I've got a 2001 zo6 that is running rich. If I start it up and immediately start driving it feels free and normal but after 15 seconds or so I can feel something set in where the car loses power and develops a slight bog to it. If I let it idle for a while it's there immediately. It varies in severity but is always there and I'll try to give as much info as I can. Idle isn't rough per se but you can tell it's not consistent and I can feel the bogging/dragging the most under 3k rpms and especially letting off the throttle and coasting in gear. When I really get on it or push into higher rpms the bogging seems to lessen or go away. The longer the car runs the richer it gets, on a short drive the scan tool I used suggested that both Bank 1 and Bank 2 fuel trims would start at about -5 at idle and work their way up to about -20 while ST's would again start somewhat normal, about -2 and climb to -10 -> this was after about 10mins of driving. Of course letting off the throttle it bogs enough to feel like the brakes are being applied a bit and I get all the gurgles and pops. The Maf reads 6-7g/s at idle and climbs to the 30's while driving, fuel pressure stays around 58-60psi, o2 sensor voltages were not pegged high or low, and no codes are stored in the DLC or shown with a scan tool. The misfire monitor showed zeros across the board.

I've done a few things to try and fix this problem already and they are as follows-----> Replaced the plugs and wires, pulled the injectors from their seats and with the ignition on they showed no signs of leaking, replaced air filter, ran TB/intake cleaner through the system, borrowed a known good oem zo6 maf, reset FT's by unplugging the battery. The only thing I have not done yet that I want to is run some techron through it but did not know if fuel quality would make the car run like this? Today I went back and torqued the plugs and got 1 and 2 clicks from the wires but to no avail the car is still bogging, not hard enough to stop the engine or create a crazy idle, just enough that you can feel the car is way more sluggish and held back than it should be.

I do have access to a scan tool and will try to answer any questions as best I can. This has been absolutely driving me nuts so any advice is greatly appreciated and thanks in advance!

 

I'm no expert, but with it starting out okay and after a short time going rich, sounds to me like it's an issue occurring during closed loop mode (which will happen as O2 sensors warm up).

Are there any modifications to the car? Any exhaust leaks? Another member recently was having an issue with poor fuel mileage and his culprit was O2 sensors installed in the wrong/opposite banks.

You could try unplugging the MAF and seeing how it runs as another diagnostic step. If it still runs poor then we can probably rule out air leaks or anything going on with the intake side of things.

 

First thing to ask is if the car is tuned (speed density)…second how long has the car been running rich ??..if it has been running rich for a while your cats may be clogged..clogged cats will normally show rich fuel trims…to check for clogged cats you can try an infrared gun on the weld rings on either end of the cats, a vacuum gauge hooked up to the intake manifold or pressure gauge installed in the upstream O2 sensor bungs…you can even disconnect the left AIR check valve and see if the fuel trims are better…now is the car running in open loop when you see the negative fuel trims ??…in OL the O2 sensors aren’t used in the fueling strategy but use ECT, IAT, MAP, and TP…now if the cats are clogged changing them doesn’t solve your problem…clogged cats are an EFFECT but not the CAUSE….I always look at MAP key on engine off for an issue like this and you should see at sea level about 29.9 inches of Hg and at idle about 10 inches of Hg…also look at IAT and ECT KOEO after the car sits overnight…should be close to ambient temperature…are the upstream O2’s switching between 200-800mv at idle and at 3000 RPM and are the downsteams steady at about 700-800mv’s ??…the last Corvette I looked at with a rich running engine were 4 injectors were flowing too much fuel…if your scan tool can do an injector balance test I would do that…maybe the EVAP purge valve is mechanically stuck open…remove the EVAP line from the intake manifold and plug the manifold fitting and look at your fuel trims.

 

At the moment the car has a Volant intake, a borla straight-through axleback, secondary air injection delete, and a catch can. Wires and plugs were replaced with OEM spec AC Delco. Checked o2 sensor location and the rears did cross over and obviously the fronts were where they should be. Unplugged the MAF and it still seemed to bog except the pops/gurgles were not as bad as I was experiencing earlier?? Sprayed soapy water around the headers and exhaust and no bubbles.

I did notice something odd but do not want to get ahead of myself here- I can definitely hear a more pronounced clicking from the passenger side coil pack area than the drivers side. If I put my ear up close I can hear the clicking but from what I could tell the passenger side was much louder. I'll give an update tomorrow when I can go more in-depth with the scan tool and run the tests C5 Diag mentioned.

 

Is the car tuned to accommodate the AIR system delete?

