Need help diagnosing an idle issue
Need help diagnosing an idle issue My car is a garage queen, and gets about 1000 miles per year put on it. When I got it out of winter storage and started it up, I noticed something unusual. As it was idling, every minute or so, the idle would raise by a 3-400 rpm. Sometimes it would drop a few hundred rpm instead of raise. I didnt think it was a big deal since it had sat so long over the winter. But then I noticed something else while driving. While driving, if I have the car in a high enough gear that the rpms are low, like 12-1500rpm, the gas pedal is a bit numb to minor throttle imputs. Its like the throttle has a dead spot.
So are these two related, and if so, what could be the cause?
Any thoughts are welcome.
Thanks
So are these two related, and if so, what could be the cause?
Any thoughts are welcome.
Thanks
The throttle dead spots, well these cars do have a few dead spots. I know on my camaro from 40-50 its a dead spot (automatic) when i had my slp loudmouth, than i replaced it with a slp muffler and no dead spot.
I would start with cleaning maf sensor.
I would start with cleaning maf sensor.
Ok, here's the latest. On a hunch, I replaced the TPS sensor since I felt that might be the issue for the dead spot in the pedal. Changing the TPS did not change anything. As I was driving it, the symptoms got much worse. Hesitation all over the rev range, erratic idle at stop lights. This got me thinking it might be the MAF as 98Boxer stated. I decided to stop at an auto parts store and clean the MAF with MAF cleaner - no change. I then decided to unplug the MAF completely, and still no change in the driveability problems. Normally, I would assume that this meant that the MAF was bad, but the computer has not thrown any codes. I assume that an unplugged MAF would throw a code, right??
TECH Enthusiast
Joined: Nov 2012
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From: NearHouston, TX
how is the fuel pressure? If you have factory rails, you can mount an inexpensive external fuel gauge on it. I did for troubleshooting just these things on mine. http://www.ebay.com/itm/CHEVY-CAMARO-SS-Z28-LS1-FUEL-PRESSURE-GAUGE-KIT-BLACK-/250538157107?pt=Motors_Car_Truck_Parts_Accessories&hash=item3a553ce433&vxp=mtr
If your engine is just sitting and idling......and the rpms increase on their own......there is only ONE thing that can cause that, the IAC valve. What is telling the IAC valve to open up is the question. If your throttle blade is just siting still its not the TPS sensor. The only way to tell if you have a bad MAF is to start the engine when it's dead cold with the MAF unplugged and see if it immediately starts surging as it idles. And NO.....bad or failing MAFs will NOT always throw a code.
You should also spray the IAT sensor in the lid with MAF cleaner.....when they get dirty or start to fail they send messy signals to the PCM which can then cause the IAC valve to open and close erratically.
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You should also spray the IAT sensor in the lid with MAF cleaner.....when they get dirty or start to fail they send messy signals to the PCM which can then cause the IAC valve to open and close erratically.
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Ok, I just started the car with the MAF unplugged, and the idle was steady, no wild surging, but I only let it run for about 30-45sec.
So a code is not always going to show up, even with it unplugged for 50 miles?
So a code is not always going to show up, even with it unplugged for 50 miles?
A sensor is causing your issues. The IAC valve is opening up to make the rpms increase.......
There is absolutely no other way an engine can increase in rpms without hitting the throttle......EXCEPT for a vacuum leak. BUT......if you had a vacuum leak it would be causing problems all the time until it was fixed.
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Last edited by LS6427; Mar 29, 2014 at 02:58 PM.
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That makes sense about the IAC causing the idle issue, but would it cause the stumbling/bucking/hesitation during driving?
You mention the IAC... I noticed last night, that if I hold the clutch in as the car is rolling, say coming down from speed to a stop sign, the idle bounces from 500-1000-500 in 1-2sec cycles until the car comes to a complete stop, and then smooths out again. IAC as well?
You mention the IAC... I noticed last night, that if I hold the clutch in as the car is rolling, say coming down from speed to a stop sign, the idle bounces from 500-1000-500 in 1-2sec cycles until the car comes to a complete stop, and then smooths out again. IAC as well?
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TECH Addict
Joined: Mar 2013
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I would start by carefully testing the fuel system pressure.....................I had similar trouble and it was rusted fuel lines. Was acting funny surging at all rpme etc. Pulled the fuel rail and it was full of rust.
