Having a hard time getting my car to idle correctly

1987 Supra with a 7MGTE on ECUMaster Black, self built wiring harness, ford style 2 wire PWM IACV

Hi all, I'm having a hell of a hard time trying to get my car to idle correctly. I've been learning all about tuning as my goal is to tune the car myself but I didn't think I would run into a road block here at one of the first steps.

Everything I can tell is pointing at too much air going into the engine. Its idling extremely lean, bouncing off fuel cut, when I increase fueling in the VE table the idle just goes up and up. Well I have done boost leak test after boost leak test on the entire intake system, from turbo inlet to intake manifold and there are absolutely no leaks. I thought this pointed to the only possible culprit being my IACV letting too much air in. No matter what setting I changed in the software for idle its still idling too high. So I blocked off my IACV completely with a rubber cap. Its still idling too high.

My throttle body is adjusted to be completely closed.... there should be no possible way its getting too much air and yet... it is? I thought maybe it was an issue of not enough fuel rather than too much air but as I said, I can adjust fueling up and it just increases the idle, which would tell me the fueling system is working. Any help is appreciated, thanks.

What is your ignition timing at idle? You can probably target like 5-10 degrees to lower the idle speed.

Have you verified your timing with a timing light? If the ECU timing doesn't equal the actual timing, you can actually be more advanced than you think leading to a faster idle. To see if it's timing related, just keep retarding the timing... is there a timing value that results in a more stable idle? This value could be a clue if your trigger offset (or whatever ECUMaster calls it) isn't correct.

Idling is actually a hard thing to do -- you have to balance the forces creating torque (airflow and ignition timing) with the load. It's a real balancing act to get it smooth. While staring out, and slightly richer target sometimes promotes a smoother idle.

Is this a new engine build, or just a new ECU on an intake system that ran / idled well previously?

If your throttle body is completely closed, what happens if you put your hand over the throttle and completely seal it. Does the engine stall? Perhaps your throttle body is worn out, has leaky shafts or a bent plate or similar.

I really appreciate the response. So I was on the ECUMaster Standalone before, but I was on the stock intake manifold, stock IACV built into the manifold and I had a intercooler that had its intake and output on the same side for the stock intake manifold.

I was trying to make the stock throttle body and intake manifold work with the setup but I was having issues with the stock stepper style IACV working with the standalone and ended up just completely disabling it which was less than convenient, I also had throttle issues (extremely touchy throttle) the way the stock intake manifold was set up. I wanted to go to a forward facing intake manifold anyways and i figured it was a good time to.

I upgraded to a forward facing intake manifold that I had custom fabricated to fit my engine as well as a 90MM throttle body. I also upgraded the intercooler so it would work with the FFIM and just so it would be bigger in case I ever wanted to upgrade the turbo. The stock IACV was in the intake manifold and wasnt working right anyways so I bought a remote mount ford PWM style IACV ( https://www.driftmotion.com/Driftmotion-Universal-Remote-Mount-ISC-p/dm3328.htm )

Base timing has been verified with a timing light. No timing drift was found even when brought to higher rpms it was steady. I was idling with a ignition timing table set to 10.00 across the entire table, should I retard the timing even more from there and see?

The idle does get a bit smoother when going to a richer fuel value in the VE table, however like I said when going to a richer value the RPM's increase as well and its already idling at around 1500 RPM.

I haven't tried choking the engine out with my hand, however the shaft in the throttle body was previously leaking but this issue was fixed when I was checking for boost leaks. One of the O-rings on the shaft wasn't seated properly. Throttle body is new otherwise though.

You should absolutely try lowering your ignition timing. Many cars idle at 0 deg advance. Try it and see what happens. Can your ECU do idle control using ignition timing as well as the IACV?

Your problem could be that the 90mm throttle body is just too big for the engine -- even it's small amount of leaking is plenty of air for 1500 RPM. I know some Honda throttle bodies sometimes have some additional material in there to help the throttle blades seal completely. When you look at one it looks like dirt is clinging to the walls where the blade makes contact. If you use solvent or abrasion to remove it, then the cars don't idle well.

Perhaps you can find what that stuff is, and treat your large throttle body with that to help seal it. OK, I've looked at bit at this thread:

https://www.evolutionm.net/forums/evo-engine-turbo-drivetrain/438430-ported-throttle-body-plate-sealant-2.html

and it looks like the stuff is Dow Corning Molykote D-321-R, here is a link to where you can buy it:

https://www.ellsworth.com/products/by-manufacturer/dupont/lubricant-products/anti-friction-coating/dupont-molykote-d-321-r-dry-film-lubricant-black-400-ml-aerosol-can/

I believe the technique is to spray a tiny amount on the throttle plate to wall junction (with throttle completely closed), allow it to dry/cure, then break the seal by opening the throttle body. Now when closed it forms a tighter seal with less air leaks.

Good Luck!

Using a sheet of stiff cardboard would be an excellent way to block off the throttle body. If the engine doesn't stall you can cintinue to chase the dirty air source. It may be a split hose somewhere or, something that's often overlooked, a faulty brake booster/servo' that is allowing air through it.

If you use something like an engine start, or brake cleaner, that is a flamable spray and spray it in small amounts around the intake components, any vacuum leak that is drawing in air will draw in the fluid and the rpm will rise/spike.

So after a bit more testing today I’m pretty sure it’s the throttle body. I blocked off the throttle body with the car running and it stalled out. I also noticed when putting a bit of pressure on the throttle plate to push it closed, the idle RPMs went down a decent amount. I’m not sure if this is the way the throttle was designed or if the plate itself is bent but I’m going to give the sealant that was linked a try. A bit expensive but cheaper than a new throttle body.

I seriously appreciate all the help and I’ll give an update on whether it solves my issue or not.

So it sounds like you might have a linkage problem. If you disconnect the throttle linkage completely, will the throttle plate return to a more closed position? if not, then I agree it's a throttle body problem (or a spring that isn't sufficient to close the throttle when the engine is running)

If the linkage is cable operated, can you increase the length of the cable allowing the throttle to close more? If solid linkage are there rod-ends or something that can be adjusted to allow the throttle to close more.

So I did a little more testing before ordering that spray just to be sure. I’m pretty certain what I’m suffering from is a cheap throttle body.

I took the throttle plate out and flipped it around and the idle increased by 200ish RPMs, then I took it out and flipped it around again and turned it upside down and the idle decreased by 400ish RPMs. Long story short I got the idle down to 1000RPMs just by what way I had the throttle plate in.

The linkage is just the cable from the pedal directly to the wheel on the side of the throttle body and I already tried adjusting the stop as much as I could. I’ll still update just in case I’m wrong but hopefully you solved the problem with that spray.

So it seems my issue was exactly what was said, too much air was leaking by the throttle. The molykote and modifying the idle ignition error table definitely helped and got the idle down to 900ish once it’s warmed up which I’m happy with. Was shooting for 800 but 900 is close enough for me. Thank you everyone!

Thanks for following up. This thread could be useful to someone in the future with a high-idle problem.