Thread: fuel pressure regulator effects on cold start

Hello. I have been chasing a cold start stall for way longer than I wanna admit and ran into something today I wanted some fees back on.

I was reading online and read about checking the vacuum line to see if there was fuel in it. There wasn't but I decided to start the car with hose completely off. Now the crazy thing is for the first time in like 3 years the car idled high like it should (usually goes up and drops like a rock to stall. Not idle at 1500 cold like it should). It stayed high till it got to 1k and then started stumbling until I plugged the leak by hand. Now with the hose off the fpr it stayed in warm up higher idle (nothing crazy but like it should) until I hooked it up and then dropped like a rock.

I plan on getting a gauge but I just wanted you guys opinion on what the car is trying to tell me. I've done smoke tests and replaced parts and set base idle/timing this is the first time it's behaved

Maybe the fpr diaphragm has a hole/isn't holding vacuum? Not sure, but you can check it it see if it holds if you have access to a might vac or similar.

oops

The vacuum line is there to balance out the fuel pressure vs the intake manifold pressure. At idle, there is higher intake manifold pressure than at Wide Open Throttle (for a great read, check out http://www.sr20-forum.com/information-library/34413-crank-case-ventilation-fully-explained-naturally-aspirated-edition.html). The vacuum line is there to help regulate how much fuel pressure is needed at different times.

Can you clarify part of your original description? You said that the idle stayed high until it got to 1k then started stumbling. How can the idle be high if it didn't get over 1k? Or did you mean that it was trying to rev higher at cold start then started stumbling after 1k?

Basically what you did by removing this vacuum line was to tell the regulator to increase the fuel pressure. As the RPMs went up, more fuel was needed, but the regulator was now maxed out, and it stumbled. This sounds like an issue with fuel delivery, bad fuel pump, clogged fuel filter, etc. But if the issue is only at cold start, and not when warmed up, then the problem may more likely be with the IAC, and I'd recommend reading up on that here: http://www.sr20-forum.com/information-library/50162-ins-outs-stock-idle-control-system.html

Also, you said Cold Start, what was the temp when you were having this problem? We need to know if this is a true cold (outside air temp) start, or if it was a warm day and just happened to be that the engine hadn't been run for a while. Two entirely different scenarios.

sorry for the delay. to clarify the high idle i mentioned for like 4 years the car would not idle above 900 rpm on its own. i know from prior sentras i owned that on a completely cold engine that sat overnight the car would fire and idle at 1500 rpm and slowly come down. mine will fire and the needle will drop like a rock if the car has been sitting. cold start for me are anywhere from 40-70 ambient degrees but it mostly does it when the temp needle is under the cold no matter the outside temp. with the vac line off the fpr the car was actually idling around 1300 on its own no input from me and this hasnt happened in years.

i changed the fuel filter recently with no change, and have tried 3 other iacv and no change. i do have a p0505 but for the life of me it wont go away. takes about 50 miles to come back after reset. i have cleaned the iacv, tried a brand new one and cleaned throttle body. ve change vac lines, tripple checked routing. put a new egr and a used air regulator valve (the one under the lowport mani with the bi-metal strip). i will try using another of the 3 iacv i have and read the threads but please let me know if any of this leads to anything else.

i also have a new bpv valve and egr and cts.

OK, it's easy enough to google the P0505 code to find out probable causes there. I'm going through some of your other threads to catch up on your issues, found this:




I have no idea if that part is interchangeable or not for the 200sx, you mentioned you have spares, are any of the others from a 200sx? If that one is isn't compatible, that would explain the error code after changing that part. Just to be clear, you are talking about this part, correct:

I think that your issue may more likely be from this part:


This one is underneath the intake manifold. When it is cold, a valve inside is open to let more air into the intake, as the engine heats up, this valve will start closing until finally it is fully closed after a minute or two. According to Ben Fenner's post linked above, the plug on this is a 5v signal that runs a small heater in the valve to warm it up. If this valve is stuck closed, that could explain your symptoms.

Your IACV/ACC with a broken plug could certainly have been an issue, but this other part seems more likely to me.

small update. got a consult cable and have been trying to set idle with it. still have cel. i did fix the cold start stalling issue finally. i have read page after page of google searches. I guess i will head back to the drawing board and recheck everything. update later its too cold for me lol.