Last edited by RockinWs6; Mar 29, 2014 at 04:36 PM.
That makes sense about the IAC causing the idle issue, but would it cause the stumbling/bucking/hesitation during driving?
You mention the IAC... I noticed last night, that if I hold the clutch in as the car is rolling, say coming down from speed to a stop sign, the idle bounces from 500-1000-500 in 1-2sec cycles until the car comes to a complete stop, and then smooths out again. IAC as well?
You mention the IAC... I noticed last night, that if I hold the clutch in as the car is rolling, say coming down from speed to a stop sign, the idle bounces from 500-1000-500 in 1-2sec cycles until the car comes to a complete stop, and then smooths out again. IAC as well?
And a lot of times you won't get codes.
Is your gas mileage the same or worse? Wen the 02s start to go bad you will run very rich......and fuel consumption goes WAY up.
Get it scanned with an OBDII scanner while the engine is running.....see what the 02 sensors are doing.......
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The car is a show car, and only gets driven 1-2000 miles per year, so I dont really track the gas mileage like I do in the daily drivers.
I have a guy coming over tomorrow with a scan tool, hopefully that will pinpoint something.
I have a guy coming over tomorrow with a scan tool, hopefully that will pinpoint something.
10 Second Club
Joined: Jul 2008
Posts: 224
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From: Columbus Ohio
How old is the gas in the car? Have you run it out and put fresh fuel in the car? Looking at data on a scan tool and seeing if anything is out of a normal range would be the next thing, especially your fuel trims, and it sounds like you're going to be doing that. Good luck.
Ok, got the Actron CP9145 scanner on the car. It showed the car running fine until it went to closed loop, at which point the ST fuel trims started falling into the negative range until the car switched back to open loop for a second to keep from stalling. That is what the idle surge was, it jumping back to open loop for a second or two.
It also showed a static reading from the MAF, even though it was unplugged. I dont kow if that is something that happens when it goes to SD mode, or what...
Everything else looked fine, all temp sensors were normal, fuel pressure was 58-60 at idle, and held at 50 when the car was shut off. Wih the MAF plugged in, it read nomally as well.
So does that sounds like O2 sensors to you guys?
It also showed a static reading from the MAF, even though it was unplugged. I dont kow if that is something that happens when it goes to SD mode, or what...
Everything else looked fine, all temp sensors were normal, fuel pressure was 58-60 at idle, and held at 50 when the car was shut off. Wih the MAF plugged in, it read nomally as well.
So does that sounds like O2 sensors to you guys?
Ok, got the Actron CP9145 scanner on the car. It showed the car running fine until it went to closed loop, at which point the ST fuel trims started falling into the negative range until the car switched back to open loop for a second to keep from stalling. That is what the idle surge was, it jumping back to open loop for a second or two.
It also showed a static reading from the MAF, even though it was unplugged. I dont kow if that is something that happens when it goes to SD mode, or what...
Everything else looked fine, all temp sensors were normal, fuel pressure was 58-60 at idle, and held at 50 when the car was shut off. Wih the MAF plugged in, it read nomally as well.
So does that sounds like O2 sensors to you guys?
It also showed a static reading from the MAF, even though it was unplugged. I dont kow if that is something that happens when it goes to SD mode, or what...
Everything else looked fine, all temp sensors were normal, fuel pressure was 58-60 at idle, and held at 50 when the car was shut off. Wih the MAF plugged in, it read nomally as well.
So does that sounds like O2 sensors to you guys?
BUT......stumbling/bucking/hesitation during driving........is almost always 02 sensors. Unfortunately with these crazy PCMs and all the damn sensors on these things......it could be something else or a combo of more than one thing. That's why it would be a good idea to see how the 02 sensors are switching with an OBDII scanner.
I wasn't sure about 3 weeks ago what was going on with my car.......it would start up fine, then I would take off and start to drive. I would then start breaking up and stumbling till about 2-3 minutes into the drive.....then BAM it would totally clear up.....probably after it reached a certain temp and the PCM switched to SD tune. But I was using RIDICULOUS amounts of gas. Josh over at Lashway Motorsports scanned my car and he said it was using 80% more fuel than normal and that one 02 sensor was totally out and the other was switching very slowly. I changed both and the car was like new. I don't know why two 02 sensors went out at the same time.....maybe from sitting 10 weeks....I have no idea. But the scan showed me it was my 02 sensors......
