Problem?

[NVMYRC51]

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,

[mike100]

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.

[NVMYRC51]

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

Craig,

[ghlkal]

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?

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

[ZZZZZR1]

What maintenance has been done to your car?

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

All original?

:cheers:

David

[mike100]

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.

[XfireZ51]

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.

[Pete]

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

[JFFerner]

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

[WARP TEN]

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,


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

[Hib Halverson]

Given the engine's calibration at idle, low speed and "tip in" is correct for the cam profiles you have and that the ignition system is in good condition...

First thing I'd do is make sure the IAC and its seat in the TB are clean and that the IAC is working properly.

Next thing I'd do is inspect the TB. If it's dirt inside, remove and clean it.

If the cal is not right, I'd still look at the IAC and the TB but after that, I'd have someone who knows calibration redo the idle and part throttle cal...someone like Marc

[XfireZ51]

I've had the opportunity to provide a tune for several motors, including mine, with larger cams. I always start tuning from the premise that the ECM is an "aid" to how the motor operates. In the case of setting idle, that means the motor idles as it should without the IAC. In other words, the motor should operate properly without the ECM as a crutch. Setting of Min Air is one of those critical baselines for getting to a proper tune.
In another thread, I outlined how I go about setting Min Air Throttle. In summary, I block off the IAC and adjust the throttle opening to just UNDER my desired idle RPM. That way I invoke a small level of intervention by the IAC in order to reach the desired rpm. By small level of IAC, I mean something around 10-15 counts even for larger cams. At lower rpms, I want to get as much air going through the throttle bore (higher velocity and vacuum) rather than bypass air from the IAC. Makes for a more stable and smoother idle. At higher rpm, you want to take advantage of the IAC opening for additional air. That pretty much matches what I have in the IAC Throttle Follower tables. However, particularly on a cammed motor, I "slow down" the reaction of the IAC to throttle changes. This keeps motor from dipping below the Stall Speed and shutting down.

[Hib Halverson]

I've had the opportunity to provide a tune for several motors, including mine, with larger cams. I always start tuning from the premise that the ECM is an "aid" to how the motor operates. In the case of setting idle, that means the motor idles as it should without the IAC. In other words, the motor should operate properly without the ECM as a crutch. Setting of Min Air is one of those critical baselines for getting to a proper tune.
In another thread, I outlined how I go about setting Min Air Throttle. In summary, I block off the IAC and adjust the throttle opening to just UNDER my desired idle RPM. That way I invoke a small level of intervention by the IAC in order to reach the desired rpm. By small level of IAC, I mean something around 10-15 counts even for larger cams. At lower rpms, I want to get as much air going through the throttle bore (higher velocity and vacuum) rather than bypass air from the IAC. Makes for a more stable and smoother idle. At higher rpm, you want to take advantage of the IAC opening for additional air. That pretty much matches what I have in the IAC Throttle Follower tables. However, particularly on a cammed motor, I "slow down" the reaction of the IAC to throttle changes. This keeps motor from dipping below the Stall Speed and shutting down.

I think your philosophy is faulty.
Without the engine controls, the engine will not run so there is no way to "tune" the engine in a manner that, without them, it would run.

"Minimum air" is not an idle setting, that is, it should not be understood as a throttle opening with which the engine can idle if the engine controls didn't exist.

Up to now, I've not heard of using the minimum air adjustment at a tuning aid.

I've always thought the reason IACs existed is the throttle bores are too large an area to use in metering bypassed air used by the engine at idle. My understudying is that some of the air the engine needed at idle came though the throttle plates' minimum air adjustment but the rest of it, and in some cases the majority of it, came though the IAC.

[mike100]

I think your philosophy is faulty.
Without the engine controls, the engine will not run so there is no way to "tune" the engine in a manner that, without them, it would run.

...

