Very Dangerous Problem Happening....looking For Help Please

[Banshee365]

I really need help guy's. This car is primary transportation and now it's dead out in the road in front of my driveway. It starts up and runs fine for 5 minutes then starts running rough, idling low, then quits and will not restart. You can smell ton's of fuel coming out of the exhaust and it just won't crank. What the heck could be going on. I'm really starting to blame the ECU.

[curiousB]

Maybe it’s not the IACV. That is solely for idling of the engine. How does the car run other than idling? If you start it and use your foot to keep revs up to 600-1000 can you get around OK without other symptoms? Too rich a fuel mix could be ECU, or other Fuel Injection factors. Also if ECU detects a problem it could have forced the system into limp mode which will intentionally run the engine on the rich side.

Do you have a way to check ECU codes?

[Banshee365]

I do not have a way to check ECU codes. Is that the port under the hood or just the one under the dash? The current CEL problem indicates an issue in the coolant temp circuit. I've replaced the temp sensor but that didn't seem to do anything. Not near as much as I've heard people on the LOC that feel like a new car after they replace their sensor. On my diesel truck you can hook up a diagnostic computer to it at the dealer and read what every single parameter is doing. You can actually see the position of any valve or selenoid. Is that possible on the '96 LS? If someone could watch the activity of the IACV and other things maybe it would be easier to pinpoint.

The car runs fine and revs off of idle right away for the first 10-15 minutes. Then it usually starts idling around 200-300 rpm and when you hit the gas the rev it it bogs down another 50 rpm for a second then revs up while some black smoke puffs out the back. I think the car has the original cats and o2 sensors but there are no codes for the sensors at all right now. When coming to a stop while it's working right it the idle stays around 800 while slowing to a stop and it downshifts to 1st and that keeps the rpms up as well. Other times the rpms will just roll on down to 200 then die at a stop. Yesterday it died before I even came to a stop. The car is hard to start afterwards. Unlike the common ECU issues, which seem to fire right back up after it shuts off at a stop. This car is hard to start again if it shuts off itself with the low rpms. It has to sit for a while as if it's flooded. When you finally get it started after letting it sit it's run rough and black smoke poors out for a second. I'm lost as to what it could be. If it's broke i wish it would just stay broke and now run fine then run horrible, then run fine, then horrible.

[Ruud]

could be that you have some cylinders misfiring or not firing at all. Have you checked the leads, coils and spark plugs?. (sorry if this advise is already given)

[91LS400pilot]

I had very rough idle at traffic lights and stop signs, hesitation, pushing the gas pedal and engine was dying. The problem was a short on the driver's side coil wire. Black electrician tape eliminated the problem. If the car is in the carage open the hood and see for any fireworks in the engine.

Just my two cents.

Good luck

[curiousB]

The car runs fine and revs off of idle right away for the first 10-15 minutes. Then it usually starts idling around 200-300 rpm and when you hit the gas the rev it it bogs down another 50 rpm for a second then revs up while some black smoke puffs out the back.

You may be due for a trip to the dealer for a deeper dive. Maybe the IACV has been giving you a head fake and something else is going on. The fact it stutters when you press the gas pedal suggests problems beyond the IACV. A code reader isn't that expensive and if you plan to maintain this car yourself it may be a good investment. Someone recently posted a recommended source and price for one. I think it was only $20-30.

The suggestion to check wires was good. I'm not sure if your model has the dual distributor that some earlier models have. I think there are cases where one bank goes dead due to bad coil, wiring, or distributor yet the car can still limp along. Maybe once you warm up one bank is failing and you have 4 cylinder with gas getting dumped into the 4 other non firing cylinders (eventually in exhaust and burning up there. My car is a little newer and has coil over plug system so no distributor.

[Banshee365]

I drove the car around some tonight and the idle never really went much below 600. I did slightly adjust the TPS as I had adjusted it before due to a high idle which I think was caused by a bad coolant temp sensor. I don't think the car ever thought it warmed up so the idle stayed high. I replaced that and the idle still stayed fine after the TPS adjustment. I thought that may be giving me some low idle issues after I replaced the temp sensor. I adjust it up and after starting it and letting it run for a few minutes, after it's already warmed up, the idle was around 900. It was definetly too high because it would slam into gear. I tweaked it down just a little and now it sits around 650 in gear and maybe just slightly higher out of gear. The car seems to have a different logic while it's in gear that keeps it at 650 it the TPS is remotely close to where it should be. But when you take it out of gear it seems like it's depending more on the TPS. Even when the idle was 900 out of gear, after it would slam into gear it settled at 650. I also noticed the car decellerating much slower than it should when the idle was high. After the adjustment it felt more normal.

As of tonight the car is running okay and the shuttering upon reving out of gear doesn't happen. That only seems to happen when it starts idling low and tries to quit. It's like it richens up and starts running like crap, didn't know if that was the limp mode. I feel I can somewhat rule out the ECU because if it dies it seems to be from too low of an idle and too much fuel. It doesn't crank right back up after a key reset like most ECU related idle problems do.

If something is wrong with the car I wish it would just stay broke and quit playing games with me. If it stayed like it ran this afternoon and tonight it's perfectly fine. But the fact that it could start acting up again worries me. If it quits in traffic like that it's VERY hard to get started and almost acts flooded out and you have to wait a while before it will crank back up. I hate cranking on this car because I don't want the starter contacts to get hot and have to tear it all down for that as well you know?

As far as a code reader. I usually just goto Advanced or AutoZone and get it read for free. As I mentioned before, the current code states an issue with the coolant temp circuit. I want to figure out how to clear that code. The temp sensor is new but the code remains. I'm planning to spend the $10 for a day to download the manuals and maybe it will have some troubleshooting procedures for that particular code to help me out.

What do you think? Thanks for the help!

[curiousB]

Maybe someone reading this can chime in on the TPS sensor. I would be concerned if you've adjusted the linkage such that you are pulling the throttle plate slightly open as a substitute for a faulty IACV. Not sure if that was what you are describing. That's the way carburetors work (now I'm dating myself). There is a screw adjustment to slightly open the plate until you get the desired idle speed.

If your only ECU code is the temp sensor then changing sensor would be proper but it could be wiring too so need to clear the code and see if it reappears to confirm that it is truly fixed.

Given the engine stumbling you’ve seen intermittently I’d keep an open mind to ignition electrical issues as a culprit. Electrical problems can be intermittent by nature and that seems to be a pattern here (as annoying as that sounds). I was going to suggest to check the air mass sensor but I would think a code would be in the ECU if this were faulty. I'd read through some of the other thread on the failing of half of the ignition due to one distrubtor/coil but not the other. Seems to be a common problem and might match symptoms you're having.