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Old 10-31-2012, 10:38 PM   #1
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Default Common Vacuum Leaks?

Im 90% sure I have a vacuum leak. To start things off I had a TPS code, so naturally I replaced the TPS. All is good right? Nope. Now the car idles at 1500rpm instead of the normal 1100(what its tuned to idle at). Unless I drive and come to a dead stop then, and only then, does the car drop down to normal. If I start the car and let it idle it still sits at 1500rpm. A few other things to note: I just put a new air filter on, at the same time I did the tps. My oil pressure needle shakes like no other. The shop broke my old one off and replaced so im guessing its just a faulty unit? Because as long as the car is given any gas at all the needle tightens right up and doesnt shake. I have no idea if thats related or not. One other really weird thing to note, when I tap the gas theres about a one and a half second pause until the engine revs. I checked the throttle and its opening perfect, the engine just lags the slightest bit. Im not sure if thats normal but Im going to replace my spark plugs so we'll see if that helps. My questions are: Does this sound like a vacuum leak? And if so, where are the most common vacuum leaks on an LT1 so I can know where to start my hunt.
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Old 11-01-2012, 09:23 PM   #2
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Nobody? I ordered an ALDL cable so I'll be able to see whats going. Just wondering if anyone has had a similar problem.
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Old 11-01-2012, 09:28 PM   #3
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Check your intake manifold bolts. When I pulled my intake to change my cam I was shocked at how loose about half the bolts were. The original owner had the intake resealed about 50k miles earlier at the Pontiac dealer.
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Old 11-01-2012, 09:35 PM   #4
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Good idea, I'll check those out. Im going to clean the Iac also. Ive checked most of the other vacuum lines today. Im thinking it might be an 02 sensor after all. Ill know soon enough.
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Old 11-01-2012, 10:37 PM   #5
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What TB are you running? What mods do you have done? What problems were you experiencing prior to changing the TPS? Which DTC was your TPS setting?

A DTC doesn't necessarily mean the sensor is bad. If your TB was slotted and adjusted during tuning, it would be reading differently if you just swapped it with a new one. When you get it scanned, pay attention to the TPS voltage, IAC counts, AFGS, and O2 readings (after they're warmed up).
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Old 11-01-2012, 10:51 PM   #6
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The TB is stock. And all of my mods are just bolt ons. Suspension, cai, exhaust, headers. No internal engine mods. I wasn't experiencing any issues, however the code was tps low something or other. I don't remember exactly.

Is there a guide I can look at to tell what all these values would be normally so I can tell if anything is off?
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Old 11-02-2012, 09:40 AM   #7
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With CAI & headers, any particular reason you have a stock engine set to idle at 1100 RPM in the tune? Does it have idle problems down around 700 (M6) or 650 P/N (A4). (You did not give year or info on the car) It would be of help isolating this to get that cable and find out what your idle MAP, IAC counts, timing and TB sensor settings are. That would point to a vac leak or something else. If an engine is not idling correctly it will effect throttle response off idle (hesitation/bogging) until the PCM transitions to correct driving conditions.
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Old 11-04-2012, 01:07 AM   #8
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Quote:
Originally Posted by aappss01 View Post
Is there a guide I can look at to tell what all these values would be normally so I can tell if anything is off?
The PCM will accept TPS voltages from .3 - .9 volts as being 0% throttle, with the ideal voltage being .64V with the throttle blades closed. The AFGS should be around 6-10 and the IAC counts somewhere in the 5-50 range (I usually see closer to 40 - 50) at a normal idle of around 700rpm, yours will be higher with yours set to 1100rpm (as questioned by bobdec, this is not normal nor ideal for the mods you have). The O2s, when heated, should be fluctuating above and below 450mV several times a second.
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Old 11-04-2012, 01:57 PM   #9
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I honestly have no idea why the car is set to idle at 1100rpm. The original owner got it tuned my Madz28. Although the PO might have adjusted the throttle set screw and made it idle higher, he was a dip****. So ill check that. If in fact it is tuned to idle at 1100rpm how hard is it to adjust it to idle at stock rpms with Tunerpro? I got the cable today, so ill run the diagnostics this week. If I saved the runs would someone give them a look over for me? I could do one at idle and one while driving.
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Old 11-05-2012, 09:04 AM   #10
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Since you said TunerPro I assume you are a '93-'95 OBD1. You can log w/TTS Datamaster EE or Freescan. Post up the .uni or .csv logs on a file sharing site and members here can help. You can PM me and I provide an email I use to receive non personal mail & logs, but it's much better to give everyone access via a sharing site, the more eyes the more info and opinions ??. Follow Firehawk's post and insure TPS voltage and IAC are OK. Using TunerPro PCM code read insure desired Idle RPM is set 550 D/ 650 P/N in an A4 and somewhere around 650-700 in an M6.. Rule of thumb.. when you read the code save the original tune on your PC and mark the file as read only before you alter and load any changes to the PCM..

