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RPMs Fluctuate at Idle

My 1998 Honda Civic DX’s RPMs are fluctuating during idle. I start the car and it is normal for about two seconds. I am assuming around 800 rpms (I don’t have a tachometer). Then it jumps drastically higher, then lower, then higher, etc.

Earlier tonight I replaced the throttle body gasket, replaced the spark plug wires, and cleaned the fuel injector cleaners. I took off several things in the process. I think the problem originates from taking apart the throttle body - not the other replaced parts.

I have a manual 2-door 1998 Honda Civic DX with a 1.6L engine (D16Y7 engine to be exact).

“and cleaned the fuel injector cleaners.” Should be: “cleaned the fuel injectors.” Sorry.

Look for a vacuum leak. A leak will cause fluctuating idle speed.


If no vacuum leak is found, then my next suspect would be the Idle Air Control (IAC).

Are you saying that this started happening after you replaced the throttle body gasket? If so then that is almost certainly the issue. Those bolts have torque specifications - did you use them? Was this an OEM-style gasket or did you maybe cut one? Did you clean the throttle body while it was off?

Other than that you look for whatever little vacuum line you forgot to hook back up or the clamp you didn’t tighten or things of that nature.

On Hondas if the coolant isn’t full, the idle will surge.

More details? I’ve never fully understood the vacuum system. The compression in the cylinders creates vacuum, which affects the throttle body/braking system/etc.? Is that correct? Where and how should I look for a possible leak?

Right. My friend had that problem with his 1993 Honda Civic. Luckily, I don’t have that problem.

I’m thinking that it might be the throttle position sensor (TPS). I bumped it several times with the fuel rail. Several Internet sites state this.

The vacuum is created by the engine sucking in air a lot faster that it is coming in. Thus the “throttle” is literally “throttling.” If you open the throttle the vacuum drops (at least briefly - it depends on what is being done with the throttle) because air rushes in. The only place air should be coming in is past the throttle plate itself. If air comes in from someplace else it leans out your fuel mixture and the car can’t run as well.

If you literally have this problem only after replacing the throttle body gasket then the problem is most likely either there or in a vacuum hose that you missed plugging back up or that you inadvertently dislodged or damaged. But you didn’t answer below when I asked. Why did you replace the throttle body gasket?

Take a can of something like carburetor or throttle body cleaner and spray the seam between the throttle body & intake (where the gasket is). If the new gasket is leaking it will suck some in and the idle should surge. Other than that, lots of the tubes and doohickeys on top of the engine are vacuum lines. Look under your hood for a schematic of the vacuum system. So you have vacuum lines and then you have everyplace where something either connects to the intake manifold (like the throttle body) and/or where the intake manifold bolts onto the engine (a gasket there too).

The idle air control valve probably bolts onto the throttle body. Did you leave that attached or pull it off and replace its gasket too?

STOP STOP STOP…Hondas are one of my specialties…Been working on them since the mid 80’s

YOu have one of the most common issues on fuel injected Hondas…ITS TEXTBOOK…

You have a problem with your AIR IDLE CONTROL VALVE…you may have unplugged it when doing your gasket for some reason…it is either unplugged, dirty with carbon and needs to be removed and cleaned OR you have air in your coolant system and it needs to be burped (most likely)…or the AIC valve failed (not too common)… Unplug the electrical connection and reconnect it to AIC valve. Did you open or provoke a coolant leak in your gasket replacement…IF SO…thats where your air bubble entered the picture.

THE AIC IS YOUR ISSUE…it needs what it needs to work…coolant bubbles will NOT be tolerated whatsoever and is BAR NONE the most common issue…

You can look up AIC valve tech support on the net…its all over the place in Honda forums…they deal with and fix this DAILY

Make sure your coolant is full and with NO bubbles…I think the civic had a bleed nipple on the coolant system near the thermostat housing BURP your system FIRST. If that isnt it…then clean your AIC valve of carbon buildup…if the slider cannot move back and forth from carbon it needs to be freed up so it can control your vacume leak ( I dont think you need to clean it…still want you to check for air bubbles…THIS IS BY FAR THE MOST COMMON REASON FOR AIC TO ACT UP… I am leaning strongly toward the Air bubbles in the system…because you are getting surging…if you unplug the AIC valve this is exactly what you will get. GO thru the checklist…and you will find the problem. DONE…problem solved. This is perhaps the one and only MOST common Honda issue…I have repaired HUNDREDS OF THEM…no problemo…its EZ and works everytime.

“Why did you replace the throttle body gasket?”

