1G 4g63 Non-Turbo Idle Surge ONLY in Neutral

[rawfuls]

Recently replaced the head gasket & rebuilt the head of a 4g63 NT '94 Talon.
Replaced the IACV as the last one was measuring out of spec (actually, just shorted the entire thing), with a used one off eBay and it has been significantly better.

The one thing I can't figure out, is why it idle surges only in neutral.
Easily reproducible by driving in any gear (video shows 1st, then reverse), then shifting to neutral, add some gas and instant idle surge. I notice this even when the vehicle is warmed up, or cold.
In the video, I show going into first, moving a foot (max), and back to neutral and it does not idle surge. Then I go into reverse, move back about 2ft, and back to neutral and idle surges.

Once the idle surges for ~5sec, it idles just fine.
When in gear, it does not idle surge.

EGR is hooked up, vacuum lines seem to be routed correctly, no cracks or leaks in the vacuum lines. No obvious cracking or leaking in the brake booster lines.
Previous thread mentioned if the ABS module is disconnected, then this would cause an idle surge in neutral; from what I can see there is no ABS module on this vehicle. Pulled both rear quarter trims and did not find any ABS modules.

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[pauleyman]

See if you are closing the idle switch all the time. A sticky or too tight throttle cable may keep it open. Set the BISS screw correctly. Any load you put on the engine may slow it down enough to keep below the 1500rpm idle cutoff. Looks like you haven't set idle correctly or you have a leak.

 

[rawfuls]

If downshifting to a stop, the surge does not appear.
Does this still indicate the idle switch?

 

[pauleyman]

You're confusing cause and effect. You need to test or log the idle switch. Check the throttle itself and cable and set the biss screw idle to factory specs. If you did not ground the timing check connector and have fiddled with the biss screw then it is not set correctly.

 

[rawfuls]

Never adjusted the BISS nor tested the idle switch so I'll start there.
Throttle cable seems to be operating nice and smooth, in addition to the throttle plate. I'll give the throttle cable a bit of lube just in case, though.

 

[92AWDHX40]

How to fix your idle surge problem. | DSMtuners

 

[BLACK'98DSM]

Is this car an automatic or a manual? This may not seem like it matters but the manuals are supposed to idle up in nuetral if the car is in motion. It's not supposed to surge like that, but it is supposed to idle higher than 1k.

Do you have a boost/vacuum gauge? That would help determine whether you have a leak or not. You can find vaccum leaks by spraying starting fluid on the possible areas.

Do your research before messing with the BISS screw. I've seen multiple poeple on this forum mess it up.

 

[rawfuls]

Is this car an automatic or a manual? This may not seem like it matters but the manuals are supposed to idle up in nuetral if the car is in motion. It's not supposed to surge like that, but it is supposed to idle higher than 1k.

Do you have a boost/vacuum gauge? That would help determine whether you have a leak or not. You can find vaccum leaks by spraying starting fluid on the possible areas.

Do your research before messing with the BISS screw. I've seen multiple poeple on this forum mess it up.

Manual, it does seem to be idling higher than normal in the video. Thinking the idle switch may not be set correctly. That seems to be a lot less dangerous than adjusting the BISS..

 

[pauleyman]

None of it is dangerous. Follow the procedures and youll be fine

 

[rawfuls]

So finally home after the holidays and planning to make these adjustments.
After test driving today, there was indeed SOME surging in gear at a stop.

It's clear the idle is not ~750rpm +- 50, so we know the idle is too high.

I need to go ahead and release the jam nut and get the idle switch to be making less contact with the throttle stop.
Then ground out the two pins, and adjust via the BISS. This will set true idle.

I guess I'm just unaware of which needs to be adjusted first, idle switch or BISS?

 

[pauleyman]

Do not mess with the idle switch unless you have to. Unless somebody messed with it before it doesn't need adjustment. Let me put it this way. I've been playing with these cars 30 years and have never had to adjust an idle stop. Biss, isc, and fiav cause the most problems.

 

[92AWDHX40]

Follow the steps to the idle surge i posted its the basics where you start at get those sorted out and check back with your updates, i found a lot of people skip from thing to thing without doing the most common steps. two decades i learned that way. my first issue was a leaking throttle body when i started back in 94.

 

[rawfuls]

The BISS cover was removed and was 2 full (360) turns out from tightened. So I have no evidence to prove that the idle switch was NOT touched... wanted to assume it was touched, but left it as is.
I shorted the two pins, idle was at about 2000 rpms, turned it a full 360 turn out from tightened, and the idle dropped to right around 800rpm. Fine adjusted to 750rpm and it seems to be happy.
So far, no idle surge when coming out of gear, but time will tell.

It's going in for a CA smog test (pass or don't pay) tomorrow so we'll see how this goes.