High Idle And Iacv Interchangability

[rx-line]

Hey guys,

I have an rb20det with intake, intercooler etc. She has a high idle, TPS seems to work properly. I am wondering if the RB20's come factory with "restrictors" before the Idle air control valve and cold start. She idles around 2500 cold. and 1800 cold there abouts.

I am wondering if the Black/Aluminum "cold start" is interchangeable with the 92 Nissan Sentra unit, as well what are my options for the IACV?

RX,

[GTSBoy]

Have you cleaned the IACV? I had to take my RB20 one apart and scrub it internally to get the black gunk out so it would function properly (at one point in that engine's long history). After that it was fine.

Have you looked for vacuum/boost leaks? If the plenum gasket is broken (or really, any other possible location where air can get in) then you get a high idle. You can't just assume that it's all sealed up.

And to answer the specific questions.....no there aren't any restrictors in the feed hoses to the cold start valve and IACV. And the Sentra cold start valve is probably interchangeable. So long as it has the same electric connector, same hose sizes and you can bolt it into place then its operating properties would likely be close enough so that it would work. They really are just an electrically heated thermostatic valve. Open when cold and close up after the heating element gets hot.

[rx-line]

I've cleaned both valves, I am going to have to look for vaccuum leaks. I would like to know first if I have the lines correct. Here is an image I drew up on paint so you may get an idea lol. I am unsure if the vacuum going to the cold start from the piping should actually go to the otherside of the cold start.

Thanks,

RX

[GTSBoy]

That layout will work. I don't think that the cold start valve orientation should matter. It's just a valve after all. It might do though, so could be worth a trial the other way around. The other thing you should do is take it off, power it up and make sure it gets warm and closes.

[rx-line]

I ran the car for about 30 mins in -10 it never hit 180 degrees but the rad was hot, I changed the orientation of the vacuum lines in the below noted diagram. It idled at about 800rpm with the IACV idle screw adjusted about half way. It would die off reving like always, So then I figured I would adjust the throttle body adjustment screw for the throttle.

Now I'm thinking the cold start is bad (I opened it up a while ago) since it never kicked down into low rpm. I think if I adjust the TB screw so the throttle is open a fair bit and adjust the IACV down with a new cold start I should see about 2k on cold and 1k rpm warm?

Correct me if I'm wrong.

Also I take it the cold start closes off when warm?

Thanks for all the help man!

[rx-line]

Also will connecting the brake booster (currently line is plugged) make it idle higher or lower?

RX

[GTRPSI]

Booster has a one way valve in line to help hold vacuum.

If the boosters diaphram has a hole, it will leak vacuum and your idle speed will increase.

Correctly working it will not alter the idle speed as its sealed.

[rx-line]

True, I had put a "check" valve in from a honda, It could leak IDK, it's from 1983. No lines are currently going to it.

RX

[GTSBoy]

The plumbing in that second sketch is crook. The cold start does indeed shut right off when hot (or at least it should if it's not buggered). When it does, you IACV will not have access to any air, so no idle control. Additionally, the cold start is supposed to be extra air on top of what the IACV lets in, to force a high idle speed when cold. Your current layout will do no such thing.

[jiffo]

If it was mine I'd be removing the cold start (air reg) completely. The AAC can control idle fine.

The air reg will eventually close even without electrical power, just from engine bay heat but who needs more plumbing.

You need to close that TB right down. It's only cracked open the smallest amount to stop any chance of jamming due to temp changes.

Idle is controlled via the AAC not the TB.

[rx-line]

"It's only cracked open the smallest amount to stop any chance of jamming due to temp changes". That was always my thought behind it, and how I set it up when we built the intake. Sometimes when troubleshooting you wonder what goes through the little Japanese engineers minds when they designed these cars.

Why is my idle so high then? with the idle screw all the way in, with the old setup it would still run at like 2k rpm. I have ran it without the "coldstart" and it still has a high idle. I'm guessing a vacuumm leak, but I've been over everything and have not seen any signs of a leak.

RX,

[GTSBoy]

Why is my idle so high then? with the idle screw all the way in, with the old setup it would still run at like 2k rpm. I have ran it without the "coldstart" and it still has a high idle. I'm guessing a vacuumm leak, but I've been over everything and have not seen any signs of a leak.

What is this plenum? Is it a Freddy? Are you sure that the flange is straight?

[rx-line]

The Plenum, is a front facing Mailbox style plenum that was built in-house. It is just the top half of the receiver which is bolted to the original bottom receiver with gasket paper between them. I can't imagine it leaking.

RX

[jiffo]

As has been pointed out, that second layout is no good.

But using that layout got your idle down to 800, so it shows the car is capable of a normal idle and the AAC looks suspect.

The factory idle circuit has the air pipe running to a tee with one leg of the tee going to the AAC and the other leg going to the air reg, and from there to the plenum. So when the air reg closes, the AAC still has an air supply.

