When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Slightly rough idle with erratic 02 sensor readings
My car is an 02Z with 22k mikes and I've owned it since 9k miles. Around 14k miles I started getting a slightly rough idle along with a P0300 random misfire code. I replaced the plugs and wires which seemed to have fixed the problem at that time. Now 7k miles later it's starting to idle a little rough again, but no codes yet. It drives fine, doesn't stall, studder or feel like it's running bad, but you can definitely tell it's got a rough idle, car even shakes a little at idle and you can hear it through the exhaust as a slight popping sound. I hooked up a decent scan tool and the only thing that really stands out are the 02S1-1 and 02S1-2 readings. They are varying between .030V to .800V while the 02S2-1 and 02S2-2 are fairly steady between .300V to .800V. The valve springs are the newer (blue) springs and I've already cleaned the MAF and IAT sensors. With a stethoscope I can clearly hear all the injectors pulsing and I'm getting consistent spark on all cylinders. Is it possible the problem is the bank 1 02 sensors even though they only have 22k miles or does it sound like bad fuel injector on bank 1? Any help appreciated especially before I fork out $ for new 02 sensors.
Steve
Last edited by killian96ss; 11-13-2018 at 10:39 AM.
Reason: Added information
I don't know the exact specs but, generally speaking, primary O2 sensors (the ones before the cats) will constantly shift between high and low. This is normal for narrow band O2 sensors, its how they work, by reading between rich and lean constantly. The secondary O2 sensors (after the cats) are only responsible for making sure the cats are working properly and should always read a consistant voltage, usually somewhere in the middle of its range. Again I'm not 100% sure what the specs are for us.
Now I'm ASSUMING that O2S1-1 and O2S1-2 means Oxygen sensor number 1 bank 1 and ocygen sensor number 1 bank 2. Unless the numbers are swaped and they are primary and secondary on the same bank but I kind of doubt it.
Maybe you had a bad tank of gas? How longs it been going on? Might be worth throwing some techron in it, unless you've already tried that.
Bad O2 sensor will NOT cause a misfire !!...The "upstream" sensors will switch between .300-800mv or so and the "downstreams" will be fairly constant around 600mv !!...O2S1-1 is bank 1 sensor 1 (upstream) and S1-2 is the downstream. Not a good idea to use a stethoscope...use a long screwdriver instead. Scope is too sensitive and might pick up the sound of the adjacent injector. What do your long term fuel trims look like at idle ??....does your scan tool show misfire counts...if not try Mode 6 !!
The gas is new, Shell 91 octane and I use a bottle of Techron in every tank. It's been idling bad for about a month now regardless of what brand gas I use which is only Shell, 76 and Chevron. I've also run water remover and extra injector cleaners tthrough the last 2 tanks to see if it would help, but nothing has changed.
I'll run it again and post my long and short term fuel trims for banks 1 and 2. I just happened to notice that the bank 1 pre cat 02 was all over the place from .030 to .800 while bank 2 pre cat 02 was fairly steady from .300 to .800 while idling. I know. 300 to .800 is normal, but .030 seems to low.
Steve
Last edited by killian96ss; 11-13-2018 at 10:40 AM.
Reason: Added information
The gas is new, Shell 91 octane and I use a bottle of Techron in every tank. It's been idling bad for about a month now regardless of what brand gas I use which is only Shell, 76 and Chevron. I've also run water remover and extra injector cleaners tthrough the last 2 tanks to see if it would help, but nothing has changed.
I'll run it again and post my long and short term fuel trims for banks 1 and 2. I just happened to notice that the bank 1 pre cat 02 was all over the place from .030 to .800 while bank 2 pre cat 02 was fairly steady from .600 to .800 while idling. I know. 300 to .800 is normal, but .030 seems to low.
Steve
Steve...usually .200mv is a "better" number but .300mv is still acceptable...So no more random misfire codes I'm assuming ??...now you said you cleaned the MAF sensor ??...has your issues surfaced after that ??...I'd look at all the connections between the MAF and the throttle body...you might have a vacuum leak there... if your long terms are higher than 10% (when you worry) run the car at around 3,000-4,000 for a few minutes and see if the short and long term fuel trims decrease....the long terms might take a little longer but they will decrease if you have a vacuum leak for sure !!
possibly you just have the designations mixed up and the sensors work like they should, i can never remember what is left/right, pre/post. btw, the rears do switch, just in a narrower range, thats why it hard to tell them appart even when logging.
