RPM WS6

iTrader: (3)

Ok, first the specs on the car:

’98 Z28 with 12,400 miles, auto trans
Stock motor, stock trans & TC, stock MAF, stock TB

It does have some minor bolt-ons, but this condition existed even with the stock air lid and before the HPP was used to change shift points and fan settings, so neither one of them have anything to do with it.

So here’s the deal; once in a while, the idle will hang slightly, just enough for me to notice that it’s high. I’m talking about ~50-100rpm too high (verified via scanner) plus it'll be slower than normal to drop after throttle input. Seems to only happen after being fully warmed and driven for at least 30+ miles, I think the times it has happened have always been after expressway drives. Other thing seems to be, that once the idle starts to hang a bit it will surge ever so slightly (only in P/N), maybe 40-50rpm or so. I’ll have more about that farther down...

Keep in mind, this condition is random, it does NOT happen all the time.

I have replaced a handful of parts on this car recently just as a matter of maintenance. These new parts include battery, fuel filter, PAPER air filter, belts, plugs, PCV valve, etc. I have also replaced the IAC motor thinking that might have been the issue. No change with any of those items.

Further details,... I’ve done quite a bit of scanning trying to observe what’s happening when the condition occurs. I can’t find anything wrong electronically.

- LTFTs usually average about +6 on both banks and are normally equal or split 2% or less, on rare occasion the split is as much as 4% but usually they are even. There is no change in LTFTs when the issue occurs.

- Front O2 sensors seem to be switching and reading as normal.

- No misfires detected.

- TPS voltage stays at 0.6v at idle and Throttle angle is 0.0-0.4% whether the issue is happening or not.

- Ignition voltage is fine, 13.7v on the scanner once the car is up to temp.

- Timing is where it should be.

- Coolant temp is fine, fans cycling as they should be.

- No DTCs.


Basically, I can’t find anything in the data stream that points to an issue or that shows up as being different when the car is idling properly vs. when it’s having issues other than IAC counts. When the idle is hanging, the IAC counts are 0 in gear. Then I put the car in P/N and after a moment or two the surge starts and the IAC counts bounce between 0 and 12 in pattern with the ~50rpm surge. If I wait long enough, they go back to 0 and the surge stops. At this point, the idle is slow to come down off a rev, and it still settles about 50-100rpm above where it should. NO surging is observed when the car is in gear.

When the car is idling properly, the IAC counts are usually in the 8-12 range in P/N and 12-16 range in gear. Again, stock TB with stock hole, 100% stock MAF.

- I can’t find any evidence of a vacuum leak, sprayed carb cleaner all around the TB, intake, PCV line connections, etc. Air bellow is tight. I can’t find any evidence of an exhaust leak either. Plus, the fuel trims do not indicate any vacuum leak or exhaust leak.

- The car drives/shifts/runs/starts fine other than this little issue. No loss of power or other driveability degradation before/during/after the issue.

Here is the odd part, to fix the issue, all I have to do is turn the car off, then fire it right back up. Then it idles perfect for as long as I drive it. What is being reset that’s fixing this? My first thought was the STFTs, but they always stay right around 0 anyway.

Most recent things I have done to try and fix the issue:

- Cleaned MAF sensor wires with some electronic parts cleaner from Radio Shack, the Q-tip I used was pretty dirty when I was done.

- I pulled the new IAC motor and cleaned out the bore and pintle of IAC, plus I cleaned the edges of the throttle blade and the TB opening where the blade seals shut.

- Performed idle relearn procedure as per GM instructions for ’98 Camaro with LS1 motor.

I drove it another 40+ miles and it was fine after that, but in the past I thought it was fixed as well and still the issue came back.

