So over a year after the last time the tuner flashed my ECM (04/11), i started having this issue (04/12)... i rarely drive the car, so I'm just now getting around to tackling it...

I possibly have two issues here... (Built 2.4 - see profile for mod list)

First of all... after clearing all the codes... the car will start up, idle normally... If i drive at all, and then pull out of gear, no throttle, the engine will hold 3000 ish rpms above 5 mph, and then below 5 mph, hold 2500 ish. I can get the idle to drop by either turning the AC/Defrost on (and then back off after the idle drops), Holding the brake and slowly letting out on the clutch until the RPMs drop (and then pull out of gear, it will idle normally), or restart the car.... after a short time of it being like this, it sets a P0507 in pending, and then above 5 mph, out of gear, no throttle, the car will drop immediately to 1500, climb to 2500, and fall to 1500... very quickly. Below 5 mph, it will drop to 1000 RPM (my commanded idle in my tune), until it warms up, which at that point it will hold 1500, until i use one of the above methods to bring the idle down...

To me... this sounds like a vacuum leak... My vacuum gauge reads between 10 - 14 inHG at idle when not moving, and 24 inHG while moving, with no throttle...

I've resealed the intake mani, the tb, checked all of the vacuum lines. I can confirm that I do not have a vacuum leak

So I started to watch the fuel trims... Cleared all codes... at idle 1000 RPM, not moving, Short term is hanging around 0% (+- 5) --- long term is hanging at -11.7% ... telling me that the ECM is removing fuel... My wideband is reading 14.7

after driving, and bringing the idle back down... back at idle 1000 RPM, not moving, Short term is bouncing between -5 and -14 ... while long term is -3.9

I then held the RPM at 3000 RPM holding the accel pedal... while doing this, Short term hung around 0 (+- 5) --- Long term slowly climbed to 16%, then fell to 13.3 % ---

I let it back down to 1000 RPM --- Long term immediately dropped to -8.6% short term still remained around 0 (+- 5)

I've also noticed my fuel economy drop by about 80 miles to a tank... my fuel pressure gauge reads about 56 psi with vacuum, and about 65 without vacuum

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So looking at the basics of the P0507... it seems to be all related to the IAC valve... I replaced it, same issue... i checked continuity of the wires, those are good...

At this point i am uncertain where to explore next... I'm thinking i should try to get my hands on another 95 LD2 throttle body and see if that changes anything... (the one i have now was bored and sleeved to 61.5 mm) ... Some of my coworkers think i should have my tuner reflash the PCM with my tune, in the event the PCM has done something stupid... i will point out that the car sat for about 4 months prior to this issue, and now the car gets started and driven two days out of the week... (with the exception of a hand full of out of town car shows that are 150+ miles away)

Any ideas?

Havent you had this issue since you put the built motor in? Id say trying a stock sized throttle body is worth a try. Ive always read that upsizing the throttle body isnt necessary unless youre boosting or youre seeing vacuum at WOT. Maybe the IAC cant compensate for the size of the TB bore. Its hard to say.

The tricky thing about the P0507 code (it would pop on infrequently on my Civic) is that its not indicating that anything is wrong with the IAC itself. Its really just saying that your car is idling higher or lower than the commanded value at the time.

Whats the profile on the cams like? IIRC you dont have anything too crazy that would mess with the vacuum that much at idle.

Im not super skilled at diagnostics so take all of this worth a grain of salt.

Have you reset the ECU after replacing the iac?



FU Tuning

Just a thought, are IAC passages in the throttle body "tuned" for stock size TB? I don't know if there is any truth to this thinking. But if the passage was to big or small for a bored TB, I would think the IAC would be fighting to adjust, but it can't.

I know that alot of gm stuff uses the same size pintle on the IAC, so I would think that there would be some differences in passage diameters to help tune the pintle for each size of TB.

Like I said, just a thought. I could be %100 wrong.

Brian (TheSundownFire) wrote:Havent you had this issue since you put the built motor in? Id say trying a stock sized throttle body is worth a try. Ive always read that upsizing the throttle body isnt necessary unless youre boosting or youre seeing vacuum at WOT. Maybe the IAC cant compensate for the size of the TB bore. Its hard to say.

