Opinions needed related to my fuel pressure loss vapor lock issue..

[Boom_ROASTED]

I've had an ongoing fuel pressure, heat soak, vapor lock issue for way too long. I work from home and don't drive the 4Runner much, but when I do I'm always running the risk of being stranded, especially when I need to drive for 30+ minutes and the destination isn't my driveway.

I think I'm nearing the end of this, but I'm hoping someone can test my logic and let me know if there's anything I'm not taking into consideration.

Tools available:

Ultragauge EM Plus
Innova 5610
Analog fuel pressure gauge attached to a schrader valve on the fuel rail.

Right now I'm thinking it's either:

1. Fuel pump and/or check valve failing to hold residual pressure after shutdown, allowing bleed-back into the tank. My understand is that these are essentially the same problem since the pump will need to be replaced in either case.
2. Weak fuel pump. The pump isn't able to get the PSI over 33. Is this reason enough alone to replace the fuel pump?
3. FPR — recently replaced but defective new unit is possible
4. Leaking injectors

The last thing I want to do right now is replace a pump when it's the injectors, vice versa, or replace all and find out there's something I didn't consider.

I'll include the tests I'm going to run today, which is what I'm hoping to get some feedback on. I'll include all of the symptom details below that if you feel like reading them.

The Logic Behind My Next Tests

With the pressure gauge on, I plan to bring fuel pressure up to 33 PSI before shutting off the engine and pinching the intake and return fuel lines. I don't believe I need a real heat soak vapor lock situation active in order to figure out what's leaking.

One issue I'm already seeing would be if the injectors are leaking. If they are leaking then I don't know how to test to see if the FPR or the pump/check valve are leaking.

Here's the isolation test sequence I'm planning and what (I believe) each result would mean:

Test 1 — Isolate the injectors first:

Shut the engine off at 33 PSI and pinch both the return and intake lines. If pressure holds then it confirms the injectors are not leaking and I can continue with the other tests.

If pressure drops, I'll have to deal with the injectors before anything else.

Test 2 — Pinch the return line (FPR isolation):

With injectors confirmed not leaking from Test 1, clamp the return line.

If pressure holds then the FPR is confirmed as the leak path. Pump check valve and injectors are fine.

If pressure drops — the injectors are good so it's either the pump or the FPR.

Test 3 — Pinch the feed line (pump/check valve isolation):

I clamp the feed line instead of the return line and watch pressure.

If pressure holds then the pump check valve is confirmed as the leak path. FPR and injectors are fine.

If pressure drops — FPR and fuel pump are both leaking.

The Symptoms

The only time I don't have a fuel pressure leak is while the engine is running. It sits at 33 PSI. As soon as I stop the engine, heat-soaked or not, the pressure starts to decrease and it will fall to 0 PSI. When the engine is cold I can start it without even realizing there's a problem. The gauge will just right back up to ~31-33 PSI.

Cold starts are perfect every time. This is only a heat-soak issue. If I shut off the a heat-soaked engine and restart quickly it's fine, presumably because the pressure hasn't dropped enough and there's still plenty of liquid fuel in the fuel rails.

If I don't restart quickly after a heat soak period with the engine off, one of two things happens:

Best case: the engine cranks and starts but immediately idles very rough, backfiring and threatening to stall. Giving it gas sometimes keeps it alive and after a minute or two of driving the issue resolves itself completely.

Worst case progression: engine cranks and runs rough for 5-10 seconds then stalls. Second crank attempt immediately stalls. Third attempt.. at this point I believe the engine is flooded, but no matter what it's not going to start. I have to wait an hour or more for the engine to cool before it will start again.

I also occasionally get a pending P0300 after successfully recovering from a rough hot start backfiring parking lot display.