I'm pretty sure 02S1-1 is pre cat bank 1 and 02S1-2 is post cat bank 1 while 02S2-1 is pre cat bank 2 and 02S2-2 is post cat bank 2. If this isn't right I'm looking at this wrong, lol. Also I made a correction to my readings above.
I'm pretty sure 02S1-1 is pre cat bank 1 and 02S1-2 is post cat bank 1 while 02S2-1 is pre cat bank 2 and 02S2-2 is post cat bank 2. If this isn't right I'm looking at this wrong, lol. Also I made a correction to my readings above.
I also forgot to mention that cleaning the MAF sensor made no difference and I double checked all connections from the MAF to the TB and all looks good, but I'll still take it for a drive and post fuel trim numbers. I also cleaned all of the plugs for injectors and coils with electrical contact cleaner and verified that the harness wasn't being damaged by rubbing near the back of the engine.
Steve
Last edited by killian96ss; 11-13-2018 at 10:48 AM.
Reason: Added information
Alright, took the Z out for a 30 min cruise today and recorded the 02 sensor data .
02S1-1 .030v to .815v
02S1-2 steady at .835v
02S2-1 .065v to .810v
02S2-2 steady at .840
LT FT bank 1, 21.9
ST FT bank 1, 0.0 to 3.1
LT FT bank 2, 24.2
ST FT bank 2, 0.0 to 3.5
Does anything look off? Both sides are almost identical, so should I move on to an injector or coil problem? Thanks.
Alright, took the Z out for a 30 min cruise today and recorded the 02 sensor data .
02S1-1 .030v to .815v
02S1-2 steady at .835v
02S2-1 .065v to .810v
02S2-2 steady at .840
LT FT bank 1, 21.9
ST FT bank 1, 0.0 to 3.1
LT FT bank 2, 24.2
ST FT bank 2, 0.0 to 3.5
Does anything look off? Both sides are almost identical, so should I move on to an injector or coil problem? Thanks.
Steve
WHOA !!...your LTFT on both banks are WAY to high !!...check engine light on yet ??...if not it might be soon !!...anything over 10.0 % is when you worry. Run the engine at an RPM around 3,000-4,000 RPM and see if the STFT start to go negative and your LTFT will start to decrease slowly...if this occurs you have a vacuum leak !! ....your PCM is "adding" fuel !!
WHOA !!...your LTFT on both banks are WAY to high !!...check engine light on yet ??...if not it might be soon !!...anything over 10.0 % is when you worry. Run the engine at an RPM around 3,000-4,000 RPM and see if the STFT start to go negative and your LTFT will start to decrease slowly...if this occurs you have a vacuum leak !! ....your PCM is "adding" fuel !!
Thanks man, appreciate the info. I can't find any leaks from the filter to the TB, but it's possible the TB gasket or manifold gasket is leaking. I just wouldn't think a 22k mile engine would have problems, but I also need to remember it was made almost 18 years ago, lol. I did notice that my LT FT numbers were around 16 when I first pulled over and plugged in my scan tool, then they slowly crept up to the low 20"s.
Thanks man, appreciate the info. I can't find any leaks from the filter to the TB, but it's possible the TB gasket or manifold gasket is leaking. I just wouldn't think a 22k mile engine would have problems, but I also need to remember it was made almost 18 years ago, lol. I did notice that my LT FT numbers were around 16 when I first pulled over and plugged in my scan tool, then they slowly crept up to the low 20"s.
Steve
Steve, like I said run it at 3-4K and see if the numbers come down...do a good visual first of all the vacuum lines...brake booster, PCV etc...double check MAF to T/B...you can spray some Brake Kleen or propane in those areas and around manifold...see if RPM changes......I prefer the smoke machine method...finds those pesky leaks very quickly...if it does look like a vacuum leak you can maybe take it to a shop if you can't locate it and get a definitive diag... better than just guessing !!!
on my 99 the rears are far from steady. they switch, slower and in a narrower range than the fronts. a while ago i got the designations wrong and bought new fronts (thinking one was getting slow), but i was looking at one rear. and i beliewe 20% ltft is max. on the ecu.
or do you call 0.3-0.8 "steady"?
btw, ls1s have a terrible idle compared to other v-8s. on my amg 5.0 i cant tell if its running or not, if i close my eyes. no clue why that is.
