Hi, I've got a 98 Civic HX. I'm getting hesitation when I accelerate, usually in 3rd and 4th gear above 2500 RPM. During the course of driving, the hesitation will go away after I accelerate a few times. I've done the usual tune-up things and it still hesitates. After reading some other threads, I think this may be due to a bad O2 sensor.

The question I have is, there are 2 O2 sensors on the HX. Which one do I change? The pre-cat one costs a few hundred less than the post-cat one, so I'm hoping it's just the pre-cat sensor. Also, is there a way to test the O2 sensor before I shell out money for a new one? My car isn't throwing any codes either, so I'm not 100% sure it's the O2 sensor.

Thanks.

 

Usually the one before the cat gets replace first. If you think about the secondary o2 get cleaner exhaust since its afer the cat. I had a problem that would hestitate around 2500 rpms, it seem like it was missing. After cleaning the inside the throttle body, replacing plugs and plugs wire, it got rid of the problem. My next step, I was going to replace the cap and rotor.

 

IF the O2 sensor (thats a big if) is causing the problem then it will be the precat O2, not the post cat O2. I would look to igntion timing, since when you floor it the computer needs to advance (i think) the timing. If for some reason the timing was not bieng advanced then you would have a hesitation problem. You can test the O2 sensor, but that requires a special scan tool and a pretty good understanding about how the O2 sensor works as well as how the computer uses O2 sensor outputs.

 

What causes the hesitation to go away after I accelerate a few times?

 

Oh, and I've already replaced the plugs and plug wires. I've also run a bottle of Gumout Regane and a bottle of Chevron Techron through the system.

 

Can't tell you b/c i don't know the problem. But if its related to ignition timing, the timing will be normal after the throttle levels out. The instant you hit the gas, the timing has to change. If like the TPS sensor is bad and the sensor is not changing its signal to the PCM quick enough then the computer will not adjust the timing quick enough and you will have hesitation. Its way complicated, i don't really understand it that well. However, i don't think the O2 sensor is the problem b/c i don't understand how the O2 sensor could cause that problem. If you do think the O2 sensor is bad, disconnect it. The computer should then run in open loop (like it does in the first couple min of engine operation). If the hesitation still occurs then the O2 sensor is still bad. Maybe a better test is right when the car starts, start hitting it for like the first min or two. The O2 sensor should not be sending signals right now. If you still have hesitation, the O2 can't be the problem since it isn't sending any signals while it is cold.

 

Thanks for your thoughtful response. Maybe to clarify a little, it doesn't hesitate until I reach around 2500 rpms. So I can accelerate smoothly in third gear until around 2500 rpms. Then it starts to hesitate. After doing this a couple of times, for example, going from a couple of stoplights, the hesitation goes away, and I can accelerate smoothly past 2500 rpms.

But yeah, the only reason I think it's the O2 sensor is because of this thread I read on this forum:

https://www.hondacivicforum.com/m_96852/tm.htm

I really have no idea. But I may try to change the cap and rotor first, since that shouldn't cost too much money.

 

You can try throwing parts at it, and after a while you will fix it. I would find a shop that has a scan tool and a scope. The scope is a really expensive piece of equipment, so if there is a shop that has one chances are there is someone that knows how to use it. So find a shop with a scope and have them diagnose the problem and tell you whats wrong. I think its something like $50 to have a problem diagnosed (i'm not too sure). Have the problem diagnosed and get a guarantee that it will fix the problem. You can either have them fix the problem or you could fix it. I would however take off the cap just to see how it looks. If its around time to change the cap and rotor, it won't hurt to do it now rather than later.