Just a quick update for anyone who may be following along my quest. Tonight I replaced the alternator with a new Stock Delco CS-144. Charging system is back in action. Unfortunately, it had no effect on my high idle situation. I didn't think it would, but I had a sliver of hope. It was wet out, so I didn't drag the scanning rig out of the house tonight. Was enough right now to know it is charging.

New radiator will be next. I have it in hand, just got to wait for a dry day that I'm off work so I can get under the car. It seems it has rained here for two weeks. Been wet out every time I've worked on it. Then, I'll get back to probing and troubleshooting the wiring for the PCM, TPS, ECT, and IAC.

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Since so many of the sensors are affected, start by checking the 4 PCM wires that connect to 2 grounds, and those grounds. This is 95 diagram but 94 should be the same: Or check the same diagrams in the factory service manual. The details of the ground locations are shown in the manual as well.

http://shbox.com/1/1995_pcm1.jpg

And the two 12 volt power supplies to the PCM’s 4 pins:

http://shbox.com/1/1995_pcm3.jpg

Factory service manual free download, courtesy of GaryDoug:

http://shbox.com/1/1995_pcm1.jpg

 

Thank you Injuneer- will do that in next few days and report back.

 

To go a step farther perhaps, use your meter to check the 5vdc reference out of the pcm. It is typically a grey wire and is present at some of the engine sensors, like TPS and MAP. It should be exactly 5.0 volts

 

Will do Gary- thanks!

 

Another small update- got my new radiator in.

While letting it run and bleeding air from the cooling system I may have discovered a vacuum leak at one of my injector O ring seals. I had sprayed cleaner around the manifold etc before looking for vacuum leaks but today I had a spray can of generic spray lubricant and it shoots a good thin stream. After my idle changed just a bit on that injector base I got the flashlight and my glasses and I think I can see the O ring pinched up in the hole. (Recall, I had them out to clean the injectors when fuel system was clogged and it would not crank).

So, pretty sure that is contributing some to the high idle. BUT, nowhere near enough to be the whole problem. My IAC is not going to near zero as (I think?) it should to combat a vacuum leak, and I do have that issue with my TPS and voltages fluctuating around still.

I will continue to accomplish the testing prescribed in the posts above by Injuneer and GaryDoug and report back.

 

One thing I'm noting here is the under hood wiring is not in good shape on my car. It's a way-high mileage car and everything is just cooked/brittle under the hood. Combine that with sitting up for 10+ years in the elements and you get electrical issues like I got.

All this rain and me working on the car in it have made me realize that the hood/cowl area leaks water onto the engine pretty badly when it rains. WTH was chevy thinking when they designed it so that could happen? Is there a seal that fails under all that plastic around the wiper area? I never noticed it leaking there back in the day.

I noted a bit of water under the hood years ago while it was sat up and began covering the intake/plenum area with plastic to keep it off the top of the engine and all the crevices around the intake bolts as I have had water seep into an engine in the past like that. But the PCM and all those plugs are right there as well and have been in that wet environment.

I still have not pulled the PCM yet to get to those plugs/connections. I am going to go try and do that. Stay tuned.

 

GM anticipated the problem, and addressed it correctly.

There should be a thick piece of weatherstrip on top of the cowl that the bottom of the hood closes against. If that’s in place and in good condition, you should never see a leak in that area. I tried running without it at the track to allow heat out from under the hood, and when it rained water could enter the engine compartment. I put the original weatherstrip back on, and even with the 27 year old car, parked outside for the last 1.5 years there are no leaks.

Before ripping things apart, do what Gary said and check the reference voltage on the gray wire to black wire in several of the sensors (TPS, MAP, A/C pressure) You can download a free copy of the 1994 factory service manual (courtesy of Gary):

https://www.mediafire.com/?40mfgeoe4ctti

..... or reference Shoebox for the 1995 PCM wiring, which is identical to 94, except for the fan relays (early 1994 oy) and the 4L60E trans harness.

4th Gen LT1 F-body Tech Articles

 

Hi Fred, the water isn't leaking past the thick 'door seal' type seal on the upper edge of the cowl piece where the hood closes down. It is intact. That plastic cowl piece around the base of the wiper arms seems to be made up of 2-3 molded plastic pieces and is held on/together by some of those plastic 'body fastener' things.