What I've Already Replaced

At the start of all this I had a few issues, replacing some of these parts resolved some, but the fuel pressure leak remains:

• MAF sensor
• ECT sensor and wiring
• Spark plugs
• Ignition coils
• Spark plug wires
• Fuel injectors were cleaned and rebuilt with new o-rings, tested to confirm they worked, but could still be leaking
• Idle Control Valve (ICV) resolved a separate rough idle issue dramatically
• Fuel Pressure Regulator (FPR) — replaced recently with a denso unit hoping the original one was defective. If it was a problem it wasn't the only problem.
• Vacuum leak — found with a smoke machine and fixed. I believe this leak that I drove with for way too long is the reason my LTFT is so high (+19.53)

I also followed multiple methods to "reset the ECU" in attempt to reset the LTFT after fixing the vacuum leak but nothing worked.

OBD2 Data

I do have some O2 sensor data recorded, but I'll leave it out for now and include it later if necessary.

Fuel Pressure Testing

I installed a Schrader valve on the fuel rail and have been running pressure tests. I've already mentioned some of this above, but here's all the details:

• After a heat soak period with the engine fully cooled, fuel pressure at the rail was 0 PSI — completely bled down overnight
• The Innova 5610's fuel pump test brought pressure up to 28-29 PSI. It could not get the pressure any higher than that, but I guess it's still high enough that the engine seems to start with no issues.
• I attempted to use the fuel pump test to bring the PSI up while in a heat-soak situation, hoping it would let me crank it and drive home, but it didn't work like it did when the engine was cold(er)
• Running the engine brought pressure to 31-33 PSI maximum — still below spec as I understand it
• After cutting the engine with pressure at ~33 PSI, it dropped 10 or so PSI within just a few minutes
• On a cold engine, pressure held at approximately 33 PSI for 25+ minutes — heat-soak making the pressure leak worse makes sense

 

[phattyduck]

Your fuel pressure should be 41psi +/-3psi... so something is not right.

You can check 'flooded' (unlikely!) state by pulling the plugs quickly after a 'failed' start. Wet (fuel coated) plugs = flooded. You'll likely find dry plugs.

My first guess here is that the fuel pump is locking up when hot. And is likely worn and not pumping enough fuel otherwise. 33psi vs. 41psi is ~10-12% less flow through the injectors, so straight away, you should see at least +10% fuel trims.

When I had an (aftermarket, slightly upgraded for the supercharger) fuel pump fail, it did nearly exactly this. High fuel trims randomly for a while, then failed hot starts.

There is basically no way to have 'vapor lock' on a fuel injected motor. That 40-ish PSI in the fuel rails will condense any 'boiled' fuel that might be in there.

-Charlie

 

[Boom_ROASTED]

Thanks for the reply!

It seems to never get over 33 PSI, at least not while idling in my driveway. There's definitely something wrong with the pump, but there's also a leak somewhere.

It's unlikely that it's leaking injectors and it's even less likely that it's my brand new FPR.. so I'm thinking the leak is on the fuel intake side unless there's something I'm missing.

Maybe vapor lock isn't the right term, but my thoughts are that after the heat-soaked engine is shut off, the leak and low pressure + heat is boiling the fuel in the rails and pushing any liquid fuel back into the tank.

 

[phattyduck]

Other minor possibilities - where is your fuel pressure regulator vacuum port referenced? Stock, it goes to the intake tube (not the intake manifold like most vehicles) through a pipe between the manifold halves. 33psi at idle would be expected if you reference the FPR to intake vacuum.

The fuel pump can lock up after shutdown if the bearings are worn - basically, the fuel is no longer cooling it (fuel drawn through it cools it, not from the outside of the body). That would normally take about 15 minutes to cool back down.

How long have you owned your 4Runner? Any history of messing with the tank and/or fuel pump?

-Charlie

 

[Boom_ROASTED]

Other minor possibilities - where is your fuel pressure regulator vacuum port referenced? Stock, it goes to the intake tube (not the intake manifold like most vehicles) through a pipe between the manifold halves. 33psi at idle would be expected if you reference the FPR to intake vacuum.

Mine is running between the upper and lower plenum.

The fuel pump can lock up after shutdown if the bearings are worn - basically, the fuel is no longer cooling it (fuel drawn through it cools it, not from the outside of the body). That would normally take about 15 minutes to cool back down.