If it happens to return again, what else can it be and what else can I try to fix it?......Or, am I just being too picky? Do most of these cars have little idle inconsistencies like this from time to time when stock? It’s been so long since I’ve had a stock LS1 that I can’t remember anymore. I had tons of idle issues with my WS6 that I spent hours trying to tune out with Edit, but that car had a cam so I expected it. I thought that a stock LS1 would idle 100% perfect all the time. Is that too much to expect?

Sorry for the extreme length, but I like to be thorough Anything anyone can contribute would be helpful, even if it's just to say that I'm crazy for thinking something is wrong here, and that I need to be less of a perfectionist.

RPM WS6

iTrader: (3)

Anyone?

gameover

your IAC's being around 0, means the IAC cannot close off enough air entering the engine to control the idle. ie. your IAC is at minimum.

When you put the trans in gear the extra load is lifting the IAC up a little but not enough.

You need to close up the throttle somehow, check the throttle is closing fully or excessive throttle blade clearance. The other possibility is a small vac leak. In anycase your problem is a mechanical one i think.

IAC's should be around say 40-50 in PN and a little higher in gear (depending what you idle RPM is set for in gear).

Chris...

RPM WS6

iTrader: (3)

Thanks for the reply.

I wasn’t aware that the IACs should be so high on a stock motor. I knew that something was wrong when they hit 0, but I thought that 10-20 was average on stock motors matching my idle rpm and running temps. So you’re saying they should be higher? My idle rpms are stock. 650 in P/N and 550 in gear. My coolant temps cycle between 207-196*F when sitting at idle, via my scanner.

I will check the throttle blade again as you suggested. I will also re-inspect the throttle cable for proper slack and smooth operation.

As I bought this car used, I cannot be 100% sure that someone didn’t adjust the TB set screw, but I HIGHLY doubt that. I bought it from the original owner and the car was totally stock with 11K miles on it. Even the factory installed air filter was still in place. No signs of anyone ever touching anything in the engine bay other than a couple GM recalls and oil changes. How can I be sure that the set screw is properly adjusted though? I guess I just need to make sure that the blade is fully closed, correct?

Also, I know it is expected for throttle percentage to usually be slightly above 0.0% at idle (I think all LS1s I’ve seen read 0.4-0.8% at idle), but what about my TPS voltage being at 0.6v? Is that too high? I have had no TPS codes, but I know I have seen other LS1s with lower TPS voltage (in the 0.3-0.5v range) at idle. Is 0.6v high enough to cause an issue?

What else can I do to check for a vacuum leak? If that happens to be the case, it must be a VERY small one. What else can I scan to test this possibility? Though the LTFTs are not ideal for WOT power as they are somewhat positive, they do not seem to indicate a problem either.

Thanks again to anyone that can help.

RPM WS6

iTrader: (3)

Other updates...

I just completed a re-evaluation of the throttle blade and cable operation. All seems well, I tested for any gaps between the blade and bore and there are none. I tested the cable for adequate slack and it is fine. I compared the set screw to two other stock LS1 TBs that I have in my “parts bin” (what I fondly call my basement) and it seems to be adjusted fine. Also, after further research it seems that a TPS reading of 0.6v at idle is normal and should also mean that the set screw is adjusted properly. In addition, I re-verified the hole in the TB blade and it is stock size (5/32”).

Other scanner reading that I forgot to mention...

MAP reading stays in the 9-10 (in hg) range at 650rpm in P/N idle. With atmospheric pressure around 29-30, this means the car is making about 20 inches of vacuum, correct? That, combined with an average of ~ +6 LTFTs that are either even or split no more than 2% on both banks at idle on a 70+*F day after an extended period of idling, sounds accurate for a stock motor on a stock tune (other than fan settings) without any vacuum leaks, correct?

Taking into account the MAP and LTFT readings, I think that the IAC readings of ~10 at 650rpm P/N idle @ ~200*F are normal for my car and not representative of a vacuum leak. Guess I could be wrong though????

2001CamaroGuy

iTrader: (6)

what he said..............you have a "leak" of some kind (around the throttle blade, PCV, etc.....).......the mods you have should not cause this problem.....

