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High Fuel Pressure, Misfires and Low Idle

Old 01-31-2019, 06:38 PM
  #16  
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From my notes/combined with FSM text:

"Warm vehicle to operating temperature

OE Spec. - 49.2 5 psi

<44.2 psi at rail, check for kink in fuel line. If no kink, but specifications for either the Fuel Pump Capacity, Fuel Pump Amperage or Fuel Pressure Leak Down Tests were not met, replace fuel pump module assembly.

If operating pressure is above 54.2 psi, electric fuel pump is OK, but fuel pressure regulator is defective.
After engine was warm, gauge read between 48 and 50 psi, this is within spec and fuel pump and regulator appear to be functioning --> Proceeded to fuel injector test"
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Old 01-31-2019, 06:53 PM
  #17  
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Originally Posted by jburg318 View Post
From my notes/combined with FSM text:

"Warm vehicle to operating temperature

OE Spec. - 49.2 5 psi

<44.2 psi at rail, check for kink in fuel line. If no kink, but specifications for either the Fuel Pump Capacity, Fuel Pump Amperage or Fuel Pressure Leak Down Tests were not met, replace fuel pump module assembly.

If operating pressure is above 54.2 psi, electric fuel pump is OK, but fuel pressure regulator is defective.
After engine was warm, gauge read between 48 and 50 psi, this is within spec and fuel pump and regulator appear to be functioning --> Proceeded to fuel injector test"

Okay so mine was reading closer to 58. I also read this is a sign of a faulty pressure regulator. I just want to be certain before I shell out $220 for a new pump assembly. On the 00 it's one unit attached to the pump.

Speaking of leak down test. I left the pressure gauge sitting for about a half hour after turning the vehicle off. After about a half hour I came back out to see it at about 20 psi. Is this normal?
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Old 01-31-2019, 07:14 PM
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Originally Posted by mattheww97 View Post
Okay so mine was reading closer to 58. I also read this is a sign of a faulty pressure regulator. I just want to be certain before I shell out $220 for a new pump assembly. On the 00 it's one unit attached to the pump.

Speaking of leak down test. I left the pressure gauge sitting for about a half hour after turning the vehicle off. After about a half hour I came back out to see it at about 20 psi. Is this normal?
The Fuel Leak Down test is more complex than that. The gauge is hooked up between the fuel line and the rail schrader valve.

Also, you just need the regulator/filter, not the whole assembly; however, since you are dropping the whole tank it is not a bad idea to replace everything at once.
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Old 01-31-2019, 10:06 PM
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Originally Posted by jburg318 View Post
From the 2000 FSM
"Fuel Pressure Regulator Operation:
The pressure regulator is a mechanical device that is not controlled by engine vacuum or the Powertrain Control
Module (PCM).
The regulator is calibrated to maintain fuel system operating pressure of approximately 339 kPa
34 kPa (49.2 psi 5 psi) at the fuel injectors. It contains a diaphragm, calibrated springs and a fuel
return valve. The internal fuel filter is also part of the assembly."

Cleaning the throttle body shouldn't change a thing I would put my money on a bad fuel pressure regulator. Your fuel trims should be showing rich as well and you should be able to smell it in your exhaust fumes. Although it looks like its cold where you are located which will also make the vehicle run a bit richer.
I haven't been getting codes for running rich. Just multiple cylinder misfire. Blinking check engine light. Does a faulty pressure regulator cause misfire?

Also no kidding it's cold over here. Can't wait to drop the gas tank in the snow
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Old 02-01-2019, 05:27 AM
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Originally Posted by mattheww97 View Post
I haven't been getting codes for running rich. Just multiple cylinder misfire. Blinking check engine light. Does a faulty pressure regulator cause misfire?

