Hanging RPM

[Glliw]

I know little of the complexties of FI, being an SU guy, but there are basic similarities. I doubt the problem is fuel ratio related. The only way any engine will run at higher RPMs is increased air supply. More fuel is needed to sustain the higher revs but it starts with the air supply. Something is controlling the butterfly and preventing it from closing. Most carbs have a dashpot,(vac. diaph.) that allows the revs to fall normally, until just above idle. Think of it as a rev cushion. Whatever is causing this problem is attached to the throttle shaft or linkage and is temp. related too. Just out of curiousity how do your plugs look? It is possible running a very lean A/F ratio that an increse in fuel will raise the revs.

Thanks for the thoughts, Mark, as well as everyone else. Though i do not think that is it. While the 280z does come with a "rev cushion", that has long since been removed as that was the first item I thought to look at.

To back up my statement on how the car performs when cold, please check out this video. The rpms act as one would think they should and return to idle in a reasonable period of time. Once the engine begins warming up, the issue will persist.

I also popped the cover of the TPS and checked its function. I cleaned the contacts with a electric parts cleaner. Everything looked cherry in there though.

Do you think one of the temp senders may be playing foul here? I didn't think any interact with the system like this, but with it only happening at warmed up and consistently then I don't know what else to think.

[siteunseen]

I think you have a huge vacuum leak, EGR. Do you have a vacuum gauge? It'd be interesting to know what it's pulling.

What about the A/C idle control? That round thing with the hook on your throttle rod. Take it off and see what happens, 3 bolts. 1 more place to put a return spring on the throttle rod is coming out of the drivers side fire wall on the top "marble" to the underside of the hood latch hole. You might could loosen the locking nut on the A/C idler hook and just turn it up? I don't know for sure. Or I'm thinking the flat piece that the hook catches is held on with an Allen screw, you could maybe take that off?

Edited April 19, 2014 by siteunseen

[Glliw]

I think you have a huge vacuum leak, EGR. Do you have a vacuum gauge? It'd be interesting to know what it's pulling.

What about the A/C idle control? That round thing with the hook on your throttle rod. Take it off and see what happens, 3 bolts. 1 more place to put a return spring on the throttle rod is coming out of the drivers side fire wall on the top "marble" to the underside of the hood latch hole.

I would think the same except the issue isn't there at cold engine temps. Also, EGR is effectively non-existent since the vacuum source to it is blocked off via the BPT block off, unless a massive leak is at the gasket surface.

I do not have a vacuum gauge handy.

The a/c idle control is disconnected and can be seen as such in the pic above as I had the same thought. Doing so had no effect.

A few days ago I did do the "yogurt cup test" by using a yogurt cup to block off the intake after the AFM, creating a closed system. I pulled off the brake booster hose and pressurized via that. During that, I could pressurize with my mouth to a point and could hear a leak, probably past the valves. It would hold the pressure for 5-10 seconds. I sprayed soapy water at each nipple connection on the intake manifold and did not see any bubbling. The AFM boot does not have any tears in it.

[siteunseen]

Take a hand held mirror and look under the intake manifold, mine had a leak where the EGR tube from the exhaust went into the manifold. I put my valve cover gasket on upside down one time and caused a vacuum leak up front. When I did the yogurt cup test through the AAR hose it parted my hair! It's something simple, I'd bet my house on it. Don't get frustrated, you'll find it, watching the videos you have a good car.

[Zed Head]

I saw the word "booster" earlier in the thread and assumed it had been covered, but does the problem happen when your foot is on the brake pedal? A bad booster diaphragm will cause a vacuum leak. Foot-on-brake might be coincidental with high RPM.

[Glliw]

Take a hand held mirror and look under the intake manifold, mine had a leak where the EGR tube from the exhaust went into the manifold. I put my valve cover gasket on upside down one time and caused a vacuum leak up front. When I did the yogurt cup test through the AAR hose it parted my hair! It's something simple, I'd bet my house on it. Don't get frustrated, you'll find it, watching the videos you have a good car.

It always is something simple. Only thing is that there are 30 simple things in the system.

Thanks again for the help, I definitely appreciate it.

How long roughly should it take the intake to leak down during yogurt cup testing? 5, 10, 30 seconds?