I'm not necessarily suspecting the catch can, but have you tried reverting the PCV system back to stock just to rule out the catch can setup?

Lastly, you mentioned "headers", but do you just have the stock exhaust manifolds? And stock H-pipe with cats?

 

Alright I apologize as I wasn't able to get as much done today as I wanted to but still got some good info. Was not able to check the Evap purge valve or do a balance test on injectors however, I was able to check everything else. I've only owned the car maybe 7-8 months and the car has ALWAYS had a burble to it, especially getting back on to throttle after coasting or after some highway miles but figured with no mufflers/stock cats this was normal? Now it has become more frequent and doesn't matter whether the car is hot or cold. This problem seems like it has developed, getting a bit worse with time over the past few months rather than just starting out of the blue. Have no idea if it has been tuned or not but the secondary air seems to have been tuned out as I do not have codes for it so the pcm has at least been messed with a little. Didn't check the temp of the cats but in between both cats there's a ton of bubbling, I'm assuming from the backfires- no leaks though. The scan tool showed 29.2 in/Hg KOEO and 9.1 in/Hg at idle(central Indiana) and the Barometric pressure read 29.2 on the tool. IAT and ECT both read 66 degrees which was about air temp, maybe a bit warmer. In open loop the short term ft's varied in between -5 and +5 at idle but never got out of the negatives after closed loop kicked in or while driving.

Here's where I think the most progress was made however- On startup all o2's read high (figured since the secondary air delete) but B1S2 showed only 670mv while B2S2 was in the low 800's. When closed loop kicked in B2S2 dropped to about 150mv while B1S2 stayed around 370mv. They stayed in this range for several minutes but as things warmed up they started to increase. ---> all of this still at idle. As the car got warmer I took on a light "parking lot" drive the voltages climbed but never saw anything more than 500mv for B1S2 while B2S2 touched the 700's after a little more real driving. Fuel trims were still in the negatives while this was going on which was odd, at least to me. The last time I had it on a scan tool I had been driving it for quite some time before so maybe it gave the o2's more time to warm up? And that day I do remember B1s2 was not moving around nearly as much as bank2, staying around 790mv and not fluctuating as much as the other sensors. The upstreams look like they're perfect though, fluctuating between about 150mv and 800mv

I will for sure get to more testing tomorrow as the scan tool I use is at the shop where I work and should be able to get it on the lift as well to be a little more thorough in looking for exhaust leaks. I still want to do the balance testing and evap test but I am curious to hear what people might have to say about the o2 sensor issue.

 

I would not worry about B1S2 or B2S2 as the downstream O2's do not really control fueling...take the car for a drive on the highway for 30 minutes and see what your O2's and fuel trims are doing and you are in closed loop...I look at total fuel trims so if the short terms are minus 10 but the long terms are plus 10 the total fuel trims are 0 which is perfect....if the downstream O2's are switching like the up streams the Cats have lost their efficiency...here is some good fuel trim videos my buddy Matt has done..it is 2 parts...also a Scanner Danner video...OH, what does your MAF read at hot unloaded idle ??...engine at operating temp with no loads ??

 

Did some more testing today and here's what I came up with. On cold start again the upstream o2's read very rich as expected but saw the voltages "plateau" with a high voltage(somewhere in the 800mv range) for a short second or two pretty often. As the car got warmer the o2 oscillations got better. Even at operating temp(195-200degrees) though I would still see plateaus here and there on both banks at the same moments. They would not last as long but were still there... As for the fuel trims, they were once again pretty low at idle, When the car got up to temp, my fuel trims improved, actually reading a tad lean but nothing worth fretting over for short terms and the long terms would adjust accordingly after a bit- all of this still at idle. As soon as I started to accelerate though she would get pretty rich. In the first few moments of acceleration, I would see anything in the ranges of -8 and -15 depending on how hard and what rpm I hit the throttle(anything under 2500 was pretty boggy). As long as I kept increasing speed/rpm though it would start to lean out a bit. After 30mins of some normal driving with the engine up to temp my total ft's would read -11. When cruising at a set speed the car behaved similarly to idle but on the rich side at -5 to -8 on short trims but would fluctuate enough for the total fuel trims to be around -8 or so, once more depending on rpm. The higher into the rpms I went the better they looked, but would usually not change much after about 4500-5000rpms and total fuel trims in this range would be very close to 0. Didn't push the car much harder than that and tried to be smooth with the throttle and braking. I can feel the drag/bogging at every rpm though, just really apparent down low.

