Beat me, kick me, kill me... just don't give me a P0171. I think it was 6 months and a dozen or so cans of carb cleaner and brake cleaner. The cleaners are good for vac lines and gasket edges.
The area where you can't count on them working is a key area though. Right where the EGR pipe goes into the manifold below the crossover pipe. You have the EGR, TB to manifold/*/C, Upper or */C gasket, and the LIM gasket. All of those possible areas, and the engine temperature is raging hot. The cleaner evaporates faster than a leak can suck it in. A massive hit of brake cleaner is the only thing which worked for me. The brake cleaner is less volatile.
P0171 is a good way to get to know your systems. About 3/4 into it, i took it to dealer. they said there was no air leaks but I needed a new fuel pump for $700.(did it myself for $100, and a sore back) Actually it ended up that the fuel pump was not in it at all. It was a combination of of a few things. The FPR was old and tired, the charging system couldn't handle stress, and to this day there is a come-and-go tiny vac leak somewhere. I'm at 100K on the '98, so when I do the LIM gaskets, I'm bettin' I'll find some tiny nitch in either the */C gasket or the lowers.
For the P0171, I'm a big fan of a cheap scantool, (electrical) multimeter, and a fuel pressure gauge. The scantool has to give you streaming data for 6 values... MAF, MAP, LTFT, STFT, O2, VLT.
The MAF and MAP readings will tell you if you have a vac problem. I can't state any hard values, but the MAF pressure and the MAP pressure should move in unison. It will also tell you when the leak occurs. Check 3 of the 4 cells which the PCM uses. Idle, accel(leration), and decel(eration. I wouldn't suggest WOT, if you're burning lean. Look for the MAP pressure to get ahead of the MAF pressure for no reason. If it only happens at idle (the MAP will jiggle) you're a winner. The IAC is wacky, or the TB or sensors on the TB aren't flush.
The LTFT and STFT are your progress report. Any time you "fix" something, you should disconnect the battery cables for about a 1/2 hour to clear the PCM. If you are burning lean, the LTFT (full range -16 to +16.4) will be high positive, probably in double digits. You might even have the dreaded steady state +16.4. The STFT will work like crazy to pull the LTFT down on throttle, but at idle it will go right back to 16.4.
The reason you get the P0171 is because the PCM notes the outcome of a burn (mostly at the O2 sensor). Over a period of time, the PCM will make your LTFT higher if it there is extra air sneaking in. Pumping more fuel. If your fuel delivery system is compromised, the PCM also pushes harder on the LTFT, and shoves the fuel pump. With LTFT (your baseline state) already high, but the engine just puttering along, when you punch the throttle, the STFT has to pile on the fuel. When LTFT + STFT is at 30 (or something), the PCM sees a dangerous lean state and throws the P0171.
At the same time, when you straighten something out, you'll see the STFT working lower and lower, which will pull the LTFT down. The LTFT takes a couple hundred miles to fully adjust, but if you hit the nail on the head for a fix, you'll see it definitely trending lower immediately.
The fuel side is easy and hard. Most everything you need to know is from the fuel pressure gauge and your nose. If you smell gas, you have a leak somewhere from the pump forward, and lousy pressure. Otherwise most you want to know about the fuel pressure you find by connecting the pressure gauge and doing a KEY ON to power the pump. It should jump somewhere in the high 40s. from there it may fall back 5 or so psi, but it shouldn't keep bleeding down into the 30s.
Hit the high number (at least mid 40s), and your pump is strong enough. If it bleeds out though, your FPR isn't holding, or the stopcheck isn't holding on the pump. The line is weak and the pressure and fuel are heading back into the tank(stopcheck) or is dribbling into the return line(FPR). A bleed out you can confirm by powering up the pump., then turning the key off and go have a beer. If you come back in an hour and most of the pressure is gone according to the gauge (and you still smell no fuel), you have to decide whether you take a shot at the FPR or the pump. The pump in a '99 and prior isn't a barrel of laughs on 2 ft jack stands in your driveway. That'* what i meant by hard.
Another thing on the scantool is the O2 sensor reading. It tells you a lot about the engine. The reading should jump back and forth between a low reading and a high reading. And the faster it does it the better the engine and EGR are working. For the cat to scrub out the two baddies from the exhaust, your exhaust has to go from a lean exhaust to a rich exhaust. But this has to take place so quickly that the engine isn't seeing the PCM do this. It just seems like a steady stream.
If the O2 sensor is moving fast and wide, you at least know the engine and EGR are working well together. If the O2 sensor readings are slow and narrow range, you either have a lousy engine, a lousy EGR, or a clogged cat. If you have a clogged cat, that'* because the engine and the EGR haven't been working well together for a long time. Which means you've been dumping gawd-knows-what back into the engine thru the EGR. If I was allowed only one reading to tell if a car was operating well or not, it would be the O2 sensor. Many possible sins reveal themselves in that hopping reading.
Of course, you go nowhere with fuel and air without a spark or the right amps to suck in the air and push on the fuel.
Heaven created electricity solely with the Bonneville in mind, and BillBoost37 to watch over it. If those charge circuit cables aren't pristine and locked down, they will get you, they will skrew you, they will break you down, and the car. Along with the ground bus plate in the door panel, they will have you chasing ghosts and false DTCs in the night, and weeping like a little girl in frustration. Just as there is Yin and Yang, just as there is Heaven and H*ll, there is the Bonneville and the electrical circuit.
(Did i overdo that ?
Danthurs wrote childproof instructions so we can test the charge stability with a $19 multimeter. Do it or you are destined to be stuck on the side of the highway at 2AM, in 20* weather, with a flickering dashboard, and an engine which starts, but cuts out every 200 yds. Living life 200 yds at a time, is no way to live. 2400 yds ago, that frowning lady in the passenger seat was all smiles and excitedly flush with anticipation. The moment is lost because last week you didn't want to take the time to remove the splash cover and make sure the ground cable to the bottom of the engine block was clean and secure?
(How 'bout that one?
Anyway.... the last note on the charge is the scantool. If you have been drawing the P0171, leave the scantool attached. When it hits, check the freeze frame and the VLT reading. It might not be a fuel/air problem, but the PCM panicing the trims and advance because it was getting millisecond shorts.
i don't know why i wrote all this, and I by no means intend it to be all-inclusive. It'* just some stuff that I figured out while i was chasing those hideous words of "Lean Burn Bank 1".