I get one weird fuel issue after another! The latest one: If I'm cruising on the freeway, and barely accelerate (maybe will get to -1 PSI, but still stay in vacuum), car will go into open loop (per the scangauge) and the AFR will drop to 10.0 and just stay there. It's not a false reading from the wideband, as the EGTs drop too and therefore confirm the AFR readings are true. The car will just remain in open loop, and the AFR will remain at 10.0 endlessly unless I take my foot of the accelerator briefly, at which point it will go into closed loop and resume running at AFR=14.7. Car still runs okay when it's doing this, there is no bucking or hesitation, but power is diminshed.

I'm running 440s, with fuel pressure of ~22 psi while cruising (they remain stable during the open loop event), EGR is unmolested, new MAP, new CLT, no apparent vacuum leaks, symtech FCD, and zero CELs. Any ideas? I need to fix this before it kills my cat.

Just got a P0172 code. My ECU is now captain-obvious, thank you yes I know I'm running rich.

I'm running 440's w/ SFMU & Missing Link and get similar results... I would describe it as 'transitioning' from vacuum to boost. I know with my SFMU, the suggested action is to drill out the inline vacuum restrictor a little bit to allow the SFMU to act more quickly in transition. What are you using to manage fuel?

I've been putting it off, I usually just step on the gas a lil harder to get through the transition to go into boost quicker
- J

Originally posted by J
What are you using to manage fuel?

I'm just using a SMFU and the 440s. I HATE this fuel setup. Stupid low fuel pressures, crappy mileage, and poorer off-boost throttle response relative to simply being NA.

The thing is, my car NEVER had this issue before. Now it happens several times each time I drive the car. And its not the "transition to boost." I can stay in vacuum the whole time. In fact, if my vacuum is INCREASING (going from -2psi to -3psi) then this can happen. At first I wasn't sure if it was a leaky injector - but all four sparkplugs have evidence of going rich (unlikely to get 4 leaky injectors simulatenously), and also that wouldn't make the car switch to open loop.

What is also new is the car idles very rich when in open loop and the engine is cold. It was never that rich before with the 440s and the same idle fuel pressure.

I'm going to turn down the gain on my boost controller and see if that helps. I turned it up not long ago, maybe that's contributing. Maybe the ecu doesn't like such a quick increase in MAP voltage. I don't know what else to do.

I recently switched to O2 too just in case it was shorting or something, causing the car to go into open loop. The wire terminal actually had melted on the radiator - I thought bingo that's it - but after the change the problem still persists.

I dunno, my setup has served me pretty well over the years... my continued education into Boost, lol. I do have a SAFC to trim my fuel @ idle, but it shouldn't be necessary at all (I would assume you would just run rich @ idle without, can't see otherwise).

What do you call crappy mileage? I get about 25mpg when I drive like an idiot :) My '99 NA topped out @ 42mpg on 0% ethanol (f*cking government) back when I had it, could drive from Miami to Nashville and only needed to fill up once in Georgia w/ ~600 miles per tank. Regardless, a bad O2 will affect your mpg, but isn't going to cause any issues. My EGR is blocked off tho (I will get poorer mpg).

'Off-boost' throttle response during vacuum won't be as zippy if you're trying to compare a low-compression engine w/ very little backpressure, to a NA. If you're trying to make up for the low end, the most you can do is a lightened flywheel/etc to push up the rpm's quicker. Is your air filter clean? lol, had to ask :p

What size turbo do you have, and how much lag is there? My S16G kicked in @ ~2500rpm if memory serves and was good up to about 5500rpm before power leveling off. My S20G-H kicks in @ ~3500rpm and is good all the way to redline. The tradeoff is the extra power compared to low lag. If I'm cruising, I'm in vacuum... if I'm moving, I'm in boost.

I reran a lot of fuel lines 1~2 years ago and sent my injectors in to VAC? (I forget the name now, injector cleaning place) because I've been running on them for 5~6 years now, and they tested in perfect condition. I'm inclined to think it's not an injector issue.

Sounds to me like your computer is playing games with you... I have a Hallman MBC, so not sure what you're doing to manipulate the MAP voltage. Because I have a SAFC wired in, I can easily check the voltage of the MAP @ the sensor and directly only the computer both, maybe checking it out would help. I've gotten bad MAP sensors before, and wouldn't rule it out... and check the wiring to make sure that's what the computer really is seeing.

I bought a Symtech FCD way back when, even stopped by Paul's house and let him hook it up (so I wouldn't screw it up, lol). But I eventually lost patience with adjusting it, couldn't get it quite right, and went back to a Missing Link.

