Does this sound like a CPS problem?

Does this sound like a problem with the Crankshaft Position Sensor?


1994 sport, all stock daily driver in otherwise great condition. 4.0, AW4, 120k
This CPS is new, installed about five months ago.
I have just tested it with an Ohmmeter, and found 100% resistance between terminals B and C through the sensor.
Unlike the last time the CPS died, the key-on OBD display is currently showing no codes.
Temperatures have been below freezing here lately.


I’ve read many CPS threads here, and I know CPS problems can be intermittent, but this is curious because there is one other preceding factor here – If the engine is cold, it will only start if the engine has not yet been turned over. If it's cold and hasn't been cranked, it’ll start right up, but if the engine has run for a minute or more, it’s dead for the rest of the day. If it does start, it runs without hesitation.



Here’s what I observed:

1. I went out yesterday morning to start the car, and it fired right up perfectly.
A minute later, I turned off the engine and walked away. A few minutes after that, I tried to start it again, but it would not catch. – Just cranking, but no fire at all.

2. So, it sat all day. Come evening, I tried again. It started right up nicely without any hesitation. I left it idling until the engine was warm, and it ran smoothly the entire time.
I then shut it off and restarted a bunch of times to test it out, and it always worked great – It’s like there was never a problem.

3. This morning, it again started up instantly. I moved the car a few feet and shut it off.
A minute later, I’m back to no start - the same problem has returned – just cranks; barely catches, coughs once and dies. Further cranking results in nothing but a dead battery.


So, considering these events, and the fact that the sensor tests okay, do Y’all think I should be considering something other than a new crankshaft position sensor?

Cold there?

Could just be a failing battery. You can enough amps to crank but not enough volts to operate the electronics. A short period of running shouldn't be enough to trigger a thermal failure--or so I would think, but I can be wrong.

I would charge the battery, then hook up a voltmeter between the battery terminals and observe the voltage while cranking when you first start it, and record that voltage. If it starts record the voltage while running. Then try and get it in a no-start condition and record the voltage while cranking with a no-start.

Tis the season to be jolly and the season for battery failure.

check all your grounds too.. My buddy just changed his cps to find that it was a ground that chafed... he spent $100 on a cps when it was a $2 wire

Okay, It looks like a fuel problem. The CPS, spark, battery & grounds all seem okay. Then I checked for fuel at the rail, and just a couple of drops to seeped out of the Schrader valve.

I’ve searched all over , but not yet seen anything that would be so intermittent like this.
The fuel pump makes its normal sound when I turn on the key. I can hear fuel being delivered to the engine when the pump runs. I got a faint whiff of unburned fuel under the hood after cranking. Tank is half full. Fuel filter is six months old.

It starts & runs great if the jeep has sat for a day.
It starts & runs great if the engine is hot.
It won’t re-start if the engine has been run briefly, but still cold.

If it does start it will run just fine - when Idling and pulling down the driveway – (But I haven’t highway tested it since the problem began).
BTW, if it matters - The day before this problem appeared, I did an oil change and added 2oz of MMO to the gas tank.


Any other suggestions?

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Any check engine light? If so, pull codes.

Your symptoms are pretty unusual. But the strategy for troubleshooting a no-start is really still the same. Right at the time it doesn't start, figure out what part of the starting equation you are missing. Spark, fuel or both. Once you know what you're missing, the suspect list gets a whole lot shorter. And because your symptoms are flaky, that makes TESTING all the more important. Don't throw parts at this. It becomes expensive in a hurry and is terribly inefficient.

You mentioned that you checked for fuel pressure and only got a few drops. This is a clue that you need to perform some fuel pressure testing. Getting a fuel pressure gauge on the schrader valve on the fuel rail would be a first good step. Take a reading after the vehicle sits overnight. Then watch it as it runs the first time of the day. Compare your numbers to factory spec. Then try to recreate the situation by starting engine and running briefly, then turning off. Watch fuel pressure numbers again. If you try spraying a bit of starting fluid into the intake when symptomatic and it fires up and runs momentarily, that is another quick and dirty way of confirming a fuel delivery problem.

If fuel pressure is in spec. and nothing abnormal is seen, then check for spark; again while symptomatic. Remove a spark plug, keep it attached to the plug wire, place the electrode near a good engine ground. Have a buddy crank the engine while you watch. You're looking for a strong, blue, snapping spark. Orange = weak spark which may not be enough to start the engine.

Good luck and let us know what you find!

Yep, I'll be getting a pressure gauge this afternoon.

The CEL hasn't come on. Checking codes with the key-on trick only gives me 5-5 (no codes).