RPM WS6

iTrader: (3)

Thanks for the reply

It seems like either an external or internal leak is the most likely culprit, but I just can’t find any supporting evidence. MAP and LTFTs do not seem to point to a vacuum leak, and TPS voltage and angle percentage don’t seem to point to the blade hanging open. I tested all around the edges of where the blade meets the TB bore and I can’t even get a thin sheet of paper to slip between the two, so I’d say it’s sealed tight. Also, TPS readings seem to discount the possibility of the blade not fully closing.

Is it possible that the TPS could be bad even though the readings look ok on the scanner?

Also, most of the time the car idles perfect, so if it is some sort of leak it must be very minor or intermittent. And all I have to do to get the idle to correct itself is turn the car off, then back on and it idles perfect. That aspect seems to point to an electronics issue, but all other signs seem to point to mechanical...

Since the IAC counts are the only thing that seem to even capture the issue I guess it comes down to one question... what could cause the IAC counts to surge at idle without any change in TPS readings or fuel trim readings? And what can happen to correct this issue that requires only a restart of the engine? The IAC motor is good, it’s brand new and two of them have done this now.

Can anyone confirm what their IAC counts are on a stock motor, stock TB, with a stock 650rpm idle in P/N (auto trans obviously)?

gameover

have you checked vacuum connections to the brake booster and any vacuum operated acessories such as vents, heater etc. ?

sorry i can't think of anything else.

RPM WS6

iTrader: (3)

Yeah, from what I can tell everything is connected properly at the back of the intake (though hard to really inspect without pulling the intake). All the vacuum acc. work fine, AC and heat blows from their correct vents, etc.

My arm is too big to actually get my hand far enough back there to feel the back of the intake, but I used an inspection mirror and everything *seemed* to be in order. Hard to see though. But wouldn't a leak at the back of the intake, such as a disconnected line, cause a much more noticeable and constant problem than this? And how can a vacuum leak be corrected just by turning the motor on and off?

I've inspected the PCV line/system a couple times now, can't find anything wrong there either.

Thanks for the help so far, if there's anything else you can think of please let me know.

RPM WS6

iTrader: (3)

Another thought, is it possibile that the MAF could be over/under reporting airflow without throwing a DTC? If that was the case, it would make sense that the issue might be "cleared" with a restart of the motor.

If the MAF was sending *slightly* bad info to the PCM (not enought for a DTC), would that cause the IAC counts to bounce and/or be "off", because all other sensors are telling the PCM info that doesn't quite equate to the MAF readings?

gameover

No the IAC control is only linked to desired idle RPM. It has no relation to the MAF.

The issue probably clears with a restart because the IAC resets to the park position and then takes a while to learn back down to 0 after the startup idle phase is over. Since you are not just pegging at 0, but hovering around it the conditions probably comes and goes as fans turn on/off, you move from PN to InGear engine temps changes etc.

imho, try winding down the set screw a small amount and see if the IACs go up. You only need to keep the thing above zero all the time (but having some margin is nice).

RPM WS6

iTrader: (3)

This evening I was finally able to get my arm all the way to the back of the intake and feel around. Also was able to get a better look with the mirror. Both visual and feel indicate that everything is hooked up properly. I also re-checked all PCV lines and found them to be in apperent leak free condition.

Thanks for the info.

Trying to close the TB blade further via the set screw is about the only thing I haven't tried yet. But I have two questions about doing that:

1) It shouldn't be necessary to adjust that screw on a factory stock TB and motor, correct? So what would have caused this particular car to "come out of adjustment" so to speak? Basically meaning, am I just masking another issue or could it really have been adjusted improperly from the factory?

2) Will dropping that screw actually close the TB further? It seems to be fully closed, shut and sealed tight as possibile. My concern is, it may start to actually open slightly on one end if I drop the screw past fully closed, correct? Therefore, would it be better to seal the TB blade hole and drill a new, smaller hole (one size down)?

