Hi all. I have a issue hopefully someone can help me with. Ive had this slight stumble when slowly accelerating and and at a steady rpm below 5k.At WOT and after 5k it runs like a champ. That tells me that its a problem with open loop and its not my tune.so thinking maybe it was my o2 sensor,i ran it with it unplugged.The car ran great with exception of a little more popping than normal due to the extra fuel that was being added because the o2 sensor was unplugged making it run . So I drove like that for a little while, and yesterday I bought a new o2 sensor and the stumble did not go away so that pretty much rules out that sensor. So what else should I check that would cause a lean misfire when the o2 is connected? My thoughts are, could an exhaust leak at the manifold (top and/or bottom) cause a noticeable stumble? Im going to check my tps as well. Any other thoughts or ideas will be great. Thanks guys for any help you can give me

 

Are there any CEL besides the o2 when it is unplugged?

Motor setup? What's it tuned on and who tuned it?

 

No theres no CEL's on. My setup is aem short ram, dc header, 5zigen exhaust, 750cc injectors, skunk 2 IM, skunk 2 valve springs and retainers. The ecu is an obd1 p72 and it was all like this when I bought the car. So im not sure about the tune. I believe its just chipped, but maybe you can tell me with a couple pictures what it looks like to you. I am planning on eventually going turbo but as of right now this is it. Now the car ran great for a while and then all of a sudden it started doing this. Thanks for the help



http://s1243.photobucket.com/albums/gg560/bmoreb18c1/ Pics of my Ecu

 

Theres the red flags

 

Really? correct me if im wrong but i thought the tune only mattered at open loop which is higher RPMs and WOT in which my car runs great. Not only that, ive had the car for almost a year and it just started doing that about a month ago.My thinking was that the readings from the o2, map, tps, temp sensor and rpm was what determind the afr

 

Please if any1 has any other ideas I would really appreciate it. I want to fix today

 

Do you even have a correct basemap for it? Put stock injectors back on

 

750cc injectors on a predominantly stock setup? of course it's gonna run like anus in closed loop. those injectors should be run on an FI application, where you're capable of flowing an adequate mass of air. as mentioned, put the stock injectors in

 

ok so than please explain how the car ran great for the past 9 months on the same injectors and just now it started to misfire. I would think that too big injectors would cause it to run rich but when the o2 is plugged in,it misses when slowly accelerating and I get good mpg's. Unplug it and mpg goes to **** and it runs fine. So to me that means with the o2 plugged in its running lean but its not giving a code. I bought the car the way it is except for s2 coilovers. If you can explain the reason it ran perfect on big injectors for that long of a period and just now started to miss from them then ill go out and buy stock injectors. Thank you for your input but im skeptical that the injectors are my underlying issue here.

 

if the o2 reads low oxygen content, the ecm will lower the injector pulse width. possibly to the point of misfiring. not saying it's indefinitely what's going on. but honestly, 750cc injectors in your setup doesn't make sense.
you mentioned exhaust leaks. do you have any upstream of the o2? because that will throw off o2 readings for sure

this isn't a tps problem, or it would occur with or without the o2 connected. could be map or even iat related. testing of those sensors is straight forward and easy enough.

 

start by checking o2 voltage though. had a friend whose o2 signal was shorting to the heater circuit. was reading 5v all the time. oddly enough, didn't set a code.

 

I do agree that 750 injectors are too big for the set up and the only reason I didnt change them out is because I haven't had a problem and im planning for a turbo in the foreseeable future. I do have a leak at the header so I will be fixing that regardless if thats my problem or not. Thanks for the info, its definitely somewhere to start. I will be checking everything you just mentioned as soon as I get the chance. If everything checks out ok I will be looking to switch injectors out. One more thing, if the ecu is tuned to 750cc injectors wouldn't it be a problem to just throw in smaller injectors without retune? What would happen out of curiosity(im assuming it would run like complete crap)? How about ordering a custom basemap chip from phearable, with my mild set up, do you think that would be ok until I get my turbo kit installed? Thank you once again man, I knew I could count on HT for some guidance!