Rough Idle until I pinch valve cover hose

[joes]

Here's what I've got. 77 280z, all stock. Was running great until one of the hoses on the fuel rail started leaking fuel. I replaced the bad hose, but have never been able to get the idle back to where it should be. I've checked and checked for leaks. Nothing I can see. But if I pinch the big hose coming off the top of the valve cover, the engine idles perfect. Not sure where to start my troubleshooting. Any comments would be appreciated.

[djwarner]

First thing to check is the PCV valve and hose for clogs. Next step would be to a compression check.

[Jarvo2]

Sounds like valve seals or a burnt valve itself letting fresh air into the combustion chamber through the head. When you pinch the hose you cut off the air source.

[Zed Head]

Was running great......

if I pinch the big hose coming off the top of the valve cover, the engine idles perfect.

The big hose opens to the air intake tract. Either through he AAR or directly through the AFM hose or both.

Are you checking with the engine warmed up or is it still cold? Did you plug the power back in to the AAR? If you didn't, it might not be closing quickly, and you're actually cutting off air through the AAR when you pinch the hose. 1977 had several hose routing possibilities, currently under discussion in other threads.

[Captain Obvious]

I have never been able to get the idle back to where it should be. But if I pinch the big hose coming off the top of the valve cover, the engine idles perfect.

I'm not sure I know what you mean when you say that you can't get the idle "back to where it should be". Are you talking quality or quantity?

By that, I mean... Are you saying that the idle quality is poor (stumbles, runs rough, etc) or are you saying that the idle quality is OK, but it simply idles at too high of an RPM?

[joes]

I'm not sure I know what you mean when you say that you can't get the idle "back to where it should be". Are you talking quality or quantity?

By that, I mean... Are you saying that the idle quality is poor (stumbles, runs rough, etc) or are you saying that the idle quality is OK, but it simply idles at too high of an RPM?

The idle quality is poor. Not smooth. Alot of sputtering but it never dies. But when I crimp the big hose, it idles perfect. There is definitely a leak somewhere. When I cut the engine off, I can hear a hissing sound near the FPR.

[Captain Obvious]

Sounds like you've got a lot of air going through that valve cover vent hose and into the throttle body nipple without going through the AFM first. That will cause a lean condition.

So, where is that air coming from? Some things to look at:

Make sure that the PCV valve and it's hose are in good shape and properly connected. (The PCV and hose are located on the underside of the intake manifold right behind the throttle body.)

Make sure you've got the oil cap installed and snug.

Make sure you've got the dipstick installed and pushed all the way home.

[joes]

Thanks everybody. Sorry it took me so long to get back but here is the good news. I went back and checked the hoses closer. The pcv hose looked good from up top, but underneath it was cracked all the way across at the clamp where it comes out of the engine. One good pull and the hose came apart. It was in pretty bad shape on the bottom side. Pretty much useless. Ordered a new one and just put it on. And now I'm listening to the most beautiful sound I've heard in a long time. A 280Z with a smooth idle. YAY!!!!! Thanks again. That hose is hidden pretty good.

[Captain Obvious]

Awesome. Glad it was a simple fix. I love it when a plan comes together.

[jalexquijano]

Awesome. Glad it was a simple fix. I love it when a plan comes together.

 

I have a similar problem with my 1972 240z. Rough idle. I have it set up at 950 rpm (Shneider 274 F Cam, Pertronix ignitor and ignition coil, remanufactured SU carbs from Ztherapy and stock exhaust). If i leave the car in the parking after warmed up for more tan 10 minutes, the rpm starts to go lower from 950 to 850 and rises back to 1000 Rpm? Arent these cars suppose to keep its idle as smooth as possible in 950 RPM? Could i have a leak somewhere?? I did a compression check and all valves are okay! Will i need to raise the car in an elevator to see if there is a leak somewhere in the exhaust or manifold? Already did the carb cleaner test around all the hoses and did not notice any change.