Also no kidding it's cold over here. Can't wait to drop the gas tank in the snow
CEL is set by very specific conditions (i.e. Voltage above 1.0 for 30 seconds during 5 warm-up cycles, etc.) and in my experience the Jeep ECU is 'slow' to react to throwing a code. Without a doubt a pressure like that at the rail is causing a rich condition, which would explain the rough and low idle and misfires. I think the rich code is -20% on the Cherokee for a specific period of time for illumination (I would have to check). The blinking is misfires (not sure if you knew that or not). To be extra sure you can get a real time scan tool like Blue Driver, but even the FSM says to replace regulator. Yes, a faulty regulator will cause misfires.

Think about it, too much fuel in the cylinders is either not being burnt or burning in the compression stroke. I would change your oil after this as it is most likely contaminated with high levels of unburnt gasoline.
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Old 02-01-2019, 01:59 PM
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Originally Posted by jburg318 View Post
CEL is set by very specific conditions (i.e. Voltage above 1.0 for 30 seconds during 5 warm-up cycles, etc.) and in my experience the Jeep ECU is 'slow' to react to throwing a code. Without a doubt a pressure like that at the rail is causing a rich condition, which would explain the rough and low idle and misfires. I think the rich code is -20% on the Cherokee for a specific period of time for illumination (I would have to check). The blinking is misfires (not sure if you knew that or not). To be extra sure you can get a real time scan tool like Blue Driver, but even the FSM says to replace regulator. Yes, a faulty regulator will cause misfires.

Think about it, too much fuel in the cylinders is either not being burnt or burning in the compression stroke. I would change your oil after this as it is most likely contaminated with high levels of unburnt gasoline.
Thank you. I managed to find a website that sells the mopar assembly $202 after the dealership quoted me $545 for a part they'll get in march. Gonna spray down all the bolts today and prep to drop the tank. Will let you guys know how it goes.
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Old 02-03-2019, 05:43 PM
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So i couldn't find an available Mopar pump. Ended up going with the deliphi. My fuel pressure hasn't changed its just less sparatic now.. still very low idle. Guarante I'll be getting a misfire code. Any thoughts?? Spend the whole day dropping this tank for nothinf
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Old 02-04-2019, 05:00 AM
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Originally Posted by mattheww97 View Post
So i couldn't find an available Mopar pump. Ended up going with the deliphi. My fuel pressure hasn't changed its just less sparatic now.. still very low idle. Guarante I'll be getting a misfire code. Any thoughts?? Spend the whole day dropping this tank for nothinf
The only other thing high pressure can be is a kink/clog in the return line. You would have to ask others, but maybe a screwed up electrical connection? The FSM says nothing of that however and maybe someone else will be of help. Additionally, the pressure gauge may be miscalibrated or broken. Source another one.
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Old 02-05-2019, 09:46 AM
  #24  
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Originally Posted by jburg318 View Post
The only other thing high pressure can be is a kink/clog in the return line. You would have to ask others, but maybe a screwed up electrical connection? The FSM says nothing of that however and maybe someone else will be of help. Additionally, the pressure gauge may be miscalibrated or broken. Source another one.
It was stated earlier that there is no return line, but rather just the pressure regulator letting some fuel release into the tank.

Is your year coil-on-plug, distributor cap, rotor, coil, and wires? I'd look that side. You aren't much off the high side.

Last edited by gat; 02-05-2019 at 09:49 AM.
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Old 02-05-2019, 10:10 AM
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Originally Posted by gat View Post
It was stated earlier that there is no return line, but rather just the pressure regulator letting some fuel release into the tank.

Is your year coil-on-plug, distributor cap, rotor, coil, and wires? I'd look that side. You aren't much off the high side.
Missed that in the FSM:

"If fuel pressure at the pressure regulator exceeds approximately 49 psi, an internal diaphragm closesand excess fuel is routed back into the tank through the pressure regulator. A separate fuel return line is not used."

