I just finished rebuilding the l98 in my 87 and if i slightly press the throttle it will start and run fine but the moment i release the throttle it dies immediately. If anyone has any ideas what may be causing this please let me know. Also I know this sounds stupid but i can not find the ALDL connector the vette is new to me and has been recarpeted and some of the plastic is missing from under the dash. I have looked everywhere fro that connector to scan the thing and i cant find it so if someone could take a picture of its location or simply a good description of where it is, it would be much appreciated

 

The ALDL connector is located above your right leg shin. About 6 inches back from the front of the dash.

 

Sounds like an IAC problem.

The Idle Air Controller motor isn't controlling the idle speed.

It's a little motor with a pintel (tip) that moves in and out to control the idle speed based on the speed burned into the car's computer PROM. It's screwed into the throttle body and has a four wire connector.

The connector could:

Not be correctly indexed with the motor

Not or not completed plugged in.

The motor may need to be reset to give the ECM a new base setting. There's a procedure for this in the Tech Tips setting, but usually the IAC is self adjusting and there's no need to run the procedure. Can't hurt though.

Everytime the engine is started, then shut down, the IAC resets itself automatically, if working properly.

The minimum airflow setting on the throttle body may be too low. Before getting into that, though, I'd check the IAC for proper operation.

If you can't find the IAC resetting procedure, let me know and I'll post it.

Jake

 

I ma curious as to what you put back in the motor,because I have the same problem.I can ease the throttle down and it will return to idle,but if I rap the throttle or just let off the pedal it dies.I hope you find out what the problem is and get it fixed.When you find out what it is could you let me know so I can try that also.


Thanks in advance

 

I agree, this has all the symptoms of an IAC failure. Make sure it's plugged in, at the very least.

 

Other things that contributes to the idle problem are:

Carbon buildup on the IAC pintel

Carbon buildup on the backside of the throttle body (TB) butterflies

Minimum airflow setting too low (many still use the 450 rpm setting when 550 would be better)

TB gasket leaking vacuum

Defective TPS

Intake manifold vacuum leak

Vacuum hose left off or leaking

Bad 02

Just to name a few

Jake

 

I replaced the IAC and still no luck with keeping it running. I cant get it to start without giving it a little throttle and then I have to keep giving it a little throttle to keep it going it will stay running down to about 1000 rpm then if I let off it will die. in the rebuild I did put on an adjustable fuel pressure regulator so i am goign to test it and see where the fuel pressure is at tonight

 

Are you sure you've got all the vac lines on, I'd at least get a propane bottle with torch connection (DON"T LIGHT IT) and start checking for vacum leaks. Just a thought.

 

I have to redeem myself i found the aldl connector it was under the carpet, I felt like an idiot becasue i couldnt find it. bad news is no codes

 

with Rick, check the easy stuff first. If you replaced the IAC, check for vacuum leak...sounds like your engine is fuel starved at idle, which could be an indication that your intake is sucking too much air for the amount of fuel being supplied..

 

I am curious, you replaced the IAC, did you set it according to the service manual?

 

Okay, here's what to do to eliminate some of the possible causes:

First you make sure that there is no carbon buildup on the backside of the throttle body butterfiles and on the IAC pintel and the IAC passage that the IAC screws into.

Fashion a jumper wire with both ends stripped Insert it into the ALDL under the dash in the top two right hand openings (pins A&B).

Next turn the ignition key to "ON" and wait about 30 seconds.

Then go to the engine compartment and WITH THE IGNITION KEY STILL "ON", disconnect the IAC connector from the IAC motor.

Turn the ignition key "Off" and removed the jumper wire from the ALDL.

Now start the engine and keep it running as best you can, even if it means manually working the throttle. Keep it running to build up some heat.

Use a torx head bit to turn the throttle body minimum airflow screw clock-wise. This will open the TB butteflies a little, allow more air to enter the engine and it should then be able to idle on it's on, but the idle speed will probably be too high.

Let the engine run for a few minutes to build up some heat; then gradually turn the minimum airflow setting screw counter-clockwise to closed the butterflies a little which lowers the idle speed. Do this gradually.

When you get to 550 RPMs and the engine can still idle on it's own, leave it there. I know, I know, the manual says 450, but in my experience 550 gives better results.

Now, assuming it will idle at 550, shut down the engine, reconnect the IAC AND BEFORE YOU START IT BACK UP, use a digital volt meter to check the TPS voltage.

