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Crank sensor?


Long Crank time?  

10 members have voted

  1. 1. Long Crank time?

    • Crank Position Sensor
      3
    • Fuel Pump / Check Valve
      3
    • Other (Write in)
      4


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This is XJ/MJ related, so here goes. I sold my '91 HO 4.0L AW4 Xj to a guy at work with an excessive crank time at startup. Probably like 5-10 seconds. He said that the shop he takes it to said fuel pump. I say the crank sensor. They say fuel pump because of the leakdown of the checkvalve, and the time it takes to build pressure. I said crank sensor, because when you cycle the key until the fuel system has built pressure(and the pump stops), and it still has the long crank time. Before I keep telling him to buy a crank sensor and am wrong...

 

So, who is right?

 

Rob L.

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Not to take sides.........

 

Mine take alot of cranking to get started, but, If I hit the gas pedal a little, it fires right up :hmm:

 

I have original fuel pump, that I know of, but replaced the CPS 3 years ago when it went under the knife.

 

:dunno:

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If allowing the fuel pump to run until it shuts off before cranking doesn't eliminate the time lag, it obviously isn't the fuel pump. It also probably isn't the CPS, or it wouldn't start at all.

 

I'll vote for bad spark plugs or dirty injectors.

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If allowing the fuel pump to run until it shuts off before cranking doesn't eliminate the time lag, it obviously isn't the fuel pump. It also probably isn't the CPS, or it wouldn't start at all.

 

I'll vote for bad spark plugs or dirty injectors.

 

I've forgotten where I heard it now, but I was told that a weak signal on startup from the crank sensor can cause the overcrank. Like it is searching for the signal, and once the engine starts and is going fast enough, the signal is strong enough to sustain.

 

Make sense? Or am I nuts?

 

Rob L.

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sometimes it takes more than just the one time waiting for the pump to prime. i'd say try it a couple times before starting. cycle key to on position for a few secs, turn it off for a few secs and repeat 3 or 4 times. see if that makes a difference.

 

I did. Makes no difference.

 

Rob L. :hmm:

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I'd be with you on plugs and wires, but its a MOPAR longblock with 30-40K on it. Everything was replaced when the new plant went in. It runs like greased owl poo, I would think if it had something to do with the ignition system, it would run like poo all the time.

 

Not saying anyone is wrong, I am just trying to explain it how I am thinking...

Rob L.

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I've forgotten where I heard it now, but I was told that a weak signal on startup from the crank sensor can cause the overcrank. Like it is searching for the signal, and once the engine starts and is going fast enough, the signal is strong enough to sustain.

 

Make sense? Or am I nuts?

I guess that might be true but, if so, it doesn't make any sense (to me) at all. There is nothing about higher RPM that would in any way make the signal stronger. The CPS is basically a Hall effect sensor. It's a magnet -- when a tooth (high point) on the flywheel zips by it, the magnetic pulse generates a VERY small electric current. I haven't studied any Physics or electrical theory for over 40 years, but it seems to me that faster would equate to less time in proximity, which would equate to a weaker signal, not stronger.

 

But, hey ... that's why we have Hornbrod around here. To explain to me why I don't know jack about 'lectricity.

 

What the heck -- it's a '91, so it's OBD-compliant. Hook it up to a scanner or DRB and see what shows up as out-of-spec.

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just let the shop put a pump in it, that way when it doesn't change anything they have to eat the crank sensor?

 

happens to me all the time! damn misdiagnosis! :mad:

 

:yes: :yes: but seriously, if he doesn't mind paying for it then let them put a pump in it and if it doesn't change anything they get to try again, it only pays the first time...

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And now for something completely different........

 

Agreed, I don't have enough experience with Jeep's (yet) to fill a bucket with BS, but...... Does the computer system look for a oil pressure threshold before turning on the ignition at start-up?

 

The description fits the behavior of two vehicles at my house, but they are Chevy's, one 89 and the other a 95.

 

Scott

(who could be just one cup of coffee short of full operating mode)

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And now for something completely different........

 

Agreed, I don't have enough experience with Jeep's (yet) to fill a bucket with BS, but...... Does the computer system look for a oil pressure threshold before turning on the ignition at start-up?

 

The description fits the behavior of two vehicles at my house, but they are Chevy's, one 89 and the other a 95.

 

Scott

(who could be just one cup of coffee short of full operating mode)

 

 

What gives???? I figured that if my idea was really stupid, somebody would at least take pity on an old phart and straighten me out. At least one smart-alec response was expected as I am really new to Jeep stuff. Come now fellas, I'm old enough to have a thicker hide and IF I can remember where I left my feelings I'll put them on my sleeve so they can get hurt.

 

Seriously, does the MJ have such a check in the ECU? Could this be a possible explanation for the original question?

 

Scott

(It's past my bedtime, I'm heading for a nitecap of my favorite old phart medicine.) :yes:

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My Comanche used to crank what I considered long, but seems normal for a Renix. Then last summer, after shutting it off, and trying to restart after 10 minutes or more it some times had trouble starting. Sometimes took 3 attempts, cranking it 15 seconds at each attempt. Since then the problem has persisted and become more frequently, until it always happened . Here's what I found out since then:

 

Sitting for a week or longer, it starts like it used to: crank 5 seconds or so and it starts up.

Sitting overnight it starts immediately. Fraction of a second

Sitting between 15 minutes and an hour or two it will crank and crank and crank. Taking up to three attempts to start. However, if I put the pedal to the floor while attempting to start, it starts like it used to, after 5 seconds or so.

Immediate restarts, within a minute of shutdown, fires up immediately.

 

And in my case I'm guessing leaky injector(s). Have been planning to upgrade them since I bought her in February of 08, just never had the money as it always needed spent on other stuff.

 

but mine is a Renix, it's supposed to crank for a few seconds before start.

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Mine's a Renix. Had the same problem since I bought it. Changed every sensor there was. No change.

One day cleaned up the ground at the block and firewall.

Added a ground from the negative terminal on the battery to the inner fender.

Truck starts faster than both my rice burners now.

Well, at least it did the last time I started it in November of 2008.

Battery is now completely dead.

If I let it charge for a couple hours it starts right up again with no hesitation.

That's with an old starter, dead battery and year old gas in the tank.

Not sure if this would help since it's an HO but you could try.

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And now for something completely different........

 

Agreed, I don't have enough experience with Jeep's (yet) to fill a bucket with BS, but...... Does the computer system look for a oil pressure threshold before turning on the ignition at start-up?

 

The description fits the behavior of two vehicles at my house, but they are Chevy's, one 89 and the other a 95.

 

Scott

(who could be just one cup of coffee short of full operating mode)

 

 

What gives???? I figured that if my idea was really stupid, somebody would at least take pity on an old phart and straighten me out. At least one smart-alec response was expected as I am really new to Jeep stuff. Come now fellas, I'm old enough to have a thicker hide and IF I can remember where I left my feelings I'll put them on my sleeve so they can get hurt.

 

Seriously, does the MJ have such a check in the ECU? Could this be a possible explanation for the original question?

 

Scott

(It's past my bedtime, I'm heading for a nitecap of my favorite old phart medicine.) :yes:

 

nope, jeeps do not use this, gm hasn't for years either. :waving:

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Thanks, that helps eliminate some of what I was thinking. That oil pressure thing is a big pain when I throw fire to the 89 Chevy. It's a 454 1-Ton, when I say start, I want to hear it ROAR, not crank for 5 seconds then ROAR.

 

There have been some good ideas on the solution to the problem. Anybody want to bet an OREO cookie on the solution? ;)

 

 

 

Scott

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