Fast idle, In search of air leaks Options

[Rand]

Been trying to nail the fast idle problem for a while. Have absolutely ruled out any vacuum leaks above the throttle body. Have replaced the injector seals. That leaves me with.... Leaking around the butterfly in the throttle body? Or somewhere else in the plenum or intake runner seals?

If I put my palm over the air intake, it does choke it down. That makes me wonder if air is getting around the butterfly (since the idle air adjustment screw is all the way closed). But I'm a noob on this detail... Please comment on this!!

I would think a crack in the plenum / air distribution box (whatever you wanna call the air box under the throttle body on a '75 2.0 Djet) would be rare. But I'm down to checking rare things on my list. I guess I don't mind taking all that apart because I haven't been there yet and wouldn't mind painting the plenum and air intake tubes anyway. Sooo.... Anyone have photos of disassembled plenum, intake runners, etc.? What will I need if I take this apart? (Seals/gaskets? Anything else?)

[echocanyons]

Stuck open AAR

[Rand]

Thanks Kelly....
But no AAR. No decel valve. No cold start injector. EVERYTHING has been sealed! Nothing left to rule out except butterfly, plenum, intake runner seals, or ???

Haven't tested the MPS yet. If I pull the vacuum hose off the MPS the idle drops. Great - except that just means it's dumping max fuel in and the rich mixture drops the idle, right? A bad MPS wouln't cause fast idle would it?

(IMG:http://www.914world.com/bbs2/html/emoticons/headbang.gif)

[lapuwali]

First off, only air leaks below the throttle cause a fast idle on D-Jet. Any air leaks above the throttle are, uh, throttled, so they don't count.

I'd check the line to the MPS (which can't be sealed), then the runner connectors, then check all of your "sealed" connections. The throttle body to plenum seal may be leaking. Your runner to head seals may be leaking. If it runs at all with your hand covering the throttle body, it's a fairly big air leak, not just a tiny crack.

[Robert21]

(IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) had the same problem, it was the valve thats by where you put oil in . the valve was open
all the time , its should open and close. don,t know the name of part. the hose goes from it to
MPS. hope it this gets rid of problem.


Robert

[Rand]

Thanks James!

Yeah, "ruling out leaks above the throttle body" sounds pretty stupid now. I guess I said that because in the past when I've talked about my "fast-idle" problem, I've had people ask me about vacuum hoses... that connect above.... (IMG:http://www.914world.com/bbs2/html/emoticons/wacko.gif)

Anyway, a clue: When I put my palm over the throttle body intake, it chokes it down to a very slow idle... almost kills it, stumbles, but does still run...

What about the butterfly in the throttle body? Does this wear, or is it an unlikely suspect?

I guess I better go ahead with taking the plenum and intake runners apart and replacing all gaskets (throttle body to plenum, intake runners to head).

Anything else?

[Rand]

Robert... I think you mean the PCV valve?

Somebody please clear this up for me. My car does not appear to have one. From what I can tell, not all 914s had them. Which 914s used a PCV valve, and which ones just vent to the intake?

Mine just vents to the intake. Which years/models/etc. use PCV valves? When does it matter if that vents to thin air vs. puke bottle vs. intake... ????

Running that to the intake... connects ahead of the butterfly... meaning backpressure at idle... Could that affect idle?? I wouldn't think blocking crankcase vent would slow idle... but I'm grasping at straws.

Thanks Robert.

[lapuwali]

If you can't kill it with your hand completely covering the throttle body opening, then you still have a leak somewhere. The only places air *should* go around the throttle plate are the AAR, the crankcase breather hose, and the idle air screw. The idle air screw still gets it's air from just above the throttle, so the hand trick should close that off, too. If you've sealed off the rest, then the air has to be coming from somewhere other than the throttle opening, so the throttle plate is not a factor. The throttle could be wide open and the engine should still die if you close off the TB with your hand.

I've no idea if cracked plenums are common (though I'd suspect a split seam would be more likely). The runner connector tubes are commonly bad, so replacing those would be a good idea. The head to runner gaskets would be at the bottom of my list, but if you're pulling the runners, anyway...

The MPS hose is the only other thing, but a serious leak here would also make it run like crap 100% of the time.

[Gint]

QUOTE (Rand @ Jan 25 2006, 05:34 PM)

Thanks James! Yeah, "ruling out leaks above the throttle body" sounds pretty stupid now. I guess I said that because in the past when I've talked about my "fast-idle" problem, I've had people ask me about vacuum hoses... that connect above.... (IMG:http://www.914world.com/bbs2/html/emoticons/wacko.gif) Anyway, a clue: When I put my palm over the throttle body intake, it chokes it down to a very slow idle... almost kills it, stumbles, but does still run... What about the butterfly in the throttle body? Does this wear, or is it an unlikely suspect? I guess I better go ahead with taking the plenum and intake runners apart and replacing all gaskets (throttle body to plenum, intake runners to head). Anything else?

