Bad Post-Cat O2 & Issues
Bad Post-Cat O2 & Issues My car has been running very inconsistent since I swapped this other LS1/4L60E in from another "stock" doner T/A. The issues the car has been having on and off are:
1) Rough, inconsistent idle.
2) High idle RPM (sometimes over 1,000).
3) Rough, late upshifts , and hard gear-by-gear downshifts at a stop.
4) Inconsistant dead spots at certain RPM ranges.
I have determined (after some testing) that this "stock" auto has to have AT LEST a 2,500 stall in it. It MAY have a shift kit as well, but I'm pretty certain that the ECU is on a stock tune. I already had 3.73 gears in the car from before which I didn't figure would effect anything major with retiring to a stock setup.
I have since replaced the alternator and found codes for a bad TPS sensor and "bank 2, sensor 2" O2 sensor, which I'm pretty sure is passenger side post-cat (correct me if I'm wrong. I borrowed a Craftsman OBD II scan tool from a buddy and went ahead and replaced the TPS sensor. I then unplugged the battery for a bit to reset the ECU. After running/driving the car for a bit, it seemed to fix the idle issues it was having and seemed to pull more consistently. It also made the code for the TPS go away so I guess that solves that issue. The "Check Engine light came back on after a whole thogh, and the scan tool showed the code for the O2 sensor "low voltage" once again. I went ahead and cleared the code just for the hell of it, but now when I start the car back up, it's running like DOG ****! Worse than ever perhaps with really high idle, and it stals, misses, and dies when I put it into gear!
I am going to replace that O2 this week for sure, but now I'm reading that the rear post-cat O2's don't really effect the way the car actually runs. Is that true??? It was finally running good it seemed, but for some reason it DID NOT like when I cleared that code.
1) Rough, inconsistent idle.
2) High idle RPM (sometimes over 1,000).
3) Rough, late upshifts , and hard gear-by-gear downshifts at a stop.
4) Inconsistant dead spots at certain RPM ranges.
I have determined (after some testing) that this "stock" auto has to have AT LEST a 2,500 stall in it. It MAY have a shift kit as well, but I'm pretty certain that the ECU is on a stock tune. I already had 3.73 gears in the car from before which I didn't figure would effect anything major with retiring to a stock setup.
I have since replaced the alternator and found codes for a bad TPS sensor and "bank 2, sensor 2" O2 sensor, which I'm pretty sure is passenger side post-cat (correct me if I'm wrong. I borrowed a Craftsman OBD II scan tool from a buddy and went ahead and replaced the TPS sensor. I then unplugged the battery for a bit to reset the ECU. After running/driving the car for a bit, it seemed to fix the idle issues it was having and seemed to pull more consistently. It also made the code for the TPS go away so I guess that solves that issue. The "Check Engine light came back on after a whole thogh, and the scan tool showed the code for the O2 sensor "low voltage" once again. I went ahead and cleared the code just for the hell of it, but now when I start the car back up, it's running like DOG ****! Worse than ever perhaps with really high idle, and it stals, misses, and dies when I put it into gear!
I am going to replace that O2 this week for sure, but now I'm reading that the rear post-cat O2's don't really effect the way the car actually runs. Is that true??? It was finally running good it seemed, but for some reason it DID NOT like when I cleared that code.
Joined: Nov 2001
Posts: 34,647
Likes: 2,573
From: Schiller Park, ILL Member: #317
You mentioned that the engine has been giving you trouble since you swapped it in. Did it ever run right to the best of your knowledge (say, in the previous car?) If so, any number of items may have been damaged during the swap that could be causing your issue, even something as simple as a vacuum leak. Honestly, the issues that you have described could certainly be caused by a vacuum leak, and one helpful thing to look for in this regard would be LTFT (long term fuel trims). If they are elevated or near max (25%) then, combined with your other issues, it would point even further in the direction of a vacuum leak. Does your scanner allow for comprehensive PCM evaluation, or is it just a code reader?
It's also possibile that this "stock" engine may not be stock, and/or may not be healthy.
Well all I have right now is a basic scan tool. The engine seems to run just fine under normal driving conditions (part and full throttle). I went ahead and swappe the O2 and the TPS and reset the battery again. This time it's running fine, but idling higher than ever! It idles just shy of 1,000 RPM's when in Park (without A/C on) and around 600 RPM's when put into gear. I can't seem to make it go down from there really. When I took it out driving today, it seemed to be running better than ever, but the "low voltage" codes for the TPS and O2 came right back even with new plugs. A friend suggested that I clean the sensor connector plugs to those thoroughly so that's what I'll try next. I guess after that, I need to start checking wires and voltages. Is there a good way to check for vacuum leaks? I had a vacuum leak coming off the EGR tube into the LS1 intake when we first put the motor in, but I replaced the o-rings there and that fixed that and the idle issues, for a while at least. A vacuum leak still wouldn't expain these low voltage codes though would it?
Joined: Nov 2001
Posts: 34,647
Likes: 2,573
From: Schiller Park, ILL Member: #317
However, if you are still getting that code after trying a couple different TPS sensors, then it stands to reason that your search should start here. Checking wiring/connectors is probably a good next step.