Maybe there is a drain hole in the 'cowl trough' that is clogged with junk or something. Or a seal UNDER the cowl piece that has degraded. When raining, water drips/pours from under the bottom edge of the plastic cowl piece right in the middle where a seam seems to be.

Now I have had a disaster happen to me in the past from a similar situation of water sitting on an engine. As a kid, I had bought a dog of a car to fix up- an 82' Z-28 with a crossfire 305 that had lobes wiped off the cam. I saved up and built a really stout old school 383 stroker with good internals, fully balanced, good heads, cammed, with a dual plane intake and a big ol' holley on it and dropped it down in there. Hooker sup comp headers and a true dual exhaust that a local wizard spent two days fabbing, backed up by a TH350 trans and a 12 bolt out back. I had to cut out all that crossfire stuff with the little 'cold air flaps' that opened up when you stomped on it from under the hood to get enough hood clearance for my induction/breather even with a low profile air cleaner and the air horn milled off the carb. That left two little 'slots' in the hood right over the engine. Nobody had ever told me before that an engine couldn't get wet. It ran like a scalded dog for months until I got busy in school and happened to park it for about three weeks. I went out to drive it and kicked it over and bent two rods and scored the cylinder on my stroker build that was already pushing cyl wall thickness to the extreme. Basically that was all she wrote. I'd spent all I had on it at 19 years old- and had to just part it out and quit as I was headed to college. Anyways, I learned to pay attention if an engine was getting wet.

​​I searched and found a couple posts on forums where others had a similar issue. There is a seal under the cowl piece, but it is an open cell foam-type seal that can and will deteriorate. There are quite a few posts where owners have had enough water leak on top of the engine in these cars to hydro-lock their engines from just a single hard downpour of rain. It seems at least a couple solved it with liberal applications of black RTV under the cowl piece. I guess I need to tear into that at some point and see what is stopped up or what the issue is. I have blasted it out with a water hose to no avail. I do not see any major corrosion/rust on anything in that area and the firewall does not leak inside the car. There was a bunch of leaves/junk in the cowl area when I started trying to revive the car. It has been sitting in the elements. Wipers didn't work when I began messing with it. They do work now that I have the charging system up to par. I went out to see if I could pull that cowl piece off real quick- nothing doing. The wipers are stuck on the shafts so hard I can't budge them. Guess I'll just keep bagging the engine and pcm until I can get a puller to get the wipers off and deal with it.

 

Injuneer, GaryDoug, I checked that 5V ref voltage this evening. It is fluctuating from 4.85V to 5.02V on my Fluke meter. Moving around pretty good. I also checked and loosened/re-tightened several of the harness/engine grounds while doing the radiator job. I have not made it to the PCM ground back by the starter yet though.

 

That says quite a bit re: the issues you're having. In contrast, the wiring on mine is still very pliable, w/ no visible cracks/breaks. Combine this w/ what you say about water getting in, and it's like a perfect storm for the electronics. You may end up having to replace much of it.

Might want to also pick up the Haynes manual. I have the FSM Gary put up, and while it's the definitive text, the Haynes is far easier to use. It covers everything from a ground-up rebuild. I paid ~$20 for mine and use it most of the time.

 

I discovered in Scan9495 that I could force the IAC to 'reset'. When I do this, it does respond and knocks the idle down. So this confirms, at least to me, that the IAC is at least capable of controlling the idle. This would seem to mostly rule out huge vacuum leaks. (I have not doubt I have a few small ones) I need to discern what it is that is making the computer not want to command it down. We have supposed is due to the erratic voltages we are seeing on the various sensors not allowing the computer to establish control. So, I will continue hunting wiring/grounding/harness issues.

Attached is a short log where I hit the 'reset IAC' function and it knocks the IAC count down to 45 and the engine idles down considerably. It is still not getting down to the commanded 800 RPM, but it is much closer.

Injuneer, if you could take a look at this log when you get a chance I would sure appreciate it.

 

The earlier log was a startup at 110-degF, with a target idle speed of 900 RPM. The PCM apparently set the 110 IAC counts to maintain that RPM. Never changed.

The new log was a startup at 200+ degF but the target idle speed is 1000 RPM, with the IAC at 67, and I can't figure why it would set a higher target RPM with a fully warmed up engine.

