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


  • edited February 2011
    "and cleaned the fuel injector cleaners." Should be: "cleaned the fuel injectors." Sorry.
  • edited February 2011
    Look for a vacuum leak. A leak will cause fluctuating idle speed.
  • edited February 2011


    If no vacuum leak is found, then my next suspect would be the Idle Air Control (IAC).
  • edited February 2011
    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.
  • edited February 2011
    On Hondas if the coolant isn't full, the idle will surge.
  • edited February 2011
    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?
  • edited February 2011
    Right. My friend had that problem with his 1993 Honda Civic. Luckily, I don't have that problem.
  • edited February 2011
    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.
  • edited February 2011
    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?
  • edited February 2011
    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 may have unplugged it when doing your gasket for some 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 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 problemo....its EZ and works everytime.
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