Fuel pump issue?

T

TJBUD

TJ Enthusiast
Supporting Member
Joined
Feb 16, 2022
Messages
40
Location
Texas
TJ fans,
I am chasing a significant power loss under load.

Background:
1998 TJ with 1500 miles on new 4.7L stroker I finished in September ‘22. It has the AX15 manual. I am running the factory tune (assuming that’s what was in it when I bought the TJ with the 4.0 in 2/22). The motor runs great and today is the first I’ve seen this problem. I have an AEM air fuel ratio gauge with wideband O2 installed preparing for a custom tune. The AFR under light Load has been near 14.7 and under heavier load closer to 13. The fuel injectors are new FIC 4 hole [email protected], 27lb@49psi that flow like Bosch 0280150947.

Problem.
The TJ Starts normally and idles fine. When I begin to accelerate, after a few seconds under load, it seems like it’s running out of gas losing power, the AFR gauge pegs lean, and the Jeep decelerates. I push the clutch in to prevent a stall, it returns to normal idle and AFR back in the 14s. If I baby it keeping throttle and load light it will slowly run up through the gears and cruise. I Limped home 50 miles like this. I stopped halfway for 20 mins (ran into store) and saw more of the same when I resumed. I started with 1/2 tank of gas and finished about 3/8–fuel is less than 1 week old. I drove the same 50 miles (other way) Monday and 20 miles running errands yesterday without incident.

Tests:
the fuel rail has a test port so I connected a Fuel Pressure gauge. With the ignition turned on but not running, it shows about 49-50psi. With the engine running it jumps to 56psi. after turning the ignition off at 56psi I read the following pressures over time:
3 mins—53 psi
20 mins—50 psi
60 mins—20psi
90 mins—14psi
120mins—12psi
Even though I didn’t get a CEL, I connected my Ancel AD310 obd2 reader—no stored codes, no pending codes. This surprised me given I thought it must have misfired. The tach stayed steady when power was lost.

Why is the regulator not holding at 49 when running? I assume the pump is faster (higher pressure) due to the higher voltage when running but the regulator should clamp it right?
Is the above leak down test normal?
I think it’s acting like fuel starvation, like the pump can’t meet the volume demand. Or fuel filter (screen) is clogged? Ideas?

I don’t have a way to watch the fuel pressure gauge while driving—if needed I’ll try to get a longer hose extension tomorrow.

Thanks y’all, TJBUD
 
I just finished reinstalling my fuel tank.

I replaced the pump and pressure regulator...I think they both have check valves.

Also welded up some rust holes, painted the skid and replaced the carriage bolts with stainless rivnuts.

But I had the same issue only faster...I'd be down to zero PSI inside of ten minutes.

The stock computer will reduce fuel out of the injectors to compensate for bigger injectors. You'll need a custom tune to fix that.

-Mac
 
Thanks MAC. Based on earlier posts by @JerryBransford since mine bleeds down slowly I would have thought the check valve in the pump was bad, but the 56psi while running makes me suspect something’s up with the regulator too. Safe thing would be to replace both the pump and regulator. Since I can find the Bosch pump (69316) but not the Bosch assembly, I’ll rebuild my assembly with new pump and regulator. My gauge seems accurate enough. Did you replace the pump and regulator as components rather than the whole assembly—not that big a deal? TJBUD
 
Yup...followed the post and took my pump assembly apart. It was an original Mopar assembly with a Bosch pump. I replaced the pressure regulator with the recommend Chinese Amazon one and a Bosch Brazilian pump I got months ago.

I was originally confused because I thought the pump would come with a spacer...and eventually figured out I was reusing the spacer in my existing assembly...which really was nothing more than a triangle of what appeared to be heavy cardboard. I'd like to find a source for a more robust plastic spacer...might just buy a chunk of fuel baffle foam.

My new pump came with a sock...and I discovered that sock had to be removed and replaced with a TJ specific one.

In removing the tank I also snapped both tank strap bolts...so consider ordering new straps.

I also replaced the fuel filler and vent hose and filler neck...all of it from Spectra.

I should have spent some time with the fuel level sender. Mine clicks the fuel light on with about 6 gallons left in the tank.

I'll probably do some more research on that and drop it again this summer.

I replaced all the carriage bolts with stainless M8 nutserts and bolts. I also put M5 stainless nutserts on the filler neck instead of sheet metal screws.

-Mac
 
macleanflood said:
Yup...followed the post and took my pump assembly apart. It was an original Mopar assembly with a Bosch pump. I replaced the pressure regulator with the recommend Chinese Amazon one and a Bosch Brazilian pump I got months ago.

I was originally confused because I thought the pump would come with a spacer...and eventually figured out I was reusing the spacer in my existing assembly...which really was nothing more than a triangle of what appeared to be heavy cardboard. I'd like to find a source for a more robust plastic spacer...might just buy a chunk of fuel baffle foam.

My new pump came with a sock...and I discovered that sock had to be removed and replaced with a TJ specific one.

