My 1997 NA1.8 runs lean at normal (40s psi) fuel pressure high speed WOT. Cranking pressure up to 60 psi is still 14.5:1. 64 psi 13:1. 65 psi 11.5:1. 66 psi 10.5:1. Not only do I need very high pressure to hit 13s:1, but 1 psi higher or lower has big impact.
On the dyno fuel pressure dropped ~5 psi from idle to full speed/load. Does this indicate restriction before regulator?
I have replaced fuel pump, fuel filter, fuel pressure regulator (now Advanced Autosports), air:fuel meter (now AEM), fuel pressure gage (AA), engine controller, and ran injector cleaner. No improvements.
Appreciate suggestions - long time lurker, first time poster
Welcome to the club. The lean '96-'97 club that is.
I have a '96 and have spent many hours and a lot of $ trying to correct the same issue. First, I think it's normal that these year cars need more than factory fuel pressure to get to the AFR sweet spot. I have literally changed every part under my hood that might impact this and still don't have it completely solved. Here's a little more info.
A healthy factory fuel pump will only maintain on a straightaway at most low 60's PSI at WOT no matter where you set it at idle. You might want to find a shop that can perform a smoke test and check for any vacuum leaks or exhaust leaks. I wouldn't mess with trying to build a smoke tester yourself. Find someone that has a real one and knows how to use it. We found a couple small leaks and corrected them. I also found small leaks at the base of the injectors as the rubber grommets that they seat into were old and hardened. Still, these cars seem to be more sensitive to intake air temp and possibly fuel temperature. On a cooler day I can now get decent AFR at 60PSI. On warmer days it's still too lean. Keeping water temp down with a big radiator may help a teeny bit. Change your engine coolant temp sensor if you haven't, but it's a bitch back of cylinder head. I've had 3-4 air flow meters on the car including a new one, no difference. 3 sets of injectors including 1 brand new set (expensive) with all 3 sets flow tested. No difference other than we did find 1 or 2 used injectors that were a bit out of range. The new injectors were spot on. You should probably get a set tested as you might find something there.
Where I left off last fall on this was observing fuel trims with an OBDII scanner on the dash. I was seeing negative long term fuel trims in the 10% range which means the ECU is pulling some fuel. The ECU doesn't know we are running high fuel pressure yet it's still trying to lean it out a bit. It may be that these early OBDII cars in open loop were simply programmed this way.
I haven't given up yet. The ECU does have some drive cycle "learning" that happens so it's possible that powering the ECU on and off (or not) and full throttle track driving doesn't satisfy that cycling. More time will be spent on this in '24. Happy to share findings with you or anyone else that is scratching their head on this.
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