You want the PCM to control idle using the IAC NOT the TB set screw...The TB should let a small amount of air in at idle, then the PCM opens and closes the IAC as needed to keep the engine at desired RPM. No other air should be entering intake at that point. If desired RPM setting is correct on your tune, and you're at 1100 I'd venture to guess you IAC is at or near 0 and the TB or a vacuum leak is controlling idle not the PCM.. If your TPS voltage is good, and desired idle is correct, and idle is still 1100 RPM then it's most likely a vacuum leak..
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Old 11-05-2012, 10:46 AM   #11
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I hooked up my cord this morning and ran Tunercat Rt to ensure everything worked, which it did. As soon as I figure out how to save the logs on Datamaster or Freescan ill post them up. I couldn't figure out how to get either of them to start so ill do some reading on those. Two things I did note on TunerCat was that my idle was showing as 800-825rpm fluctuating pretty rapidly, while my physical gauge cluster was reading 1100rpm. So the problem must lie within the gauges themselves? And also, my left 02 sensor is in fact bad.

Edit: Yes the car is a 1995 Z28. Sorry forgot to mention that earlier.
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Old 11-05-2012, 03:02 PM   #12
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Alright I recorded part of my drive home today. Sorry for all the idling at the beginning, I got stuck in the parking garage. My Left 02 sensor is at the correct mv and my right is extremely low, like 45mv, but its my left thats throwing a code? In TunerCat it says that my left 02 sensor is "Not Ready". Any how, here is a link to the .csv log: https://www.dropbox.com/s/0nyq9oj53130b1m/Drive1.csv

Anyone who can take a look at it and be of help that would be really awesome! Thanks guys.
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Old 11-06-2012, 10:24 AM   #13
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You file is empty.. no data. An O2 at .45 volts (450 Mv) is not switching, PCM puts 450 Mv on the sensing leads and as soon as the O2 heats up and starts switching (100-900 Mv) it will override that voltage. Your O2 is totally bad, possibly open heater or burnt wires. I have had that problem once, an open heater and with headers the car ran like crap, bucking, stalling, etc. You need to fix that before chasing other failures. Swap the O2's left to right to isolate to O2 or circuit problem.
By the way I assume you are using TunerPro RT, not TunerCat as TC does not log or display active engine sensor data. What did you IAC read at idle ?
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Old 11-06-2012, 11:32 AM   #14
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Ah yes TunerPro Rt, sorry for the confusion. I'll upload the other file and see if that one works.
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Old 11-06-2012, 11:44 AM   #15
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Alright hopefully this one will work for you, its the uni file. My IAC count at idle was 49. Ill swap the O2's from left to right and see if anything changes. So the 02 at 45mv is bad? Just trying to clear up your post. Heres the link to the file: https://www.dropbox.com/s/nma1vprnljgdmzb/Drive1.uni
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