Slow starting (already replaced the fuel pump with a Bosch unit)

SeizeTheKarp

SeizeTheKarp

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Jun 17, 2021
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Ridgecrest, CA
Hello all,

I am being plagued with reoccurring slow starting. initially i assumed it was the fuel pump per the many threads on the topic and since the bosch unit for my 2004 (67726) isnt available, i predicted having to go with store brand pumps so i installed the Genright access panel so wouldnt have to drop the tank frequently when they undoubtably die. heres where it gets weird. i didnt have the new pump on hand so i put the tank back in and magically no more slow starts. this lasted for a few weeks then started up again so i pulled the assembly out and replaced just the fuel pump itself with the bosch one which is still available. again i got a few weeks of the jeep firing right up and now its slow starting again. i guess its possible the check valve in the brand new fuel pump died but im kinda thinking that isnt the case. any suggestions of what to investigate?

I plugged a scanner in to measure fuel trim and o2 sensor readings for the heck of it and long term at both banks at temp is 32% im going to have somewhere verify that because that seems unrealistic without throwing a code. my o2 sensors are reading like i may have a bad one so ill probably replace those. from my understanding a bad 02 wont cause slow starting so im at a loss of where to begin.

cheers
 
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When the engine is cold the fuel injection system is operated in Open Loop where the ECU provides preset information for fuel injector pulse duration and the O2 sensors are not involved.
Once the engine warms up and the O2 sensor heating elements are warmed sufficiently fuel injector control shifts to Closed Loop where the fuel injector pulse duration is controlled by the O2 sensor signal sent to the ECU.
If the problem starting occurs when the engine is cold; it is highly unlikely the problem is with the O2 sensors.
If there were a problem with the O2 sensors the engine may idle rough, gas mileage would be down and performance may be lacking.
If the engine needs to crank for more thank a several seconds before starting; this could be the check valve. You can test that by cycling the ignition switch off and on allowing the fuel pump to raise the pressure to approximately 49 psi.
 
CharlesHS said:
When the engine is cold the fuel injection system is operated in Open Loop where the ECU provides preset information for fuel injector pulse duration and the O2 sensors are not involved.
Once the engine warms up and the O2 sensor heating elements are warmed sufficiently fuel injector control shifts to Closed Loop where the fuel injector pulse duration is controlled by the O2 sensor signal sent to the ECU.
If the problem starting occurs when the engine is cold; it is highly unlikely the problem is with the O2 sensors.
If there were a problem with the O2 sensors the engine may idle rough, gas mileage would be down and performance may be lacking.
If the engine needs to crank for more thank a several seconds before starting; this could be the check valve. You can test that by cycling the ignition switch off and on allowing the fuel pump to raise the pressure to approximately 49 psi.
Click to expand...

Correct. I was aware of this. The bosch fuel pump is less than a month old.
 
When you said you "replaced just the fuel pump itself", do you truly mean just the pump or the entire assembly that has the float, pump, pressure regulator, housing, etc...? If it's truly just the pump, it could be the pressure regulator
 
Have you checked the fuel pressure at the rail and monitored the gauge after you turn off the Jeep to see if the pressure bleeds down?

-Mac
 
Jerry Bransford said:
Does the slow start happen even after the engine has only been off a few minutes, like after getting gas?
Click to expand...

the first time around (prior to dropping the tank) it only did it when it was cold basically textbook check valve issues. now all it takes is me running into tractor supply for a couple of bolts.

Rob L. said:
When you said you "replaced just the fuel pump itself", do you truly mean just the pump or the entire assembly that has the float, pump, pressure regulator, housing, etc...? If it's truly just the pump, it could be the pressure regulator
Click to expand...

well... i said just the pump so i meant just the pump. pulled the assembly appart swapped the pumps out and put it back together. i have considered regulator. im picking up a pressure guage today.
 
SeizeTheKarp said:
the first time around (prior to dropping the tank) it only did it when it was cold basically textbook check valve issues. now all it takes is me running into tractor supply for a couple of bolts.
Click to expand...
That now sounds like it's caused by a bad fuel pressure regulator. Slow starts after sitting a longer period of time like overnight is typically caused by a bad check valve inside the fuel pump. Slow starts after the engine has only been off a few minutes is typically caused by a bad check valve inside the fuel pressure regulator.
 
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Jerry Bransford said:
That now sounds like it's caused by a bad fuel pressure regulator. Slow starts after sitting a longer period of time like overnight is typically caused by a bad check valve inside the fuel pump. Slow starts after the engine has only been off a few minutes is typically caused by a bad check valve inside the fuel pressure regulator.
Click to expand...

that makes sense to me. ill know for certain after work today when i can test at the rail. my guess would be that the entire assembly was on its way out and i fixed half of it only for the other half to start acting up. either way, im more than likely going to stick that bosch pump in a new assembly if i end up getting one
 
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so.. i tried every store in town. no one had a fuel pressure kit with the right fitting for my jeep since i dont have the schrader valve on my fuel rail. i drove the jeep around for most of the day and now it gets a pretty noticeable hesitation when i press the gas. is this another symoptom of a dying pressure regulator? i know it can be indicative of a bad 02. since i couldnt check the opressure myself i think im just going to throw stuff at the wall. new fuel pump, new 02 sensors since i dont think theyve ever been replaced in 185k miles going with ngk of course.
 
