Laddis

Idle varying between almost stall to 1200rpm both hot and cold.

When removing the hose at the vacuum Y-piece (where the 3rd hose goes to the break), the idle settles smoothly to approx 600+ rpm.
Blocking the Y-piece suction with my finger causes the rpm to drop again and engine starts hunting up and down again.

Can someone explain which direction the air goes in these hoses? I have read that air is taken from after the MAF but before the throttle, then passes trough the ICV to feed it into the plenum after the throttle, but why is this connected to this Y piece on the 'outside'?

And, are these symptoms of faulty ICV , faulty MAF or faulty J-tronic box?

Laddis

yardpro

I have the same issue. I have not however checked any vacuum stuff

OTR18WHEELER

600 rpm is too low at idle, does seem like a ICV issue, may not need replaced, just cleaned. This solved my idle surge issue, it used to almost die after a romp of the throttle while in park, and then would search and finally level out. I cleaned the ICV with PB blaster, and then shot it with a liberal amount of silicone spray. Idle surge problem solved.

jcorenman

Removing the hose from which side of the Y-fitting?
And blocking which side?

Yes, any of those. It could be ICV/ISV, but since it is idling better with a disconnected vacuum hose then the MAF might be more likely. How many miles (km's) on the MAF since new or rebuild?

And correct, air is taken from the rubber boot between MAF and throttle (normal air pressure), through a venturi in the Y-fitting (to create suction), and then to the intake plenum (roughly 0,5 bar negative pressure at idle). This airflow is part of the idle air (along with the ICV) and also generates vacuum (suction) for the brake booster via a venturi.

So disconnecting this hose allows additional air to enter the intake, without being metered by the MAF. This "un-metered air" causes the mixture to become more lean, so if the mixture is too rich then the idle would improve-- but normally it would become too lean and the engine would stall.

Connecting a Sharktuner would quickly identify the problem, otherwise the best option is to swap parts with another car (87-95). Start with the MAF, then LH (best to swap yours into another car which runs and idles well). Try cleaning the ICV or replace with a new one, also check fuel pressure (make sure it stays at 3 bar or so for 20 min's after shutting off the engine), check for air leaks.

I hope this helps.

Cheers, Jim

Laddis

Thanks for answers.
Looking at PET the hose from the "after MAF before throttle" outlet is T-out in a strange looking bent T. One hose from this T goes to the ICV and the other goes to the Y piece on the outside, where again it is connected to the intake manifold. I can understand that the fist hose regulates additional air into the plenum through the ICV, but what is the mission of the T'ed out hose which again is connected to the manifold through the Y piece. There must be a constant "false air" through this connection, And there shouldn't be anything that is preventing it from getting air when connecting the hose back on. Or is there a checkvalve or something in the first T piece?

jcorenman

Here's the PET drawing, with numbers. The "bent T" is #8, which supplies (metered) air to both the brake venturi (via hose #10) and the ISV #2 (via hose 11). Those are the two circuits I mentioned, and both provide air to the intake and bypass the throttle plate-- but not the MAF.



The first path goes through the brake venturi (not numbered here, the "Y" piece) and then to the intake via hose #12 to the plenum side-cover. The purpose of this is to generate vacuum for the brake booster.
The other path goes through the ISV #2 to regulate idle, then to the lower plenum via hose #15.

The third leg of the "Y"-- shown empty here, see the brake-booster pages-- goes to the brake booster via a check valve. The brake booster is basically a closed canister, no net airflow-- except when brakes are applied.
But just sitting there idling, there will be airflow through the straight part of the "Y", but not the leg to the brake booster.

So disconnecting the hose where you did allows air into the intake without being metered by the MAF. So less air through the MAF means less fuel, making the mixture leaner. And when you block it, then all of the air now flows through the MAF, more air means more fuel, and the mixture goes rich again (same as with hose connected, all air is metered).

So what does this mean? Something is causing the mixture to be rich. As before, likely MAF, possibly LH. Not likely to be the ISV, or an air leak-- that would cause a lean mixture. When I had this trouble it was the MAF.

It could also be the O2-sensor, if your car has one. Euro cars with no cats and no O2-sensor will have a "CO-pot" to adjust the idle mixture, located near the ECU boxes. A mis-adjusted CO-pot would not explain a sudden problem, but a disconnected one might. Has anyone been missing with the wiring around the CE panel?

the flyin' scotsman

another potential issue that results in erratic idle is the hose #11 splits where it connects to the ISV. Any easy test for this is to spray a NON-flammable brake cleaner under the intake with the engine running and observe any change to the idle speed..........any change confirms that theres an point in the system that's allowing unmetered air into the system.

good luck