Been watching my STFT creep up and up as the months go by. Now it's at the point (+21) that it's triggered a PO171 and P0171 three times. Probably because of the cooler weather that's finally moved in. I've also had random P1491 codes well before this for some time, and they all now appear randomly after I reset the CEL.

So I decided to check hoses for vacuum leaks. I'd already used a propane torch to do this once, with zero results. This time I went plugging things up at the manifold, and using a vacuum pump to check for hose leaks on the other side. The only thing that made any change to STFT, and very instantly, was disconnecting the FPR. The reading went from +25 to -1! I tried drawing a vacuum on the hose, and it held fine, but nothing changed as far as how the engine reacted. 0 vacuum to 20 in/hg. So 99% chance the FPR is the problem.

So I'm guessing this means the FPR is shot, and not doing anything now but plugging the hole in the fuel rail? The car still runs decently, though it occasionally breaks up randomly at low/mid RPM then recovers. But wouldn't a bad FPR that's already making things run lean at idle choke the car significantly at higher RPMs?

 

As you said, it appears the FPR is the issue. The computer has been working hard to fight it, so I would think replacing it and resetting the computer will then reteach it how to working with a functioning FPR vs the broken one now in there.

 

How the FPR is supposed to work is,
with vacuum @ the FP Regulator the fuel pressure is lower
without vacuum @ FP Regulator the fuel pressure is higher

So it sounds like your FPR is actually working. Did you try and apply vacuum directly to the FPR with the car running? If it is working the STFTs should start to go back toward the +side.

Odd that when you disconnected the FPR it changed the AFRs emediately as it should, But then when you applied vacuum to the hose nothing happened.

 

Things to check with a P0171 code:

• A faulty fuel pressure regulator - if your fuel pressure changes when you disconnect it, it's probably working
• A weak fuel pump - You will need to hook up a pressure tester to the fuel rail and actually test for proper fuel pressure.
• A clogged fuel filter -
• A faulty ECU
• A vacuum leak
• Faulty injector(s) - Pull your spark plugs and read them, if you have one that is much lighter (lean cylinder) you could have a bad injector.
• Faulty oxygen sensor(s)

 

You tried drawing vacuum to what hose?

The hose on the FPR goes to the intake manifold. You should have vacuum on the manifold side and nothing on the FPR side.
The issue is more than likely that the FPR is leaking and when manifold vacuum is pulling against the FPR diagram,it is sucking in fuel. There should be no fuel at all in that hose from the FPR to the manifold.

 

I applied vacuum to the FPR via the hose that attaches to the intake manifold. Didn't lose any vacuum, but it also didn't change the fuel trims at all. Absolutely nothing changed. Didn't have much time to work on it since it started raining halfway through. I'll try testing via the hose coming directly off the FPR if I have a chance tomorrow.


I unplugged the hose that goes to the FPR at the end that attaches at the intake manifold. I plugged the port on the manifold with my thumb and drew pressure via that hose.

I didn't smell any fuel when I pulled the hose, though I didn't get the chance to pull the small hose that comes directly off the FPR because it started raining. Gonna try that tomorrow, along with picking up a fuel pressure test kit from HF.

 

Drawing vaccum on the FPR will not do anything. You will increase fuel pressure,but injector pulse width is still the same,as it is computer controlled.

If the FPR is the culprit, the only failure is leaking. It will leak fuel into the intake manifold, and that is what will increase the trim


Remove the hose and check at the FPR itself. Draw a vacuum at see if it holds.

 

This kind of goes against everything I know about tuning fuel injection and the results adjusting fuel pressure. The injector pulse widths change as you adjust fuel pressures. This is why a set of injectors that flow 210cc @ 80% duty cycle and 34psi will flow 240cc @ 80% and 44psi. Now, in saying that, I agree that the ECU makes adjustments to maintain a stoichiometric AFR of 14.7:1, It does this by altering the injector pulse widths and you will see those adjustments immediately in your Short Term Fuel Trims.