If I shut off the engine and crank again shortly after that, it fires back up without an issue. I'm not sure exactly what my time limit is before it's too late, but if I wait ~20 minutes (time to run in and out of a store) I'm likely to be stranded for an hour or more. The longer I wait the likelier I am to get it to crank again. Even if the PSI drops to 0, as long as its cooled down.

It's always the same though, and I don't know if it's multiple issues or I'm making it harder on myself, but the first time I crank (after those 20 minutes) it will idle horribly rough for 5-10+ seconds before stalling. Sometimes I'm lucky and I can keep it alive by giving it some gas and after a bit of revving and backfiring I'll be able to get it moving. Once I start driving it starts to run normally again within a minute.

The times that I can't keep it running it will stall. If I crank again it just stalls immediately.

How long have you owned your 4Runner? Any history of messing with the tank and/or fuel pump?

I've owned it since the beginning of 2019 and until now I've never had a reason to touch the tank. The only thing fuel related I've done was rebuilt the injectors.

Speaking of injectors, and the fuel pump, do you have any thoughts on the pressure leak?

and thanks for this. I appreciate the help.

 

[subzjc]

I can tell you don't want to believe there is an injector leaking

however that is what I believe is happening. You can fire back up in a few

minutes because not a lot of fuel has flooded the cylinder yet.

After waiting long enough you can refire because the fuel has had time to

evaporate. If you have a borescope or could borrow one you could look pull

the plugs and watch for the fuel leaking into the cylinder. Are you getting

a misfire code for any particular cylinder? That way you could narrow it down.

Just my opinion, Good Luck.

 

[Boom_ROASTED]

I can tell you don't want to believe there is an injector leaking

however that is what I believe is happening. You can fire back up in a few

minutes because not a lot of fuel has flooded the cylinder yet.

After waiting long enough you can refire because the fuel has had time to

evaporate. If you have a borescope or could borrow one you could look pull

the plugs and watch for the fuel leaking into the cylinder. Are you getting

a misfire code for any particular cylinder? That way you could narrow it down.

Just my opinion, Good Luck.

oh no I definitely accept it could be the injectors and the flooding makes sense with the symptoms along with my shit gas mileage. I'd much rather replace injectors than deal with dropping the tank, but for all I know right now, the injectors and the pump are both causing problems.

It's been a long while, but I know I've seen a 301, maybe a 302, and possibly another, but the majority of times I've had gotten a code after a rough idle that I was able to ride out, it was a 300. I have a boroscope and I've looked at 1, 3, and 5 after a long drive but I didn't see any fuel or wet spark plug. I'll have to find the time to check the other three.

An issue though.. I don't seem to have this problem if I just take a quick ~5 minute drive and I suspect the injectors would be leaking after a drive no matter what. I dunno. Maybe more heat does make that leak worse.

 

[JustTroItIn]

I don't know the answer, but I have a similar problem and can at least share what I have done that has not fixed it. This is on my 2000 Limited. If I shut down when hot and let it sit 5~ minutes, it will take much more cranking to start, but it always starts. It has been this way since I bought the truck four years ago.

I have replaced:

Fuel Filter
Fuel Injectors (Motor West)
MAF
An entire engine: replaced with used engine with 130k. Why is a different story.
Fuel Injectors again: Motor West is a stand up company. I contacted them for replacement O-Rings and they made me an offer I couldn't refuse to just exchange mine for another set.
Both coolant temp sensors replaced on new engine.
Fuel pressure regulator was tested with gauge and held pressure over time. Did not replace.

The damn thing still has the same problem.

I had purchased new fuel pump as well as a lot of other "dropping the tank" parts prior to the engine issue. I intend to do that job soon.

 

[phattyduck]

I'd much rather replace injectors than deal with dropping the tank, but for all I know right now, the injectors and the pump are both causing problems.

A few weeks ago, I dropped the tank, changed the fuel pump and had it back in and running in about 2.5 hours. That was in a parking lot with beer breaks with a heavy gas tank skid. It isn't that bad as long as it is near empty... since this was unplanned, I had about 1/3 of a tank and it was still doable with a few tools and a couple friends standing around the truck cracking wise and helping a bit. Doing the injectors is probably similar, but just a different 'type' of mechanic work.