Just an update on my idle problem. I replaced the intake gaskets and TB gasket. I also put a new MAP sensor in since it's nearly impossible to replace without pulling the intake. Here are a few pics.
I have to say replacing the gaskets was far easier than I thought it would be. It probably took me about 2 hrs with all the cleaning too. Now for the bad news. My idle problem didn't change at all. At least I know all the gaskets are good, lol and I'm positive I have no leaks from the filter all the way to the heads.
My long term fuel trims on bank 1 & 2 are still around 20.0 after a 30 min drive. If raise rpms to 3k-4k while in N the numbers do lower to about 14.0, but even held at 3k-4k rpm they start going right back to 20.0. Short term trims are -1.0 to 3.0 on both banks. I don't think it's an injector problem, but obviously something isn't right. What still stands out to me are the pre cat 02 sensor readings on both banks. They are switching between .030 to .800. Btw .030 is not a typo! They should be at least .300 right? .030 seems way to low, too lean. Any more ideas?
Here's a few other readings at idle.
TPS 4.7
MAF .80
MAP 9.7
IGN ADV 16.0
All of these readings are basically the same as before.
Steve
Last edited by killian96ss; 12-06-2018 at 09:47 PM.
Reason: Added information
If those LTFT’s remain high under load it’s usually a MAF or fuel delivery issue...if you have a fuel pressure gauge I’d do a “snap throttle” fuel pressure check...it should not drop !!..does your ‘02 have the external fuel filter or integral in the tank ??...I think they went to the FFS system at the end of 2002...maybe you have a clogged filter...as far as the MAF the number you quoted was pounds per minute ??...what does that number look like when you do a snap throttle ??...I’ll include a pic of my idle O2’s...at higher RPM the frequency will increase !!...mine go below 200 Mv...no worries !!
My Z has the external fuel filter. I saw it last time I was under the car. I do have a new GM filter to install, just haven't gotten around to it probably cause I figured the car only has 22k miles, but who knows maybe it's bad. I also have a fuel pressure tester, so I'll hook it up and see what I get. What is normal pressure, like 60 psi?
My Z has the external fuel filter. I saw it last time I was under the car. I do have a new GM filter to install, just haven't gotten around to it probably cause I figured the car only has 22k miles, but who knows maybe it's bad. I also have a fuel pressure tester, so I'll hook it up and see what I get. What is normal pressure, like 60 psi?
Steve
Ok, first thing your MAP number you saw at idle...9.7...is that psi or voltage ??...my '01 MAP at idle is 4.35 psi and 1.05 volts. With key on engine "OFF" MAP at sea level should be around 103 kPa (4.9 psi or 30.41 inches/hg)...The MAP should read your atmospheric pressure at your location...get it from the weather bureau...Your MAF looks OK at idle... .book number is .50-.90 lbs/min (4-7 grams/sec) is OK and 25.0 lbs/min (190 grams/sec) on a snap throttle !!...fuel pressure number for my '01 is 55-62 key on engine off. And like I had said see if it drops when you snap the throttle. If it does decrease replace the fuel filter and retest...I'd be worried more about VOLUME than pressure since the car has low mileage and has been probably sitting in the garage more than it has been driven. I don't know if your scan tool can do a fuel injector balance test but if you do you can tell if those injectors are clogged...small pressure drop equals low flow...you did say they were "clicking" but you may not have the flow through them...maybe consider having them cleaned professionally.
What does the spark look like if you compare each bank? Do the coils on one bank produce a stronger or weaker spark than the other bank? Something to compare. I would read the coil supply voltage and do a resistance check on the coil ground wire for each bank.
There are two fuses for each set of coils. You can use a DC Volt meter and test the test slots on top of each fuse to chassis ground to verify that you have the correct FULL Battery voltage on the fuses. Low or weak voltage will result in a weak spark.
Pin "A" on all the coil packs is COIL GROUND and it goes to G-107 and that is on the back of the Drives side cylinder head. Its two black wires with a white stripe terminated in a single eyelet.
11-12-2018, 06:52 PM
Slightly rough idle with erratic 02 sensor readings
