I am chasing an issue on an 03 Subaru. I have a general question about fuel pumps & regulators.
If the fuel trims are really good, is it reasonable to assume that the fuel pump and regulator are fine? I can borrow a gauge and plumb it into the system but I'm wondering what the odds are of having an issue and not seeing the computer trying to correct it via the fuel trims?
Tom
"I'll never know it all but I'm willing to settle for knowing where to find the answer!"
tmcquinn wrote: I am chasing an issue on an 03 Subaru.
What issue are you chasing?
I have a general question about fuel pumps & regulators.If the fuel trims are really good, is it reasonable to assume that the fuel pump and regulator are fine?
That is a reasonable assumption. If fuel pressure is low, it would be reflected with positive fuel trim. Likewise, if fuel pressure is high, it would be reflected with negative fuel trim. ( Assuming closed loop. )
"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer." I have learned more by being wrong, than I have by being right.
If the fuel trims are really good, is it reasonable to assume that the fuel pump and regulator are fine?
Probably? :silly: In principle, yes, good fuel trims suggest a good fuel supply.
In practice, I have seen marginal pumps or regulators that generate 75% of the specified pressure and still keep trims happy enough. The problem only gets truly exposed under high load or hot soak conditions.
A shop I used to work at sold MotorVac injector flush services, which required the technician to set fuel rail pressure as needed during the service. I used that to experiment with different fuel pressures on several different vehicles. It was very surprising how much you could change fuel pressure (at idle, anyway), and not see a total fuel trim change went beyond +/- 10%. I remember having an Equinox turned to 15 PSI less than spec and still kept humming along like nothing happened. :silly:
Like Chad said, I think the symptoms in question would determine if I'd take the time to install a fuel pressure gauge or not.
It's a P0420 - catalytic converter below efficiency threshold. 2003 WRX. It's such an issue on these cars that they sell parts to 'fool' the downstream O2 sensor and stop the code.
Not many miles on the engine, A/F sensor, O2 sensor, and Cat. No vacuum leaks, no exhaust leaks. I'm going to have a buddy drive me around this week with my good (junk to most of you) scanner and check a couple of things. I may have more questions when I get more data. I am trying very hard to do my own homework and not just send out a forum mayday every time I have one of these head scratchers. The hard learned lessons seem to stick in my brain a lot better and I have my son driving another car while I slowly figure this one out..
"I'll never know it all but I'm willing to settle for knowing where to find the answer!"
If the fuel trims are really good, is it reasonable to assume that the fuel pump and regulator are fine?
Probably? :silly: In principle, yes, good fuel trims suggest a good fuel supply.
In practice, I have seen marginal pumps or regulators that generate 75% of the specified pressure and still keep trims happy enough. The problem only gets truly exposed under high load or hot soak conditions.
A shop I used to work at sold MotorVac injector flush services, which required the technician to set fuel rail pressure as needed during the service. I used that to experiment with different fuel pressures on several different vehicles. It was very surprising how much you could change fuel pressure (at idle, anyway), and not see a total fuel trim change went beyond +/- 10%. I remember having an Equinox turned to 15 PSI less than spec and still kept humming along like nothing happened. :silly:
Like Chad said, I think the symptoms in question would determine if I'd take the time to install a fuel pressure gauge or not.
Well, the only symptom so far is a P0420 code.
The fuel trims look good so I just wanted to see if that was an indication of proper fuel pressure. There's a very long list of things that people claim can cause this one so I'd like to pursue it as efficiently as I can. There is no real hurry on this one and I am going to fix it somehow! I should probably start a new thread with some of the questions I have about how the catalytic converter works.
"I'll never know it all but I'm willing to settle for knowing where to find the answer!"
tmcquinn wrote: Well, the only symptom so far is a P0420 code.
The fuel trims look good so I just wanted to see if that was an indication of proper fuel pressure. There's a very long list of things that people claim can cause this one so I'd like to pursue it as efficiently as I can. There is no real hurry on this one and I am going to fix it somehow! I should probably start a new thread with some of the questions I have about how the catalytic converter works.
Gotcha.
In that case, as long as the total trims are less than 10% +/-, then I wouldn't sweat fuel pressure at all.
Oddly enough I have seen twice where people just changing the quality of fuel they put in their car has gotten rid of this after they have changed O2 sensors and cats. They stopped buying fuel at Flying J or Holiday or whatever it was and started buying ethanol free at Chevron. After a fuel tank fulls the check engine light never came back on. It’s possible it was just a coincidence as well. I don’t see why ethanol or even fuel quality would seem to effect only the Subbies though when every other vehicle on the road seems to run the same fuel without any issues. Unless the programmed strategies in the ecu are a lot more stringent on what it looks for.
I wouldn’t think low fuel pressure would cause that issue unless you went out on a limb and said that the fuel wasn’t atomizing well enough? Pretty far stretch though I would think.
Let me just do one post and hopefully address the questions you folks have asked. Thank you for your time.
Top tier, 93 octane fuels only. I too have experienced issues with cheap fuels in the Subarus.
I was thinking of high fuel pressure or a stuck injector. It seems like the 3 causes of premature cat failures seem to be a constantly rich mixture, oil in the exhaust, and coolant in the exhaust. I just thought I'd ask for clarification on the possibility of high fuel pressure, which I thought would likely show in the fuel trims.
Between the waves in the downstream O2 and the fact that the cat was cooler on the downstream end, I believe the cat was bad. I will be trying the oxygen storage test in Paul's video soon so I will have something to compare to if I end up here again.
I had some people tell me privately that I was lucky to get that 20,000 miles on my $250 cat. I guess this doesn't prove much but the fact that the recycler gave me $130 for the failed Subaru cat and automatically lowers it to a range of $10 to $30 for aftermarket cats, supports the 'cheap junk' theory. I don't know for sure. The OEM cat was $1800 and they give me their good price as a volume customer.
I have a muffler shop near me whose work has always impressed me. They recommend Walker cats and fabricated one to replace mine. It's too soon to declare victory but I have to assume that an 80,000 mile warranty implies that there is more of the good stuff in these than what I'm replacing, which had a 25,000 mile warranty. To each his own but Rock Auto will never get another penny from me. I found their warranty policy completely unacceptable. To even begin the process I needed to buy another cat from them. I understand that the mfgr can't just give away their product to someone until they verify that they unit wasn't ruined by a poorly maintained engine but I'm unaccustomed to paying another fee and hoping to get my money back.
"I'll never know it all but I'm willing to settle for knowing where to find the answer!"
All of my experience with aftermarket cats says they're junk. They either don't make it out of our warranty period without a failure, or don't fit correctly in the first place.
Yeah, I suspect you may be right. The recyclers certainly agree. I'm hoping that the allegedly better ones, Walker & Magnaflow, are exceptions. I just couldn't get myself to pull the trigger on the $1800 cat for a 2003. I'd probably save money putting on another $250 unit (I did not) every 20,000 miles. Well, if I value my time at zero...
They had to use what they called a 'high flow' cat. No CEL now but the fuel trims are positive and double what they were before. I'll test for vacuum leaks on some nice day soon. Maybe a little less back pressure, too. But now I'm just making wild guesses. Still learning. They day I finally feel like I have really learned the OBDII system will be the day the news says 51% of the cars on the road are now electric.
For whatever reason, I'm not getting notified of replies to this topic. Not sure if it's just me or everyone.
(Edited to remove my blithering about maybe the battery was disconnected. Clock and trip odometer indicate it was not.)
"I'll never know it all but I'm willing to settle for knowing where to find the answer!"