However, based on the FSM, the pressure he is reporting is well out of spec and points to a faulty regulator "(4) If operating pressure is above 54.2 psi...". Unless of course the pressure gauge that was sourced is not calibrated, then yes, there is something else wrong. It currently appears that something else is wrong based on his results.
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Old 02-06-2019, 02:02 AM
  #26  
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I agree with you that 58 p.s.i. is high, but I can't figure how higher pressure in the rail translates to low idle. Maybe misfires from preignition... I'd think that low idle and misfires is more likely an issue of electrical side, than fuel side.

It is also hard to figure how two different pumps fail at the same pressure of 58 p.s.i. I'm wondering whether the voltage is a bit high to the fuel pump and at maximum the pump can overpower the regulator to that pressure.
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Old 02-06-2019, 12:26 PM
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Originally Posted by gat View Post
It was stated earlier that there is no return line, but rather just the pressure regulator letting some fuel release into the tank.

Is your year coil-on-plug, distributor cap, rotor, coil, and wires? I'd look that side. You aren't much off the high side.
No it's a 2000with the distributorless and coil rail. I checked the coil rail with a meter and everything seems fine. Replaced my plugs with NGK since that's factory for those Jeeps. I really think it's fuel related Because it only happens at idle. Jeep runs fine after given gas. Occasionally misfire will get too bad (at idle) that the CEL blinks putting the Jeep in limp in mode. But whenever I reset the codes it drives like a cjamp
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Old 02-06-2019, 12:32 PM
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Originally Posted by gat View Post
I agree with you that 58 p.s.i. is high, but I can't figure how higher pressure in the rail translates to low idle. Maybe misfires from preignition... I'd think that low idle and misfires is more likely an issue of electrical side, than fuel side.

It is also hard to figure how two different pumps fail at the same pressure of 58 p.s.i. I'm wondering whether the voltage is a bit high to the fuel pump and at maximum the pump can overpower the regulator to that pressure.
I'm thinking it's severe heat soak. I've noticed this only happens at idle, when the engine has warmed up. Cold start the Jeep idles fine and then slowly starts to idle poorly until it starts misfiring. I bought a huge roll of reflective insulation for $18 at lowes. I'm gonna wrap the fuel rail and injectors as soon as it stops snowing. Regardless, I've heard that it doesn't always fix the misfire, but improves idle. I just need to get it to Idle well enough to pass inspection will let you know how it goes
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Old 02-06-2019, 12:43 PM
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I believe that some also put insulation over the intake header to further insulate the heat from the exhaust and block from the injectors. Jeep sells fluted cardboard to wrap the injectors. You could rig multiple layers of insulation.

We're you ever able to take the reading for pressure with a different gauge to ensure that is was 58 p.s.i. It just seems so strange to me that both pumps happen to have the same value over pressure. Is there a big dent in the rail right around the Schrader valve perhaps?
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Old 02-08-2019, 05:17 PM
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UPDATE: So i tried the insulation thing, spent an hour wrapping everything up only to find a jeep that still idles like garbage. Temporary inspection expired today meaning I can no longer drive it, so I threw in my 6 year mechanic free towel and brought it to a local shop.

They compression tested it and compression checks out. Mechanic then says the issue is the coil rail. "Night and Day" after replacing it, ready to be picked up. Head down to the shop, pay my bill, get in the Jeep. WILL NOT START. Mechanic comes out. NO CONNECTION TO THE CPU can't read the codes. Doesn't know why. Say's he can't look at it until monday. If it's something he did he'll fix it for free but if the issue is elsewhere we'll see what happens.

Total Bill:
$100 Compression Test (Outrageous if you ask me)
$120 Coil Rail (Not too bad because thats about what the part goes for)
$21 Inspection (Failed since it hasn't completed it's drive cycle.

Not once have I had an issue with the Jeep starting until I brought it to a shop. I'm gonna try to get my money back for the coil rail, or at least get my old one back and keep both parts. I said I'm gonna go to the shop tomorrow and look at it myself in the parking lot tomorrow. Any thoughts? I'm absolutely pissed over here, all I needed was that inspection sticker!
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