Reading this topic has everyone jumping around, so I'm attaching some historic technical information for the typical C4 owner. Individuals need to understand when dealing with C4 engine issues, there are 2 operations "Open Loop" and Closed Loop"

C4 Engine Information!

 All EFI systems rely on sensors to report engine conditions to the ECM so it can make intelligent decisions based on the engine’s performance. Each of these sensors reports on a particular engine condition. It is then up to the ECM to gather all this data and condense it to deliver the proper fuel and spark to the engine. The cool thing about this is that current automotive computers can make these decisions thousands of times per second! Try that with your hand calculator.
For example, let’s take a look at a typical late-model TPI- or LT1-style fuel injection system. We’ll deal with only the sensors that have direct input for fuel injection, omitting the emissions-oriented sensors for now. To start with, all EFI systems need an ignition trigger. This is fairly simple since it’s virtually the same as the hookup for a normal analog tachometer.

TPS: The throttle position sensor (TPS) hooks directly to the throttle blades to tell the ECM via a voltage signal not only how far the throttle is open or closed but also the rate at which the throttle opening is changing.

MAP: The manifold absolute pressure (MAP) sensor reads the intake manifold pressure (or vacuum), converting this pressure reading into a voltage signal that the ECM can use. In speed-density systems, this is a critical sensor. If the MAP sensor fails, the system usually reverts to a limp-home mode.

MAF: Certain TPI, LT1, and LS1 Chevy production engines use a mass airflow (MAF) sensor to measure the amount of air the engine is using. This sensor is located upstream of the throttle body and uses what is called a hot-wire sensor. A very small wire strung across the airstream is electrically heated to a certain temperature. As air passes over the wire, it cools. The amount of current required to maintain the wire’s temperature then represents the amount of mass airflow.

O2: Oxygen sensors (O2) are often employed in EFI systems to report the amount of free oxygen in the exhaust. This is a convenient way to measure rich or lean air/fuel ratio conditions in an engine at part-throttle. EFI systems employing an O2 sensor are generally referred to as operating in closed loop. If the engine does not use the sensor, the system is said to be operating in open loop. Closed loop refers to the fact that the O2 sensor readings allow the computer to measure the result of combustion. Low oxygen readings indicate a rich condition, and higher oxygen content in the exhaust is a result of lean air/fuel conditions. Even MAF-equipped engines operate in closed loop, since the O2 sensor’s feedback allows for very tight control over part-throttle air/fuel ratios.

Unfortunately, these factory-type O2 sensors only operate accurately within a very narrow range of the air/fuel ratio scale. They are most accurate when reading ratios hovering around 14.7:1, which is the “ideal” ratio for emissions where hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) are at their combined lowest levels.

Coolant: This is a fairly simple but important sensor. For the engine to run properly at low temperatures, it’s critical that the ECM be aware of the coolant temperature. Low-temperature input, for example, will trigger fuel enrichment outputs from the ECM to act like a choke for cold-start and warmup situations.

MAT: Inlet air temperature affects the air/fuel ratio, so a manifold air temperature (MAT) sensor is needed to help the computer compensate for changes in air density based on temperature. All EFI systems offer compensation tables for inlet air-temperature changes.

Knock Sensor: All late-model GM engines are also equipped with a knock sensor that identifies engine knock or ping. When the knock sensor informs the ECM that it senses knock, the ECM can retard timing to prevent engine damage. Several aftermarket EFI systems also employ a similar knock sensor system.

Cam Position Sensor: Sequential fuel injection systems require a sensor to tell the ECM where cylinder No. 1 is in relation to top dead center in order to trigger the fuel injectors to deliver fuel at the proper time. This sensor is usually some type of magnetic pickup located either on the camshaft drive or on the damper (similar to a racing-style crank-trigger sensor

 People need to understand there are two different engine management functions:

The engine starts out as Open Loop, and then if all conditions are met, it will switch to Closed Loop!

 OPEN Loop is when it is fueled direct from the fixed tables in the tune

 Closed Loop is when the ECM has control of the fueling and adjusts it to engine requirements

That usually happens once the engine is up to operating temperature(more than 160 degrees) and it is getting a signal from the O2 sensor. The closing of the loop causes the ECM to go into Enrich/Enlean mode that constantly changes the fuel injector pulses to lengthen or shorten to satisfy the O2 sensor.

 

When you have engine related issues, you need to identify if your running Open Loop or Closed Loop before trying to diagnose any engine related problems!

 The next question is, are there any stored error codes???

Open Loop Operation Basics:

o        Before anything else, check to make sure that you don't have any vacuum leaks and that your throttle position sensor and idle air control sensor are working and properly adjusted. Check your ignition timing to ensure that it's within factory spec and to ensure that you have a constant spark. These failures can easily cause a rough idle and loss of performance, and they're common enough to warrant checking before anything else.

