sorry, this is gonna be long....

I have an 88 TII with no mods other than a cone filter. I just recently replaced all of my major vacuum lines in hopes of eliminating my poor idle. Yes, the same old idle problems that are posted on here all the time. Vac lines didn't fix the problem. So the car still bounces between 500 and 1000 rpm and sometimes stalls when I'm stopped.

I went searching, not only the forum but also the FSM. The FSM makes little sense to me but from the forum I have learned that the TPS adjusts the actual idle speed. You can just turn the screw and it will change the RPM that the engine will idle at that load, right?

The BAC controls the air fuel mixture at idle depending on different loads right? So if I had a pulsating Idle, the BAC would be the thing to adjust wouldn't it?

Well I read the section in the FSM for adjusting the BAC and it doesn't make a whole lot of sense to me. It says how to inspect, remove, replace, and check the thing but don't you adjust the BAC when your car is idling rough? or am I way off track here?

Sorry for posting what seems to be a question that's already been asked, but after searching for "bac adjust" "tps adjust" "pulsing idle" "rough idle" etc. all day, I think I just need to post a new thread

 

on a t2 the bac is on the turbo side of the upper intake, its got 2 coolant hoses going to it. theres an adjustment screw on it (might have a cap on it) this adjusts the idle speed.

the idle mixture screw by the airbox on the strut tower adjusts the idle mixture.

the tps is what the ecu uses to know that the car is idling, it also uses the tps for light load/rpm drivability

 

*****I went searching, not only the forum but also the FSM. The FSM makes little sense to me but from the forum I have learned that the TPS adjusts the actual idle speed. You can just turn the screw and it will change the RPM that the engine will idle at that load, right?********

Not right. Drop that idea. The TPS on a series four should be set to output approx one volt on the green/red wire with a fully hot engine. Never use that screw to set the idle. EVER.

******Well I read the section in the FSM for adjusting the BAC and it doesn't make a whole lot of sense to me. It says how to inspect, remove, replace, and check the thing but don't you adjust the BAC when your car is idling rough? or am I way off track here? ********

BAC stands for Bypass Air Control. It bypasses air around the throttle plates to increase the idle. Little or nothing directly related to fuel. Screwing the screw counterclockwise gives more air, turning the scew clockwise gives.........yes.

The pulsing idle, imho, is probably a combination tps setting problem combined with a improper adjusted idle and a little variable resistor muckup.

You need to install the initial set coupler with a fully warmed up engine.

Set the timing to spot on with the idle speed under 900 for sure but closer to 750 the better.

Now set the idle speed using the bac screw. That with a little adjustment of the variable resistor should give you close to a 750 idle speed.

If the idle is still bumpy while doing this.....consider the acv's anti afterburn diaphram might be busted/leaking air into the intake manifold. Something it should not do at idle. If suspected being a leaker....pull the Blue plug off the Relief Solenoid. Did the idle change noticably??? IF it did then the acv is leaking unauthorized air into the intake manifold.

All the above should have been done with the initial set coupler installed.

Remove the initial set coupler while idling. The rpms might/will pump up and down for a moment or two and settle down to a even 750 rpms and should not cycle up and down if the tps was set to output one volt to the ECU.

I don't think it's possible to tell someone how to see the idle on this car unless they have mucked around with things and know what they are and their names and a clue about what they do in life. But you gotta start somewhere. It's the march of ten thousand mile thing that Chairman Mao was talking about a few years back.

 

Ok, I think I understood most of that. but what is the initial set coupler? and how do I adjust the variable resistor?

 

The variable resistor is only idle mixture.
Is for the guys at high altitude.

Grounding the initial set plug shuts off he BAC so it won't obsure the mechanical idle setting.

 

initial set coupler is by the battery;

s4: green 2 pin connector, jump the 2 wires together
s5: green 1 pin, ground said wire

i adjust the variable resistor to the highest idle speed, like you would on a carb. if its maxed out either way it usually means you have a vacuum leak