Still battling P0174

I don't believe it can, I don't think there's a sensor for fuel pressure, but it can see commanded and actual, IIRC? Bubba has that scanner, maybe it has some bells and whistles I'm not aware of.

Once again, they give great advice, but just touching on one thing really quick.

Unless I'm mistaken, path of fuel delivery lines is Pump -> Filter (optional) -> Driver side fuel rail+regulator -> crossover to passenger fuel rail -> Passenger fuel rail+regulator -> Return to tank.

If accurate, the passenger fuel rail would be the farthest delivery point before fuel returns to the tank, and be at risk for the lowest pressure. That means, if @Fragman is spot on and you're showing the 1st stop (driver rail) is near-lean, and passenger rail is so lean it's throwing a code, a weak pump could point to that **OR** and the pump isn't working in high-speed mode/defaulting to low-speed mode (because fuel pump relay is stuck in low-flow, or resistance on the hi speed direct circuit is corroded or otherwise problematic).

There are ways to test the theory. You could bypass the resistor so the fuel pump can't enter low-speed mode, and measure resistance on the high-speed line. You could test the fuel pump relay to verify it's not stuck or sticking. I'm sure there are other ways.

 

Alrighty then...

In order to just post two pics, the coolant temp was ~187.

 

The ambient temperature is 120 degrees where you are? The intake air temp should be the outside air temp. If it reads higher than it really is, then that would mean more air going in than it thinks and it would need to add fuel, but I would not expect 25 percent. But still....

If your fuel pressure regulator on the passenger side was stuck open, it would reduce fuel pressure and that would get worse as engine revs increased.
If your fuel pressure regulator was stuck close, you'd prob get negative fuel trim at idle and it would improve as the throttle opened.
So, I'm probably leaning away from the FPR, at least for now.

Reason is this. The FPR has a spring loaded diaphragm in it. It's natural state is to be closed, essentially 'pinching' off the fuel return line and therefore increasing pressure in the rail.

Enter engine vacuum. This is caused by the pistons sucking in. When the throttle is nearly closed, that generates vacuum. As the throttle opens, the air comes in more easily and vacuum drops. The engine vacuum (via that hose on the FPR) sucks on the top of the diaphragm in the FPR, pulling it against a spring and opening it, allowing fuel to flow more freely out the return line. Which is what you want at idle, otherwise the fuel pump is pumping more fuel than the injectors are injecting, increasing fuel pressure. As RPM increases (or actually throttle opening), vacuum goes down, the diaphragm closes, pinching off that return hose and maintaining fuel pressure for the injectors delivering more fuel.

At least that's my understanding of how it works. I wrote it out to 'show my work' so if I am wrong about how it works, the helpful folks here will correct me!


Fuel pump issues tend to get worse as engine revs increase as the ability to keep up gets worse. Yours is improving slightly.

All that said, I'd still measure fuel pressure. You'll need a gauge and an adapter. Autozone may loan them, not sure

You either have unmetered air or a fuel delivery issue. It's just not possible for all injectors to go bad, so I'm thinking air is getting in there. Something that 'global', I'd expect it to get a little better as throttle opens more as suction goes down (as throttle plate is open further). That's what you are seeing.

You want to spray propane or something like that round the manifold, You're looking for the SHORT TERM fuel trim to go negative, not the long term to get better. That would confirm a leak.

That would be my next step.

 

It was hot as balls yesterday... hot as my balls. I drove her around my little town to warm her up, then sat out in the street with the engine running, hood closed, hooked the scanner up and did the readings. That temp doesn't surprise me, given what I see on my track Miata when she's heat soaked and not moving.

I'll try to get to checking the manifold for leaks this weekend. Not happening tonight.

 

Do the readings in the table in post #25 confirm or deny what I've quoted above?

 

Post 25 deos not contain readings with enough engine speed to make any determination. My post was based on your later #32 post.

 

The next question is, do I need to stop driving her until I figure this out?

 

My normal go to for finding vacuum leaks is to spray starter fluid and listen for an rpm change. I found none, even when spraying underneath the intake manifold from the front.

The CEL returns more often when it's warmer, and it's been hot, than when the temps are cooler, which seems backwards. If the mixture is right on the edge of being lean, wouldn't the thinner, hot air be better, since less fuel would be required? Makes me think fuel pump, since the heat affects mechanical clearances.

I have a Harbor Freight fuel pressure gage somewhere in the garage. The FSM states I need to do it at the fuel banjo, but I've watched some You Tube videos and it seems like folks are doing it wrong.

 

Shameless bump...

 

For vacuum leaks, I made a small smoke machine which has been much more reliable than spraying starter fluid for me.
With that said, the easiest and most reliable method has been pulling vacuum lines and inspecting for cracks and replacing anything that has any crack showing.
It sounds like you are being lead down the right path as far as actually testing fuel pressure, but if vacuum comes into the equation I would inspect and replace hoses.

 

Gentlemen,

I'm hopeful everyone had a Happy Fourth and you all still have all your fingers, eyes, and hair.

I apologize up front for asking what appear to be similar questions repeatedly, but I'm still looking for answers.

I ran four bottles of Techron and 93 octane gas through her in hopes it would help with the P0174, but the most significant impact I had ON THE CODE was replacing the hoses on the passenger side that go to the power steering assist. I actually thought I had the -174 conquered. I had several days and several local miles without a CEL, and the truck was running great. Unfortunately it came back. Then, I replaced the fuel injectors with the 12 hole upgrade, and now the P0174 comes back almost immediately after clearing it.

