SkipI

I have a question regarding the idle process for an 89 corvette. Here is what my car is doing... When I start the car cold (sit overnight) the idle speed is 1000-1200 rpm. My drive to work is pretty much a 15 minute drive at highway speeds that involves only two quick stops. When I get to work the car is still idling at 1000-1200 rpm and at 185°F. If I stay stopped for about 10 seconds after the engine is warmed up, the idle will drop to 600-700 rpm. So, if halfway to work I stop for 10 seconds, the idle drops to 600-700 RPM and that setpoint stays there until the engine gets cold again... all subsequent stops will idle at the 600-700 rpm setting. I did go ahead and vacuum test all the vacuum lines and I made sure the throttle plate is not sticking. I would tend to believe that it is the ECM doing this because if it was a sticking throttle plate or vacuum leak, it would not go away after I stopped for 10 seconds and STAY gone. I am beginning to think that this is how the car is supposed to be. Comments???? :crazy:

fixvet

How about cleaning your IAC? If that's dirty and sticking alittle it will effect your idle speeds. I cleaned mine and the throttle body and my '88 returned to running very smooth.
Good luck

SkipI

Ooops. I should have also said that I have cleaned the throttle body and cleaned and then replaced the IAC valve. This is sort of a funny story on this... I pulled the IAC out to clean/inspect it and read here that you could turn the key on to see if the IAC moved. Well, I had the diagnostic terminals shorted (which sends a full out signal to the IAC) so when I turned on the key, the pintle fully extended and then shot across the garage. :lol:

RonRed89

I am not a mechanic, however I'm having a similar problem on my 89. I would check out your TCC Soleniod as well as Vehicle Speed sensor. I believe that the reason mine does this is because of the incorrect VSS in the car (did a gear swap, haven't corrected the speedo yet). Since the VSS sends data to the computer which is then used to lock/unlock your TC, you might want to check/test both of those sensors.

Just my $0.02.

Ron

89vette

This seems to be a common problem on 89 cars for some reason. Mine sometimes does the same exact thing. It will idle high for no reason. I plan on replacing the IAC valve over the winter to see if that cures the problem. Try running your car with the AC on. Mine does not idle high with the AC on.

dlmeyers

Head for Barnes and Noble, Watson's "Modifying/Tuning Chevrolet Fuel Injection", around $15. Several sections deal with poor idle and possible causes. Superb detailed trouble shooting procedures. Gets you good with multimeter. Not sure what the specs are for your model year as far as base idle speed, and adjusted idle speed for accessory operation, ie Air Conditioning and power steering sensor input. Looks like somethings telling the puter to add more fuel.

dlmeyers 90 coupe zf6 3 speed shocks

86VetteBill

Boy, is this a familiar story with no apparent answer. :smash: :smash:
http://forums.corvetteforum.com/zerothread?id=398788

Not a stuck / worn TB shaft. It's definitely electronic. I can start the car after work, where I hop onto the highway right away and drive in traffic that doesn't come to a complete stop for the few seconds that are needed to let the idle come down from 1200-1300 to the normal 700. I'll continue to drive the 25 miles to my kids' house and it's won't idle normal until I come to a complete stop for a few seconds. Once it has come down to normal, it won't hang high again, until the next cold start - I think - pretty sure of this last part.

It's definitely an electronic thing. :cuss

I've also thought that it's the VSS, but I haven't checked it out yet. Nor do I know what I'd test on the VSS - yet - or where it is.

89vette

It might be a faulty temp sensor. The motor might think its colder than it is. Just a huch though. I'd have to do some research to find out for sure. As for the VSS, if you had no signal you would set a code and your car would want to stall at every light. I know because this happened to me a few years ago. I would remove the TB and thoroughly clean it. Then remove the plug over the idle set screw and perform a minimum idle air set. Let me know how it works out.

SkipI

I have recently cleaned the throttle body and set the idle speed in accordance with directions I found in another thread. So, if you start your vette cold and drive at highway speeds 10-15 miles, your idle speed instantly goes to the low setpoint? I am beginning to wonder if the problem I am observing is normal.

PeteL

It's not normal at all. Mine idles fine.

There is a C4 tech tip by Lars with very good instructions on minimum idle setup. I would add that the only way to clean the TB properly is to remove it, and remove at least the bottom cover.

My idle started to get a little flakey. When I took it apart to clean it, I found too much slop in the shaft to TB. Replaced the TB, no problems.

Pete

grumpyvette

start sequence
L-98 Engine Start Sequence

When you start an L-98 engine Corvette, a series of events take place that causes the engine to run. Knowing the sequence will help you troubleshoot no start conditions.

Fuel Rail Pressurization:

When you first turn the key to the “on” position, the fuel pump will run for 2 seconds pressurizing the fuel rails. There is a Shraeder valve on the passenger side fuel rail near the rear of the engine and if you measure the pressure there after the pump runs, you should see between 40-42 pounds of pressure. The reading will go to 38-40 pounds nominal once the engine is running.

Initial Crank Action:

If you then rotate the key to the start position (assuming the anti-theft system has not disabled the starter), the engine will rotate.

Once the oil pressure has reached 4 PSI, the oil pressure switch will close allowing the fuel pump to run. (Note that you should have a black oil pressure switch/sender. It is mounted behind the distributor on the driver’s side and if it is not black, it is suspect due to a run of bad units that stayed in the GM parts pipeline for some time).