In removing the tank I also snapped both tank strap bolts...so consider ordering new straps.

I also replaced the fuel filler and vent hose and filler neck...all of it from Spectra.

I should have spent some time with the fuel level sender. Mine clicks the fuel light on with about 6 gallons left in the tank.

I'll probably do some more research on that and drop it again this summer.

I replaced all the carriage bolts with stainless M8 nutserts and bolts. I also put M5 stainless nutserts on the filler neck instead of sheet metal screws.

-Mac
Click to expand...

Can you elaborate on the fuel sender issue, I have a complete replacement fuel pump assembly but now the gauge indicates 3/4 when full not 4/4 like the old fuel pump (which had a bad pressure regulator), anyone have ideas how to fix ?
 
Jayramble said:
Can you elaborate on the fuel sender issue, I have a complete replacement fuel pump assembly but now the gauge indicates 3/4 when full not 4/4 like the old fuel pump (which had a bad pressure regulator), anyone have ideas how to fix ?
Click to expand...

Haven't found it yet...but allegedly there's a procedure to check and possibly adjust the ohms...I imagine you check at both ends of the swing. I think I heard it was in the FSM but I haven't found it yet.

-Mac
 
TJBUD said:
TJ fans,
I am chasing a significant power loss under load.

Background:
1998 TJ with 1500 miles on new 4.7L stroker I finished in September ‘22. It has the AX15 manual. I am running the factory tune (assuming that’s what was in it when I bought the TJ with the 4.0 in 2/22). The motor runs great and today is the first I’ve seen this problem. I have an AEM air fuel ratio gauge with wideband O2 installed preparing for a custom tune. The AFR under light Load has been near 14.7 and under heavier load closer to 13. The fuel injectors are new FIC 4 hole [email protected], 27lb@49psi that flow like Bosch 0280150947.

Problem.
The TJ Starts normally and idles fine. When I begin to accelerate, after a few seconds under load, it seems like it’s running out of gas losing power, the AFR gauge pegs lean, and the Jeep decelerates. I push the clutch in to prevent a stall, it returns to normal idle and AFR back in the 14s. If I baby it keeping throttle and load light it will slowly run up through the gears and cruise. I Limped home 50 miles like this. I stopped halfway for 20 mins (ran into store) and saw more of the same when I resumed. I started with 1/2 tank of gas and finished about 3/8–fuel is less than 1 week old. I drove the same 50 miles (other way) Monday and 20 miles running errands yesterday without incident.

Tests:
the fuel rail has a test port so I connected a Fuel Pressure gauge. With the ignition turned on but not running, it shows about 49-50psi. With the engine running it jumps to 56psi. after turning the ignition off at 56psi I read the following pressures over time:
3 mins—53 psi
20 mins—50 psi
60 mins—20psi
90 mins—14psi
120mins—12psi
Even though I didn’t get a CEL, I connected my Ancel AD310 obd2 reader—no stored codes, no pending codes. This surprised me given I thought it must have misfired. The tach stayed steady when power was lost.

Why is the regulator not holding at 49 when running? I assume the pump is faster (higher pressure) due to the higher voltage when running but the regulator should clamp it right?
Is the above leak down test normal?
I think it’s acting like fuel starvation, like the pump can’t meet the volume demand. Or fuel filter (screen) is clogged? Ideas?

I don’t have a way to watch the fuel pressure gauge while driving—if needed I’ll try to get a longer hose extension tomorrow.

Thanks y’all, TJBUD
Click to expand...

How is the fuel pressure issue coming?it does seem like a regulator issue. Your pressure shouldn't be 56 psi for a 98?
 
Last edited:
RickyD, I replaced the fuel pump with Bosch 69316 and and regulator with Mopar 52100053 (Rock Auto substitute for 4798301). It stays right at 49-51 psi all the time. The symptoms cleared up And I consider it fixed. There was a parallel Thread started by another member and I never went back and closed this thread. TJBUD
 
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TJBUD said:
RickyD, I replaced the fuel pump with Bosch 69316 and and regulator with Mopar 52100053 (Rock Auto substitute for 4798301). It stays right at 49-51 psi all the time. The symptoms cleared up And I consider it fixed. There was a parallel Thread started by another member and I never went back and closed this thread. TJBUD
Click to expand...

Glad that worked out. Good afr's up to redline now? I also got fuel injector connection 24lb injectors for my russ spec 4.6l so I'm curious.

I regret not having him put LS valves in my edelbrock head and using his modified 235 cam that you have.pulls well off idle? Better top end than my 231 I'm sure
 
Yes now fuel pressure stays same all the way up past 5000 while driving. Fuel pump/regulator were a problem. Just back from tuner this week and AFR stays rich during acceleration and hills past 5000. Was running lean on factory tune even with good fuel supply and scaled up injectors so tune was needed. Pulls hard down low and when timing hits above 1500 it pulls real hard all the way past 5000. Very happy. TJBUD
 
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