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IMRC Autopsy Findings

New 1/4/05

First, thanks to Scott and Dan for their timely advice, and to everyone who contributed to the wealth of knowledge on the web site.

After removing my IMRC motor and securing the secondaries in the open position, I have several observations and a couple questions to share.

IMRC motor removal was, as expected, a piece of cake; literally a 5-10 minute task. The infamous cable clip was intact and popped off easily when prodded, after which most of the cable came sliding right out of the housing revealing a frayed break. A fancy yellow zip tie is now holding my secondaries open.

Next I cleared P1518 with my trusty Harborfreight code reader and took a quick test drive which confirmed that my missing ponies are now back in the barn, and Mr. MIL is nowhere to be found. As Dan predicted, low end pull seems unchanged even with secondaries open below 3400. Forced downshifts seem to produce a more instantaneous surge onto the powerband than before IMRC failed. This could be due to elimination of the 1-2 sec lag while the IMRC chugs its way along the cable; it could also be nothing more than a failing memory module in my butt dyno.

Disassembly of the IMRC motor was straightforward, with the following caveats: 1. Flat blade screwdriver was not effective at prying the cover off until LFH (BFH's little bro) got involved. 2. Only three of the five screws atop the printed circuit board need to be removed. The two on either side of the connector plug can stay in place. 3. The white plastic allen screw must be removed before the PCB can be removed 4. The epoxy sealing the connector to the IMRC case requires gentle hammer taps to break loose.

At this point I could clearly see that the cable had broken about 3/4" from its anchor on the IMRC pulley. (See pic #1) The pulley itself seems to be undamaged. If I can get a cable made, I don't see why this IMRC wouldn't work like new.

Note: The cable seems to have frayed and broken precisely at the point where it would chafe against the flange of the housing when in the closed position. I'm wondering if this has more to do with the cable failure than gummy butterflies (mmmmm....gummy butterflies!). I worked the butterfly lever by hand several times and felt nothing more than normal spring tension. The watch spring is so strong that I think you'd need a hell of a lot of carbon to produce any meaningful additional strain on the cable.

This brings me to my questions:

1. This cable is smaller in diameter than a typical bicycle cable. Where might I find a suitable replacement?

2. If I elect to punt on the repair and just replace the whole unit, where's the best place to get one?

3. Why replace it at all? I am seriously questioning the value of the "two stage intake" on this motor. The concept was sound on the V6 SHO, where intake velocity on the separate primaries was enhanced by the length of the runners, but on the dumbed-down V8 intake the butterflies seem superfluous.

Last, some observations of IMRC design:

Several months ago there was a thread discussing the possibility of wiring a manual IMRC control. One of the unsolved problems IIRC was how to trick the PCM into thinking the IMRC was in the "correct" position when the driver had manually commanded otherwise, so as to avoid throwing a code. Pics 2 & 3 clearly show how the IMRC signals butterfly position to the PCM. It's nothing more than switch which gets bumped open by a cam on the pulley hub when the IMRC reaches the fully closed position. What's more, the IMRC does not generate any signal of its own to the PCM. Pins 4 & 5 simply complete a circuit back to the PCM, through the aforementioned switch.

Ergo, the PCM can be fooled by a jumper between these pins (or their external leads, naturally). In practice, it would be a little more complicated because your "jumper" would have to know what the PCM is commanding and either open or close the circuit accordingly, but there's no stressing over millivolt modulation or signal processing.

Final bit of useless information: The IMRC motor is geared to the pulley at a reduction ratio of 19.68:1. This explains why it takes an eternity for the secondaries to open. I imagine this gearing is required get enough torque out of the tiny motor to overcome the butterfly spindle spring tension.

Thanks again, and good night.

Mike Testa '97 VW, 138k Welded, zip-tie intake mod

This echo's my findings last spring. I honestly felt the car was at least as strong from a stop, if not stronger, with the secondaries open all the time vs connected to the IMRC. In talking to Nimz, and based on my feelings about it, I was seriously thinking about removing the butterfly's all together. They seem to have little to no value for increasing torque below 3400 rpm's, and would provide a decent amount of restriction above 3400.

If we make it to the spring Lapeer outing, I'll see about getting some runs in with the IMRC intact and disabled (open) and see what, if any, difference it makes.

Dave Garber

I would trust chassis dyno before I would value Lapeer results.


My Red 97 has had its Secondary Plates (Surgically Removed) hanging in a Baggy on a Pegboard wall for a Year now. No IMRC Motor plugged in Either. It HAS thrown a 1518 code once or twice in the last year but can go for months without doing so. Performance wise I only notice a slight deviation in the Idle every now and then - I would guess the IAC is struggling a bit with the lower velocity air changes - almost a little "Lope" but not like a coil going out. Kind of sounds cool with the Side Exhaust. Low end? Nothing different. Gas Mileage? Never Looked - When the Little orange pointer aims at the "E" I resolve to stop for however many gallons of Fuel will fit after one or two clicks.

IMRC = Meh . . . Take it or Leave it. The little Map light seems to be the biggest difference to me.

Regards Eric Lehmann

The first time the IMRC cable snapped on me, I zip-tied the butterflies open, and I didn't get all my HP back. Apparently, once everything settled into place, my zip-tie job wasn't taut enough and the butterflies were stuck in about 3/4 open position. After tightening things up a bit, a TCCA member PMed me to say he had come across a junkyard Duratec IMRC motor that he had snatched up earlier and stored away on a shelf. I was able to score the part for shipping cost. Two of the three mounting holes on the housing are the same as the SHO unit, and the cable and operating parts of the unit are identical. I had to use longer screws/bolts to hold the IMRC motor down, since the Duratec unit has the mounting holes cast into the body, whereas our SHO units use a thin steel mounting plate instead. As luck would have it, the two top appearance cover bolts are interchangeable w/ the IMRC mounting bolts for this purpose! Once I had it all back together, the IMRC worked as it should, and I was very happy to have everything back to "stock", even though there was no noticeable change in performance or mileage from my zip-tie job.

After about a week, the cable clip snapped (understandable as they are fragile and this clip had been removed and reinstalled at least once.) I bought the $12 clip from NAPA and the throttle cable stop necessary to attach it. Everything was fine for about 8 months, at which point the cable stop must've worked loose or something, b/c I lost the IMRC action again and got the P1518 code. After trying to retrieve the cable clip, which still sat on it's post, I ended up in a desperate search to find it when it went flying despite my best efforts with the needle-nose pliers. It's somewhere in the mess of intake tracts, where it will stay since I no longer care to deal with this IMRC business. The secondaries are held open with a piece of chrome plated steel wire (about twice the thickness of a paper clip) which should theoretically never let go. I have no intentions of ever returning the IMRC to the stock configuration. After hundreds of miles w/o it, then with it back again, and now hundreds of miles w/o it yet again, I have absolutely no inclination to have the stock setup back. I don't get a single MPG or any noticeable difference in power.

Paul Nimz has done 1/4 mile passes both with and without the IMRC and he has confirmed that the IMRC does not change the 1/4 mile times.

Dan Carman
 '97 PG

I've been w/out a working IMRC for quite some time and have driven both with open all the time and closed all the time. Medium-High load acceleration below 3400 suffers enough to be noticed w/the 'flies open. Between ~3,000-3,500 at WOT it's actually stronger with the 'flies open compared to the stock operation. On a cold morning with a working IMRC there was a noticeable "bump" in power when they opened, but now it's more of a nicer much smoother power transition into the mid-range through 3,400.


'97ES Orlando/UCF




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