I also did the injector balance test as well. This was my first time actually doing a balance test so bear with me... My coolant temp gauge in the dash showed about 135 degrees when doing this, do not know if engine temp matters for this sort of thing but figured it can't hurt to mention it. My readings for the initial pressures and after the injector pulse were; CYL 1- 55--->24 CYL 2: 54--->24 CYL 3: 54--->23 CYL 4: 54--->23 CYL 5: 54--->23 CYL 6: 54--->24 CYL 7: 54--->22 CYL 8: 54--->23. I searched a little bit but could not find the specs for this test. Also, do not know if this is worth mentioning but after hitting the injector pulse ok and letting the pressure drop, the reading for each injector was one psi lower initially. I could watch the psi increase very slowly for about 10-15 seconds until it would come to a stop, never increased more than 1.5psi. I did this test twice for peace of mind with the slow psi increase. Ran the engine for a bit after the first time and let it set for a while before I did it again to get the same temp and ensure everything was tight. Numbers were the same.

When the engine is up to temp idling my MAF is reading about 6.2 g/s

 

It’s a little hard trying to diagnose this without having your scan tool in front of me…I would check back pressure on the cats since you say on acceleration it’s “boggy”…I don’t know if Autozone has something like this to rent out but it screws into the upstream O2 sensor bungs…does your oil dipstick smell of fuel ??…your injector balance test is fine.

 

I called some Autozones and O'reilys and no luck.... Can the cheap-O amazon backpressure testers be trusted? Also pulled the dipstick and no evidence of fuel on it or a fuel smell. Some other pieces of info that might help is a few months ago(before the problem really got bad) was driving in stop-n-go traffic after driving on the highway and got some really hard backfires and gurgles that lasted until I reached my destination. Anytime I let off the throttle it sounded like thunder coming out of my exhaust- has never done that since. Also, about a week ago I was highway driving doing a consistent 60mph for about 20 mins when the car started dragging so hard that I thought I needed to stop and pull off. I could feel it in the pedal that it didn't want to accelerate but after a couple of spirited "pulls" it went away and reverted to its usual lighter dragging state. I feel like these relate to the cats but when the car is up to temp my rear o2's read a steady 820-ish mv and do not oscillate like the fronts. And also just to say I did I put the old pcv system back on and disconnected the battery to reset FT's and no change.

 

A loss of power or "dragging" or "lagging" like you mentioned is a pretty good symptom of clogged cats...like I mentioned you could disconnect the driver side AIR check valve and run the car at idle and at higher RPM's and see if the O2's and fuel trims look different compared to the right bank with the scan tool...you can try smacking of the cats with a mallet and see if you can hear the substrate coming apart...you can also go to Harbor Freight and get one of their Infared guns and point the gun at the weld rings on both ends of the cat...the downstream weld ring should be 50-100 degrees hotter than the front weld ring...the pressure gauge is more accurate though...not more than 2 psi max pressure at 2000 RPM...BTW the rear O2's do not oscillate like the front...if they do you have problems !!...video below...if the cats getting clogged the upstream O2 sensor will go rich above 800mv's...if it does you'll see the fuel trims go negative like yours...if it is plugged up you'll have to find the cause first before you install a new one.

 

May be onto something here then!! When I was first diagnosing this issue I could have sworn that I got another click from the spark plug wire to the coil pack on cylinders 6 and 8 but chalked it up to my mind playing tricks on me. (If you push hard in the wrong places sometimes you can hear/feel the wires crinkle a bit and thought that was the case) Although that would not explain why it would be rich on both banks except... When I replaced the spark plugs a few weeks ago noticed that cylinder 5's spark plug was loose and had some blow-by on the plug. It wasn't crazy loose per see but noticeably looser than the others. The blow-by was not mistakable though. One of my corvette friends suggested maybe the valve cover gaskets were leaking onto the plugs but found zero signs of that being the case- nothing on the wires and heads were dry. When I changed plugs and wires I did notice a slight improvement in the idle but car still drove the same but has never dragged that hard on the highway ever since. Is it possible that those issues could have caused this and now I am just dealing with the cats? I apologize for not mentioning this earlier!!! Will make a harbor freight run tomorrow to test the cats for sure!

 

Well if you start getting these secondary ignition misfires and if that's what it was for a period of time you start sending that unburned fuel into the cats and that's when they start getting clogged...I wouldn't use just the temp reading to verify it's clogged...I can think of 5 tests I could do and the infared gun may be the least accurate but you can try.

