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I'm trying to nail this down but unfortunately without any success. Jeep has around 27-9 psi pressure at rail at idle and vacuum connected. With disconnected vacuum it is around 36psi. I think this is main issue why my fuel trim numbers are 5-8%. Things done so far: 

  1. Fuel filter has some 3k miles on it. Bypassed fuel filter and pressure is still the same.
  2. Cleaned and tested fuel injectors. All tested good.
  3. Checked vacuum lines
  4. Fuel tank had to be dropped for skid replacement, so I replaced fuel pump at the same time /w new strainer
  5. Replaced fuel pressure regulator with both o-rings

Biggest difference happened when I replaced fuel pressure regulator. Vehicle would start around 30psi and will slowly drop down after warm up. I can't see any fuel dripping from regulator but I can smell fuel odor in vacuum line elbow. Smell was there even with the old regulator, I thought that it had shot diaphragm and replaced it. I really don't know where to look next, as I can't really see any fuel leaks and fuel consumption seems to be around normal...

 

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  • Moses Ludel changed the title to 1995 Jeep Wrangler 4.0 Low Fuel Pressure
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Hi, Peter...You've been thorough and systematic.  Normal pressure after warmup, vacuum attached to the regulator, should be around 31 psi.  Cranking psi should be 8-10 psi higher.  You do have low pressure though not extreme. 

I would run a fuel pressure test on the engine side of the fuel filter, using a Tee to prevent spiking up the pressure.  A gauge alone at the end of the filter will create a dramatic pump pressure spike that can damage a fuel test gauge or worse.  This would be the same as pinching the return line to get a pressure spike for testing purposes; pinching the return line should never be held for more than three seconds.  The spike will reach around 75 psi.

If the regulator is set for its normal 31 psi release, you should get at least that much pressure at the fuel filter.  This will help indicate whether you have the right pump pressure.  The other concern is volume:  the pump should deliver at least 1 liter of fuel per minute.

Check for fuel pressure bleed down with your pressure gauge on the rail before and after the engine shuts off.  Note the idling pressure when you shut off the engine.  Normally, the pressure will drop another 20 psi in 30 minutes.  If a greater drop or quicker drop, the pressure regulator or fuel pump check valve could be leaking.  Both the pressure regulator and fuel pump need to hold pressure at least while the engine is running.  (I can explain how to test the fuel pump check valve if necessary.  The test is somewhat straightforward.)

Note:  If pump pressure and volume are each low, one possibility is that you may have installed a 1987-90 YJ 2.5L pump with the lower (TBI) pump pressure range.  Compare your part number on the receipt or box to the 4.0L MPI fuel pump's pressure rating and application. 

As for fuel trim, one diagnostic approach involves an S.U.R.&R. injector cleaning kit.  Use of the kit would serve two functions:  1) obviously, cleaning the injectors and 2) allowing you to manually regulate the fuel pressure directly at the rail.  You could watch the fuel trim with the pressure set to precisely 31 psi and see whether that cleans up trim.  If so, the result would be from injector cleaning and precisely regulating the rail psi.  Here is the kit I'm describing, I find it useful for a variety of reasons if you can justify the cost:

https://www.4wdmechanix.com/curing-an-engine-knock-with-surr-tools-and-sea-foam/

If you do not invest in the tool, concentrate on the fuel pressure and also the fuel volume.  To run right, you need both pressure and fuel volume.  A lack of either will reflect in low psi and possibly fuel trim issues.  Fuel trim issues can also be related to an O2 sensor going out or a generic aftermarket replacement sensor's calibration—I always use a genuine Mopar or NTK replacement O2 sensor that matches the OEM.  

Other trim issue causes include an exhaust air leak before the O2 sensor, so make sure the manifolds are secure with no gasket leakage or exhaust manifold cracks.  To be clear, I'm guessing you're concerned about 5%-8% long term trim?  This would not be bad for an engine with higher mileage.  Verify the engine's compression if the trim gets beyond 10%.  I would run a cylinder leak-down test on at least the lowest compression cylinder to evaluate the engine's internal condition.

Let us know how this works out.  We can pursue the issue further...

Moses 

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Moses, thanks for your detailed and quick response. I forgot to mention that this engine was rebuilt a year ago. O2 sensor is brand new NTK. All other sensors have been tested either with OBD tool or with scope and seem to operate properly. In some cases I had spare ones that return same values.

Based on part number I installed proper fuel pump. I tested for bleed down and it held pressure for more than 30 mins. I will preform the test at fuel filter. Can I simply remove fuel filter and put tee instead of it?

What bothers me is that with both old and new regulator there is fuel odor in vacuum line to the regulator. It is not extremely strong but definitely present, so I'm not sure if that is a sign of leak... With old regulator (factory installed one) long ft numbers were around 3-4%, while with new one that went higher. I'm not 100% sure but I think that with old regulator fuel pressure was close to 30psi. I replaced it because I could smell fuel odor in vacuum line and because I replaced fuel pump before that but pressure stayed below 31psi. I could not see any fuel dripping out of it.

