88psi Fuel Pressure & Hot Start Issues

T

Testrun

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Joined
Sep 28, 2023
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25
Location
Hollywood, FL
Good evening all. I've been lurking here a bit and decided to ask some of the pros on here what the heck is wrong with this thing. No codes except a few previous from 02 sensor, evap, crank pos sensor, but those codes disappeared awhile ago and never came back.

Startup cold flawless.
Runs flawless.
Shutdown and start up right away, flawless.
Shutdown wait about 15 min will only crank.
Shutdown wait 30 min will start and run flawless and sometimes may start a tad weak then run flawless.

I like throwing a few parts at things before diagnosing too much if I think the parts need replacing anyway.
1. This has a new unknown pump that I swapped and was supplied with when I bought it do to a poor fuel level sending unit. Pump has maybe 3k miles on it. It is loud and seems to be louder than when I 1st installed it.
2. New Crank shaft sensor
3. New Coolant temp sensor
4. Bosch 4 hole injectors


Fuel pressure test is at 88psi at idle and will hold around 88 psi. From what I understand with the 4.0L is FP should be around the high 40s and/or low mid 50s. So I am wayyyy over that. Could this be just a FP regulator issue? The pump is noticeably loud and if I use the pressure relief valve on the FP tester the pump will quit down a bit as the pressure is released (while running).

Best Options? Replace whole fuel assembly? Or replace FPR on top of tank? (The metal one). Shall I replace both, but with a bosch pump and whatever FPR?

Any help would be appreciated. I am kind of a Toyota and Rotary fan, but I want to support to my home c and run a little American, but this thing is 10x more of a pain than my 94 RX7 lol...

Help!!! I promise I will let the forum know what fixes it, if I ever get there. Seems to be common these days not to finalize a thread.
 
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The only thing I can think of that would cause an excessively high fuel pressure, which I've never actually heard of happening, is if the fuel pressure regulator was stuck closed and unable to release excessive fuel pressure by dumping gasoline directly back into the tank. You sure that fuel pressure gauge is accurate and reading in psi?
 
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Jerry Bransford said:
The only thing I can think of that would cause an excessively high fuel pressure, which I've never actually heard of happening, is if the fuel pressure regulator was stuck closed and unable to release excessive fuel pressure by dumping gasoline directly back into the tank. You sure that fuel pressure gauge is accurate and reading in psi?
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I used 2 separate gauges to verify it. :(
I always heard decreasing instead of increasing, although I am sure it is possible.
 
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I don't think the pic went through. It is a 94 with upgraded Hitachi Twins... probably around 360-380 to the wheels. Has about 64k miles.

Screenshot_20230928_194649_Gallery.jpg
 
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One of the sexiest cars ever designed. So sexy on fact they had to install a rotary in it to keep it from being too awesome 😄
 
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SkylinesSuck said:
One of the sexiest cars ever designed. So sexy on fact they had to install a rotary in it to keep it from being too awesome 😄
Click to expand...

My 2nd gen ran 200k miles on it seeing redline every day never missing a beat.. It isn't until you force air that you start having issues and even then they can be extremely reliable... lot of common misconceptions out there. 86 CU that can push 700HP, yeah...... I am sure anything less than a jet engine becomes a hand gerande at those pressures. Good tune, lube, and cool temps I expect this to last a awhile.... I hope lol
Speaking of pressures, what about my TJ? Lol I am thinking FPR
 
So I waited until it would only turn over and not start. I purged the fuel rail and it was vapor. So I am obviously getting vapor lock.

Could this be the result of the higher fuel pressure? Or a failure of the fuel to return back to the tank?

I will try to just swap out the fuel pressure regulator and if that doesn't work swap out the pump.

Or could this be a "check valve" issue that is on the assembly? Sorry guys I am a bit new to the Jeep world and never really dealt with excessive pressure, only lower pressure.
 
Testrun said:
So I waited until it would only turn over and not start. I purged the fuel rail and it was vapor. So I am obviously getting vapor lock.

Could this be the result of the higher fuel pressure? Or a failure of the fuel to return back to the tank?

I will try to just swap out the fuel pressure regulator and if that doesn't work swap out the pump.

Or could this be a "check valve" issue that is on the assembly? Sorry guys I am a bit new to the Jeep world and never really dealt with excessive pressure, only lower pressure.
Click to expand...

When I was buying my 97 the fuel line had wooden clothes pins on it. I asked the owner what fur ? He said it was vapor locking and that fixed it ! So it would be cheap enough to give it a try. A old timer told me they did that long ago to fix that issue. Here's some pictures there are 2 more you can't see on the fuel line under the exhaust manifold.

28588.jpeg


28587.jpeg
 
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Testrun said:
So I waited until it would only turn over and not start. I purged the fuel rail and it was vapor. So I am obviously getting vapor lock.

Could this be the result of the higher fuel pressure? Or a failure of the fuel to return back to the tank?

I will try to just swap out the fuel pressure regulator and if that doesn't work swap out the pump.

Or could this be a "check valve" issue that is on the assembly? Sorry guys I am a bit new to the Jeep world and never really dealt with excessive pressure, only lower pressure.
Click to expand...