I do have spark even when it won't start.

Grr. The only places in town, Kragen & Napa, tell me that they don't carry fuel pressure guages.

What else might work in a pinch?

Nothing. If you can limp to a shop have it checked, or order a gauge online.

Please do go and buy a NOID light, about $3. It is the only way to check for sure if the injectors are getting a firing pulse, meters and test lights won't cut it.

Cold = metal contraction. Renew all of your grounds, make sure to clean the metal down to a bright surface, then treat it with a corrosion inhibitor you can get at any hardware store, then make sure the connections are tight.

Just for grinz and giggles, mark your relays, and then swap them around.

Wow. It turns out that it was just flooded.

Apparently, it dumps in a lot of extra fuel at start up. If I didn't run it long enough to burn off the excess gas, it would prevent me from re-starting until it all evaporated on it's own(about 6-8 hours).

Problem solved by pressing the gas to the floor when starting. That tells the computer that it's flooded, and prevents additional gas from being injected.

UltimateG said:

Wow. It turns out that it was just flooded.

Apparently, it dumps in a lot of extra fuel at start up. If I didn't run it long enough to burn off the excess gas, it would prevent me from re-starting until it all evaporated on it's own(about 6-8 hours).

Problem solved by pressing the gas to the floor when starting. That tells the computer that it's flooded, and prevents additional gas from being injected.

Click to expand...

It shouldn't be flooding like that.

Glad you figured out the problem, but what is causing the excess fuel?

I have absolutely no effing clue. I don't even know where to start looking to figure it out.



Could this have anything to do with it?
The day before this all began, I had just arrived home from a couple of weeks in Sacramento. This problem arose on my first attempt to start it at high altitude.

Sacto = Sea level & Warm weather
Home = Elevation 6500 feet & freezing.
How long does it take for the computer to compensate to changes in atmosphere?

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I think that with one fresh start in high altitude that you would be okay.

If symptoms return, check your throttle position sensor and fuel pressure. Having to tip in the throttle to start the vehicle can sometimes point to the TPS.

UltimateG said:

How long does it take for the computer to compensate to changes in atmosphere?

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Click to expand...

Instantly, everytime you cycle the key, before you crank the engine over it measures the atmospheric pressure and adjust accordingly.

There is nothing wrong with your Jeep. This is a common thing with alot of cars and it will vary from engine to engine.

When you start your car in cold weather it dumps in alot of fuel. Basically consider it an automatic choke. Then if you only move a very short distance and shut it off, you've basically washed off all the oil off the cylinder walls. It's doesn't take much as there's very little oil on the wall to begin with, but it's needed for compression and no oil is going to get on the wall because it's cold and thick and won't make it to the cylinders. The next time you go to start it, it's probably cold out again so your dumping in a ton of fuel again, but this time you've reduced or elimintated your compression, then it's flooded. Don't forget the pistons have alot of cylinder clearance when the engine is cold as their aluminum and the cylinders are steel, so their week in the compression dept until it's at operating temp.

I just had this happen to me on a customers car last week and I've seen it many times before. On alot of cars when you crank it over, you would think the timing belt is broken as there is no resistance/compression at all.

How to prevent it? Avoid those very short run times in the cold, if you have to, let it run for a while before shutting off. Or use a block heater. In any case you want to try avoiding this as it's hard on the engine.

BTW...you need to change your oil again, you've contaminated it with alot of raw fuel..

Thanks Sero. I'm glad to know that I'm not the only one out there with this.

Strange, I've done this many times in the past - Every time it snows here, I have to start the car, move it to the curb, and then move it back again when I'm done snowblowing - Which is the sequence I went through on monday, causing this malfunction. This is the first time It's prevented me from starting. Ohwell.

So, how serious is my need to change the oil? I just changed it a week ago! I probably attempted 30 unsuccessful starts this week.


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Yes that is a bit strange that it's doing it now and not in the past. Perhaps it's the oil you changed? The fuel adative? Maybe it's a bit cooler out than previous years? Your not too far from me so it can't be much colder this year than last.

Take the oil cap off and sniff the crankcase to see how gassy it is. If it's strong smelling I would change it. Just a dump and fill, your filter doesn't need changing.

Older spark plugs or even relatively new ones can become fuel saturated. Think about replacing them. Or else a good high speed run to get them good and hot for a while.

winterbeater said:

Older spark plugs or even relatively new ones can become fuel saturated. Think about replacing them. Or else a good high speed run to get them good and hot for a while.

Click to expand...

X2, I forgot about that.