The distributor will send a string of pulses to the ECM (Engine Control Module) in response to the engine being rotated by the starter. These pulses continue as long as the engine turns (both starting and running) and if they are not present, the engine will not run.

ECM Reaction:

If the ECM sees oil pressure greater than 4 PSI and the reference pulses from the distributor, it will energize the injector drivers which will begin pulsing the injectors on for 4 ms (milliseconds) periods. (In the L98, all injectors on one side of the engine fire at the same time followed by all injectors on the other side firing at the same time. On the LT-1, the injectors are fired individually at the appropriate time).

The ECM will also pull in the fuel pump relay in effect paralleling it electrically with the oil pressure switch. (If the fuel pump relay fails, you can still normally get the car to start and run unless you can’t make at least 4 PSI oil pressure. This is a “limp home mode” feature put in place to allow for a fuel pump relay failure).

The ECM also monitors the TPS (Throttle Position Sensor mounted on the throttle body assembly) and wants to see .54 volts at this time. If it sees appreciably more than 0.54 volts, it will assume the engine is flooded and the driver has pressed the accelerator to the floor to clear the flooded condition and restrict the fuel flow as a result. (.54 volts during start and at idle from the TPS is very important to both starting and run performance.)

Assuming the ignition module is good (meaning there is a spark of sufficient intensity to ignite the fuel), the engine will “catch”.

Engine "Catches":

When the engine catches, the MAF (Mass Air Flow sensor mounted just ahead of the throttle body) sends a signal to the ECM advising that air is flowing and also just how much air is being pulled through to the intake manifold. The ECM takes note of the amount of air being consumed and adjusts the injector pulse width to around 2.2 ms nominally so as to attain a proper air/fuel mixture to insure combustion. (This is how the 1985 through 1989 L-98 works. For information on the 1990 and 1991 L-98 variant, see the Note below).

The engine should show an initial idle speed of around 900-1100 RPM and then slowly diminish to 600-700 RPM unless the air conditioner is on in which case it will run at around 800 RPM.

If this does not happen, the Idle Air Mixture valve (located on the throttle body) may be misadjusted. Alternatively, there may be a leak in the intake manifold or another vacuum leak may be present. Listen for hissing sounds---there should be none.

ECM Mode:

The engine will now be in Open Loop mode meaning that the ECM is controlling the air/fuel mixture by referencing values stored in memory.

Once the Oxygen sensor (mounted on the exhaust pipe) reaches operating temperature of several hundred degrees, the Manifold Air Temperature (MAT) sensor shows an intake air temperature of more than 140 degrees and the Engine Coolant Temperature (ECT) has reached 160 degrees, the computer will switch to closed loop mode meaning the Oxygen sensor’s output is examined along with the MAT and ECT outputs and the ECM adjusts the injector pulse widths (more “on time” or less “on time”) to constantly strive for a 14.7:1 air/fuel mixture which is the best mixture to hold down pollution.

Note that prolonged idling can force the computer back into open loop mode.

Note: In 1990, the MAF was eliminated from the engine in favor of a speed/density system. This system uses a sensor called the MAP sensor which measures the Manifold Absolute Pressure (hence the name MAP) and compares it with the atmospheric pressure outside the intake manifold. This information, coupled with the Manifold Air Temperature, Engine Coolant Temperature and Engine RPM is used by the ECM to determine the amount of air entering the cylinders. It is a different way of reaching the desired 14.7:1 air-fuel mixture ratio but functionally is like the MAF system in that the ECM uses the feedback to control the "on time" for the injectors.

Corvette used this approach in the 1990 and 1991 L-98 engines and in the 1992 and 1993 LT-1 engines. With the 1994 model C4, they went back to the MAF system. Note that MAF based systems are far more accurate since they measure air flow directly whereas the MAP system infers air flow indirectly. A multitude of things can throw the calculation off and Corvette returned to the MAF system beginning with the 1994 C4 (with a MAP backup). From a troubleshooting standpoint, the MAP operation comes into the sequence the same place that the MAF does.

Summary:

If you have a no start condition or if the L-98 starts and then dies, check the above items in sequence to see if all the events are occurring as required.

A Scan Tool makes this job much easier and is a highly recommended troubleshooting aid for these sorts of problems

http://www.diy-efi.org/diy_efi/oem/gm/tunetip.html#idle

BUY THIS BOOK
Corvette Fuel Injection & Electronic Engine Control: 1982 through 2001
by Charles O. Probst

89vette

No mine is a littel different. When the vehicle is moving the idle is higher. Try this by coasting with the clutch in or in neutral. You will see under normal conditions, your idle will be higher than normal stopped idle speed. On mine sometimes when I shop at a stop sign, the car is idling at 1200 or so. If I come to a complete stop, it will go to 800 which is where I have it set. Lately it has been staying at 1200 even when its stopped.

After reading this thread, I had some time today and it was a sunny day so my son and I went and bought a new IAC valve. Turns out the old one was not tight and the threads were cracked. So far so good. It is idling like it is supposed to. I know you said that you replaced the IAC valve and did a minimum idle air set. If you have access to Diacom or other good diagnostic software, you could see if anything jumps out at you. I not sure what to tell you to try next.

86VetteBill

Hey Grumpy - VERY nice write-up :yesnod: :cool: :cheers:

I really need a scanner to see what the 'counts' are.

grumpyvette

I did not write that up,:nono: its taken from a vette trouble shooting site! but its good info! :D