gaedbo

Maybe it is time to try the SCAN 94/95 software. It would give you specifics that could be analyzed, and possibly lead you to the correct solution.

It is Free Software available from this website.
Go to the LT1 Base Engine Tech part of this forum and select the Sticky - Free 94/95 LT1 Scanning Software - PCM/ABS/SRS - For 93 Also (Except SRS)

It sure couldn't hurt, and would be invaluable in the future to diagnose problems.

You will need a cable and a laptop to use it. One of the places you can get a cable from is http://www.aldlcables.com/products/aldlobd2u.asp

BLUEDEMON95

Where do I get the software from the. Site?

gaedbo

The way is in my previous post, but here is a more specific route.

Go to 1967-2002 Specific Technical Discussions section of this website.

Select Computer Diagnostics and Tuning.

Go to the Sticky labeled Scan94/95 : Free OBD1 scan app for 93/94/95 LT1. Read some of the information before you click on the link to download it.

The mediafire link to the website that has it is there.

Injuneer

https://www.camaroz28.com/forums/com...95-lt1-874306/

BLUEDEMON95

alright, i have the software downloaded, im just waiting for the cord to come in the mail, should be here tomorrow, if not tuesday

BLUEDEMON95

Ok, my cord is on route to be delivered in the next few hours, once it gets here I'm going to hook it up to my car, based off of my symptoms, what should I look at specifically?

Injuneer

Run a data log, and upload the .csv file in a post. I'll review it for you.

BLUEDEMON95

here is the scan, for a good portion of the scan i had it in park, the iac is whats allowing the rpm to be so high because a few times during the scan, i hit reset the iac position, and it brought idle down to normal, i also did this when i had it in drive and the car dropped to about 400 rpm and almost stalled until the iac opened back up. I know about all the dtcs except for the fuel injection circuit, the others were caused by me unplugging the sensors one by one last week trying to diagnose

Injuneer

You have 4 DTC's.

Clear the codes. MAF, TPS are the result of unplugging. Did you unplug the ICM at any point?

18 - Key on/Engine off check for 12V on the pink wire at each injector harness connector. Connector off the injector, check resistance across the 2 injector pins. Should be 11 - 16 ohms.

22 - Were you moving the throttle at any time during the data log? Data indicates it was not touched.

42 - This code can be set by a faulty Opti.... no ICM activity during 84 revolutions of the crankshaft while cranking the engine.

The RPM data is totally out of control. For example, the data indicates the RPM jumped from 1900 to 6000 and then back to 1900 in a fraction of a second. But MAF reading did not vary much at all, nor MAP. That tells me that the engine was not reaching 6,000 RPM. And at no time were you touching the accel pedal? Only time I've seen anything like that is a problem with the RPM signal from the Opti. Hate to say this, but the instances where I have seen this problem before mostly involved an MSD Opti. One of them had just been returned to the owner after MSD rebuilt it, and it had an optical sensor problem

Above is preliminary. Let me take some more time to look at this.

BLUEDEMON95

Ok, to answer your question during that scan I didn't press on the throttle, towards the end of the scan I put it in drive but had my foot the brake or else the car starts moving by itself. I reset the dtcs after I ran that scan, none of them came back on today.

Injuneer

Spent some more time on it. Definitely a problem with erratic RPM readings. RPM spikes for a single frame of data, as high as 7,000+ RPM, in a fraction of a second, and immediately drops down to the same point it was at before the spike. That, for some reason, causes the PCM to start opening the IAC valve. I think that is because as RPM increases, the PCM wants to put more air through the IAC to prevent the engine from stalling if the throttle blades suddenly close. The RPM readings are spiking so frequently, it keeps the IAT near 160 max reading most of the time.

It is obvious that the RPM spike is not real, because the engine simply can't change RPM that fast, and there is no response like increased air flow, or increased MAP when the spike occurs. An engine can't be running at 7,000 RPM (beyond the rev limiter) without a major increase in throttle opening, major increase in air flow, and a significant increase in MAP (= decrease in vacuum).

I have seen this before three times, but never as bad, twice with an MSD distributor, and once with an AC Delco. In all cases, replacing the Opti cured it. But your data indicates a much more severe problem than I have seen before. If it's OK with you, I am trying to get MSD to review a copy of the file. There is a guy on LTxTech.com that works for MSD, and in the past he has offered to forward any problem file to MSD. So far he has not responded to me.

Now.... let's make sure the RPM spikes are something being caused by something like a high voltage, pulsing wire too close to the Opti harness. A plug wire, running in parallel with the Opti could cause the problem. And we can't rule out the PCM.

There is another strange thing I have never seen before, in reviewing hundreds of these logs. When the PCM sees the RPM increase, it advances the timing, which is normal. That's right off the spark advance tables. But all of a sudden, when timing is in the 18-35 degree range, there is a single frame of data indicating advance is only 1 degree. About a second later, the PCM responds by retarding the ignition timing (knock retard). It is conceivable that that advance of 35 degrees, due to the false high RPM spike is just too much even at idle, and some knock retard is required. But that odd 1 degree reading may be the result of a PCM problem. Can't be sure, because I have never seen it before.

A case of split BLM"s also showing up at idle. Left (driver) side is adding about 8% extra fuel, and the passenger side is subtracting 4%, although the numbers are drifting up and down over a wide range. I think the RPM spike issue needs to be resolved before working on the fuel issues.

I'll let you know it I am able to send the file to MSD, and whether they have any comments on it.

Injuneer

MSD engineering has your data log. They asked if you could run another log with the IAC disconnected. I think it would be better if you disconnected the IAC when the IAC counts are in the range of 20 - 40 counts. Under that condition it would likely be closer to correct idle speed.

But if you can't get the RPM and IAC counts down that far, just unplug the IAC motor. Apparently MSD thinks that might eliminate the RPM spikes. Be interesting to see if that is correct.

BLUEDEMON95

I didnt get it on a scan, but I unplugged the IAC, and started the car up. It did what it normally does, which is goes up to 1100 rpm, then drops to 900 rpm. about 10 seconds after it dropped, it went back up to 2500 rpm with the same bouncing. I sprayed around the intake manifold just to make sure that it was completely sealed, the engine rpm stayed the same. One thing to note, on the MSD distributor, one of the vacuum ports was on top instead of both being on the bottom, what would happen if i accidentally swapped those two lines?

Chimera96

On the MSD opti the vent nipple on top goes to the vac hose from the intake elbow and bottom nipple goes to the line with the check valves to the vac port on intake manifold.

IDK if they are reversed if that is any issue

Injuneer

I don't think the direction of vent air flow thru the unit will make any difference.

Is there any way to run aunt her log? We have to look through rows of data to try and find single instances of irrational RPM readings. Then try and tie those RPM readings to some other sensor or parameter. Also need to compare the idle RPM with the programmed target RPM.

When the coolant temp is low the program raises the idle RPM, then slowly reduces that as the coolant warms up. It uses the IAC to accomplish that. But with IAC disconnected, the throttle isn't moving, and there are no vacuum leaks, how can the RPM change? Gotta look for clues in the rest of the data. Timing changes, etc.