For the sake of discussion, let's assume I did the injector swap correctly. I've swapped injectors on other cars before. It's not rocket surgery, just tedious. Does the fact that the P0174 is now more persistent AFTER replacing the injectors provide any insight as to what might be wrong?

Thanks,

 

I've replaced all the vacuum hoses I can get to. There are a couple that go back behind the intake manifold and disappear. There's also a hose that goes under the front of the manifold and disappears. I have not replaced those.

 

As I understand it, you aren't having any issues or symptoms with these codes? Or did I miss something here. Reason I am interested is I am having engine management issues with P0171 and P0174 codes.

 

So what issues and symptoms were you having to diagnose along with the codes? How difficult a job was it to replace all of the seals?

 

What specific issues were you experiencing along with the codes??? What do you mean by massive amounts of Techron to clean out the gunked out injectors?

 

Gents,

I got the P0174 AND P0171 yesterday, along with P0420 and P0430. That's never happened before. That's also the first time I've ever seen the P0171. The truck is running fine.

Also, Torque tells me with the engine warmed up, at idle (~700 rpm) in Park the manifold vacuum is ~ -24.4. Is that normal?

Any thoughts would be greatly appreciated.

 

I had the CEL and it idled like crap. The dealer ran Techron through it so not sure what their process was.

 

Were you blowing the P0174 and P0171 codes?

 

Gents,

The local shop that does all my inspections and the stuff I don't want to do could not do a fuel pressure test, so I took it to the local dealer. I asked them to do a diagnostic for the P0174 and P0171 codes. The printout I received states the fuel pump is putting out 25 psi, where the minimum should be 44. I need a fuel pump.

IF I get the 77020-0C061 pump assembly from Toyota, do I need anything else, like gaskets or seals?

IF I get a Denso pump from O'Reilly, do I need anything else?

Thanks much,

 

Ok, so ... that's what @Fragman and I had been getting at, me in reply #27 and #31 at the top of this page, maybe even earlier.

If you buy the fuel pump from O'reilly's, you're going to pay 2x as much, to the point you may as well buy the OEM assembly by then.

Here's the thing. Toyota did some weird stuff with pumps across the years. I'd need to see what the offering is for your year/model. Some years, you could buy the pump, other years, the assembly, I've seen weird results. And they stopped making the pump for the '00-'02 at this point, I think.

That said, at the age of the vehicle, if there is an assembly, and the assembly includes the fuel sender, I'd recommend replacing the assembly, not just the pump. And I'd go Denso (if just the pump), or OEM buy (if the full assembly). EDIT: And the reason I'd replace the full assembly is simply because the senders are known for developing corrosion, which leads to dead spots in reading, causing the gauge to drop suddenly to "E" when it passes over them; if you can replace the pump, the sender, and the structure holding all of it, and all its seals, you'll be good for another 20 years without having to drop the tank for anything. Worth it, right? Considering the effort involved and all.

Job is easier to do if there's less than 1/8 tank of gas given the quantity the tank holds. I want to say it holds 24 gallons total, and IIRC a gallon of gas is around 5-6lb, 6 I think. So just having 6 gallons or a quarter tank, you're dealing with 30-40lb or more over head with the tank and fuel and all the accessories. If you have a scrap 2x4, 2x6 or something, floor jack with that board to hold the tank up, detach everything, then drop it, slide it out. May be easier if truck is up on stands, if your jack goes high enough.

 

Something else to think about too. We're getting close to another holiday and there is a good chance that there will be another sale for the OEM parts online. Anything from 15-25 percent off the already discounted online prices. I've consistently found those times to end up being close to and often cheaper than aftermarket crap for a lot of parts.

 

I'm definitely looking forward to a labor day sale, I've got a few things I need to pick up! You're right, it'd be a great time to pick up a pump assembly. @poormxdad, I'm happy to look up the p/n if you want to add it to the PM chain we have going.

 

Finally got a new fuel pump installed.

I attempted to do the ECU relearn. I’ve done it several times as I’ve replaced items (intake manifold gaskets, fuel injectors, vacuum lines, etc.) while trying to solve these codes. Every time it was the same. I’d start the engine, it would catch, then almost die, then the rpms would go up pretty high, higher than normal fast idle. It would do some weird sheet for a couple of minutes if I didn’t take her for a ride. However, when I tried it after the fuel pump install, the engine did die. Upon restart, it didn’t do any of the usual rpm shenanigans. I assume since it shut off, the “timer” for having the battery disconnected was reset, but that’s just a guess. I attempted it a second time. Same thing. It started, ran for a split second, then died. I disconnected the battery again for half and hour and tried a third time. This time, I kept the starter engaged until she was running, but again, she didn’t do any of the rpm changes I had seen in the past.

How do I KNOW when the ECU is in relearning mode? Can I force it using my Autel scanner?

Thanks,

 

Disconnect the negative battery for 10-15 minutes. The truck will absolutely begin a relearn.

 

shifty,

What you're essentially telling me is I just did one. Why didn't the recent relearns act the same as the earlier ones? Was it because of the new fuel pump? If so, that would make everything make some sense...

Unfortunately, it gets better.

Now I have a P0441. I let the guys know that did the fuel pump swap that I now have a code related to their work. I cleared the code, and if it comes back, I expect them to troubleshoot.

What could they have done, or not done right, whilst replacing the fuel pump assembly, that would cause the P0441?

 

Normally I associate P0441 with faulty gas cap, or other similar style of small leak. Basically you have some small air leak somewhere in the fuel system.