'll try testing the fuel injectors tomorrow, and look into to Bluetooth OBD scanner, that seems like an invaluable tool to be able to access that kind of detailed information about the car's performance at any time.

BlueRidgeMark

Nobody has asked The Big Question yet?


What brand of CPS was used?

Our beasties are famous for not liking aftermarket sensors, particularly the CPS. Aftermarket units are known for being bad out of the box, or failing very quickly. You should use only NGK (a.k.a. NTK) sensors. If you used something from Autozone or Advanced, or somewhere like that, it's probably crap and should be replaced with the correct unit. You can get the good stuff from the dealer with the Mopar name on it, or from NAPA with the NTK name on it for about a third of the dealer price. Probably some online sources, too.

By the way, did you mean Crankshaft Position Sensor, or Camshaft Position Sensor?

'Nuther question: any black smoke?

wgebb

I meant Crankshaft Position Sensor. The brand I put in was Standard Motor Products - it was the only one I could find online that had the correctly shaped connection. I'm not sure of the brand that was put in by the mechanic two weeks ago. Also no, no black smoke.

BlueRidgeMark

Okay, since the CPS was not OEM, it's immediately suspect.

Yes, the aftermarket stuff really is that bad.

drewactual

not reading but portions of this thread (i didn't, I read it all) the first thing i'd figure is a CKPS (crank position sensor), with the CPS (cam position sensor) landing distant second.

you said you used a sensor with marrying connection- which isn't certainty you've got the right one... but... let's pretend it is- you said you were pulling some resistance on the sensor, which indicates a lack of internal integrity right off the bat...

the other animal I would look at, if'n I were you, and I ain't, but if'n I were- I'd look at engine/computer grounds, and make sure they are good... a bad ground can create all grades of funky issues that will keep you running in circles until it dawns on you to check them... been there... several times... done that.... more times than i care to admit.

wgebb

Good to know about the quality of aftermarket sensors. Hard to believe I've had 2 new ones that have failed the resistance test..

Can anyone direct me on where to get an OEM one? (Also unsure what NGK/NTK is?) I tried looking at moparonlineparts.com bu they didn't seem to have CPS.

Thanks

Update - I found some here at
[URL="http://www.4wheelparts.com/eSearch.aspx?Ntk=primary&Ntt=Mopar+56027280+Cranks haft+Position+Sensor"]
but it seems none are exactly for my '96 XJ. The one at the top says its good for Grand Cherokee, would that work in my vehicle?