The TPS voltage is checked with the ignition key "ON" but the engine "OFF".

You'll need three jumper wires to test the voltage. The wires go between the TPS which is mounted on the throttle body and the connector. Cut off a little of the wire insulation about midway on two of the wires, this is where you'll be connecting the digital voltmeter.

DON'T LET THE EXPOSED WIRES TOUCH EACH OTHER.

On the wires I fashioned, I cut one wire (for the meter connection) near the TPS and cut the other wire (for meter connection) close to the connector. That way they're far apart and can't touch.

One wire goes top to top, one goes middle to middle and the other goes - well you get the picture.

You'll be connecting the digital volt meter to the top and middle wires. Doesn't really matter which meter lead goes to the top and which to the middle. If installed one way the meter reading will have a (-) sign next to it, if you swap the leads from the meter to the jumper wires, the (-) will not be there.

MAKE ALL THESE CONNECTIONS WITH THE IGNITION KEY "OFF" TO PREVENT THE POSSIBILITY OF A VOLTAGE SPIKE.

The reading should be .54 mV plus or minus .075, so anything from .615 (on the high end) to .465 (on the low end) is still within specs. Most seem to get the best results with the setting a little on the high end, like .60.

You can go even higher, but at some point a code'll get set for the voltage setting being too high. I've gone as high a .67

Once the reading is set correct - Oh, forgot, you just loosen the two screws on the TPS and rotate the TPS UP to increase voltage and down to lower it - tighten the screws, checking to make sure the voltage didn't change as you tightened the screws. If it did, just reset it and re-tighten.

Now turn the ignition key "OFF", remove the jumper wires and reconnect the TPS connector.

Fire the engine and drive the car at normal throttle settings at about 35 to 40 MPH to allow the ECM to learn the new settings.

Let me know if this helps. If you still have the problem there are other things we can do to find the cause.

PS: I forgot to include the jumper wire part, but I edited this post to include it. Sorry; gettin' old.

Jake

 

Jake,
Thanks a ton that was a great post, and i got as far as bringing down the idle with the engine hot but i dint reach 550. The lowest it would go was 1100 and then it would start to miss and then cut out. Not good i know. My question is should i continue o and test the voltage at the TPS or do you have another suggestion

 

Yes, check the TPS voltage.

 

Could also be a bad MAF sensor or a bad 02 sensor.

Check the MAF sensor first. Try first disconnecting it. If the engine will then idle, the MAF is bad.

You can visually check the MAF by looking into it at the thin wires inside. One of them could be broken.

Could be a bad MAF power relay too. There are two for the MAF, a Power Relay and a Burn-Off relay. Both are on the firewall behind the battery on the 86, so probably the same location on your year.

If there is no change with the MAF disconnected AND if the wires are not broken, plug it back in, start the engine and tap the MAF underside with the handle of a screwdriver.

If the engine stumbles, the MAF internal electronics are bad.

You can check to see if the engine is going closed loop by inserting a jumper wire into the ALDL terminals. The Service Engine Soon light will flash at different intervals when the system is in Open Loop and in Closed Loop.

I'll have to look up the intervals you'll be looking for to be more specific; perhaps someone that knows off the top of their head can post it and save me the trouble.

Jake

Keep me posted.

 

This would have been a whole lot easier if you had a data logger, a program like Diacom or a ScanTool, etc.

That way you'd be able to see exactly what the sensor readings are and we'd have the answer in short order.

without one, all we can do is one step at a time.

I have a gut feeling we're getting close though. We now just need to know about the MAF, the 02 sensor and whether the ECM is controlling the ignition timing.

Jake

 

I don't remember the specifics, but a year or two ago, one Forum member had a similar problem and it turned out he'd connected the fuel lines wrong at the fuel rail.

I didn't think that was possible, since they're different sizes, but that's what he did.

We'll need to get around to verifying that too if the other things check out.

You did say you aren't getting any SES codes, right?

Jake

 

Right no codes

 

So, let's get an update:

List the things you have checked and/or changed so far. We need to know where we stand.

Jake

 

well unfortunately i am not going to be able to get to it until monday. I have to go out of town. I really wish I was taking the vette. But i did get as far as jumping the ALDL and then adjusting the throttle body to an idle at about 1000 RPM I didnt get to checking out the tps but first thing monday i will. Thanks for all the help