- J

Well I used to get 32-33 mpg every tank, and >36 if strictly freeway, so around 400 to a tank. Now I get 270 to a tank. I do have lower comp pistons than stock, but I have a gut feeling that it's more related to the poor injector performance at low fuel pressures more than anything.

I changed the O2 sensor, it works fine. My EGR is attached and functional. My air filter is filthy but I don't feel like changing it. I have a 16g on the log manifold. Boost controlled with Greddy e01. MAP clamped with a FCD (that's really the only variable that recently changed, before I was using the emanage to clamp volts to 4.64V.) The MAP sensor is brand new.

What gets me is I've had this setup for years, and never had this issue before. I just can't see what's triggering open loop! I thought it might me a TPS shorting out and telling the ECU I'm in WOT, but that doesn't seem to be occuring per the scan gauge. I ordered a blue tooth OBDII reader for my android phone so I'm going to start logging things.

Yep, yep... I'd check that MAP/FCD then, and also check it at your computer too if at all possible! You can get brand-new sensors that are bad. For a long time, whenever I went to a junk yard, I'd just pull out any map sensor I'd found and test them for use when I was first getting going (was having map issues). If you suspect your TPS, how is your idle, any surging?

I've still got a Palm III w/ PocketLogger if you can believe it, and is prob one of the best things I've ever bought off here (very useful). More air = more power on the filter ;)

- J

No surging. I had a bad tps begore and it would cause hesitation on the freeway. This is different. What also is weird is how rich it runs on a cold start in open loop, afrs at 11. That is also new

I think you are right about checking the FCD. I got a wifi OBDII logger for my android phone and it shows that the transition back to closed loop occurs when manifold pressure goes into higher vacuum. I'm still not sure if an increase in mani pressure is what causes the initial transition to open loop. I always thought that too high of voltage in the MAP caused fuel cut. My 99 seems to get fuel dump instead!

Now I gotta find my instructions for the symtech FCD...

EDIT: Played around with the little dial on the FCD today (really difficult to dial in, I need to drill out the hole on it more b/c sometimes I can't tell if the screw is even turning). It appears I an make the rich condition occur more frequently, but can't eliminate it entirely. blah

Well today I completely disconnected the FCD and disconnected the intercooler piping, to take the turbo aspect out of the equation. And it still goes to open loop on the freeway while cruising, if the vaccum moves past -3.0 PSI to -2.9 psi. The car goes back into closed loop if I back off the throttle just a hair so the vacuum increases to -4.0. I logged everything, but can't find any sensor that's tripping the switch to open loop. I thought there might be noise on the TPS wire giving a fast false "WOT" signal to the ECU, but even with 0.1 second sampling I never catch one of these events.

Well, I would not assume it to be the TPS, but I wouldn't count out the MAP sensor or wiring harness itself either... it's been a long while, but don't remember the MAP voltage being reported via OBDII. That sensor can make a word of difference is only reason why I keep going back to it. If memory serves, your computer needs to see no more than 4.7v or it'll throw a hissy fit (and go too high and it'll throw a code), and that sensor needs to be 'perfect' through the entire range. There's a lot of heat behind the head which is where that wire is run.

Not sure the symptoms of a CAM/Crank position sensors, don't think I've ever needed to replace one, but something to check. Regardless, there's not 'that many' sensors to check.

- J

MAP sensor wiring is fine. I've logged the MAP and it's stable when the issue occurs, nothing weird occuring with it.

What I have noticed is that 200 to 300ms before the car goes open loop, the TPS voltage changes slightly. It doesn't go up to WOT voltage or anything, but I see no other sensor logs changing. EDIT: The front O2 sensor voltage jumps the INSTANT that the car goes open loop, but I don't know if that's expected behavior b/c the combustion really went rich within 1 ms or if the ECU fiddles with the O2 voltage. Not sure how to read the rear O2 logs.

I changed to a new TPS, just in case. Did not solve the issue.

Still logging the MAP sensor voltage output, and it is absolutely not the cause.

I don't know what else could possibly be pushing the car into open loop. The only old sensors on the car now are the crank and cam sensors. But if they were the cause, I'd expect sputtering and misfires.

I'm wondering if the O2 harness is shorting out. I can't explain why it would only occur intermittently as I'm accelerating or when cruising on the freeway. But I don't know what else could throw the car into open loop at this point. :wallbash

Not sure what to tell you, but am interested to hear what you find should you correct the issue... I know I went absolutely nuts one time trying to trace an electrical issue, and it really helps to have spare parts/sensors lying around. Whenever I head to the junk yard, I tend to pull the sensors 'just in case'... handy to swap out.

- J