Backfire

[twizdt]

About 3 days ago i installed a new 3" downpipe due to mine having a hole in it. I removed the cat and cut off the rear o2 sensor while i was under there. From manifold to muffler it is 3" straight piped. I am now getting a very bad backfire during acceleration. Any ideas on what i could have done?

 

[gofer]

...and CUT OFF the rear o2 sensor while i was under there.

found it. you cant eliminate your rear o2 unless your running dsmlink, and even then you've got to set it up correctly to do it.

 

[twizdt]

There are alot of people though who remove their rear o2 sensor and all it causes is a CEL.

 

[gofer]

a lot of people don't recirc their bov either, does it mean its the correct thing to do?

the ecu uses the front o2 sensor to control AFR's and the rear o2 sensor to check the efficiency of the catalytic converter. you will throw a CEL and since your running 3" turbo back your going to have a little hiccup back fire every once in awhile. now if your shooting flames 10ft and it sounds like a stick of dynamite blowing up then your running way too rich.

 

[twizdt]

yea i cant really go over 25 mph or give anything other than a light tap on the gas to accelerate or it backfires rediculously bad.

could it have anything to do with the fact that im using a 99' ecu in my 95' and the running rich and lean became noticable after doing the exhaust?

 

[gofer]

looks like its just you and me on this man, nobody else is chiming in! ive ran a 95 EPROM, my cars 97, & a 99 ECU in my DSM and its been fine, swapping ECU's won't have an affect on AFR's. putting on a bigger exhaust wouldn't change anything either as far as AFR's go, your turbo would just spool faster and possibly creep unless you've still got the t25 (stock) turbo on it still.
did you literally "cut" the wires going to the rear o2 or did you simply disconnect it at the connector? and is that ALL that you've done to the car since it started running ridiculously rich?
ive looked at your mods in your profile and there doesn't look like much, if you could update that thing as best you can we might be able to diagnose the problem your having.

 

[twizdt]

I have not done any mods since i got the car. My profile is wrong. Havent had the car long at all. Its still the stock turbo. I literally cut the wires off of the o2 sensor. I pulled my ECU out and it looks fine. I have an OBX air/fuel gauge and it has always been sporadic. moving from red to green and inbetween the whole time i drive. But since i did the exhaust when it hits red i can feel my car lagging and when i put it into the green it backfires. Green=Rich, Red= Lean.

 

[gofer]

oh no.
you made the same mistake i did when i first got my car bud, you bought a narrow band AF gauge that does nothing but show lights. i hate to break it to you but sell that thing to a honda kid and get a wideband o2 sensor. you can't use the narrow band AF gauge to do anything except look cool.
heres what you need to do since you CUT the wires! remove the drivers seat and then the center console where your shifter is. then lift the carpet up where the front right portion of the drivers seat would be. you'll find a hole in the bottom of your car where the rear o2 sensor was... trace your cut wires back to the connector and unplug the rear o2 sensor wires that you cut.
after you do that try driving your car and see what she does...
its acting almost like you've got a really bad boost leak somewhere. when you did the exhaust did you remove anything from the engine bay? intercooler pipes? intake? anything?

 

[twizdt]

I removed nothing but the exhaust. I already made a rear o2 thing and installed that. check engine light gone but still backfire bad. If i barely touch the gas im fine but like if i hit boost its crazy. at idle i can rev fine and it blows off fine. Recirc BOV btw. Stock.

I didnt buy the gauges either. They were in the car when i bought it.

 

[BoredDSM]

Here are some things that should have been mentioned.

#1. You need to check your ECU and make sure its the right year. Changing between the 99 and 95 ecu could have cause you to have the wrong firing order.

#2. You can run without a secondary O2 sensor without dsmlink as long as you properly bypass it. To find the how to go to VFAQ Site - Visual Frequently Answered Questions. Go to DSM modifications and it is under exhaust. It is the last link at the bottom.

Once all those things are checked and completed, come back and let us know if the problem persists. From there you can further diagnose.