Another thing worth mentioning - when I first installed new regulator, on idle it was sitting at 37psi with vacuum line either connected or disconnected. I checked line multiple times but it was ok. Then after some driving it went down to current pressure. Could this be that this regulator is bad from the factory? Maybe it was not calibrated properly.

I will again do smoke test. Last time I did it only place I could see smoke coming out is throttle body shaft but I guess that this leak can be compensated by IAC motor. If I hook up jeep to OBD tool, I can see that IAC steps are 7 on hot idle, not sure if this is normal.

 

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7 hours ago, Peter Rutesic said:

Moses, thanks for your detailed and quick response. I forgot to mention that this engine was rebuilt a year ago. O2 sensor is brand new NTK. All other sensors have been tested either with OBD tool or with scope and seem to operate properly. In some cases I had spare ones that return same values.

You're being very thorough, Peter, thanks for clarifying.  

Based on part number I installed proper fuel pump. I tested for bleed down and it held pressure for more than 30 mins. I will preform the test at fuel filter. Can I simply remove fuel filter and put tee instead of it?

You could put the Tee at the tank side of the filter (inlet) and get a very accurate fuel pump pressure reading.  A post-filter Tee check would add whether the filter is clogged or not.  To save time, it's cheaper to replace the filter...I'm hesitant about running unfiltered fuel to the rail and injectors, a taboo as early as the Bosch Jetronic fuel distributor era.  Bypassing the filter would leave only the pump module sock as protection.  That only works to a degree.  I'd keep filtration in place.

What bothers me is that with both old and new regulator there is fuel odor in vacuum line to the regulator. It is not extremely strong but definitely present, so I'm not sure if that is a sign of leak... With old regulator (factory installed one) long ft numbers were around 3-4%, while with new one that went higher. I'm not 100% sure but I think that with old regulator fuel pressure was close to 30psi. I replaced it because I could smell fuel odor in vacuum line and because I replaced fuel pump before that but pressure stayed below 31psi. I could not see any fuel dripping out of it.

Outside of a defective pressure regulator, there could be another reason for gasoline scent in the vacuum line to the regulator.  You may think that with MPI and no EGR, the vacuum line from the intake manifold to pressure regulator should have no fumes.  The intake is air only, right?  Not right...The fixed orifice crankcase vent tube from the valve cover to the intake manifold and the EVAP canister vacuum source tube each attach to the intake manifold.  According to the vacuum schematic in the FSM, the pressure regulator vacuum line and the vacuum source for the EVAP canister share a fitting on the intake manifold.  Gasoline fumes from the EVAP system could be present and wafting into the regulator vacuum hose.  (Either EVAP or the CCV/fixed orifice tube could bring fumes into the intake manifold.  EVAP would be raw gasoline fumes;  CCV/fixed orifice tube would be crankcase fumes.)  The only other way gasoline fumes would be present in the intake is from a leaking intake valve(s)—which is unlikely with a fresh engine.

To pinpoint whether the pressure regulator is weeping fuel into the regulator vacuum port, I would disconnect the manifold vacuum line to the regulator.  With the engine at idle and fuel rail pressurized, I'd take a dry hand vacuum pump and suction/draw down the regulator to 20-plus in/hg manually.  Zero the vacuum pump and repeat this procedure several time, then check inside the vacuum pump for any signs of raw gasoline or fumes.  No fumes and no fuel would mean that the scent of gasoline is not coming through the regulator.  In this case, the scent you smell at the pressure regulator is fumes wafting through the vacuum line to the regulator.  The fumes source is the intake manifold, either the CCV/fixed orifice tube's crankcase fumes or raw fuel fumes from the EVAP/canister system.

Another thing worth mentioning - when I first installed new regulator, on idle it was sitting at 37psi with vacuum line either connected or disconnected. I checked line multiple times but it was ok. Then after some driving it went down to current pressure. Could this be that this regulator is bad from the factory? Maybe it was not calibrated properly.

YOU MAY HAVE A DEFECTIVE PRESSURE REGULATOR, IT WAS STUCK HIGH INITIALLY THEN DIDN'T FIND A NORMAL SET POINT.  If pump pressure and volume are okay, the culprit could be this pressure regulator.

I will again do smoke test. Last time I did it only place I could see smoke coming out is throttle body shaft but I guess that this leak can be compensated by IAC motor. If I hook up jeep to OBD tool, I can see that IAC steps are 7 on hot idle, not sure if this is normal.

Smoke tests are great.  (I have a 25-year-old Vacutec machine.)  Scope testing is phenomenal.  (Much more about oscilloscopes this year at the magazine site.  I'm preparing to do an HD video series on scopes, their use and their place.)  The fume concern sounds like EVAP or crankcase fumes.  In moderation these fumes are normal...If excessive or when raw gasoline is present, check the functions of the EVAP canister and EVAP system.  Even the simplest item in the EVAP stream can raise havoc, including an improperly sealing gas cap.  Your long fuel trim is off some but may correct as you work through this.  If the current pressure regulator is under warranty, get a replacement after you confirm that the fuel pump pressure and volume are correct.

Keep us posted...

Moses

 

 

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