Fuel Problems

o        A clogged fuel filter won't necessarily cause a serious drop in pressure, but it will cause a reduction in fuel volume. Ultimately, it is this reduction in volume that causes the engine to drop power, and the rapid fluctuations in pressure that can result from a clogged filter will cause a loss of idle quality. Of course, you may also have the opposite problem; your engine could be getting too much fuel if one or more of the injectors are hanging open and constantly leaking fuel into the engine. This latter will result in a constant or intermittent miss and a raw fuel odor in the exhaust.

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Limp Home Mode

o        Fuel injection systems typically operated in what is called "closed loop" mode, meaning that the engine's sensors form a loop of information regarding air going into the engine and the quality of air and fuel coming out. If any of the engine's sensors fail, the computer will go into "open loop" mode, a very conservative state of tune wherein the computer resorts to its best guess on how the engine should run. The result is a noticeable loss of power and possibly idle quality.

EGR Stuck Open

o        Your Corvette uses an exhaust gas recirculation system to feed used exhaust gases into the intake to reduce combustion chamber temperatures and thus nitrogen oxide emissions. The EGR valve regulates how much exhaust gas goes back into the engine, and carbon buildup in the unit can gum it up and stick it open over time. This will ultimately result in a loss of performance, bad fuel economy and a rough idle.

Spark Failure

o        A malfunctioning electrical system or intermittent spark will also cause the above-described symptoms. A loose or frayed ground strap, burned or split plug wires, bad ignition coil or failing alternator can result in reduced ignition power. "Spark scatter" happens when ignition timing randomly jumps up or down a number of degrees, and it's usually the result of a worn and stretched timing chain. A small-block Chevy's distributor gets its motivation directly from the oil pump, so spark scatter can also be the result of a worn-out oil pump or distributor drive gear.

So, you may want to ensure the engine is running OK, and with-in spec before the engine switches to closed loop!

ChiTownAL

Not all  L98's are equal. The following differences between a 90 and an 85 will all affect emissions / tuning, and therefore idle quality.

1985 utilizes a one year only, slow baud rate ECM. While certainly adequate, it's processor speed is substantially slower than the "165" ECM used in the 86-up. ( I can't recall the actual baud rates, so I won't guess at the differences. But it's like 4X faster for the 165).  The P/N changes for 90 because of the under hood location of the ECM, but the internals and programing are the same as the 165 ECM.  Faster up-date and processing rates are more stable. This is a major difference between 85's and all other L98's.

 Aluminum vs Iron heads. And chamber design. Timing can be more advanced for Aluminum heads, because chamber temperatures are lower than iron heads at idle. The heat is carried away faster. Advanced timing improves idle quality  and off-idle response.

90 has pre-cats that light-off a 3-way catylist. 85 has a two-way catylist, and no pre-cats.  System can enter closed loop sooner after cold start, and idle richer in general as main cat will stay effective at idle because of the pre-cats. Closed loop is more stable (usually) and richer idle is smoother.

And another big one in favor of the 90: The 90 and 91 L98 are speed-density systems. 90-91 have a MAP sensor that measures intake manifold vacuum. The 85-89 have a Mass Airflow Sensor. The MAF system is not as stable at idle as the MAP system. This too is a big difference. The early MAF systems (on all GM's that have MAF) don't idle very smoothly.

There's some reasons why "all L98's can't idle smoothly".  Even though the engine RPO is common to both 85 and 90, there are some significant differences in the hardware.












|UPDATED|7/31/2013 11:23:42 PM (AZT)|/UPDATED|

Wow Laurie - I feel like we're in the coolest school ever!  This page is going to be one I'll refer to for future reference.  Thank you all of your very useful info!  I'm even happier that I too have a '90!  My idle isn't particularly smooth as glass, but compared to my 2006 Impala SS‘s newer V8 it’s very similar feel.

Wow Dave!  I knew about the MAF vs MAP but I didn't know the rest, thanks I've been educated.  If I have a problem the car is coming to you!!! :)

|UPDATED|8/1/2013 1:07:33 PM (AZT)|/UPDATED|

Lots of great information here!  There are so many changes between 85-91.  Dave, thank you for pointing out these differences.  With no codes and no other operating issues going on, it would seem that the issue is simply that there is just an inherently less-than-smooth idle to the earlier L98 engines.  Even so, the more I drive it, the more I become attached to it.  

I have just the coolant temp sensor and maybe the ignition coil to replace in my search for the Holy Grail of a smooth idle. Everything else is set right to the manual so it is what it is. I won't be too disappointed if it's not perfect since it's the nature of this animal. Spanking the throttle gets an instant result so no complaints.