Problem?
 

I have a 16,000 mile, 90 Z. It has a Haibeck chip, B&B Catback, K&N with open airbox lid, and other than that she is stock. Runs like a dream....most of the time. The last two times I have driven her, she has died unexpectedly when letting off the gas and coming to a stop. Starts right back up and it does not happen all the time. Any ideas? Clogged or dirty injectors maybe?

Craig,

Re: Problem?
 

I feel that the leaky throttle body and/or PVC air can be a little too much causing the idle control to go into a non stable oscillation trying to run at x rpm during coast. My car has a leaky TB like so many do and this in itself will not stall it out, but once I got some bad gas and it really acted up until I burned off 90 percent and refilled.

Check for vacuum leaks and if possible, scan the ALDL data stream to see what the IAC (idle air control) count value is.

Re: Problem?
 

Ruled out bad gas but I will check the other things.

Craig,

Re: Problem?
 

Craig, mine does this too sometimes. Since Marc did some of the work and created the chip, I asked him about it. He says he has seen this issue where the car will die on decel coming to a stop. I wonder if it has something to do with the tune. Stock, my idle would remain high when coming to a stop and stay high for a number of seconds before settling down. Now, the LT5 settles down quickly as I think it should. But perhaps there are certain conditions that cause the tune "problems." Anybody else experience this?


Gary

Re: Problem?
 

What maintenance has been done to your car?

Injectors / coil packs / spark plug wires / fuel pumps?

All original?

:cheers:

David

Re: Problem?
 

I found the headers exasperated this most on mine. Not stalling (that was bad gas), but just a little oscillation hunting 200 rpm up and down...might be a gain in the code that could be changed or some pid process...

seems just as easy to either fix the throttle body seal and/or to restrict the pcv line to the plenum some to compensate for the calibrated leak that is now too much. On the pcv line, I had t-eed it to a filter venting to atmosphere and the vacuum line going to the plenum was restricted with a small coupler to the rubber line. pcv vents as much as it needs, idle air more under control.
you can also change the idle target speeds a little higher or lower in the tune.

Re: Problem?
 

The LT-5 is an "unstable" engine in that it reacts quickly to input. The stalling when you let off Throttle is why the factory cal keeps the rpms higher(using IAC) to slow down the rpm drop. If it slows down too quickly it just drops down to the point of shutdown. It gets worse in a modded motor.

Re: Problem?
 

If all is good
This should never ever happen on a stock camed motor.

When you adjust for the hanging rpm issue you have to make other changes.
This is hot rodding 101 if your hot rodding you should know this if not education costs money.

Easy fix
Pete

Re: Problem?
 

Two years ago and about 5k miles, I had similar problems with my '90, but only during the summer months while running the air conditioner in heavy speed traffic. The problem began occurring more frequently, then the engine began missing to the point the dual mass flywheel made sounds like I was about to lose a connecting rod. I guess the ac placed an extra load on the engine causing it to slow to a shut down. I ended up replacing the fuel injectors, coils, ignition wires, plugs, vacuum hoses, etc and haven't had a problem since.

And so it goes....Jim

Re: Problem?
 

Craig, I ran into the same problem on my 95 about a month or two after Marc Haibeck did his 510 package for me. I remember him attaching his computer to the OBD port and making a physical adjustment near the throttle in the front of the engine but did not recall precisely what he did. So I asked him and he responded with the following; hope this helps:

Hi Bob,

The adjustment that we made is the, "idle air flow". Basically the air flow through the throttle body at closed throttle needs to match the table that the calibration chip uses to control the IAC servo motor. Engine's with stock cams can use the OE table values. The OE IAC count at idle with a hot engine is 10 IAC servo steps. On your engine the IAC count was 30 at idle. That's because my larger 63 mm throttle body is tighter and leaks less air than the OE throttle body. There is another concern, that the IAC count could go to zero. Then the idle speed would go too high. I initially left the IAC at 30 because running out of counts is a more common problem than die out if the IAC count is not perfect. After your engine exhibited the die out problem. I adjusted the primary throttle plate stop screw to open the throttle until the IAC count on the scanner was 10.

An engine with big cams might idle at 850 rpm. Because of the additional air flow requirement, the IAC count is typically 50. In that case the table in the calibration chip needs to be adjusted to what the engine wants, that is 50.

When I figured this out ten years ago I probably spent 20 hours analyzing data to understand it. At first one might think that the problem is related to fueling or spark advance. I spent most of the time determining that I was barking up the wrong tree.

Regards, Marc