Higher fuel pressure wouldn't cause vapor lock...it actually helps by increasing the temperature required for the fuel to evaporate. While certainly interesting/concerning, it may be a red herring.

If it was only high during the heat soak incidents I would say the pressure was resulting from the vapor lock instead of the other way around, but since it does that even when running, I think they're unrelated.

I think you're on the right track with the FPR being the cause of the 88psi but I don't think it's going to fix your vapor lock issue. I would focus on the heat soak first.

Do you have any heat shielding under your fuel rail?

PXL_20230721_175620125.jpg
 
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macleanflood said:
Some sort of heat sink, perhaps?

Old wives tale IMHO...or in this case old mechanics tale...

-Mac
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I see where it can help. I dont have the fuel rail/injector insulation kit installed (yet) and in almost 40k miles never had a problem. I wouldn't call it a tale if it works !
 
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freedom_in_4low said:
Higher fuel pressure wouldn't cause vapor lock...it actually helps by increasing the temperature required for the fuel to evaporate. While certainly interesting/concerning, it may be a red herring.

If it was only high during the heat soak incidents I would say the pressure was resulting from the vapor lock instead of the other way around, but since it does that even when running, I think they're unrelated.

I think you're on the right track with the FPR being the cause of the 88psi but I don't think it's going to fix your vapor lock issue. I would focus on the heat soak first.

Do you have any heat shielding under your fuel rail?

View attachment 461485
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Thanks for the response. I have a beat-up one under the #3 inj. I am ordering the DEI kit off Amazon unless you recommend something else. I can see if that will help.
Why would it just start out of nowhere? Maybe the added pressure is heating up the pump? "Straining it"? That may be the few degrees difference resulting in this? As stated before I can hear a difference if I purge some pressure off whilst it is running. I can hear the pump quiet down and instantly get louder when I stop purging the pressure from the rail. Hope that all makes sense.
Any recommendations besides the DEI kit?
 
macleanflood said:
Some sort of heat sink, perhaps?

Old wives tale IMHO...or in this case old mechanics tale...

-Mac
Click to expand...

Testrun said:
Thanks for the response. I have a beat-up one under the #3 inj. I am ordering the DEI kit off Amazon unless you recommend something else. I can see if that will help.
Why would it just start out of nowhere? Maybe the added pressure is heating up the pump? "Straining it"? That may be the few degrees difference resulting in this? As stated before I can hear a difference if I purge some pressure off whilst it is running. I can hear the pump quiet down and instantly get louder when I stop purging the pressure from the rail. Hope that all makes sense.
Any recommendations besides the DEI kit?
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Mine is an 06 which came with one and just bought the official Mopar part this summer because mine basically fell apart while I was working on replacing the valve cover. It was under $30. The DEI kit has wraps for the rails and the injectors themselves which definitely won't hurt.

It makes sense that the pump will make more noise when working against more pressure. The tendency for liquid to vaporize more easily at lower pressures makes me doubt the connection. I don't think the extra work by the pump is going to result in any significant temperature increase because it's submerged in fuel inside the tank so it has lots of cooling.

Mechanical components go bad out of nowhere and maybe that's what happened to your FPR, but I don't have a good explanation for why the vapor lock would have started out of nowhere unless something changed about your lifestyle or driving habits and you're suddenly making a lot more 15 minute stops. Did you recently start eating gas station sushi a lot more often?
 
freedom_in_4low said:
Mine is an 06 which came with one and just bought the official Mopar part this summer because mine basically fell apart while I was working on replacing the valve cover. It was under $30. The DEI kit has wraps for the rails and the injectors themselves which definitely won't hurt.

It makes sense that the pump will make more noise when working against more pressure. The tendency for liquid to vaporize more easily at lower pressures makes me doubt the connection. I don't think the extra work by the pump is going to result in any significant temperature increase because it's submerged in fuel inside the tank so it has lots of cooling.

Mechanical components go bad out of nowhere and maybe that's what happened to your FPR, but I don't have a good explanation for why the vapor lock would have started out of nowhere unless something changed about your lifestyle or driving habits and you're suddenly making a lot more 15 minute stops. Did you recently start eating gas station sushi a lot more often?
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Gotcha. We really haven't changed anything in our daily routines.
The pump assembly was changed out about 3k miles ago with a cheapy unbranded assembly.

Would you just swap the whole assembly out again? Plus the heat mitigations of course. I can work some.custom stuff in there as I am no stranger to heat with a turbo rotary around lol.
 
Testrun said:
Gotcha. We really haven't changed anything in our daily routines.
The pump assembly was changed out about 3k miles ago with a cheapy unbranded assembly.

Would you just swap the whole assembly out again? Plus the heat mitigations of course. I can work some.custom stuff in there as I am no stranger to heat with a turbo rotary around lol.
Click to expand...

I would try the heat stuff first because I think that's more likely to address the actual driveability issue.

Did the no-start issue and the extra pump noise start immediately with the pump replacement?

I would swap the whole assembly with a Bosch if possible. I don't remember how the years break down but I know some of the older model years aren't available as a full assembly from Bosch anymore. If that's the case for your 03 I'm not sure what to suggest as an alternative. I put a Delphi in my Silverado in 2019 that has been working just fine since.
 
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