OK. I am baffled and admit up front I have no clue to this. Engine is 1994 LT1 in a 36 Chevy street rod. About 60k mi. It is controlled through the original PCM which has been modified to get rid of VATS, EGR, Speed Control and been tweaked a bit for added horsepower. ALDL is hooked up but I am not getting any response from it on a code reader. So I can't provide trouble codes.

The engine and 4L60e are out of a wrecked 94. It sat for about five years while I was building. When I was ready to fire up it with the modified pcm hooked up it went according to the book. The car only ran briefly as I was still finishing up several things. When it went to paint they were able to run it in and out of the shop with no problems on idle.

It has since come home and is now about finished but it has taken on an ugly habit of starting just fine but the idle never drops back. It tries to after a few seconds of high idle at start up. When it tries to idle back there is a little rough idle and goes back to high idle and then to about 2000 rpm where it stays despite heating up to 180. In fact it eventually goes up to almost 3000rpm.

Since I can't get trouble codes from my reader this super fast idle issue is driving me nuts. When I was checking wires to the IAC and TPS I noticed when I tugged slightly the idle tried to slow some but went back up in a few seconds. I found a frayed wire next to the TPS connector and replaced the tps connector with one from a spare harness on hand. No help.

Everything appears to be hooked up OK to the ALDL except there is a wire in the EBCT slot I can't account for. Not sure why it is there and have not traced it out yet. Hard to imagine it would stop the pcm from sending trouble codes though.

My next step will be to remove and clean the IAC. But I am not keen on doing that unless there is a good diagnostic-based reason for it. So far without the ALDL sending out readings I have no diagnostic base.

Any help/insights etc. very much appreciated. Either on why ALDL isn't putting out or why it idles fast then even faster when warm.

 

Is the tan wire connected to pin "M" (lower row, right corner) of the ALDL connector? With an OBD-I reader, that's the only data point required. Ground is pin "A" (upper row, right corner). See Shoebox's wiring diagram for the 94:

http://shbox.com/1/DLC_wiring.jpg

Note the the EBCM and SIR signals also connect to pin "M".

There are no diagnostics for the IAC motor. If you have a scanner, you can read the IAC "counts", which represents the position the PCM is directing the IAC to take, but there is no feedback on the actual IAC position. By watching how the counts respond to the excessive RPM, you may be able to determine whether the IAC motor is working. It almost sounds like you have yours wired up backwards, and its moving in the opposite dirrection than it is supposed to. Perhaps the "frayed wire" was one for the IAC???? Again, from Shoebox:

http://shbox.com/1/iac2.jpg

 

The DLC is wired just the way shoebox shoes for 94s.

Re. the IAC and TPS. I might not have made the point clear enough but when the engine was installed and first fired up a year ago it started at about1200 rpm and within ten to fifteen seconds was down in the 500-700 rpm range. So it was working OK then. The engine was started maybe a dozen times for various purposes, mostly to test the alternator hookup which revealed a faulty alt. It then sat for almost a year after painting and during upholstery and resolving a bunch of typical "almost finished" issues.

So it started and idled down to the proper rpm originally. Now it doesn't and the wiring hasn't been changed. That is what is so puzzling plus, it clearly tries to lower its rpm about ten seconds after starting but something is taking over and boosting the rpm.

Re. IAC Unless I have my connectors wrong the triangular 3-pin connector is for the TPS and the four-in-a-row connector at the front of the TB is the IAC control. It was definitely the 3-wire connector that had the frayed white wire. As I mentioned, the connector has been replaced with no change in the idling behavior.

I can sure understand how you might think I had the IAC wired backwards given the increasing rpm as the engine warms up and I will double check but I am thinking the fact it behaved correctly initially meant it is wired right and some other gremlin is causing the odd behavior.

The priority of things is to get a signal to the DLC so that is what I will go back to later this morning. Thanks very much for your input and the references to shoebox.
Jerry

 

There is no "white" wire that connects to the TPS in a stock wiring harness. Yes, its a triangular pattern, but the three wires are black (ground), gray (+5V) and dark blue (signal). I realize you may have a custom harness, so I have no way to know if the colors have been altered.

