I have a 1985 Nissan Maxima SE with 208,000 miles, Manual Transmission
On hot starts, the engine begins hunting aggressively (rpms up and down) and usually stalls out after a couple seconds, sometimes immediately, or is able to stay alive on its own until it smooths out in about a minute. Holding the gas down a little will also help keep it alive until it smooths out. Car will run perfectly after the hunting smooths out. It does not occur all the time, it seems to be random but only after I've drove around for some time and leave it off for at least ~10 minutes. New fuel pump, new fuel pressure regulator, new cylinder head temp sensor, same problem.
However recently, I found out that when I have this problem and disconnect the vacuum line that runs into the fuel pressure regulator, the problem disappears instantly. Wondering if I could leave this vacuum line disconnected? Or if I will have to replace the solenoid valve that the vacuum line runs into from the pressure regulator? Problem is that I cannot find the same one online, unless I can use similar ones that aren't exact.
Thank you.
Considering the car is 40 years old nearly everything is suspect. Do you have leaky ancient vacuum hoses? Were the replacement parts OEM? What is your actual fuel pressure? How old is your fuel filter? Are your injectors clogged? Is your AIC valve working properly?
@chucktobias Vacuum hoses are good, replaced a couple, just one area still where I hear hissing near the EGR valve. OEM replacements (except for fuel pressure regulator, that is from Standard Motor Products). Yet to do a check on the fuel pressure. New fuel filter. Already cleaned the IAC valve and tested the resistance and all looks to be good with that. I have suspected the injectors, but the car runs near perfect, just this problem and old suspension - and after discovering the vacuum to the fuel pressure regulator affects my issue, I'm just hoping to narrow it to this area, although I do understand many different things can affect one.
You can't necessarily rely on hissing to locate vacuum leaks. Smoke test or carb cleaner test is better. An aftermarket pressure regulator is suspect. You also need to check operation of the IAC valve to make sure that cleaning it was good enough to ensure that it works. Definitely check fuel pressure against factory specifications, both with the regulator vacuum attached and disconnected.
@chucktobias Did a smoke test as well, showed some coming from the EGR valve under the diaphragm. Didn't get anything from carb cleaner test, will have to do again. On my list is definitely a fuel pressure test. Will check on the IAC valve today and also try carb cleaner test again. I still suspect something with this vacuum and the solenoid valve that it come from to the pressure regulator. I just wonder if it is okay to leave this vacuum disconnected and drive it for a day and see how it behaves. Also thank you for your responses.