Hey guys, I am new to thirdgen, but you guys have helped me so much over the years with your specs and data. I have a 1983 Z28 with a 355 roller in place of the 'ole 305. The engine is bored .030 over with pop ups and eagle crank and rods and compression running close to 10.5 with a comp 292H magnum cam with .501 intake and exhaust and 110 degrees lobe separation with 244/244 duration and headers and exhaust, I also have crane 6 to 1 roller rockers.

Now the fun part.....I switched from carb to TBI with a 454 throttlebody, painless wiring, all new sensors and a C.A.T.S. tuner.

Where I am at the moment is that I can't get my beast to idle under 1000 rpms. I have my baseline idle at 600-650, my T.P.S. is at .50 volts and I have already tuned the chip to my low vaccuum by changing my MAP sensor parameters. My problem is that I can get the car to idle at 750-800, but as soon as I drive it and come up to a stoplight, It wont go below 1000-1100. I have a snap on scanner so I can read the data and the only thing I can see is my IAC steps are way too high. When its idling high, the IAC steps are anywhere from 40-65. Every once in awhile when they go down to 15-25, the car is idling correctly. I swapped in a 3 wire oxygen sensor so the car will go into closed loop faster, also. What is strange Is that I have the IAC steps to coolant temp set to 24 steps when it comes up to temp. I have also set the coolant temp to idle speed set for 800 rpms. The only other strange thing I am running into is sometimes it will stumble at idle and drop down to 600 and then back up again(surging). It does it very occasionally and just really whenever it wants to. I have checked my time for vacuum leaks and haven't found any. Timing is set to 0 with the connector disconnected. I have been fighting this thing for weeks and I just can't figure it out. I am open to suggestions, please just fire away!

 

And let me throw this one out too, what should a cam that size idle at? I have been reading so much data lately that I can't hardly rememer which way is up........

 

You didn't mention which ECM you used. Have a look at this thread:

https://www.thirdgen.org/techbb2/sho...hreadid=308613

Lots of good info found here:

https://www.thirdgen.org/forums/diy-...uide-book.html

RBob.

 

On the Cam I would go to cam with 220 to 226 deg durations and LSA of at least 112 degrees to make idle a bit more manageable. From description: 292H Series
This cam is for the serious street/strip type car. An aftermarket torque converter should be used as well as headers, intake and increased compression. The 292H has a radical, rough idle.

 

I am running a 7747 ecm and the beast is a 5 speed, I forgot that part. Now one other question is have is: I dont have a VSS hooked up. From what I am reading, It looks like the IAC uses input from the VSS To let it know how fast its going and thus, idle. My borg warner T5 is cable driven. If this could be a problem, does anyone know of an adapter for this T5 to be able to convert to a electronic signal so I can hook up the VSS? And BTW, that was an awesome writeup about IAC control!

 

The VSS isn't required for idle control. It does help with the IAC throttle follower function. If that '83 had an ECM (CCC or x-fire), there should be an optical buffer box in the dash cluster. Look for a brown wire from the dash harness near the ECM.

If not, TPI Parts has a 2K PPM inline VSS.

RBob.

 

Excellent! This car did have an ecm originally, and I put the 7747 in its physical place(really handy). Even though this vehicle only has a mech speedo, are you saying that because it had an ecm there should still be a provision for a VSS via that brown wire?

 

Okay, I found the brown wire that led to the old ecm from the dash cluster. Now, can I just hook it up to my 7747 via the VSS signal wire in my wiring harness. And if so, the 7747 ecm should read the input from the speedo and I should be able to see the speed on my scanner, correct?

 

You got it, to ECM pin A10.

RBob.

 

You guys are awesome! Thank You so much!

 

Is the 7.4L TBI wired correctly? My understanding is what you are seeing in ALDL is the commanded. Not the actual.

If you do a key on engine off and do a visual does it appear to close then open fully?

If you were to command zero-five steps and do visual would it be fully closed with engine warm idleing?

I once had mine miswired.

 

Hmm, very good question. I purchased the adapter from TPI parts, there is always a chance that the adapter could be wired wrong. I am going to check on that tommorrow. Also, back to this VSS thing. I did find the brown wire and I am going to hook it up to the A10 pin on my 7747 ECM, now two more questions: is this wire brown all the way up to the speedo cluster? The reason I am asking is because the only brown wire leading to the back of my speedo is not OHMing out, does it change color or does it lead to a relay or switch first? Two, does anyone have a pinout for the speedo connectors?

 

The brown wire comes from the buffer box, usually yellow and mounted in the back (towards the front of the car) cluster.

RBob.

 

I looks like the buffer does go through the cluster printed circuit board. Here is a picture of it:

https://www.thirdgen.org/forums/post...44-post37.html

The yellow box is the buffer, and is on the right-hand side when the cluster is installed. The three wires going over to the speedometer head is for the optical pickup.

RBob.

 

Okay, hooked up the VSS brown wire and still no speed reading with my scanner. Is that brown wire brown all the way under the dash from beginning to end or does it change color? The reason I ask is I ohmed out the brown wire and I cant get a reading from the connector on the dash cluster on any of the wires. Think I might have a broken wire somewhere...............

 

I have no idea if this is correct or not, there should be a connector (C207, pin K) that has the VSS signal. It goes to pin 8 of the cluster connector. Pin 9 of the cluster connector is ground for the VSS buffer.

The wire is brown on both sides of C207.

RBob.

 

Finally slowed down enough to be able to play with the car a bit. I checked all the wiring and found that my original speedo IP connector wasn't pinned at all for the vss signal. So I stole the pin from the choke pinout and put it in the vss signal wire pin and now I have a speed reading on my snap on, life is good!
Now, time to try to adjust idle and IAC steps and timing, wish me luck!

 

Ronny, good call on the incorrect wiring; the IAC adapter I bought was wired wrong. Now more questions. The IAC now works according to how many steps the ecm commands. My problem is that now I can't get the IAC to go below 50 steps, which is really strange considering the IAC steps to coolant is set to 1 step(I am using tunercat) and my Target Idle RPM vs Coolant temp is set to 800 rpm. What is even stranger is that with my snap on scanner, each time I enter DTC retrieving mode, the idle goes up and when I exit, the IAC steps go down a little. Each time I enter and exit DTC mode, the steps will go down a little farther, thus the idle goes down. Is there another adjustment in tunercat that I am missing or a step I am missing completely?

 

See post #9 of the IAC writeup linked above. I wrote a correction and a better explanation of how the '7747 idle control works. Has to do with the kick down steps and the steps vs coolant table.

Increased idle speed in DTC mode is normal.

RBob.

 

Okay Rob, hopefully one last question on this matter. In tunercat, I don't have kickdown steps. What do you think could be another name for "kick down". In my ECM constants, I have "IAC park or neutral offsets", could this maybe be it?

 

The kick down steps is at location $5D8. Can use a hex editor to check/change the value there. The IAC P/N offset sounds like the steps that get subtracted out when the shifter is placed into P/N position.

RBob.