TPI swap backfiring in closed loop

Hello,

Been banging my head on this one for awhile. Installed 350 tpi in a cj7 all stock. In closed loop it back fires through the exhaust at certain rpms. I ran datamaster and all logs look good. open loop runs great. O2 is heated and installed about 12" past collector. Can this be to far? The values I see from o2 are swinging from 50 to 900mv. All smog removed in chip as well as engine. EGR still on. Thought the egr solenoid was the problem. Changed that same thing.Blocked off egr samething. injectors all ohm out at 16 ohms tps,cts, mat and iac all check good. Tried 2 different ICMs 2 different ecms. 2 different proms with pretty much same tune. I am about to lose my mind. Main question is does anyone know if distance of o2 from header can be an issue closed loop runs perfect. Thanks in advance.

Quote:

---

O2 is heated and installed about 12" past collector. Can this be to far?

---

the distance past the collector isn't important at all, what's important is how far it is from the exhaust valves.

are we talking 12" past the collector with longtubes, or what?

if it's fairly far back, there can be two problems, it doesn't get hot enough to read properly, and at low airflows you have to slow down the integrator a lot, or it'll be all over the place.

if the o2 is switching properly and you're getting lots of cross-counts, i'd call it good enough where it probably isn't causing your problem

Quote:

---

In closed loop it back fires through the exhaust at certain rpms. I ran datamaster and all logs look good. open loop runs great.

---

are you sure about that? are you sure that if you force it to stay in open loop (unplug the o2 sensor) it doesn't start acting up eventually?

it could be a coincidence that when it's warm enough to enter closed loop is about when it's warm enough that the problem starts occuring

or the different fueling in open loop is masking a problem.

i would definitely suspect a spark misfire. perhaps you have a bad signal from the pickup in the distributor due to a wiring problem (harness to close to plug wires?), which seems to be common with swaps.

Hmm my harness is very close to plug wires. Hedman conversion headers. About 5 inches longer than stock camaro manifolds. But I thought heated o2 should be fine. I'll try running it with o2 unplugged. And raise harness because it's practically on top of the distributer

Unplugged tps to force in limp mode. Ran great. Has to be feedback issue from o2.

Thanks for response. Greatly appreciated

Are you running a VSS? If I am not mistaking it will never go into Decel Fuel Cutoff without one.

Running a vss and it reads close to my speedo in datalogs. Does the backfire at stop and driving. I'll look into my harness.

Quote:

---

Originally Posted by steveo

are you sure about that? are you sure that if you force it to stay in open loop (unplug the o2 sensor) it doesn't start acting up eventually?

it could be a coincidence that when it's warm enough to enter closed loop is about when it's warm enough that the problem starts occuring

or the different fueling in open loop is masking a problem.noticed

i would definitely suspect a spark misfire. perhaps you have a bad signal from the pickup in the distributor due to a wiring problem (harness to close to plug wires?), which seems to be common with swaps.

---

I unplugged the o2 and same thing. Moved harness up away from distributer. same thing.

I noticed in datalogs map voltage going down to .7 when slowly getting on throttle and thats when it would backfire like crazy. put vac gauge on and vac is increasing. inj bpw very low below 1. wtf would cause that. no vac leaks of any kind

I unhooked the inj and plugged in noid went from slow flash to almost full on at light throttle. So I am assuming distributor issues.

I tried to 2 different distributors with different icms

i will ohm out wiring to ecm from dizzy this weekend.

Any inputs based on this data would be greatly appreciated.

Where do you have your map sensor vacuum line connected? The map sensor has the lowest voltage output at highest vacuum. As throttle is opened the vacuum should drop going to sensor. Have you checked for vacuum leaks?

Brian

Map voltage at idle 1.5 at idle. 4.8 with key on eng off. Vacuum connected to right port. It needs a tune. But I can't drive it really to datalog. Stock except dual 2.25 exhaust. Need a tune close to datalog. Limp mode runs ok. I'm chasing wires this weekend. Thanks for the response. Leak checked none.

Ok so I ohmed every wire to the ecm. all looks good. Still starts backfiring light loads. 20 to 30 on map. If I get on it and go full throttle its ok. Engine is a fresh rebuild so I really don't want to do that. Next question--- WOULD A STOCK TUNE BE OFF ENOUGH TO CAUSE A BACKFIRE OUT THE PIPES??? ENGINE IS STOCK EXCEPT 2 1/4 DUALS WITH HIGH FLOW CATS. IDLES PERFECT. RUNS SMOOTH AND STARTS TO BREAK UP AT ABOUT 2800 TO 3000 RPM. THEN BACKFIRES NOT RUNNING RICH IN FACT I THINK ITS LEAN. PUT IN A 3RD ICM, EVERY SENSOR IS TESTED and GOOD AND READ OUT WELL ON SNAP ON SCAN TOOL AND DATA MASTER. Im about to lose it.

You are going to have to datalog this thing and post the log. We need what BCC (calibration / letters off memcal) you are using. If you can give the list this info, we can tell you more about what your problem is. It is hard to help you when there is no data to look at.

Brian

This may sound to simple. But with a new engine you may have some spark plug fouling issues. Stick a new set of spark plugs in it and see if the problem goes away. I had a similar issue as you when I rebuilt my TBI engine. I went thru a couple of sets during the engine brake-in.

Thanks for the responses. This weekend I put the original distributor back in with a new pick up coil and my 3rd icm. It runs perfect!!!!!! I will still post the datalog and see if there is room for improvement. Since I bought all the stuff to burn chips and datalog. It runs real strong so who knows may not need a tune. Thanks again. The distributor that was giving me issues was an ebay 70 dollar special. Billet with red cap. It was well built and very close to an MSD dizzy. Had 2 different ICMs on it and ohmed out pick up coil. All good. Not sure why this distributor was causing backfire.