The tricky thing about the P0507 code (it would pop on infrequently on my Civic) is that its not indicating that anything is wrong with the IAC itself. Its really just saying that your car is idling higher or lower than the commanded value at the time.

Whats the profile on the cams like? IIRC you dont have anything too crazy that would mess with the vacuum that much at idle.

Im not super skilled at diagnostics so take all of this worth a grain of salt.

ive had the higher rev issue since the motor was put in (04/10) but it took 2 years to pop the p0507 code. It started about a month after i upgraded the throttle body.
The cams are .430 lift .222 duration. i always assumed my low vacuum numbers at idle were related to my cams...

Addicted to meth wrote:Have you reset the ECU after replacing the iac?

Negative... I don't have access to a Tech 2... i suppose i could unplug the battery before i head to work today, and it should be reset by the time i get home.

Slowzee wrote:Just a thought, are IAC passages in the throttle body "tuned" for stock size TB? I don't know if there is any truth to this thinking. But if the passage was to big or small for a bored TB, I would think the IAC would be fighting to adjust, but it can't.

I know that alot of gm stuff uses the same size pintle on the IAC, so I would think that there would be some differences in passage diameters to help tune the pintle for each size of TB.

Like I said, just a thought. I could be %100 wrong.

it's possible... wouldn't everyone with bored tbs have this issue though?

I would slap a stock TB on there and see what she does.




PRND321 Till I DIE
Old Motor: 160whp & 152ft/lbs, 1/4 Mile 15.4 @88.2
M45 + LD9 + 4T40-E, GO GO GO

Wrench Monkey wrote:*snip*

Thanks man, but i have that with alldata... problem is that i don't have a tech 2 to actuate the IAC... i've circumvented this by continuity tested the wires back to the pcm and replacing the TPS... my issue isn't there...

I think my next step is going back to a stock 95 TB and seeing what happens...

I have a couple things i need to do first... my battery is taking a @!#$, so im about to warranty it with optima and get a new one... I need to replace my rear tires and pass rear shock/shock mount... I have a car show (3 hours away) coming up in less than 2 weeks, and the motor runs and idles decent with the code... my fuel economy sucks... but on the interstate i still get decent mpg... city driving on the other hand, i lose about 80 miles to a tank....

after the show i'll throw my other TB on and see what happens...

My coworkers are thinking i'm at the point where i need to update my tune.... i still think i have a mechanical issue.. and i don't want to bug my tuner until all mechanical issues are settled...

you should try another new or known working IAC i have a feeling the one you have is broken or the computer is dying. A broken wire would be constant. A vacuum leak should be constant that is unless you have a collapsing hose. Another way to DIAG the problem would to disconnect any vacuum line and cap off the port while the problem is happening to verify you do not have a vacuum leak.

Another thing. GM IAC's can come extended to far out and when you go to install them you compress them down and damage them in which case if you did recieve and extended IAC you would have had to commmand it closed with a scan tool. You don't have any access to any scan tool with BI-directional control?

i've swapped to a known good IAC, and nothing changed... ... i do not have immediate access to a scan tool that can control the IAC... i'm looking for my spare stock 95 TB. next week i might be able to ask my buddy at the chevrolet dealer next door to borrow the tech 2 during my lunch hour...

All i have access to right now is a Hyundai Hi-Scan using the Carb OB2 option. (our Hyundai GDS does not connect even with carb ob2 selected)

I would hook up the evap smoke machine to the intake tube to verify there are absolutely no vacuum leaks then. You could have a hose that is collapsing causing this issue. Also if you have any way to load test the wires that would be extremely helpful as well.

If i were to suspect a specific vacuum leak if it does have one i would guess in the Air/Oil seperator tube.

my oil/air separator vents to atmosphere... i don't have the return tube linked to the intake...

I'm wondering if my vaccum is right... at idle its between 10-14 ... while moving with no throttle, out of gear, its 24

That is low vacuum. I would hook a mityvac to it.

Vacuum at idle should be closer to the 19-22 range.

even with .430 lift .222 dur cams? my idle jumps between 900 - 1200 at all times from it loping...