You're going to have to make your own TB gasket. BTDT, not hard. You want reasonably thick gasket material.

[Rand]

QUOTE

The idle air screw still gets it's air from just above the throttle, so the hand trick should close that off, too. If you've sealed off the rest, then the air has to be coming from somewhere other than the throttle opening, so the throttle plate is not a factor.

Ok. So I guess I will go ahead and take the throttle body, plenum, and intake runners apart. I'll test the plenum for leaks. Clean and paint. Reassemble with new gaskets/seals.

Anything else???

Thanks!! (IMG:http://www.914world.com/bbs2/html/emoticons/pray.gif)

[Rand]

Thanks Gint!
Should I make gaskets from rubber? (Not cork for air seals, right?)

[SirAndy]

QUOTE (Rand @ Jan 25 2006, 03:54 PM)

I would think a crack in the plenum / air distribution box (whatever you wanna call the air box under the throttle body on a '75 2.0 Djet) would be rare.

i have seen cracked plenums. in fact, i'm looking at one right now ...

here it is, underside of the plenum, on one of the two bolt-holes, hairline crack that started at the weld ...
(IMG:http://www.914world.com/bbs2/html/emoticons/welder.gif) Andy

[Rand]

Thanks Andy.

Ok, I'm taking it all apart. Somebody stop me if there's something else to check first.

Will test plunum for cracks/air leaks and reassemble with new gaskets.

Hopefully my problems will be solved... At any rate I'll know what it looks like under there and have a damn good looking bright red intake system when I'm done! (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

Anything else before I dig in?

Please advise on gasket/seal materials.

Thanks gang!

[Bleyseng]

QUOTE (lapuwali @ Jan 25 2006, 04:12 PM)

First off, only air leaks below the throttle cause a fast idle on D-Jet. Any air leaks above the throttle are, uh, throttled, so they don't count. I'd check the line to the MPS (which can't be sealed), then the runner connectors, then check all of your "sealed" connections. The throttle body to plenum seal may be leaking. Your runner to head seals may be leaking. If it runs at all with your hand covering the throttle body, it's a fairly big air leak, not just a tiny crack.

yeah (IMG:http://www.914world.com/bbs2/html/emoticons/rolleyes.gif) . look at these areas....

[Rand]

Geoff... If I fail on this round, I'll be crying at your doorstep for help! (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

Please elaborate on the "runner to the MPS." What, the vacuum hose? If I pull that off, the idle drops to normal. But I assume that's because no vacuum at the MPS puts it in full rich mode... which means my idle only drops because it's drowning in a mondo-rich mixture, right? And then???

[Gint]

QUOTE (Rand @ Jan 25 2006, 05:53 PM)

Thanks Gint! Should I make gaskets from rubber? (Not cork for air seals, right?)

No, not cork. Although I suppose that would work.

FLAPS will have gasket material. You want paper; about as thick as a CD, give or take. That's what I used last time. It's about the same thickness as the stock gasket. No issues with it, and the that was one of the best running 2.0/4's I've ever had. I'm going to have to do it again myself on the "Budget 914".

[Rand]

Ok, I'm gonna dive in... a big sheet of paper gasket material on hand...

Two topics still not addressed, so I'm looking forward to comments on the "runner to MPS" or "PCV valve..." mentioned previously. ****???****

Thanks all...

[lapuwali]

The "runner to MPS" is the hose to the MPS, and has to be in place and not leaky for the car to run at all (as in actually go, not just idle in the driveway). The PCV valve, if there is one, is on the top of the oil filler tower, and there's normally a breather hose attached to it. If you've sealed that off at the plenum, no worries.

[Robert21]

(IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) i have a 1973 2.0 motor , the MPS hose goes to PCV valve , all i know is
i got another PVC valve , car runs great now . cheap fix.

hope it works for you .

[Rand]

The vacuum hose between the plenum and MPS is good.

I don't think my 75 has a PCV valve. I'm not sure which models had or didn't have them. But if I completely plug the port where the oil breather hose connects to the airbox, idle doesn't drop. So I don't think that could be it in my case... unless there's something else I don't understand about how the PCV valve could affect idle.

With the idle-air adjustment closed tightly, and the butterfly squarely closed, it's still sucking pretty hard when I put my palm over the opening. Blocking it pulls the idle way down below normal and makes it stumble. Is it normal for that much air to still be getting past the closed butterfly?

(Before anyone asks... no, I don't run it without the air cleaner! Except that few seconds just long enough to block the throttle body intake to see what happened to idle. (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif) )