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I've got a Impala 1969 with swapped LT1 over here in Sweden. The engine was broken when I bought it and I had it rebuilt and stroked to 383 with an aftermarket cam (https://www.lunatipower.com/voodoo-h...4-270-278.html). Some parts from 1993 and some from 94. PCM is from 1994.
Having some trouble with the idle rpm, checked for vacuumleak. Couldnt find anything I have about 17-18 Hg/inch. There is a BBK 58 TB with a IAC housing from what I think is a 1993 Corvette. Measured the old IAC valve and had the right resistance according to http://shbox.com/1/iac2.jpg Tried out the old IAC valve by making a manual IAC valve with a bolt and a rubber grommet thats fitted in the IAC housing and the old Valve is connected but outside of the housing.
In EEhack the IAC count goes down but the valve doesnt move at all.
Therefore I ordered a new valve that arrived today. Fitted and resetted with out luck. Still high idle.
(I wanted the original 17112193 but it was over a month delivery time)
Now I tested with the manual IAC Screw. Start the car and set the manual IAC-screw to 1000 rpm idle the IAC count goes to 0 on EEhack. Then the new IAC is opened max (shortest pintle length, furthest away from seat in the TB-housing) And when I close the screw to get 600 rpm idle the IAC count goes to 160 on EEhack. Then the new IAC is max "closed"(longest pintle lenght, closer to the seat in the TB-housing)
How I understand it should work: Lets say PCM wants 800 rpm idle in park
If idle RPM is higher than 800, IAC count goes down toward 0, Valve trying to reduce air by closing toward seat and reduce rpm
If idle RPM is lower than 800, IAC count goes up toward 160, Valve trying to increase air by opening more from the seat and increase rpm
Do I get it right? Does the new valve operate in reverse of what it actually should?
I checked the wiring to the IAC. I think when they switched connector from the 1994 to the 1993 square one, they might have mixed up the order of the letters.
The order right now in the connector: A= BLU/WHT B= BLU/BLK C=GRN/WHT D=GRN/BLK
A=GRN/BLK ->C5 "B" Low B=GRN/WHT->C6 "B" High C=BLU/BLK->C2 "A" Low D=BLU/WHT->C1 "A" High
Is it that simple they have reversed it and therefore it operate in the wrong direction? Is it that simple that I can just reverse the connection and it will work in the right direction without harming PCM?
Or will I have to buy a new IAC housing and a proper 1994 IAC valve to solve this?
My review of the factory service manuals for 93 and 94 confirms the pinouts you quote at the end of your post. Your analysis of which direction the pintle is supposed to move is correct. I can't answer with certainty, but the fact they have the colors of the wires reversed for "A" and "B" makes it appear the wires just need to be switched.
Yes, they changed the connector, and the method of attaching the IAC valve from 1993 to 1994. 1993 - screw-in, square connector. 1994-1997 bolt-in/flat connector. And they also changed the PCM (actually from ECM to PCM) design significantly, so the pinouts on the two computers could be reversed for a reason.
Worth a try rewiring before buying a new bottom plate.
A=BLU/WHT->A4 “A” HIGH
B=BLU/BLK->A10 “A” LOW
C=GRN/WHT->A11 “B” HIGH
D=GRN/BLK->A5 “B” LOW
Since this is a DC circuit LOW (black stripe wire) is ground, and HIGH (white stripe wire) is +??volts. Polarity would likely affect the direction of movement.
My little experience of dc voltage is the same as your conclusion!
Wanted to try it but were too afraid of messing up the PCM before I checked with the experts!
I will of course double check with the pins on the PCM connectors to be sure this is the case.
So as long as I dont mess up the ground and positive cables it should be cool without harming the PCM?
I also have a question regarding the pintle length, its possible to adjust the length by pushing the pintle spring backwards and gentle screw it cw/ccw to increase or decrease the working area of the pintle.
The manual that came with the new IAC valve stated that the pintle should not be longer than 28 mm from the bottom the IAC valves own housing to the tip (See attached photo) But it seems that this is to short for our TB cover housing(It will not reach the seat when count is 0)
Should the pintle be able to close completely(not to far so the valve will get damaged) when the count is 0 and just take air from the bypass hole?
The reason im asking is that I have done the drilling mod (So far 4mm) on our TB to get a better idle and trims.
Last edited by Flashmus; Jun 28, 2022 at 07:11 PM.
Not sure about the length. Is that specification in the fully extended position (160 counts) or in the fully retracted (0 counts) position?
And I don't know if the 93 internal geometry is the same as the 94-97 geometry.
In any case, the general guideline is not to pull or push, a USED valve pintle because it can cause damage. Shoebox indicates this is not the case with a new valve:
This is what was stated when installing it first time, I guess the reason was it would not touch the seat before the threads were all the way in so the valve didnt get damaged.
What count position it was out of the box i cant tell. The instruction also had 3 pics of different TBs that i think was for a TBI engines.
When you say push/pull do you mean screw it CW/CCW to adjust it?
There is s little gap on the backside of the pintle tip were a little pin from the spring will fit to lock the pintle from unscrewing/adjusting itself
I thought about measuring it fully extended (count 0) outside the housing and then measure the internal lenght in the IAC cover housing to be sure and adjust it gentle to be able to almost close fully.
But im afraid I will get the wrong operating range and the wrong IAC count for the drill mod.
The IAC valve was originally connected as it should be on a 1995 LT1. This was not the same as on a 1993 LT1 as previously mentioned in the thread.
I connected it so that respectively coils high and low matched with each other in both diagrams. (See attached photos)
I now have the correct idle with a IAC count of 80 with a 4mm(10/64) bypass hole.
IAC Table 1992
Regarding the pintle length the only important stuff I found out were that the pintle should not be longer than the housing to risk "crushing" the wormgear when installed.
After a IAC reset it uses the seat in the housing to "home" the pintle and and then go back to the proper length and work distance.
If this is the same with the later LT1 IAC valve I cannot confirm.
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