 

If you have a vacuum gauge It's pretty easy to test for a clogged cat. Once the car is warmed up the vacuum gauge will read around 21 in hg. The needle.should be steady. You then slowly bring the engine to 2000 rpm. If the cats are clogged the vacuum will drop. If they are not clogged there will be some small increase in the vaccum. With clogged CATs the engine will usually run hotter than normal.

 

Connected the vacuum gauge to the hose coming off the booster/check valve and at idle it did read 21 in/hg maybe a tad less. When I first touched the throttle it would go down a bit to around 18-19 in/hg (less than 1000 rpms), but after slowly bringing the revs up it climbed back up to about 23 in/hg when holding the throttle around 2k rpms. After letting off the throttle my readings jumped a bit to maybe 25 in/hg. From what I understand this seems normal? ( made an edit to this as the first test I did not hold the rpm steady). The needle held steady at idle and was as steady as my foot allowed it to be when up in the rpms The car doesn't seem like it's running that much hotter either, it actually comes to temp quicker at idle rather than driving, although it's about 45 degrees out right now. Also smacked the cats around a bit with a rubber mallet and didn't get any sign of material bouncing around in there.

 

I was able to snag some pictures of the scan tool readings but wasn't going to post them as I wanted to actually set it up right to make it easier for everyone to read the next time I had the tool and take more pictures then. I'm stumped and just want to give as much info as I can... (I apologize for the glare and not setting this up right but when I get back to the scan tool I will set it up to more frames/sec and give proper voltage range references+better idle frames) Not very clear markings so I will do my best to describe what happens when- The dark line represents a stop at a stop sign so I'll start there. Shifts occurring at 1470, 1490, 1510, and 1570- each time I brought it up to roughly 3000rpms, maybe a bit less in the later gears. after 1570 is cruising speed for a while(2000ish rpms 5th gear). I should have let this go on for longer- at 1680 you can see I come to a stop and it begins to idle again. Looking at it now I'm realizing just how bunched up the o2 readings are under load, height-wise and width-wise, then it returns to normal 200-800mv oscillations at idle. Between 1570 and 1600 I do not think the sensors got out of the 400-500mv range... You can see my ft's lean out as I accelerate and then drop back down a bit when coasting. I think cleaning the intake system, MAF, and all of the other tests/cleaning/torquing I have done have made the trims a little less rich overall, obviously not seeing the -20's as before but this was also a short drive with temps in the desired range (mid-high 190's) but still feeling the drag...

 

Fuel trims should not lean out as you accelerate...if they do it's either a fuel delivery or MAF issue...I do a VE or volumetric efficiency test when faced with this possible issue...I use a separate specialized scan tool that can tell me if the engine is breathing properly and I'll know just going a block...if you give me some numbers and let me know what the max MAF is in grams/second and RPM as you hit WOT I can plug the number into my calculator...these are the numbers I need:

1) RPM at WOT ***
2) MAF at WOT ***
3) Temp
4) Baro in inches Hg
5) Humidity

 

Alright here's what I came up with... The max MAF reading I got was 297g/s at roughly 5600rpm(this wasn't exact as I did this a few times and got a little variation but just past 5500 seemed to be the sweet spot) at WOT. The temperature outside was 51 degrees(IAT showed 59 while driving), Baro was right at 30, and the humidity was 36% according to a quick google search.

 

Your VE is over 80% so your engine is “breathing” properly !!…close to 300 grams/second is also a good number at WOT.

 

I found the problem!!! Or at least most of it... Was working in my garage and heard a big PSHHHH come from the engine bay. I'm used to hearing some air escape after shutting off the car but this was 15-20 minutes after the fact? Checked all vacuum lines I could see without getting too invasive and turned my attention to the booster... The clamp holding the hose to the booster was not tight enough to create a good seal with the check valve! Have no idea if it was the factory clamp or not but with a little force I could wiggle the hose around pretty good and pull the line right off with no issues while the clamp was on! Put a fresh hose clamp on and drove it and the difference was NIGHT AND DAY. There is still a bit of drag but the car no longer bogs or drags after letting off the throttle. This also probably explains why the car was fine at idle but got leaner(vacuum leak) the more I pushed the rpms and the fact that everything tested fine for the vacuum test as I used that hose. I bet if I cleaned the MAF(cleaned MY maf but not the "loaner" on it now), and checked the other lines for improper fit/looseness the car would run flawlessly. Also want to run some techron through the system and change the fuel filter(probably not needed but it's cheap and might as well with 65k on the dash- the deeper I go into the car I realize how poorly it was taken care of by the old owner). Will give an update tomorrow after all this so I can verify my results with scan tool data!!