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too much cranking to start the engine? Losing gas pressure??


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Hi Jeep Comanche enthusiasts!!! I'm requesting some advice related to a situation with my Comanche 4.0L 90'. It usually take some time and tries to start the engine, and i observed that the gas pressure release valve don't hold pressure at all. Could be a pressure return valve worn out, or what could be? When the fuel tank is full, the engine usually start faster than with the tank in half or near empty. I'll appreciate any advice to locate the problem. My MJ is my DD (daily driver) and love it so much. Last week I swapped the bench with nice OEM buckets (same year!) and I love to drive it more every day. I'll be waiting your comments...

 

Jeep On!!!! :cheers: jamminz.gif

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It is possible that your fuel pump is going south, but more then likely it's not. I would replace your fuel filter first. Normally when a fuel pump goes bad... it will start to cut off after starting, or it will run like pooo pooo. By this I mean will have no power and take you putting your foot all the way to the floor to keep it running.

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This is a difficult question to answer, because you didn't tell us exactly how long your MJ cranks before it fires. Most people think the early (1987 through 1990) 4.0L XJs and MJs crank "too long" before starting. The fact is, they require a long crank period because the computer doesn't "remember" anything and it requires a few revolutions of the engine for the ECU to register sensor input and calibrate itself. Since what it is calibrating is primarily (I believe) CPS (crankshaft position sensor) input, and since CPS signal strength degrades over time (until the CPS fails), a very long crank time may be a warning of approaching CPS failure rather than a signal of fuel system problems.

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  • 2 weeks later...

My MJ cranks from 5 to 7 seconds to start when it happens, sometimes longer, but with a full tank it fires in 3 seconds or less, always. The CPS can't be, because I changed it a couple years ago. Also, the fuel pump can't be, because my MJ should be on a tow truck if it was the reason. Any suggestion???

 

This is a difficult question to answer, because you didn't tell us exactly how long your MJ cranks before it fires. Most people think the early (1987 through 1990) 4.0L XJs and MJs crank "too long" before starting. The fact is, they require a long crank period because the computer doesn't "remember" anything and it requires a few revolutions of the engine for the ECU to register sensor input and calibrate itself. Since what it is calibrating is primarily (I believe) CPS (crankshaft position sensor) input, and since CPS signal strength degrades over time (until the CPS fails), a very long crank time may be a warning of approaching CPS failure rather than a signal of fuel system problems.
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i don't remember if anyone suggested this, but turn the key to run before starting the truck to 'prime' the pump. then release back to off, turn to run again and re-prime it a second time, then try to crank it, and see if that doesn't improve crank time.

 

only thing i can think of to try and help out, if it still takes a while with the system primed up good then idk. i know that when i swapped tanks and was FLAT empty, i put in a good 3 gallons and had to prime the system probly 5 times then crank on it for a good 5-7 seconds cause it was bone dry.

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and i observed that the gas pressure release valve don't hold pressure

 

My thoughts

 

If I understand you have tested the fuel pressure bleed off and your saying that after the truck is off for any length of time you no longer have any pressure in the fuel line. If that’s what your saying that would indicate the fuel pump is not holding pressure and that can cause long crank time as it fills the fuel line back up. This is a function of a valve in the pump. The pump itself may be just fine but you will have the long crank time due to empty fuel line on start up. As for the "full" tank improvement, that would be due to the level of gas in the tank being higher than the fuel line on the frame and keeping it from draining back. But when the gas level in the yank drops to level or below it will drain back?

There is a test for this in the repair books to test if the fuel system is holding pressure and the side affect is longer than normal crank/start time when the valve in the pump has failed. This sounds like what you may have.

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Eagle is correct about the early 4.0 comanche's computer does not remember anything once the key is cut off. The factory also issued a harness upgrade due to signal degradation in the CPS circuit. This is the only upgrade that I have not done but I will be doing it soon, as my truck failed to start the other day even after recent replacement of the CPS a couple of months back. The following is a reference and part number to Chrysler's fix for the problem. Although I beleive even with this modification the 87 to 90 4.0's will still crank longer than the newer models. If you go this route I think the dealer charges about $100 bucks but it includes the new sensor as well. I'm going to try it without the sensor since I just replace it in May.

 

 

Subject: Crankshaft position (CKP) sensor and harness replacement on 1987–90 Jeep

vehicles with the 4.0L engine

Symptom: No start, cranks okay

Source: Chrysler service bulletin 18-51-88

The symptoms described above could be caused by a weak signal from the CKP sensor. A new CKP

sensor and harness is available to correct this problem. The new CKP sensor is available in a kit, part

number 83100066. The kit includes the new CKP sensor, and a sensor patch harness for connecting the

new sensor directly to the engine control module (ECM).

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I think Comanchedude is right, even though MJCanadian could be saying the truth too. My MJ have 17 years, there's no doubt about it. But for an engine well cared and with near 100K miles on it ( very few miles for 17 years, don't you think??), its probably that the problem could be in its original fuel pump valve wear. I never imagined that the pump could have a fuel pressure holding valve. I'll do the pressure holding testing next time I do some work with my MJ.

 

I can assure that this forum is very instructive, thanks to all those guys who help with their comments.

 

LONG LIVE THE JEEP!!!!!!

 

and i observed that the gas pressure release valve don't hold pressure

 

My thoughts

 

If I understand you have tested the fuel pressure bleed off and your saying that after the truck is off for any length of time you no longer have any pressure in the fuel line. If that’s what your saying that would indicate the fuel pump is not holding pressure and that can cause long crank time as it fills the fuel line back up. This is a function of a valve in the pump. The pump itself may be just fine but you will have the long crank time due to empty fuel line on start up. As for the "full" tank improvement, that would be due to the level of gas in the tank being higher than the fuel line on the frame and keeping it from draining back. But when the gas level in the yank drops to level or below it will drain back?

There is a test for this in the repair books to test if the fuel system is holding pressure and the side affect is longer than normal crank/start time when the valve in the pump has failed. This sounds like what you may have.

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