For simplicity I'd remove the air reg.

Your brake booster vacuum should come from a separate nipple.

[rx-line]

So I have my air regulator hose (Cold Start) plugged off, I adjusted my throttle body to just above close (sticky zone) and the tps sensor to .47 volts. the car runs at about 1500rpm with the idle screw almost all the way in ( 1 and 1/2 turns out). I turned my fuel pressure down to about 30psi with the vacuum line on and has an AFR of 11.8 warmed up @ 1500rpm no load. I check the Bov and it doesn't open at all during revs or off revs so no leaks there.

However the car wants to stall out after any revs above 2000rpm.

I do not have AC or Power Steering, I will check for vacuum leaks with propane another time but I am doubtful.

How can I get my revs lower without the quitting after revs? is the IACV to blame?

RX,

[rx-line]

So I have my air regulator hose (Cold Start) plugged off, I adjusted my throttle body to just above close (sticky zone) and the tps sensor to .47 volts. the car runs at about 1500rpm with the idle screw almost all the way in ( 1 and 1/2 turns out). I turned my fuel pressure down to about 30psi with the vacuum line on and has an AFR of 11.8 warmed up @ 1500rpm no load. I check the Bov and it doesn't open at all during revs or off revs so no leaks there.

However the car wants to stall out after any revs above 2000rpm.

I do not have AC or Power Steering, I will check for vacuum leaks with propane another time but I am doubtful.

How can I get my revs lower without the quitting after revs? is the IACV to blame?

RX,

Solve this problem and I'll send you 20$ on paypal lol

[GTRPSI]

Find the vacuum leak and problem solved, you have air entering from somewhere.

When can i have my $20?

Jokes aside, start blocking things off and noting the idle, while also spraying around gaskets etc and listening for RPM changes.

Somewhere airs getting into it......just a matter of finding the leak source or partially jambed open cold start, could even be a cracked hose.

Start picking things off one at a time.

[rx-line]

Find the vacuum leak and problem solved, you have air entering from somewhere.

When can i have my $20?

Jokes aside, start blocking things off and noting the idle, while also spraying around gaskets etc and listening for RPM changes.

Somewhere airs getting into it......just a matter of finding the leak source or partially jambed open cold start, could even be a cracked hose.

Start picking things off one at a time.

It holds 20lbs vacuum at ilde, so a vacuum leak would cause it to run less? I got it to run about 1100rpm cold by doing the unplug set to 850rpm and plug back in (IACV) method. I alos adjusted the throttle plate all the way down adn set the tps again. I would say the no AC/Power steering (Or wiring for it) may be causing the high idle, and to compensate the idle screw is in far and causing it to die after revs. Odd that it revs to 1250rpm after being up to temperature. Correct me if I am wrong here.

I have another question for you folks about wiring.

I did not receive the lower harness (starter,neutral switch, speed sensor etc.) with the engine/trans and am curious as to the repercussions of not having these wires going anywhere.

Wire#

18- (Fuel Pump relay) - I use the fuel pump harness for the mazda

43 - (Ignition Start Signal)

53 - (vehicle speed sensor)

104 - (Fuel Pump Volt Control)

I assume one of these is causing it to run 10.8 afr's on idle even with reduced fuel pressure, My guess it would be the Vehicle speed sensor. I am unsure what color or even which pick up on the transmission is the speed sensor as both have green wires coming form em and the pinout says it should be yellow/green... not green/yellow....

RX,

Edited March 22, 2015 by rx-line

[rx-line]

The neutral switch and reverse switch are not wire as well.

[jiffo]

Pin 18 won't cause any grief to the ecu, but you need to somehow control the fuel pump and using this pin is the best way.

When the engine stops, your fuel pump must also stop/time out. (You don't want fuel pumping in the event of a crash)

Pin 43 starts the timer for the fuel pump. It also re-sets at every IGN start signal. (I always believed the ecu would not continue without this signal, must have been wrong on that one)

Pin 53 input is vital once you get the car moving. The ECU will operate OK without a speed signal but after a while it gets confused. It see's RPM, TPS, AFM and so on but no speed. It thinks you're sitting still and free revving. You will have inconsistent power delivery without a speed signal input as the ecu will eventually pull timing.

(Many, many smarter blokes than I have been fooled by this one)

Pin 104 is for the fuel pump control module and won't have any effect if left unconnected.

Pin 44 (neutral switch) won't effect your engine as it's a Normally Open switch when in gear. But if you were using HICAS etc then you wouldn't be able to do self checks without this input.

Reverse switch has no input to the ecu.

You need to do thorough testing for vacuum leaks if it still idles so high. These are a tiny engine and only need a small amount of air when idling, so it doesn't take much of a leak to raise the idle.

How many inches Hg does it pull when idling and have you got your IGN timing as correct as you can with these crazy RPM's?