 

[twizdt]

I have had the 99 ECU in it since i got the car. It was running rich and the CEL light was on from it but no backfire or anything. I made one of those things that replaces the rear o2 sensor.

 

[gofer]

Here are some things that should have been mentioned.

#1. You need to check your ECU and make sure its the right year. Changing between the 99 and 95 ecu could have cause you to have the wrong firing order.

#2. You can run without a secondary O2 sensor without dsmlink as long as you properly bypass it. To find the how to go to VFAQ Site - Visual Frequently Answered Questions. Go to DSM modifications and it is under exhaust. It is the last link at the bottom.

Once all those things are checked and completed, come back and let us know if the problem persists. From there you can further diagnose.

the car wouldn't even start if your firing order was wrong. if the car ran rich before the exhaust install its just rearing its ugly head since you don't have a cat. restricting the exhaust. if you got the car with the gauges how do you know what else is installed on that thing? if theres a bigger fuel pump you might be over running the stock fuel pressure regulator going into boost. my guess is a bad boost leak to be honest with you.

for what its worth... heres the firing order BoredDSM was talking about.

You must be logged in to view this image or video.

 

[twizdt]

its always nice to learn something new. i bought the guy from joe_sobo on here. ITs a stock fuel pump. So my first task is to get the 95' ECU? And another thing. When i first put the exhaust on it didnt backfire like this. It seemed like the computer started adjusting for no cat and rear o2 or something. backfire started on second start up. when i drive down the highway with steady gas. It lags from fuel cut or running lean.

 

[twizdt]

Im assuming the last thing for me to try is get a 95' ECU for my car? I have been out of work so much because of this car. .

also will a MD309947 ECU work? Its from a 95' FWD Turbo.

I also noticed if i let the car run for a while before driving it, its not as bad. LIke after the car is warmed up. The backfire is still bad to where i wont drive it but i can get through gears at 10% gas without backfire.

 

[velocitysport31]

you can run a 99 ecu in your car with no issues. 99 ecu's are also flashable. Check your firing order, Also you have to do a pin swich at the ecu, I doubt that your rear o2 is causing you to back fire. Check your plugs make sure there gapped right, make sure you dont have any boost leaks. Also if your running rich you could have a bad coolant temp sensor or front o2 sensor. Make sure all your vacuum lines are good and are not leaking.

 

[twizdt]

Lately it seems like it has been taking a while for my car to warm up. The gauge will show my engine being right but the heat takes forever. Could that be the cooland temp sens? Is it hard to change? I could knock that out and find out if thats it.

What is the pin switch you are talking about and what does it do?

Just went out and realized that i was low on coolant so i picked a gallon up. Took 3/4 the gallon and its starting to run a little better. Can build boost now with not as bad but still bad backfire. What should i look for now? Will low oil have anything to do with it?

And by any chance would the battery being bad have anything to do with it?

And the codes i have for the car are PO446 and PO1105

The running lean is really bad when i get to 5-6 lbs of boost. It seems really jacked up over that.

 

[twizdt]

Bought a new alternator since mine was bad. Still no change. PO1105 code went away though. Should i just go down to a smaller downpipe? Like a 2 inch or something?

 

[gofer]

fix the problem don't just put a bandaid on it. its not the exhaust, im telling you that car has had something changed in the fuel system and you don't know about it. bigger injectors or bigger fuel pump, the PO1105 is your fuel pressure solenoid or more specifically "open or shorted condition detected in the baro read solenoid control circuit"
the PO1105 code should have gone away when you installed the alternator because you disconnected the battery from ground when you did it? after a few drive cycles or going WOT and backfiring really bad it should rear its ugly head again.

 

[twizdt]

Where should i go from here? joe_sobo is who i bought this car from. Thats his screenname. He said everything is stock. What is the pin switch thing that velo was talking about? does that have something to do with it? I dont see how something got messed up in the fuel system just by putting a 3" downpipe on. My dad says it seems like its getting to much air and not enough fuel.

As soon as i get into boost the A/F gauge goes crazy and so does my car. Usually it says its lean and jumps over to rich occasionally.

something else i noticed today was when im trying to get into the boost and it starts running all crazy that the boost gauge will show the boost jumping. like the needle is twitching.

