Runs Rough After Sitting 10-15 mins...

[NiiChan GT-R]

Hi, maybe I can get a bit of additional insight into my issue

75 280z, stock EFI. Starts up wonderfully from cold.

I can turn the car off, and start up again within about five minutes, and it's fine

If I leave the car to sit for about 10 - 15 minutes, then it will start up, and run extremely roughly for a couple minutes, occasionally dying, before evening out eventually. It seems like if I move the AFM wiper slightly counter-clockwise, the situation alleviates somewhat, which indicates that adding fuel is helping.

I would think that that would point away from any issues like a weeping injector, bad thermocline valve keeping my csi open too long, etc....

I'm sort of leaning toward it being the auxiliary air valve closing too late? I cleaned the connectors, and that didn't seem to make much difference, so maybe I should try plugging up the hose at both ends, and seeing how it runs?

If anyone has any additional input into what could be causing this problem, any help would be appreciated

[siteunseen]

I'll say try and put a fuel pressure gauge in the fuel line after the fuel filter because it sounds like the 36psi is dropping after 15 minutes, especially if it smooths out after the rough idle which gives it time to reestablish the pressure. Could be the fuel pressure regulator or the check valve in the pump, lots of possibilities. Good luck.

[EuroDat]

It could be the check valve in the pump leaking back to the tank.

A simple check would be stop the engine and wait 10-15 minutes. Before starting it remove the air cleaner and use the handle of a screwdriver to open the AFM. Turn key to on (DONT start the engine) let the pump build up pressure. Then remove screwdriver and start engine.

If the engine runs normal, then you are getting leak back. If it still run rough, your problem is not in the check valve.

Chas

[Captain Obvious]

If I leave the car to sit for about 10 - 15 minutes, then it will start up, and run extremely roughly for a couple minutes, occasionally dying, before evening out eventually.

The hot restart complaints of the fuel injected Z's is a common discussion and there are a lot of previous threads about it. Try a couple keywords like "hot start injector" in the search box and I bet you'll get some good info.

It's not the AAR... The AAR might cause an idle speed change, but it won't affect mixture.

[NiiChan GT-R]

Thanks for the suggestions...

So doing a search, it sounds like check valves/leaking injectors are a culprit typically... I tried starting the car this morning first thing, with the pump lead unplugged, and it wouldn't start up, so I guess I'm losing pressure overnight. Though I do need a guage to confirm with certainty, the car should run for a few seconds if I had pressure, right?

I'll try running the pump before startup in about another ten to fifteen...

Is there any particular indicator I should look for, to tell whether its a pump check valve, or an injector leaking?

Thanks.

Edited February 2, 2013 by NiiChan GT-R

[Zed Head]

Winter blend gas vaporizes easier, hot injectors, liquid vaporizing in injector tip before injection, lean mixture results. That's the best theory I've seen so far. Personally, I think that one or two injectors are in a hot spot, get overheated, and stop functioning properly.

Fuel pressure leaking in the rail down doesn't matter anymore after the engine starts and the fuel pump re-pressurizes it. Certainly not after a few minutes (if you actually timed it you'd probably find that a few minutes is actually about 30 seconds at most). Many people have this problem and I've not seen a permanent fix from anyone, once they get it. Mine got better with a good set of Bosch injectors, and was non-existent all summer, but came back this winter (winter blend gasoline probably). You can spend a lot of time trying to fix it, and it will probably never go away completely.

How long have you had the car and when did this start happening?

[NiiChan GT-R]

I've had my car since 2009, and I drive it daily to work, so I'm pretty familiar with it.

I first noticed the problem creeping ip a few days ago, but didn't have any further issues until yesterday. I've tested it repeatedly a few times, and still seem to have it

[hr369]

I installed the 280zx injector fan and it cured the problem of hot start/rough running. I first started by replacing the fuel pump check valve

hoping it was the cause and they're cheap. I then put on a known good fuel pressure regulator from my other car. That wasn't it either.

So in the end I concluded it was my injectors leaking. I've since tried seafoam in the gas tank and chevron gas with techron

and i've noticed it starts/runs better without using the injector fan.

I've heard of some people crimping their fuel return line with vice grips to prevent gas from going back to the tank, then letting it sit for 15/20 minutes or so and try restarting. If it still is hard starting its definately the injectors. Problem with that is if you may have damaged your rubber fuel line by crimping it.

my 78 loves the ethenol free gas. It revs smoother and starts easier but its 4.37 a gallon here in hawaii so I mix it with my costco gas

whenever i get down to the marina gas pumps. Try finding ethenol free in your area and see how yours runs

[siteunseen]

A mechanic clamped my return line with needle nosed Vise Grips and Popsicle stick looking pieces of wood, without damaging the hose.

[NiiChan GT-R]

...Bit of follow-up, running the pump for ~thirty seconds prior to warm start totally works.

So I'm guessing its the check valve, otherwise a leaking injector would still prevent the car from running smoothly, and I don't seem to be losing pressure from the fpr/return line.

Check valve ordered Hopefully that'll put this at bay for a while...

[Captain Obvious]

running the pump for ~thirty seconds prior to warm start totally works.

Excellent! Glad it seems to have been that simple.

Out of curiosity, thirty seconds works... How about twenty? Five?

[Wade Nelson]

A mechanic clamped my return line with needle nosed Vise Grips and Popsicle stick looking pieces of wood, without damaging the hose.

Get some needle-nose vise grips and put rubber hose over the jaws.