A poor or missing reading from the rear O2s is definitely not the source of your idle problems, but the fact that you are getting multiple low voltage codes on sensors that you've already tried replacing may indicate a wiring or PCM related issue.
Ok, I cleaned the plug to the TPS with CRC spray and did a "TPS reset" buy unplugging the TPS, turning the key to the on position for 30 secs, then taking the key out and plugging the sensor back in, and finally starting the car and letting it run reach running temps. That seemed to cure the idle issues and made that code go away (for now at least). The car now idles just over 500 RPM in Park and at about 300 RPM in gear (all with A/C off). I noticed today as the car was running that the side that is throwing the "low voltage" 02 code (passenger side) has an exhaust leak from a worn weld on the back of the cat. Could this cause that code to be thrown as well?
The car is idling a bit high again. It's running fine and everything, but idle in Park is around 700-800 rpm and just under 500 in gear. Do I maybe need to play with the idle screw and reset the TPS to that? It's not throwing any idle codes anymore. Also, how would I test for a possibly vacuum leak?
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Joined: Nov 2001
Posts: 34,647
Likes: 2,573
From: Schiller Park, ILL Member: #317
Ok, I cleaned the plug to the TPS with CRC spray and did a "TPS reset" buy unplugging the TPS, turning the key to the on position for 30 secs, then taking the key out and plugging the sensor back in, and finally starting the car and letting it run reach running temps. That seemed to cure the idle issues and made that code go away (for now at least). The car now idles just over 500 RPM in Park and at about 300 RPM in gear (all with A/C off).
650rpm in park/neutral
550rpm in gear
A 300rpm idle in gear is way too low. But it may have still needed to do an idle relearn at that point. It will eventually relearn on it's own even if you don't follow the process though. It's simple for the late model LS1s, I think it's just a matter of letting it reach operating temps, then idle in gear for 5 mins, then in park for 5 mins, then shut it down and restart. With an early LS1 car, I believe you also need to let it idle in and out of gear with the A/C on.
The car is idling a bit high again. It's running fine and everything, but idle in Park is around 700-800 rpm and just under 500 in gear. Do I maybe need to play with the idle screw and reset the TPS to that? It's not throwing any idle codes anymore. Also, how would I test for a possibly vacuum leak?
There is the standard pratice of using carb cleaner, etc., to spray around various areas that may have vacuum leaks. But if you could get a hold of a comprehensive scanner, then you could also look at the LTFTs - if they are highly elevated (in addition to your idle issues) then a vacuum leak is more likely. You could also inspect the PCV system, since this can often be the source of vacuum leaks on these engines.
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Verdad Gallardo I know these 4L60E's are electronically CONTROLLED, but don't they still use vacuum to actually shift? What I'm saying is, could a vacuum leak cause those hard, laggy shifts? I know it did in my old Mustang that was fully vacuum controlled at least.
I went and got the exhaust leak on the back of the cat welded up and cleared out the code and it seems to be running a lot better. It idles pretty much within spec, but it will occasionally want to go up a little. That may be because it is still learning idle since I haven't driven it much since my last post. It still shifts kinda funny too, which is why I was asking about transmission vacuum. If not, I guess its just the bigger stall in there making it act funny. Car runs pretty good all and all though.
I went and got the exhaust leak on the back of the cat welded up and cleared out the code and it seems to be running a lot better. It idles pretty much within spec, but it will occasionally want to go up a little. That may be because it is still learning idle since I haven't driven it much since my last post. It still shifts kinda funny too, which is why I was asking about transmission vacuum. If not, I guess its just the bigger stall in there making it act funny. Car runs pretty good all and all though.
Joined: Nov 2001
Posts: 34,647
Likes: 2,573
From: Schiller Park, ILL Member: #317
I know these 4L60E's are electronically CONTROLLED, but don't they still use vacuum to actually shift? What I'm saying is, could a vacuum leak cause those hard, laggy shifts? I know it did in my old Mustang that was fully vacuum controlled at least.
I went and got the exhaust leak on the back of the cat welded up and cleared out the code and it seems to be running a lot better. It idles pretty much within spec, but it will occasionally want to go up a little. That may be because it is still learning idle since I haven't driven it much since my last post. It still shifts kinda funny too, which is why I was asking about transmission vacuum. If not, I guess its just the bigger stall in there making it act funny. Car runs pretty good all and all though.
I went and got the exhaust leak on the back of the cat welded up and cleared out the code and it seems to be running a lot better. It idles pretty much within spec, but it will occasionally want to go up a little. That may be because it is still learning idle since I haven't driven it much since my last post. It still shifts kinda funny too, which is why I was asking about transmission vacuum. If not, I guess its just the bigger stall in there making it act funny. Car runs pretty good all and all though.
A dirty MAF can cause some weirdness with the shifts. The aftermarket stall with stock tuning can also sometimes lead to some weirdness.
Last edited by RPM WS6; May 16, 2012 at 11:31 PM.
