Bogging and backfiring out the air intake.

Alright, I have almost had it with this. Every once in a while (1 in 3 starts about) I'll start to accelerate out of my driveway or a parking spot and the engine will just start to bog down, It'll lose acceleration like it's about to stall, then it'll backfire in the front of the car (the air intake i presume) and the car will launch forward a bit and drive perfectly normal like nothing ever happened and it won't do it again until it chooses to, the next time I start the car (cold or warm).

I just recently took it to a mechanic and they couldn't duplicate the problem (go figure) but I had them motorvac the fuel injectors and I have noticed more response in the throttle but it still does it. I've changed the TPS, spark plugs, wires, air filter, and I've seafoamed the TB as well as put fuel system cleaner in the gas tank. I plan on pulling out the IAC valve tomorrow and see if I can clean it out a bit, and if that doesn't work, I'm going to check out the O2 sensors. Is there anything else I could check out that might be the cause?

There are no codes.

It's a 1997 I6 Cherokee Country 134,500 miles.

Thanks.

I posted this on another board awhile back, cant remember which one now, but I had similar symptoms on a 98xj.

From a cold start in the morning, you could sit in it, start it up perfectly, even rev it up and no problems. Leave out the driveway and start going down the street and in about 30 seconds to a minute into driving, right around 1500 to 2000 rpm it would start to bog down and buck. If you maintained steady throttle input...ie did nothing about it, it would backfire out the intake. If you "patted" the pedal when you noticed the rpm dropping and power loss, you could get through the trouble spot. I would have to do this about 3 or 4 times before it would do right again. It would then be fine the rest of the time that the engine was running. If you shut the engine off for any amount of time, you would have to go through the procedure again. Sitting in the driveway from a cold start in the morning and just let it sit there and idle until the engine was completely hot, it would also stumble in idle, but it would take much longer to get to the stumble point...i seem to remember about 4 minutes of idle before it got there. What I noticed with my nose during the idle test is that the exhaust smelled rich.

My thinking at the time was that the engine was fine during open loop, preprogrammed fuel injection conditions...ie engine warm up, but as soon as the computer decided things were ready to go to closed loop mode and manage air/fuel ratios on its own, thats when it had trouble. Since the jeep had high mileage and was in need of a tuneup, I did that first with no effect. I needed a muffler about this time, so I had the muffler shop put a new one on and a cat at the same time, because now the cat was fractured from the backfiring and rattling in its shell. No effect. Cleaned the throttle body and checked every sensor that I could. Everything looked fine and I had no trouble codes.

Sat back and thought about it more and tried to realize what the computer needed to have in order to go into closed loop mode. I replaced the coolant temperature sensor with no change, but it wasnt until i changed out the upstream o2 sensor that the problem was solved. In my case the 02 sensor was putting out the correct readings, but was "slow" in its responses. I was testing every sensor I could with a high dollar digital volt meter and it still missed the o2 sensor problem...hey, it was giving me valid numbers so it must be good....wrong. I think in my case, I would have had to have an oscilloscope and a A/F meter to catch it right away, but you work with the tools you have available

I'm not sure what I would have replaced next if it hadn't fixed the problem, but MAP and TPS were on my "thinking" list, even though I had checked them previously. What I cant remember is if it started off doing this only intermittently or if it just started doing it like I described one day.

I think you are on the right track. Hopefully you can get it figured out before the backfiring damages the cat or something else.

Dave

Hmm, took a look at the IAC today and well... should I take it off from where it's attached directly onto the TB or that black thing that the harness is attached to? Either way, it kinda looks like a pain. I bought a before the CAT O2 sensor today and I'm gonna install that tomorrow. I would have bought the one that goes after, but all the auto-parts stores have the universal one. Is there anything bad about getting a universal O2 sensor?

ChairOhKey said:

Hmm, took a look at the IAC today and well... should I take it off from where it's attached directly onto the TB or that black thing that the harness is attached to? Either way, it kinda looks like a pain. I bought a before the CAT O2 sensor today and I'm gonna install that tomorrow. I would have bought the one that goes after, but all the auto-parts stores have the universal one. Is there anything bad about getting a universal O2 sensor?

Click to expand...

Typically a universal O2 sensor is just a similar sensor to the O.E. one with one noticable difference. Rather than just plugging it in you have to splice the wire from the sensor into the wire leading to the main harness. As far as I know there is no functional difference between the two.

Also, is the sensor before the cat the upstream one?

LazarusMJ said:

Typically a universal O2 sensor is just a similar sensor to the O.E. one with one noticable difference. Rather than just plugging it in you have to splice the wire from the sensor into the wire leading to the main harness. As far as I know there is no functional difference between the two.

Click to expand...

Is it as easy as splicing stereo harnesses? Or is there something different about it?

ChairOhKey said:

Is it as easy as splicing stereo harnesses? Or is there something different about it?

Click to expand...

It should just be one wire. Nothing special. I have never done this personally, but I have sold an a$$load of universal O2 sensors in the 8 months i have worked at Autozone, and no one ever complains about them.

My best guess is MAP sensor. It will cause the symptoms you describe and the back fire is a pressure pulse that may shock it into working again.

x2 on the MAP sensor or connecting MAP vacuum line.

Should I still take a look at the IAC valve?

No..

My 99 does very similarly, but I've never noticed a backfire out of the intake. Will be watching to see what you determine to be the problem.


-Hershey

Changed the MAP sensor and the upstream O2 sensor and seafoamed it for good luck. It hasn't done it... yet, but we will see. I can only hope for the best that this finally worked.

Well, I've driven it a few times already and no signs of it. I'll see when I take it out again in about an hour or so. *crosses fingers*

Mine was doing the same thing and it was the upstream O2 sensor.

I have the same problem and have replaced plugs,wires,cap+rotor,downstream o2's,IAC,TPS, checked fuel pressure(even when rough running occured) swapped the computer for ha has with NO kind of luck. I had officially given up, until i read this thread... DEFINITELY going to buy an upstream o2 ASAP, will definitely post results! thanks for the info!

Well , after pulling out alot of hair trying to solve this problem I took the advice and replaced the upstream o2 sensor even though it wouldn't throw a code AND sure enough it solved the problem THANKS!!!!!!!!