Maybe there is something wrong with the throttle body itself? Bore out of round? Something just a hair out of spec maybe?

Thanks again.

wrencher

iTrader: (2)

I totally agree w/ gameover here. The IAC counts are low thats where I'd look. The only other area I might look at not suggested is the EGR vavle. Is the pintle hanging up a little, seen it on low milers B-4. But you can clear the problem by shutting it off & restarting it, so that would make me kinda lean away from that.
Question, when problem occurs is the canister purge solenoid on (duty cycle) & vent solenoid open(normal position)> AKA it purging at the time of the complaint?Have had some real funky idle issues w/ GM's that where associated with the canister purge system. Is there any thing else in the PIDs that are off during the complaint anything, STFT? EGR pintle postion, A/C request P/S switch input, I'm looking for anything to go on that can influence the PCM's idle controll strategy. + anything that would also change on a shut down /start up senero.
I still dont like the fact that it is under 20 IAC counts though. Where is the air comming from? Maybe disconnect the vacuum line to the purge solenoid & see if it changes your symptom?

QuickSilver2002

One other thing to check. Scan you evap valve operation and see if it correlates with the problem.

Some of your info (after 30 minutes of driving...) seem to possibly point in that direction.

I've seen some idle issues relate to evap operation. It can be a vacuum leak of sorts, but it is not really suppose to take place at idle, but it always turns off for a while after a restart.

Just one more thing to check.

RPM WS6

iTrader: (3)

Thanks for the replies guys...

Wait, the part I bolded above, you said it can be cleared with a restart? That’s exactly what I’m able to do though, so why would you lean away from that? Or was “can” a typo when you really meant can’t? Either way, what should the EGR read at idle?

Little info on my EGR... I was lucky enough to get all the paperwork associated with the car when I bought it. Looks like an EGR recall was performed by a GM dealer years ago.

I’ve never looked at any of the above info concering the purge system. How can I verify that? I am using AutoTap as my scanning software, can it read that info? If so, are there certain values I should be looking for, or just “on/off or open/closed” type info? Or is this something that has to be checked manually?

Is the vacuum line you’re referring to the dark gray hard plastic line coming off the driver’s side of the intake just behind the throttle body? If so, how do I pull that line without breaking it? Looks like it has some sort of special connection method.

What is involved in just replacing the EVAP components that might be problematic? What parts do you replace when there is a problem with the system? If it’s easy and not too expensive I’ll just swap it out and see what happens. I should be getting a DTC if it's bad though, correct? Last I checked, the EVAP test was showing complete. I'll have to check again.

I can’t seem to find any other readings that are off. I have not checked EGR pintle though. What should it read at idle for a stock LS1?

Looks like EVAP is the one area that I haven’t really looked at yet. But like I posted above, what exactly am I supposed to scan for, what sort of value *should* I be seeing (most specifically, at idle)? I’ve never messed with the EVAP system on any of my LS1s, I’m totally clueless as to what is normal operating parameters for it or even how to replace it. Only thing I can say for sure is that I’ve never had any DTCs for it.

I really do appreciate everyone’s time on this.

wrencher

iTrader: (2)

I meant that the EGR is not really a likely candidate. Due to the fact that you say you can shut the engine off while problem is present. Then upon restart the complaint is gone. It would seem that anything mechanical is ruled out & it is prolly PCM control related. (on/off output?)I am not familair w/ autotap & its capabilities. But there are alot of PIDs for the emission controls that I see left out of 'performance scanners'.
The EGR pintle position in % also in volts.
The commanded EGR position.
Fuel tank pressure sensor in volts
canister purge solenoid duty cycle in %
canister vent solenoid /off <normally open componet
You'll see that as purge duty cycle % goes up it has an influence on STFT's. How much is the concern, had few damaged charcoal canisters cause problems.
If you disconnect the intakes vacuum line from the purge solenoid, it would eliminate it as a culprit. After a good drive in 'modified' form.