The IAC counts do drop a bit, but nothing significant happens until line 242, when the scan time jumps by almost a full second. Before this is was running at about 0.12 seconds between lines. When that happens, everything changes. Did the engine stumble, of possibly shut off for a second? The target idle speed drops from 1000 to 800, the actual RPM drops to 800 and continues to drop. The IAC drops to 39 and starts to open up to get the RPM back to the 800 RPM target. But there is bad data on line 242. The target RPM shows 3188,... that's the number you see when the engine isn't running. IAC counts indicates 16. MAF flow drops by 50%, but the injector pulse width changes very little. MAP suddenly increases from low/mid 30's to 48kPa, and then increases above 50kPA. Have never seen anything like that before.

Seems like the PCM hiccupped. Or... is that when you did the IAC reset, using Scan9495?

Bottom line, the voltage on every resistance based sensor is still jumping all over the place, and the system voltage ranges from 11.7-14.0 volts. IAC counts stay at 43 because to the PCM, the engine isn't idling - once the throttle moves above 0%, the PCM cannot control the RPM. And the throttle is constantly jumping around between 0-5%, and very little of that time is a 0%. Excludes the part where you apparently opened the throttle to 31%, pushing the RPM to 4,400 RPM. Note tht when you did that, the PCM responded correctly by opening up the IAC counts as the RPM increased, and lowering them when the RPM decreased. It just isn't dropping below 43 because of the throttle showing a non-0% so often.

I can't judge whether the system volts jumping around is due to an actual fluctuation in the voltage at the alternator/battery, or a problem of the PCM having faulty reference voltage regulation. After you replaced the alternator, did you measure the voltage at the battery and at the alternator? I'm in preview mode and can only go back and look at some of the earliest posts. In any case, might need to pull Gary back in here.

 

Injuneer, that portion that it shows the target idle at 1000 RPM was when I was using the idle testing commands in the Scan9495 actuators screen where you can command it to different idle speeds to check function. I forgot to mention I was messing around with that.

I think the PCM did hiccup, I saw a momentary red blink on the green 'Connection' tab in Scan 9495. But I had been commanding different idles and then reset the IAC.

Through much of that test the idle was low enough to at least be acceptable. I could easily drive the car no problem how it was idling last night.

The readings and voltages jumping around is just insane. I don't think a break in a wire, or even an intermittent connection would cycle so much or behave like that.

Jeez, I really hope it isn't the PCM taking a dump. My LT1 Edit is locked to that VIN # so if I have to replace the PCM I lose my ability to tune. I don't know if carputing would work with me on it, or if they are still even reachable.

Do you, or Gary have any idea what the voltage regulation scheme/components look like in these PCM's? I'm not adverse to some board-level discrete solid state power supply testing/repair if that is an option. Now if it is chock full of SMD chips, then all bets are off.

I did not measure output of the alternator at it's terminal. I just saw my charging came up on the dash and in the scan and called it good. It's a brand new Delco unit. I will check it as soon as I can.

Could the resolution signals from the opti cause any havoc like this and yet it still run? It is an opti I swapped new into it back in about 2000. It was a new unit from gmparts direct at the time and is the vented 95' version I put in when I did the HotCam swap. At this point it has sat on the car for 21 years half of it out in the rain in the yard. I am gobsmacked that I haven't had to change it so far right now.

 

I have no idea what is inside the PCM with regard to voltage regulation. Obviously, the input voltage from the battery, via the ignition switch is going to vary, but not to the extreme in your data logs. The system (input voltage as seen by the PCM) is undergoing significant changes 6 to 8 times per second. You are fortunate to have a very high scan rate compared to many logs I get to look at. It highlights the dynamics of the situation that might be covered up at a lower scan rate.

Normally, it seems the system voltage starts high, as the alternator tries to replace the battery energy consumed by the starter, then tapers off to the 13.0-13.5 range. But some setups never recover, and operate at lower voltages closer to 12 volts. The PCM appears to be able to accommodate these gradual changes in order to maintain a steady 5 volt reference.

I suspect Gary knows exactly what is inside and how it works. He has a complete LT1 engine simulator, complete with PCM, Opti, sensors etc., and he has successfully identified and repaired problems in external modules like the Opti optical cam position sensor, body control module (1996 and up), and more. Odd thing is, when he created Scan9495 he had never owned an LT1. He has since picked up a boat with an LT1 engine, though.