gthill13

I have been having idle issues for a while and have a quick simple question. I am running AEM EMS4 with MP62 so realize idle is precarious to begin with. Checked for vac leaks and found nothing obvious. I cleaned IAC, but now wondering if the unit is bad. When I first start the car it takes about 10 seconds to stabilize around 1500 rpm. Within 30 seconds it goes to 3000 rpm and goes back and forth abit but stays high. When I unplug IAC it stays stable at 1500. Is this a bad IAC?

ByteVenom

Where is your throttle body? Where is your IAC getting air? What year car?

The idle stabilizing after 10 seconds is probably your ECU beginning to control things.

If your TB is after your supercharger, then your IAC should be mounted to the dummy TB at the intake manifold. If you're running the TB at the intake manifold, then it sounds like you have unwanted air coming in somewhere. Do you have a bypass valve?

gthill13

It is intercooled, and it does have the standard bypass valve (I think it is a ford part). This is the classic BRP MP62 that probably hasn't been made for 10 plus years now

Assuming there is a way to just pull the IAC and hook it to 12 v to see if it opens, or maybe the voltage should be 5?

Godless Commie

iTrader: (2)

I had an MP62 for 10 years, it was a decade of decadent abuse, much experimenting, some head scratching, road trips and track outings.
There were idle issues for a while in the beginning.

An IAC valve is designed to react and control idle speed by "leaking" some air in a pretty limited volume, i.e, the intake manifold.
When you relocate the TB all the way to the inlet side of the SC, that volume is multiplied by.... a lot.
You therefore exceed the design parameters of the IAC, and the poor thing starts chasing its own tail - never catching up with that much volume.

Long and short, you need the Dual Throttle Body mod if you want to keep the MP62 and your sanity.
The difference is so mind blowing, you will be actually mad at yourself for not having done it sooner.

Please search the DTB mod using my name both here and at m.net and you will land on some pretty helpful threads.

Also, do not exceed 11 - 12 psi, do not ever use a 150 mm crank pulley, and do reduce your redline down to 6K for track use.
These precautions will mean not spending money on a replacement SC. I have been there.

I have also crossed over to the land of the turbo. I currently have two of them under the hood. They get along nicely, and produce great boost from 1050 rpm all the way to redline.

x_25

I did the DTB setup on my JRSC supercharger setup. It helped a ton with idle but I had some odd handoff issues between the two throttle bodies.

What ECU are you running? You can ditch the IAC and do a static idle. With some trickery in the idle areas of the timing and fuel maps you can get it quite stable.

scottns

iTrader: (1)

When I did the dual TB my charger just whined all the time. I messed with cracking the TB opening and just could not get it to quiet down. It was so annoying I didn't enjoy driving it and went back to a single TB. My idle hangs a bit still when I come to a stop but settles down and becomes stable around 850-900. Maybe I'll try the suggestion below in the Spring (getting close to putting the car away for the Winter).

RunninOnEmpty

iTrader: (5)

I don’t think it’s the IAC and you still have a vac leak somewhere. The IAC is meant to attenuate idle in small increments, not really accommodate for 600rpms worth. My experience has been the IAC trying to adjust too radically and causing swings. I think you should be idling at least around 1000 before anything.

Do a smoke test, general scans of connections isn’t enough. One time I had a leak at the throttle body butterfly bearing. I would’ve never found it otherwise.

gthill13

Running AEM EMS4. When I unplugged the IAC it actually idled perfectly.

gthill13

Agreed. I have done a pressure test before but never a smoke test. Can this be done DIY or is it really only a shop task?

gthill13

I found a vape pen smoke test on youtube and will try that

Godless Commie

iTrader: (2)

That's only normal during steady state idling.
That merely tells me you have adjusted your throttle plate hard stop to allow a reasonable amount of air into the IM.

Any change in engine condition (load, decel, temp, etc) is relayed to the IAC to make the necessary corrections, but by the time the IAC increases or restricts airflow, there is a yuge, bigly delay for that correction to take effect because of the enormous, tremendous volume of air in the IC as well as the IC plumbing waiting for its turn to enter the IM.

The DTB mod, when implemented and adjusted correctly, ensures the IAC operates as designed, and the engine always returns to the preset RPM level. No drama.

Godless Commie

iTrader: (2)

The trick to adjusting the pre-SC TB is pretty straightforward actually.

Bring the engine up to temp, and manually open the IM TB. You can secure it to keep it open. Use zip ties, whatever.
The engine will idle faster, but not much.
Then, adjust the SC TB to a level somewhat higher than your regular idle speed. If your actual idle speed is 950 RPM, set it to 1500 or so at steady state idle.
Release the IM TB. Idle will return to normal.
If there is any moaning or whine from the SC, close the SC TB at very small increments. All noise will magically disappear at one point.

There is a crucial step n all this: When the IM TB is fully open, SC TB should be fully open, as well, otherwise you will never achieve full throttle.
There should also be little to no cable slack between the two TBs.

RunninOnEmpty

iTrader: (5)

It can be done by yourself, esp. with a vape pen. With a cigar I'd sometimes get confused where the smoke came from. Then the room gets smoke and I'd have ti wait till the room cleared. Sometimes it's easier to break the sections up. For example, I put a PVC cap on the IC outlet pipe just prior to the IM. That allowed me to test the intake piping only. Then I did the IM second. Troubleshooting is a little easier, you can control pressure and where thr airs going.

RunninOnEmpty

iTrader: (5)

Scott, I just switched over to dual throttle body and got it dialed in. The noise is subjective, but I found it dramatically quieter after a few tweaks. I noticed that it’d have more whine when at idle, and completely quiet coasting to a stop under heavier vacuum. I interpret this as the SC still compressing more air than necessary against the TB on the IM.

Therefore:
1. Try to minimize the pre-SC throttle crack. You want as much air as possible pulled through the bypass without causing too much drag.
2. Ensure your bypass actuator is working properly under vacuum.
3. Ensure the BOV is opening properly under idle.

Point 3 was key for me. Transition was pretty terrible on my first mockup. The moment I put the BOV on the shortest vacuum reference possible - and separated from the other SC-related vacuum lines, the results were a lot better. BOV is tapped into the boost sensor line at the IM throat. Actuator bypass valve line was teed from a separate line on the back of the IM.

Also, if you’re futzing with the actuator bypass valve, many people remove the brass restrictor on the nipple for the feel of instantaneous boost. Or if removing the vacuum hose, sometimes the brass restrictor gets pulled out with it, and gets lost. My experience was that you need to keep it in. It provides just enough delay to work well with the BOV, even on a separate reference line.

Before the adjustments, I got annoying, mid-throttle Power transitions, as if the BOV wasn’t opening fast enough. I’d either get these really loud BOV sounds from a plumb back (That sounded like an open air BOV) indicating lotS of boost was being relieved. In other scenarios, when I’d release throttle and roll back on it, I’d get this wall of jerky power, as if pressure was still building against theTB butterfly, and shot right in as I babied the throttle back open.

gthill13

So I did get it to a shop and they found a few small vac leaks and it is running a little better but still too high. Would a bad boost actuator and bypass valve potentially contribute to this issue as well. Pretty sure that both are the originals making them about 16 years old. Hoping to pull the actuator this weekend to test. Not sure how to test the bypass valve.

scottns

iTrader: (1)

If you have a small hand pump or something you can put pressure on the bypass valve. If it holds your good.