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He did say that the o2s were switching very slowly compared to his Impala SS that he hooked it up to to show me the difference. We were not sure if that was because they were bad, or because they are in long tubes and farther away from the heads.
I too thought it would be wierd for both o2s to go at the same time, but like you, my car sits for long periods of time.
So whats the recommended brand for O2s? I heard to stay away from Bosch, and looked at ACDelcos website, but they have multiple part numbers for the same application.
I too thought it would be wierd for both o2s to go at the same time, but like you, my car sits for long periods of time.
So whats the recommended brand for O2s? I heard to stay away from Bosch, and looked at ACDelcos website, but they have multiple part numbers for the same application.
Teching In
Joined: Apr 2006
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From: N. Ky
I'm the guy with the scan tool. These O2 sensors were switching extremely slow as they were almost holding steadily around 0.6 to 0.7 volts. I haven't scanned a car that had long tube headers on it and if they ready any different than the stock manifold cars.
Seeing as they both were so slow I figured they were bad but with the high price of some O2 sensors I just hate to just throw parts at something in hopes that it fixes it. I'm more use to tuning carburetors and reading spark plugs than scanning ECM's
Seeing as they both were so slow I figured they were bad but with the high price of some O2 sensors I just hate to just throw parts at something in hopes that it fixes it. I'm more use to tuning carburetors and reading spark plugs than scanning ECM's
OK, here's the latest. I just got done replacing the O2 sensors, and the car runs much better, BUT thats because it never goes in to Closed Loop. It stays open the whole time, even after being at Operating Temp, and after a short drive.
Does anyone know what O2s are in what position? My scanner reads four O2s, 11, 12, 21, and 22. Which are which? The reason I ask is because the rear O2s were tuned out after the long tube install. On the scanner, three of the O2s read pretty steady at .445, while one is around .900 and fluctuates with throttle changes, while the other 3 do not. Could one of the new ones be bad as well? would that cause the car not to go to closed loop?
Does anyone know what O2s are in what position? My scanner reads four O2s, 11, 12, 21, and 22. Which are which? The reason I ask is because the rear O2s were tuned out after the long tube install. On the scanner, three of the O2s read pretty steady at .445, while one is around .900 and fluctuates with throttle changes, while the other 3 do not. Could one of the new ones be bad as well? would that cause the car not to go to closed loop?
Here's the scanner readings. Car was at operating temp and sitting at idle after a short drive. Before the drive I had unhooked the battery to reset the computer.
MIL STATUS........OFF
ABSLT TPS(%).....0.0
ENGINE (RPM)......801
CALC LOAD(%).....2.0
MAF(LB/M)..........0.78
MAP("HG)............8.3
COOLANT(F)........196
IAT(F)................84
IGN ADV(DEG)......22.0
LT FTRM1(%)......-1.6
SECOND AIR........ATMOS
ST FTRM1(%)......0.0
LT FTRM2(%)......-1.6
ST FTRM2(%)......0.0
VEH SPEED(MPH)..0
FUEL SYS1...........OPEN
FUEL SYS2...........OPEN
O2S11(V).............0.920
ST FTRM11(%)......0.0
O2S12(V)............0.445
O2S21(V)...........0.445
ST FTRM21(%)....0.445
O2S22(V)...........0.445
OBD2 STAT........CA
also, I noticed when cruising around being easy on the throttle, the ignition advance was in the 40s. Is that right?
MIL STATUS........OFF
ABSLT TPS(%).....0.0
ENGINE (RPM)......801
CALC LOAD(%).....2.0
MAF(LB/M)..........0.78
MAP("HG)............8.3
COOLANT(F)........196
IAT(F)................84
IGN ADV(DEG)......22.0
LT FTRM1(%)......-1.6
SECOND AIR........ATMOS
ST FTRM1(%)......0.0
LT FTRM2(%)......-1.6
ST FTRM2(%)......0.0
VEH SPEED(MPH)..0
FUEL SYS1...........OPEN
FUEL SYS2...........OPEN
O2S11(V).............0.920
ST FTRM11(%)......0.0
O2S12(V)............0.445
O2S21(V)...........0.445
ST FTRM21(%)....0.445
O2S22(V)...........0.445
OBD2 STAT........CA
also, I noticed when cruising around being easy on the throttle, the ignition advance was in the 40s. Is that right?