 

Just pulled vacuum on the FPR directly from the small hose attached to it. No leaks. At 0 vacuum, I get 0 STFT. At 10 vacuum, I get 15 STFT. At 20 vacuum, I get 23 STFT.

Also took a look at the O2 sensor readings, and while my scanner doesn't have the greatest sampling rate (can't see every peak and valley every time), I could easily see the readings start to swing rich under light acceleration, and full rich at full throttle, even at higher RPM. Guessing fuel pressure is OK if it can handle WOT up to around 6K.

I have a new FPR coming in tomorrow (if FedEx is on time), so I'll try that first. Should I bother checking fuel pressure? I've heard these cars have a non-standard thread on the test port, making many kits useless, but supposedly HF/Pittsburg 92699 fits.

 

If your STFT are moving with the vacuum and there is no fuel in the hose, then I would say that your FPR is working. But that doesn't mean you have proper fuel pressure. Not sure if that kit comes with the proper fitting, but you should be able to hook it up somewhere???

Have you checked your spark plugs?

 

Increasing fuel pressure alone should not affect injector PW. PW also needs throttle and MAP input(mainly MAP), so him applying vacuum directly to the FPR should normally not change trims or PW.

OP I agree with Ghost that you need to check fuel pressure. You also need to check the MAP readings.

 

OK, installed a new FPR just to ease my mind, and got the fuel pressure test kit (yes, HF 92699 fits perfectly, with some teflon tape). Before the new FPR: vacuum applied = 38psi, no vacuum applied = 43psi. After the new FPR: vacuum applied: 35psi, no vacuum applied 45psi. All within spec, and no change in the fuel trim.

Also did some digging with my scanner. Nothing out of the ordinary except when the engine bogs a bit on moderate/heavy load (only once at the beginning). I see the STFT and O2 sensor readings bottom out for a second, then catch back up. I also monitored TPS, MAP, ignition, and engine load, and they aren't affected by this. Their readings look normal overall.

I also got a vacuum cap kit and proceeded to block off every port on the manifold individually. No change.

No exhaust leaks anywhere back to the cat area.

Gonna try to build one of those makeshift smoke testers (glass bottle, tire schrader valve, hoses in and out, smoldering sock, tire pump) and see if I can find any gasket leaks on the intake manifold. Really the only other thing I can think of beyond a bum computer.

And yes, all ignition components are relatively new, within the past 8 months, even the ignition coil.

 

Geez I apologize for being so dense. For some reason I thought you were chasing a rich condition, when I reread this thread I realized you're chasing a lean condition. I would've steered you away from the FPR long ago had I realized sooner.

Ok lets start again. Here is a really simple,but very effective way to check your fuel supply/feed back system. Connect your scan tool, find a stretch of road and drive your car at full throttle for at least 30 seconds. Graph only the rear O2 sensor while doing so. The rear O2 should read .8v or higher for as long as you are at full throttle and should not drop out, even a little.

*** What is the year/ make/ model of the vehicle***

 

It's a 96 Accord LX manual.

Not sure if fuel pressure is the issue, as the car is usually in high STFTs (20-25) at idle and low RPM vs. lower while cruising at higher RPMs (10-15) or full throttle (0). If anything, it seems like a vacuum leak, though I'd think the MAP system would be able to compensate. Could a bad/dirty fuel injector cause an issue like this? Running out of things to check.

I'll give the full throttle thing a try tonight. Can it be in 5th the whole time (25 to whatever I reach), or only at high RPM running through the gears? Either way, I'll definitely be going fast after 30 seconds...

 

MAP based systems are speed density and will only run lean with a vacuum leak in a very narrow,specific condition(specifically a leak at a manifold runner)

99.9% of the time a vacuum leak will not cause a speed density system to run lean. Im certain you have a fueling issue. Either supply or feed back related.

 

Pull the rail, crank it, and watch the test patterns....then clean it up. It does not sound like they are clogged.