I was cleaning the throttle body several months ago and noticed that the gasket was in bad shape. I finally got around to ordering a new one. Finally installed it a few days ago.

"Take a can of something like carburetor or throttle body cleaner… "

I’ll do this today. I also got Diagnostic Trouble Code (DTC) P1509. It’s a problem with the Idle Air Control Valve (IACV or IAC valve) circuit. I understand that it means something electrical.

I don’t think it’s the coolant. I ran the Diagnostic Trouble Codes (DTC) and got a P1509 - a problem with the Idle Air Control Valve (IACV or IAC valve) circuit. Any idea?

Yes, the idea is that you inspect the wiring & harness for the IAC valve. It probably sits right there on the throttle body. It would have been easy to disturb things. If it “looks” ok, put a voltmeter on it and make sure that it is getting power. This also would have been something to clean & provide a new gasket for when you did the throttle body.

Problem fixed! I replaced all the vacuum hoses, installed the old throttle body gasket, and tightened the hose clamps. I don’t know the cause of the problem. The vacuum hoses (and new hose clamps) were a few bucks. So it was a cheap fix. The DTC code also went away.

Thanks for all the help!

Excellent news. Thanks very much for reporting back to let folks know. Most people just disappear never to be heard from again.

Hello Honda_Blackbird, I am as new as you can get on this blog, new member as of 5 minutes ago! I am here because I have the exact problem you appear to have extensive knowledge on. My 1995 Civic VX 5sp with 153540 mi. developed a surging issue after it is warmed up, after doing some research I replaced the IACV with a new DENSO IACV but the problem persists. After reading the post I am responding to I will get a can of carb cleaner and check for vac leaks and I will find the place you recommend to purge any air bubbles from the cooling system, I already checked and the reservoir is at the proper level and when I remove the radiator cap the coolant is right there with out any air gap.
Oh, I should tell you I purchased this from the original owner who took very good care of it, Mobil 1 every time from the first oil change, garaged most of its life. Love this car!!

I have a question for you about another topic, could a voltage inconstancy to the coils cause the problem, I ask because my idle surge seems to be tied to motion. As I approach a stop and coast while in gear, I put it in neutral around 1200 to 1500 rpm, it settles to 1k or so and I get a surge of about 200 rpm as I roll to a stop, once at a complete stop the idle settles down to a perfect VX idle around 800 than as a VX will do after about 2 or 3 minutes it settles to about 600. It is the difference between motion and stopped that has me stumped, does not act like a vacuum leak.

Full disclosure is that this started after having what is called The Revelco anti theft device installed, the site is and they connect it to the coil circuit and a few other places they will not divulge for security reasons. Is the coil circuit sensitive to any minuscule voltage irregularities ?
The engine runs perfect other than that, I had the valves adjusted to spec, new plugs,wires,cap,rotor and timing done just prior to the Revelco installed 44 mpg Hwy.

One guy whose name has been lost to forum changes got the oddball suggestion above that I would try: Check your coolant level. Older Hondas with low coolant will surge when the coolant is low.

That is what is called a ’ Clue ’

Ha, the dreaded idle surge, yes…OK…here goes… SOME Honda models deleted this air bleed nipple…and some have it. I dont know the why’s of it all…but I do know that if you have a bleed nipple the process is much easier to accomplish. Follow your top radiator hose to the engine/thermostat housing… Is there a bleed nipple anywhere on that housing? There will be only one bleed nipple under your hood IF you have one at all and the location of it changes over time…till it disappears completely.

The vehicles that do not have a bleed nipple are a bit more trouble to burp, but it can be done by jacking up the front of the car and putting it on stands. You warm up the engine with the rad cap off and allow it to burp itself.


In EITHER instance you need to be sure that your overflow reservoir is functional… be sure the hose leading to the radiator goes into the res and into the coolant so that it has nowhere to pick up any air bubbles…The tube cannot be cracked and it must be immersed into liquid inside the res. During normal Hot and Cold cycles this tube will burp your system provided it is immersed into liquid coolant in the reservoir. But it will take several Hot and Cold cycles to accomplish…and I mean… HOT…and Completely COLD cycles.

If you provided me pictures of your engine I may be able to spot your bleed nipple for you… You SHOULD have one… I’m hoping that you do…

Your engines idle is affected NOT by the motion of your vehicle…but from the use of your brake pedal… You can recreate this by running the engine and repeatedly pumping the brakes… This uses engine vacume for the power brakes…and thusly affects the IACV by stealing/altering vacume and affecting idle speed.

SEND ME PICS FROM UNDER THE HOOD PLEASE… If you have a nipple I should be able to spot it… Send 3 good pics