I think he was referring to the idle control scheme and not the fuel injection and ignition functions.
Assuming you could disable the iac control in software or even just removing the component and blocking the air bypass, the engine should be able to maintain a low idle speed on just mechanical means with the min throttle opening set on the throttle stop screw.

[Pete]

I've been tuning GM stuff for 30 years.
What Dom is saying is correct.

If i remember correctly (man going back 30 years) on the old TPI motors you had to disconnect the IAC to set idle speed.

Holy cow when you guys type you write dictionaries my two typing fingers would fall off.:)
I like short and to the point.

Have fun.
Pete

[Pete]

I think he was referring to the idle control scheme and not the fuel injection and ignition functions.
Assuming you could disable the iac control in software or even just removing the component and blocking the air bypass, the engine should be able to maintain a low idle speed on just mechanical means with the min throttle opening set on the throttle stop screw.



BINGO!!!!!!

[Pete]

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,


If your injectors are the original factory units, i would replace them.
I can 99.9% guarantee you they are bad.

Good luck
Pete

[XfireZ51]

I think he was referring to the idle control scheme and not the fuel injection and ignition functions.
Assuming you could disable the iac control in software or even just removing the component and blocking the air bypass, the engine should be able to maintain a low idle speed on just mechanical means with the min throttle opening set on the throttle stop screw.

Thank you mike100. And if you read my other post, you'll see that the IAC passage can be blocked without removing the IAC. No need to disable ECM IAC controls. I tuned a Xfire TBI for 8 years, and it had 2 IACs, one for each TBI.
AND you had to balance the two TBs w each other.

[NVMYRC51]

Ok, quick update and more info. My injectors have been replaced, they are not the stock units. My car has had a chip in it for quite sometime and never had a problem. I have driven this identical set up; chip, catback w/x-pipe, K&N with open airbox top pretty much since I bought the car about 18 months ago. I drove it with zero problems during the Caravan this year to Bowling Green and back without a single stall. Now it has not gotten any worse and only happens coming down from a higher engine speed. From a lower speed or coasting easily it does not shut down and when it does it always starts easy. I am under the impression, as somebody mentioned, that it could be the IAC and/or dirty throttle body. I have only driven my car twice since I last posted this question and they were both short drives. I will look into the IAC suggestions today if possible and report back. I do hope to attend the ZR-1 gathering in May so maybe I can put some faces to names. Thanks guys.

[Paul Workman]

It sounds like you're in good company. My motor is ported and afterward I started encountering the same occasional stall you and others mention. It only happens occasionally, AND when coming down from high speed (HOT) mostly. Sure is annoying when you're coming into a hairpin turn on a mountain run and suddenly the engine shuts off when I shift gears as I enter the turn.:jawdrop:

[XfireZ51]

Ok, quick update and more info. My injectors have been replaced, they are not the stock units. My car has had a chip in it for quite sometime and never had a problem. I have driven this identical set up; chip, catback w/x-pipe, K&N with open airbox top pretty much since I bought the car about 18 months ago. I drove it with zero problems during the Caravan this year to Bowling Green and back without a single stall. Now it has not gotten any worse and only happens coming down from a higher engine speed. From a lower speed or coasting easily it does not shut down and when it does it always starts easy. I am under the impression, as somebody mentioned, that it could be the IAC and/or dirty throttle body. I have only driven my car twice since I last posted this question and they were both short drives. I will look into the IAC suggestions today if possible and report back. I do hope to attend the ZR-1 gathering in May so maybe I can put some faces to names. Thanks guys.

If possible see if you can get a scantool on it and let us know what is the IAC count, RPM and kPa at Closed Loop idle. With key ON, what is TPS v?

[NVMYRC51]

Alright! Upon closer inspection it seems the airbox had come loose from the hose connecting the box to the horn. I hooked up, retightened, and drove it without incident. So I am hoping that is it but I will put some miles on her tomorrow after I double check and possibly clean the IAT and TB assembly. Fingers are crossed.