I've had zero luck with getting the parts I want in town and usually order from Amazon, Rock Auto or Mopar Parts Giant.

You can buy an aftermarket rail with a test port. I was thinking about permeantly plumbing in an oil filled pressure gauge just so I had something to glance at quickly when doing trail diagnostics.

I'd do one or the other and do some test driving. You're probably right and they both need done... but there's some value in knowing what fixes it.

Good luck and stick with it!

-Mac

P.S. You're probably within reasonable driving range of Black Magic Brakes...Mr B probably has everything you need in his secret bat cave.
 
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macleanflood said:
I've had zero luck with getting the parts I want in town and usually order from Amazon, Rock Auto or Mopar Parts Giant.

You can buy an aftermarket rail with a test port. I was thinking about permeantly plumbing in an oil filled pressure gauge just so I had something to glance at quickly when doing trail diagnostics.

I'd do one or the other and do some test driving. You're probably right and they both need done... but there's some value in knowing what fixes it.

Good luck and stick with it!

-Mac

P.S. You're probably within reasonable driving range of Black Magic Brakes...Mr B probably has everything you need in his secret bat cave.
Click to expand...

Yeah thats the plan. Ill probably end up doing just that. It would be awesome if Blaine was willing to take a look at it. Hes about 3 hours away and idk if my jeep can make it that far in the shape its in lol plus the cost of gas would probably be the same as ordering everything lol
 
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SeizeTheKarp said:
Hello all,

I am being plagued with reoccurring slow starting. initially i assumed it was the fuel pump per the many threads on the topic and since the bosch unit for my 2004 (67726) isnt available, i predicted having to go with store brand pumps so i installed the Genright access panel so wouldnt have to drop the tank frequently when they undoubtably die. heres where it gets weird. i didnt have the new pump on hand so i put the tank back in and magically no more slow starts. this lasted for a few weeks then started up again so i pulled the assembly out and replaced just the fuel pump itself with the bosch one which is still available. again i got a few weeks of the jeep firing right up and now its slow starting again. i guess its possible the check valve in the brand new fuel pump died but im kinda thinking that isnt the case. any suggestions of what to investigate?

I plugged a scanner in to measure fuel trim and o2 sensor readings for the heck of it and long term at both banks at temp is 32% im going to have somewhere verify that because that seems unrealistic without throwing a code. my o2 sensors are reading like i may have a bad one so ill probably replace those. from my understanding a bad 02 wont cause slow starting so im at a loss of where to begin.

cheers
Click to expand...

Check your vacuum
 
A little update, the jeep i throwing a 171 code O2 sensors for bank one look ok think. Upstream is bouncing between 0.8 and 0.08 and downstream seems steady at 0.1. Fuel trim for both banks at idle is 32%
 
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Upstream O2 sensors are part of the closed loop engine control.
Downstream sensors only monitor the cats (not part of the closed loop control).
I though tI read that fuel trim should be in the 5% range (though someone with more knowledge may want to weigh in)
 
N757KF said:
Upstream O2 sensors are part of the closed loop engine control.
Downstream sensors only monitor the cats (not part of the closed loop control).
I though tI read that fuel trim should be in the 5% range (though someone with more knowledge may want to weigh in)
Click to expand...

Youre absolutely right. The difference between up and down just indicates cat life. Normal trim should be close to zero in an ideal world. 32% is crazy high. O2 sensor reading appear fine though.
 
SeizeTheKarp said:
Hello all,

I am being plagued with reoccurring slow starting. initially i assumed it was the fuel pump per the many threads on the topic and since the bosch unit for my 2004 (67726) isnt available, i predicted having to go with store brand pumps so i installed the Genright access panel so wouldnt have to drop the tank frequently when they undoubtably die. heres where it gets weird. i didnt have the new pump on hand so i put the tank back in and magically no more slow starts. this lasted for a few weeks then started up again so i pulled the assembly out and replaced just the fuel pump itself with the bosch one which is still available. again i got a few weeks of the jeep firing right up and now its slow starting again. i guess its possible the check valve in the brand new fuel pump died but im kinda thinking that isnt the case. any suggestions of what to investigate?

I plugged a scanner in to measure fuel trim and o2 sensor readings for the heck of it and long term at both banks at temp is 32% im going to have somewhere verify that because that seems unrealistic without throwing a code. my o2 sensors are reading like i may have a bad one so ill probably replace those. from my understanding a bad 02 wont cause slow starting so im at a loss of where to begin.

cheers
Click to expand...

I forgot to list what has been replaced.

Within at most the last 15k:

-Cats; magnaflow carb compliant ones,
-Tps; mopar
-Iac; not mopar but will get mopar one
-Plugs; autolite double platinum
-Coilpacks; mopar i think been a while
-Fuel pump assembly; store brand
-Fuel pump; inside new assembly bosch
-Water pump/thermostat; store brand
-Throttle body/iac gaskets; felpro
-Power steering gearbox; used oem
-Power steering pump; store brand
-Radiator hoses; gates i think
-Battery; optima
-Belt; idk
 
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