Sorry about all the edits im just trying to be as specific as possible. Unfortunately noone around where i live messes with these cars.

 

[rarson]

The first thing you should have done upon noticing misfiring is to check the spark plug gap. Hell, while you're at it, you might as well just put new plugs in there. The lack of a rear O2 sensor is NOT going to make the car misfire.

The '95/'96 ECU uses an inverted CAS signal compared to the '97-'99. Apparently it could cause some roughness if you use the inverted signal, but I've tried it both ways on my '97 and didn't really notice a difference. I doubt that is your problem.

Your A/F gauge is worthless. Narrowband O2 sensors are dumb; they cannot tell you how rich or lean your car is running. However, if your O2 sensor is bad, it's possible that it's not switching properly and preventing your ECU from going open loop. You might want to think about replacing the O2 sensor.

I would do that after eliminating all possibilities of boost leaks, and if all that doesn't work, you can send your injectors off to get cleaned and flow-tested, to make sure that not only are they the right injectors, but that they're not leaking or clogged.

Is the stock plastic BOV still on the car? How much boost are you running?

 

[gofer]

this is going to be my last post on here trying to help until you do the basics like we said days ago.
read through these threads because this is what you need to do.
IC Pipe Tester

http://www.dsmtuners.com/forums/newbie-forum/15800-how-do-i-make-buy-use-troubleshoot-boost-leak-tester-merged.html

since your new to these cars and their symptoms just start with the basics, its probably a boost leak is all. something simple and not a big deal you just have to take the time to figure out where its leaking and thats something we can't do for you.

what rarson is talking about (open loop/closed loop) is that when your cars idling/not under a load the ECU is in whats called "open loop" mode. what it does in open loop is take the input voltages from the front o2 sensor and tries to keep the AFR at a 14.7:1 or stoich reading. in closed loop the ECU ignores what the front o2 sensor is saying and runs off the AF maps on the ECU itself. what happens when your front o2 sensor goes bad (or becomes disconnected or cut) is that the car gets "stuck" in closed loop since its not getting any inputs from the front o2 sensor and won't idle properly or even want to stay on at all. i know this isn't your problem because if it was "stuck" in closed loop under hard acceleration your car would feel fine since it wouldn't be using the front o2 to maintain a steady AFR it would be using its preset AF maps. it would just idle REALLY rough or not at all...

take a shopping list to home depot or lowes and build a boost leak tester (BLT) for about $20. once you eliminate the possibility of it being a boost leak then post up the results/what you found.

 

[rarson]

what it does in open loop is take the input voltages from the front o2 sensor and tries to keep the AFR at a 14.7:1 or stoich reading.

That's closed loop. The ECU goes to open loop at WOT and ignores the O2 sensor voltage. The reason for this is what I stated before: narrowband O2 sensors are ridiculously inaccurate in determining how rich the fuel mixture is. They basically just switch voltage around stoich.

 

[gofer]

That's closed loop. The ECU goes to open loop at WOT and ignores the O2 sensor voltage. The reason for this is what I stated before: narrowband O2 sensors are ridiculously inaccurate in determining how rich the fuel mixture is. They basically just switch voltage around stoich.

im an idiot! sorry, rarsons right i guess those sweet tea vodkas still haven't worn off yet from last night. closed loop and open loop explanations above are backwards. if you really want to get a 100% understanding of closed loop/open loop operation is read this... fueltrimupdatepoints [ECMTuning - wiki]

get that BLT done.

 

[twizdt]

Okay ill do a boost leak test tomorrow. i just dont see how only putting a 3" exhaust made me have a boost leak but ill do it anyways. yes stock BOV btw. Tried driving the car home tonight and i couldnt. running so bad had to leave it in a parking lot.

after the car runs for about an hour it runs fine unless i try to get into boost. but first starting it for the day it wants to stall out after warming up it only misses and stuff under boost.anything over 0 on my gauge and the car is garbage .

 

[joegamma1]

Did you make sure your MAF is plugged in tight?