I'll buy a cheapo vacuum gauge and a mirror here shortly and follow this write-up: http://www.atlanticz.ca/zclub/techtips/vacuum/

[Glliw]

I saw the word "booster" earlier in the thread and assumed it had been covered, but does the problem happen when your foot is on the brake pedal? A bad booster diaphragm will cause a vacuum leak. Foot-on-brake might be coincidental with high RPM.

It sure does, but it also happens when my foot isn't on the brake so I don't see a correlation there. In the videos above, I am not on the brake at all. Just run it up in 2nd gear and put the clutch in.

I just did the vacuum test. I tee'd in at the brake booster line, at the firewall mount to try and include as much of the system as possible. From my perspective, I look pretty solid from a vacuum stand point. It hold at -18in.Hq at idle with maybe a 1/4 to 1/2 in. Hg fluctuation towards positive values. When blipping the throttle, it spikes to 0-ish, and returns down to -20 and settles back in at -18, just as it should.

If there are any vacuum leaks at all, they are very minute and I do not think they are a driving factor here.

I also inspected the underside of the intake manifold. I did not see anything obvious under there.

[Zed Head]

To siteunseen's point on the EGR- there have been cases where the internal passages of the EGR have rotted out. If the tube at the intake manifold from the exhaust manifold is open, and the EGR passage in the manifold is rotten, it would be a vacuum leak, even if the vacuum nipple is blocked or even if the top of the EGR has a plate sealing it.

In the same vein but a bigger stretch, the internal-hex head nuts in to the EGR passage could be loose. Very unlikely though since they're really hard to loosen.

[Glliw]

To siteunseen's point on the EGR- there have been cases where the internal passages of the EGR have rotted out. If the tube at the intake manifold from the exhaust manifold is open, and the EGR passage in the manifold is rotten, it would be a vacuum leak, even if the vacuum nipple is blocked or even if the top of the EGR has a plate sealing it.

In the same vein but a bigger stretch, the internal-hex head nuts in to the EGR passage could be loose. Very unlikely though since they're really hard to loosen.

Wouldn't that show as a vacuum issue on the gauge that I just tested with though? I would have to be below -15 in. Hg to begin to think I had a strong enough vacuum leak to create this effect I think. That and the whole works fine at cold temps part pushes me away from thinking vacuum.

[Captain Obvious]

I'm unclear still if it's vacuum, electrical related, or strictly mechanical...

Warm it up to the point where it starts hanging. Once it's warm enough and starts exhibiting the problem, open the hood and rev the engine by grabbing the throttle linkage all the way back at the pivot point by the firewall. Then drop the RPM's slowly by releasing the linkage gently. The object is to get it to hang.

Then once it's hanging, grab the linkage right at the throttle body and manually force the throttle body butterfly closed.

Does it still hang while you're forcing the throttle body closed, or do the RPM's drop like they should?

http://www.classiczcars.com/forums/fuel-injection-s30/48078-sticky-throttle-body-hanging-idle.html

[Glliw]

I'm unclear still if it's vacuum, electrical related, or strictly mechanical...

Warm it up to the point where it starts hanging. Once it's warm enough and starts exhibiting the problem, open the hood and rev the engine by grabbing the throttle linkage all the way back at the pivot point by the firewall. Then drop the RPM's slowly by releasing the linkage gently. The object is to get it to hang.

Then once it's hanging, grab the linkage right at the throttle body and manually force the throttle body butterfly closed.

Does it still hang while you're forcing the throttle body closed, or do the RPM's drop like they should?

http://www.classiczcars.com/forums/fuel-injection-s30/48078-sticky-throttle-body-hanging-idle.html

I will double verify this tomorrow, but I have done this already with no effect as I recall.

[Zed Head]

Wouldn't that show as a vacuum issue on the gauge that I just tested with though?

The reading on the vacuum gauge tells about the state of the various engine components - valve timing, valve condition, ignition timing, valve seals, rings, power balance between cylinders, etc. If the engine is running, it doesn't tell you anything about vacuum leaks in to the manifold. With a vacuum leak, the engine RPM can increase, actually increasing the intake vacuum or keeping it the same.