Vacuum leak symptom

[jthames]

Car is a 92 Spider, only mod is the Sqadra chip. Could surging idle (from 600 - 900 rpm) when warm be a vacuum hose/vacuum leak issue? It runs perfectly when cold. Runs perfectly when warm, too, just not at idle. It did the surging exactly the same way before the chip. Any thoughts?

[Tifosi]

Might just be the idle is set wrong. It's happened more than once where it gets set without the engine being fully up to temperature. (as in driven for several miles, not idled til the gauge reads temp)

Another possible idea is the aux air bypass is partially stuck open. You'll get a wicked surge once the engine is actually warm.

And way out there on the edge, a worn or bent throttle plate screw. (they do get beat down over time, and it's n ot even beyond the realm of possibilities that it may have backed off a 1/4 turn or so)

To check for vacuum leaks, get a propane torch, and with the engine running, haze the gas around all the vacuum lines and the brake booster. It'll kill the engine, or make it run really ragged when the gas permiates the intake system through a leak.

It'll run pretty ugly or outright quit if there's a vacuum leak and the propane gets sucked in.

[jthames]

Thanks--here's another question--how do you set the idle? Is it the throttle body / butterfly set screw you mean? I am leery of adjusting the AFM--that screw is for the spring tension, and I know from past experience of other Bosch cars that tweaking that too much can screw up the EFI.

[Tifosi]

You don't touch the AFM to set idle.

Regular adjustment is done at the nut w/locknut right on the plenum next to where the auxillary air valve connects. (looks like just a big hose that does nothing)

Loosen the locknut, (the lower/bigger nut), back the other nut out for higher idle, screw it in for lower idle.


The long way around it entales the throttle stop screw located next to the linkage on the throttle body. (not a common adjustment, but not beyond the realm of 'need' after many years use)

1) get engine fully warmed up (you'll be wasting much time and definitely be setting it wrong if it's not fully up to temperature)

2) with engine off, unhook throttle bypass hose (above) at the plastic tube and block off bothe holes

3) refire the engine and using the stop screw, (where linkage is on throttle body) set idle so the engine barely runs (about 500-600 rpm)

4) shut down engine and check throttle postion sensor (plastic thing on throttle body opposite the linkage) to ensure that the 'throttle off' contact is still being made, adjust as neccisary. (ideally you'll hear a soft 'click' in the sensor just as you come off idle when moving the linkage by hand and then right as you return to idle. There's also a test that can be done with a multimeter which is described elsewhere in the forum)

5) reconnect throttle bypass hose, fire it up and use it to adjust idle til it falls in the 950 +/- 50rpm range

[Tifosi]

Secondary note:

To check that aux air valve without tearing everything apart you'll need:

Vice grips and something to cusion the jaws

Engine fully warmed up


After warmup and when the surging is happening, take the grips with cushioning and clamp the aux air hose shut between the valve and the plenum.

If idle drops a bit and/or then steadies out (no surges) then the valve is stuck partially or fully open and is in need of replacing.

[jthames]

Awesome instructions--thanks!

[DaveC]

Check this thread out.

http://www.alfabb.com/bb/forums/show...=motronic+idle

Your 92 has Motronic with an Idle Air Controller on the Plenum. The L-jets had Aux. Air Valves.

Dave

[jthames]

Thanks DaveC--

[Gubi]

Indeed, you have no aux air valve, and no way to set the idle either. The idle is controlled by the Motronic computer. Tifosi's instructions are valid for L-jet spiders but are incorrect for a Motronic Spider.

[Tifosi]

WHAT!?!

How did I miss that? They of course are absolutely right.


Crap. Really sorry for the mis-information jthames, I'm kinda stuck in a 'injected = L-jetronic' loop

Thanks for the corrections DaveC & Gubi!! :wave:

[jthames]

No worries--I am still at work, so no chance to break anything yet!

[jthames]

I tightened a buch of hose clamps, lubed the VVT plunger as suggested, and wire-brushed my plugs--no more surging idle.

Now if only I hadn't dropped the idle air control valve's black plastic gasket down into the engine bay and lost it for 2 hours, the repair would have taken only 45 mintes!

Anyway, thanks again--it was a vacuum leak.

[Greg Gordon]

Like the old L-Jet system, most Motronic problems are caused by air leaks.
Greg,
www.hiperformancestore.com