Odd starting issue - JeepForum.com
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post #1 of 9 Old 07-30-2021, 07:58 AM Thread Starter
kerian159
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Odd starting issue

The other day I went out to drive somewhere and the Jeep took a bit longer to start than normal (normally it fires right up with no issues), once it cranked up everything was fine so I went on my way. Get back into Jeep after leaving and it does it again this time a bit longer but again once it starts it's fine. Next time I crank it up it started without any problems so I'm at a loss as to where to even start to diagnosis this issue. Only other thing that may be related (maybe) is sometimes it feels like when I start driving I'm not at full power but only for a moment. The engine is a stock 4.0 with over 200,000 miles so I'm sure it's getting a bit tired and a refresh/replacement is in the works but in the mean time if it's something I can fix I'd rather do that :-)


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post #2 of 9 Old 07-30-2021, 11:13 AM
CJ7-Tim
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A faulty check valve is very likely based upon the described symptoms.


When a 4.0L OBD-II Jeep won’t start on the first try day after day, a good suspect is a failed check valve on the fuel pump. The fuel pump check valve is supposed to hold pressure in the fuel line while the engine is off. The check valve is part of the fuel pump and located inside the gas tank. A failed check valve allows the fuel to drain back to the tank. This results in one or more of the following symptoms:

SYMPTOMS

• Longer than normal cranking times
• Never starts on first try, almost always starts on the second try
• Rough idle for a few moments of idling
• Rough running for first ˝ block of driving
• Little or no fuel pressure at the fuel rail test valve similar to this -

1- Key in on position; fuel pump primes for 2 seconds then stops. 0 PSI
2- Cranking over the engine. 2-3 PSI
3- Engine starts and idles for 20 seconds. < 5 PSI
4- Engine idles and fuel pressure creeps up to 45-50 PSI.


POSSIBLE CAUSES

• Failed check valve on the fuel pump in the gas tank.
• Leaky fuel injector(s) may be the true cause of your problem, but more likely they are simply contributing to the check valve issue.

Testing of the fuel injectors should show if any are faulty and are allowing fuel to drain into the cylinder. Either problem can allow heat soak to vaporize the remaining fuel in the fuel rail, and you may be dealing with a bit of vapor lock as well, especially in warmer weather.

1. Connect the gauge at the fuel rail and start the engine. The gage should read 49 psi plus or minus 5 psi.

2. Turn the engine off and immediately clamp the rubber fuel line ( if equipped ). Watch the pressure gauge and see how long it takes to loose pressure.

If the pressure remains above 30 psi for an extended period of time then the problem is in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably one or more leaky injectors.

There are a few solutions to the problem –

1. Turn the key to ON for 5 seconds and let the fuel pump prime. Turn the key to OFF, turn the key back to ON for 5 seconds, start the engine.

2. Crank the engine for 3-5 seconds, pause, and crank the engine again. It will almost always start on the second try.

3. Replace the fuel pump assembly in the gas tank. The check valve is not a separate part and cannot be serviced. This is both expensive and time consuming because you have to remove any hitches and skid plates and then remove the gas tank from the vehicle.

You can use solutions 1 or 2 for as long as you want to, they do not harm anything and the fuel pump will continue to function for many more years.


If you do replace the fuel pump assembly, purchase a high quality OEM style pump assembly such as Bosch or Carter. Some cheap aftermarket fuel pump assemblies, like Airtex, and Precision don’t hold up and will fail completely within 12-18 months.
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post #3 of 9 Old 07-30-2021, 11:21 AM Thread Starter
kerian159
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so if I were to go and pull the tank and fix it correctly (assuming it's the fuel pump which is the way it's feeling) would this be the Bosch part #?

https://www.napaonline.com/en/p/BSA6...nput=fuel+pump

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post #4 of 9 Old 07-30-2021, 11:30 AM
Wallyb4
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Try turning the key to the on position and let it sit there for a few seconds, will it start right away? IF so then like CJ7-Tim stated the fuel is leaking back into the tank.
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post #5 of 9 Old 07-30-2021, 04:44 PM
CJ7-Tim
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My 2000 fuel pump is going on 12 years with a faulty check valve.

AFIK, the fuel pump and pressure regulator are separate assemblies.

“We the people are the rightful masters of both Congress and the Courts,
not to overthrow the Constitution but to overthrow the men who would pervert the Constitution” Abraham Lincoln, 1859.
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post #6 of 9 Old 08-17-2021, 04:12 PM
danj15
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I went through the same issues, baffled me for a while, turned out to be my crank sensor
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post #7 of 9 Old 08-17-2021, 04:32 PM
CJ7-Tim
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The fuel pressure and check valve function can be tested, as can the CrankShaft Position Sensor.

Diagnose and test before buying parts.

“We the people are the rightful masters of both Congress and the Courts,
not to overthrow the Constitution but to overthrow the men who would pervert the Constitution” Abraham Lincoln, 1859.
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post #8 of 9 Old 08-18-2021, 05:57 AM Thread Starter
kerian159
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My issue feels like the Fuel Pump as the Sensor was replaced with a good OEM one last year.

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post #9 of 9 Old 08-18-2021, 08:06 AM
CJ7-Tim
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Without data from testing, everything else is feelings, opinions, and just like that time my step-brother's girl friend's next door neighbor's Uncle had some kind of Chevy with the same symptoms.
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“We the people are the rightful masters of both Congress and the Courts,
not to overthrow the Constitution but to overthrow the men who would pervert the Constitution” Abraham Lincoln, 1859.
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