98 honda civic fluctuating idle

have a 98 honda civic ex with the d16y8, have replaced the map sensor and iacv and still have a fluctuating idle only when car is warm or at operating temperature. it has dc sports racing headers into a straight pipe then exhaust tip. does not have any o2 seniors on it(wasn’t my choice was like that when i got it) will idle up and down between 1700 and 2200 rpm also after replacing iacv the car jerk when going at low speeds or letting off then getting back on accelerator. any suggestions will be appreciated

No O2 sensors?

How is the computer going to know what the fuel mixture should be?


that was my first thought on the thing took to a honda guy said that shouldn’t affect the idle so i went with it but like i said when the old iacv was unhooked it would idle fine just would be jerky put the new one on and went back funky and still jerks

Your vehicle has the OBDII engine management system.

It relies on the signals from the O2 sensor once the engine warms up to adjust the fuel/air mixture.

Either find a new vehicle or a new mechanic.


my next question would be can i add the o2 sensors even with having the dc headers?? theres no place for them any way to trick the vehicle into thinking it has it? first time ever dealing with this issue so its all a learning experience at this point

@ jpearce91 This is not Twitter so there is enough space for sentence separation and punctuation.

Start with a new mechanic… on the other hand, if it was the “service advisor” that told you that, realize that “service advisors” are generally not knowledgeable, they only pretend to be. They’re there to do the paperwork and prevent you from talking to the actual mechanic.

The way this system works is that it takes a number of sensor signals, including the crank speed sensor, the mass airflow sensor, the throttle position sensor, the temp sensor, and the upstream oxygen sensor, and runs them all through an algorithm that determines how much fuel to add to the engine’s incoming airstream. It then sends that “demand” signal to the injectors as a squarewave. The wider the “pulsewidth”, the more time the injector is open and the more gas goes in. The sensor signals are collectively referred to as “engine demand” signals. If you remove one, in your case the oxygen sensor, the algorithm is incomplete and the ECU sends out an incorrect signal. While it struggles to figure out why the idle isn’t stable, it wanders.

The guy that told you that the removal of the oxygen sensor won’t affect the idle clearly doesn’t know this. And since it’s so basic, I have to assume he’s either the “service advisor” or slept through a whole lot of classes.

since it is straight piped and no cat converter, should i have both put on or just the upstream? i know they normally rely on each other to make sure the converter is doing its job but since it doesn’t have one wasn’t sure if if would make a difference or not.

also it is throwing codes for the vtec. it says vtec system b1 failure/ vtec system failure.
not sure if that may have something to do with it or not.

My opinion is to stear clear of these cars that have had racing/ or hot rod parts installed.

My first thought is how hard was this engine pushed by the previous owner and has he shortened the life it by driving hard.
They don’t put this stuff on the car to drive to church each Sunday. They are out there flooring it and pushing it as far as they can.

The same goes for those big tire on trucks.
I always wonder how hard the owner pushed it to go through mud…or was he mud racing.
That’s a lot of extra weight and torque put on an engine and tranny.

Kind of like seeing a 2 inch hitch ball on a Ford Festiva. Think What did this owner force this little car to do.

I never even look at a vehicle for purchase, that has been customized.


The ECU only uses the downstream sensor to monitor the cat converter performance. It relies on only the upstream sensor to help meter fuel. However, it’ll give you a constant Check Engine Light without both… and a converter in the middle.

I haven’t messed with the Honda vtec system, however my limited understanding is that it uses the “engine demand” inputs to operate a spool valve that directs the engine’s oil to move the engine camshaft to use a different set of lobes, a “high speed” lobe. It uses the oil like hydraulic fluid. Perhaps a full set of “demand signals” is necessary for the algorithm and the ECU is telling you that one is missing. I’m guessing a bit here, hoping someone more knowledgeable will follow up with a correction and an explanation… or a verification. Sometimes I guess just to get somebody else’s thought processes in gear.

right now the vtec is the last of my worries. just want to get this idle fixed going to go tomorrow and have some o2 sensors engineered to work with the headers and pipes it has. the light doesn’t bother i can pull the bulb out. i have access to a code tool so i can check if regularly for any codes. I’ve got the list down to just the 2 o2 sensors, vtec codes, and egr code.

Aha! An EGR code!
The EGR system prevents cylinder temperatures from getting too high under load and causing unwanted pinging or preignition and elevated NOx output. The it does this by opening a valve that allows a bit of inert exhaust gas to displace a bit of intake air, reducing oxygen to the cylinders. It has the same effect as closing the vents on a woodstove, it cools the fire.

Exhaust gas is inert (it won’t feed the fire) because its oxygen is already bound up with carbon, having formed carbon monoxide (CO) and carbon dioxide (CO2). Combustion is the process of the hydrogen and carbon atoms in the hydrocarbon (HC) molecules (gasoline) separating and bonding to the oxygen atoms in the air. Normal combustion happens when the HC is put in intimate contact with oxygen and sufficient heat energy is added with the sparkplug to begin the process. Pinging and preignition happen when heat in the cylinder becomes great enough to cause an ignition process to begin without the aid of the sparkplug. In preignition it simply happens by itself, in pinging it happens by itself but as a secondary origin in addition to the spark… and the two flamefronts crash.

If the EGR valve is carboned up, and since they pass exhaust gas they do get carboned up, it could be remaining open and allowing exhaust gas to be drawn into the engine at idle, when it should NOT be. That will cause an uneven idle.

so then would the o2 sensors still cause this? if it is the erg valve is bad?

only reason i ask is because i have had vehicles in the past that threw egr codes and they didn’t have any idle issues

The oxygen sensor can cause a carbon buildup on the EGR valve only in a second-order manner, by causing excess carbon due to improper fuel mix, but not directly.

If an EGR valve sticks open, it’ll usually affect the idle.
If it sticks closed, it won’t affect the idle… but it CAN cause pinging or (worse) preignition knocking. And it will trigger a CEL.

will more than likely add both o2 sensors tomorrow then and pull the erg apart and clean it along with the tb and hope at this point

When unsure if the EGR valve is leaking I have cut a piece of cardboard (single layer, not corrugated) to fit in place of the EGR gasket to serve as a temporary block off plate. It should only take a few minutes to see if the idle stabilizes.

Pcv was bad replaced now idles fine also needed intake manifold gasket thanks for all the advice

Sincere congratulations. I never would have thought of that. I never seem to check the simple stuff. :smile:

And sincere thanks also for the followup post . A happy ending is always great to hear.
Happy motoring,