If leaking injectors is the issue, you can try 'clear flood' mode. Flooring the gas pedal while starting actually turns OFF the injectors while cranking. Maybe try that for ~5 seconds when you have this issue before trying to start normally. That should clear the cylinders fairly well if it is a leaky fuel injector issue.

-Charlie

 

[Boom_ROASTED]

I don't know the answer, but I have a similar problem and can at least share what I have done that has not fixed it. This is on my 2000 Limited. If I shut down when hot and let it sit 5~ minutes, it will take much more cranking to start, but it always starts. It has been this way since I bought the truck four years ago.

I have replaced:

Fuel Filter
Fuel Injectors (Motor West)
MAF
An entire engine: replaced with used engine with 130k. Why is a different story.
Fuel Injectors again: Motor West is a stand up company. I contacted them for replacement O-Rings and they made me an offer I couldn't refuse to just exchange mine for another set.
Both coolant temp sensors replaced on new engine.
Fuel pressure regulator was tested with gauge and held pressure over time. Did not replace.

The damn thing still has the same problem.

I had purchased new fuel pump as well as a lot of other "dropping the tank" parts prior to the engine issue. I intend to do that job soon.

Man, that's rough! Did you get the denso injectors from Motor West or have yours rebuilt? Also, I'm curious, when you get the 4Runner to start do you have a rough idle? Threatens to stall? That first time I turn the key I don't have any issue cranking. It's keeping it running that's my issue.

 

[Boom_ROASTED]

A few weeks ago, I dropped the tank, changed the fuel pump and had it back in and running in about 2.5 hours. That was in a parking lot with beer breaks with a heavy gas tank skid. It isn't that bad as long as it is near empty... since this was unplanned, I had about 1/3 of a tank and it was still doable with a few tools and a couple friends standing around the truck cracking wise and helping a bit. Doing the injectors is probably similar, but just a different 'type' of mechanic work.

If leaking injectors is the issue, you can try 'clear flood' mode. Flooring the gas pedal while starting actually turns OFF the injectors while cranking. Maybe try that for ~5 seconds when you have this issue before trying to start normally. That should clear the cylinders fairly well if it is a leaky fuel injector issue.

-Charlie

I have a feeling that dropping the tank is like any other thing where you just gotta use the right tools and know what you're in for, but I'm still in the place where I've never done it unfortunately. I'm remembering the first time I ever did my own brake pads. First side took 2 hours. Second side took 15 minutes.

I haven't had a chance to mess with the 4Runner any in the last couple days, but I'm nearing the point where I say **** it, put in some new injectors, and see how it goes. I've been down to the fuel rail so many times before new injectors is really just a financial issue. I'm thinking of going with the Denso ones though. Maybe pay Motor West to refurbish my oe ones and keep them around just in case.

 

[2TH4IST]

I don't have said issue. But I have dropped the tank and done injectors. I'll drop the tank any day. It seriously doesn't take long, unless rust exists on your truck. Injectors are easy too. I thank toyota for that design.

I also run the motor west injectors. 33psi isn't not in spec. A fuel pump (denso) is cheaper then injectors. I'd do the cheaper first.

The oem injectors are known to have a coil failure issue. Hence why motor west doesn't sell oem injectors for the 5vz.

 

[JustTroItIn]

Man, that's rough! Did you get the denso injectors from Motor West or have yours rebuilt? Also, I'm curious, when you get the 4Runner to start do you have a rough idle? Threatens to stall? That first time I turn the key I don't have any issue cranking. It's keeping it running that's my issue.

The injectors are the Denso. Yes, it has a slight stumble on startup when this happens, but has only stalled on me a hand full of times in four years. The higher the ambient temperature, the more I will have to crank it before it starts, and the more likely it will stall. I almost never experience the issue during winter in Michigan. If it does stall, it will start up and run on the next try. I never have to touch the gas pedal.