If we reject the theory that its a wiring problem, we have to think of ways air flow would increase as the engine warms up. You indicate "speed control" has been programmed out of the PCM, but if you mean "cruise control", that is not handled by the PCM. There is a separate black box with a second throttle cable for the cruise, and the only interface is the VSS signal from the PCM (pin B8). But you obviously wouldn't have installed the cruise box if you didn't want it.

Does the engine rotate or move significantly in the motor mounts? Enough to put tension on the throttle cable?

Again, grasping at straws, the idle speed is elevated with low coolant temp. Is it possible you have a faulty coolant temp sensor? Max idle per the stock tables is not 3,000RPM even below 0*F.

 

Injuneer, Thanks for helping me. Just gotta keep plugging away at it until I solve it. I will be working this afternoon (PDT) on trying to get a signal into the DLC (called an ALDL in my factory 94 Camaro/Firebird manual)

Engine mounts are new and don't allow enough flex to bind the accelerator cable. I have checked the cable repeatedly to make sure it is not the cause.

I am using the factory temp sensor on the front of the water pump to send coolant temp data to the pcm. I suppose it could be bad but the fan is responding to 195 and 215 settings for low and high speed fan setting so it would seem like the sender is OK. I have a single spal fan and spal controller and they are wired according to spal's instructions for using OEM temp sender.

Re. the other temp sender in the driver side head: The car has VDO instruments so I'm using a VDO sender with the correct resistance for the temp ga. in the instrument panel. I don't think that sender has any connection to the pcm.

My attention is focused on the fact the engine tries to go to low rpm shortly after a cold start. That is the correct behavior but it never gets there. As the rpm drops slightly something takes over and moves the rpm back up. I'm thinking it is the IAC as I see no movement of the throttle body blades. But I am at a loss to know what drives the IAC to act backwards.

My attention was also drawn to the fact when I jiggled the bundle of wires containing the wires for IAC and TBS the engine tried to go to a lower rpm but only for a moment. I couldn't hold the wires steady enough to maintain the downward rpm and back up it went. That suggested a bad connection/connector in one or possibly both. An eyeball exam only showed the light colored wire frayed. You are correct that the harness has been reworked and original wires may have been replaced in some cases. It is also possible what I thought was dirty white was light grey.

Someone mentioned a bad 02 sensor might be a cause. I know the 02 monitors rich/lean fuel burning. But would it cause the pcm to try to control too rich by leaning it out which could cause higher rpm? Any way to check the 02 sensor without the code reader?
Jerry

 

The PCM controls the A/F ratio by altering the short term and long term fuel corrections, which in turn alter the injector pulse width.

There is a case where the PCM increases the IAC counts, and that is when the vehicle is moving. It raises the IAC counts and elevates the "idle" speed when the VSS tells the PCM the car is moving (but not to 3,000rpm). The idea is to get enough air flow through the IAC passage that the engine will not stall if the throttle blades close suddenly. This function is controlled by a table in the PCM. Is it possible you have a problem with the VSS input to the PCM? How are you driving your speedometer?

 

Car is not driveable yet. Just sitting in my shop trying to get it roadworthy.

Speedometer is VDO electronic and is driven by VSS on 4L60e transmission. Don't think VSS should be sending out any signal.

 

FIXED! Cause of wandering too-fast idle on start up turned out to be a bad IAC. Put in a nearly new one from an Ebay ad and the car starts and idles like it should. Must have been a rogue IAC acting on its own.

Can I pat myself on the back re. my prediction earlier that it would be the IAC?

Haven't solved the mystery of why no data is forthcoming from the DLC but suspect it was wired wrong when the harness was redone by an expert. For starters I pulled the PCM out where I could get to it and went looking to make sure pins and wires were in the right places. Got to the blue pin set and no Blue 20. Someone said it wasn't needed to get TCs from the pcm but I cobbed the white/blk wire and pin from my junk harness and put it in. Stopped there for the night. Will jumper it in the morning to the DLC and see if that does it. Next wire is the ground which the manual shows as a chassis ground instead of a pcm ground. I will jumper it to a chassis to test for ground. Blue 30 the tan wire that carries serial data (?) is hooked up on the pcm side but I haven't tracked it to the DLC yet. Time to do that tomorrow.

Thanks Injuneer. I appreciated your help.
Jerry

 

Glad to see you were able to get it fixed..can we see some pictures?