QuickSilver2002

EVAP is a strange animal. I'm just starting to understand it myself.

The pid to watch is % duty cycle. Not sure if that is available in autotap or not.

The EVAP valve is releasing an unknown AF mixture directly into the intake and the general idea is for the PCM to only open the valve when in open loop (where the fuel trims can adjust). It typically keeps the duty cycle real low until the throttle is open a decent amount.

RPM WS6

iTrader: (3)

I think I can get most of the above info with my scanner. I’ll have to scan the PCM again and check for the above fields.

Problem is, it may take quite a while for the issue to pop up again. But in the mean time I can scan those values while everything is operating properly and see what they show. Maybe something will be close enough to “out of spec” to be noticeable even when things seem to be running fine.

If everyone is certain that my IACs being around 10 in P/N at full operating temp is too low even when the motor is idling perfectly, then it stands to reason that the problem is probably there all the time but for some reason it just gets worse on occasion after long drives causing IACs to drop to 0, then a restart clears it but still IACs are lower than they should be.

I’ll do some more scanning, looking at the areas you mentioned above. I guess after that I need to try disconnecting vacuum to the canister purge solenoid and see what happens to the IAC counts.

So assuming I pull the vac line and IAC counts go up to normal (say 20+), what components exactly would I need to replace to fix the issue? Again, I am completely unfamiliar with the EVAP system.


I’ll do more scanning this weekend. I will check my atap and see if I can get duty cycle %.

In the event my scanner doesn’t support that field, and since I seem to have exhausted most all other possibilities, maybe I should just change the valve and lines at the intake just to see if that helps. Do you know if it comes as a kit or are the lines and valve separate parts to order?

In case I go that route, can anyone tell me if there’s anything special I’d need to know about replacing the EVAP valve & intake vacuum lines? What’s the procedure? I’ve never done that before.

Thanks again guys

RPM WS6

iTrader: (3)

I did some reading on EVAP system, to try and understand how it works and what might be wrong causing my issue. Seems to me, that the only way an EVAP system problem could cause an idle issue but *not* set a DTC is if the vacuum line from the purge valve to the intake had developed a condition causing a tiny vacuum leak on the intake side but not losing enough vacuum to affect EVAP operation. Is my assumption correct? If it is, I’m once again back to looking for a vacuum leak. Guess I'll check the EVAP vac line next, but if not from there, then from where else? And how is the engine restart “clearing” it?

RPM WS6

iTrader: (3)

I tried to scan some of those EGR and EVAP areas mentioned above.

I found three different fields for EGR that seemed to give the needed information:

1) Linear EGR position normalized (defined by a-tap as the *actual* pintle position): 0%

2) Linear EGR position desired (defined by a-tap as the value commanded by the PCM): 0%

3) Commanded EGR pintle position (a-tap did not give a definition on this one): 13.3%

There was one field for EVAP canister purge solenoid, it read: 18.8%

All of the above values were in P/N idle @650rpm at full coolant temp (196-207*F).

I'm a little confused about that last EGR figure. A-tap says that "Linear EGR position desired" is showing what the PCM commands (0%), but then there's that other field that's actually called "Commanded EGR pintle position" and it reads different (13.3%). Plus, a-tap gives no definition for that field.

I checked and the EVAP test and EGR test are both "complete".

So how do those numbers look?

Anyway, that's all the info I could get past what I've already scanned previously. IACs still drop as low as ~8-10 in P/N after idling for an extended period of time (20+ mins) but I haven't had the idle hang/surge issue anymore so far.

I re-checked everything around the intake, TB, and vac lines for leaks and still can't find one. I even changed the PCV valve again. LTFTs at idle are about +4-5% in P/N and +7-8% in gear. STFTs are around -3-0%.