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86 Civic Si Erratic idle, high tach reading

Discussion in 'Civic and CRX - EW' started by ewgoforth, May 5, 2008.

  1. ewgoforth

    ewgoforth New Member


    I have an 1986 Civic Si, it's currently my only running car, but I don't drive it much. It has a couple problems. The main one is that the idle is erratic. When it's cold it's not too bad it fluctuates a little bit, but after it warms up it goes up and down up and down, fluctuating between 1200 and 1800 rpm. The temperature normally runs about 1/5 to 1/4 of way up. If it idles for long enough on a warm day (temperature reading close to 1/2 scale) the idle smooths out considerably.

    The coolant level seems fine. From what I've been able to find out online it sounds like it's a temperature sensor or the IAC. Where is the IAC physically located on this car, relative to the throttle body?

    I've tried spraying a canful of really strong carburetor / throttle body cleaner into the throttle body, but it didn't help.

    The other strange thing I've noticed is that the tachometer seems to read high most of the time. Typically when I'm cruising at 75 mph it's reading around 4500 rpm.

    Every once in a while it will seem to read, what I'm guessing is a more realistic 3500 rpm. The engine sounds more like it's actually turning 3500 rpm when it's reading 4500. It's not that the tach is varying when I'm cruising at 75 mph, it stays steady. I'll start up the car, drive it normally a few block until I get on the Interstate and then in fifth gear on the Interstate it will be reading 4500.

    Another time, I'll start the car, get on the Interstate and it will be reading more like 3500 rpm at the same speed. When it's reading high I really have it in fifth gear (I've checked) and I'm sure the clutch isn't slipping.

    The car seemed to have lost a little power over the years I've had it, I think I know why now. The other day I needed accelerate quickly from a stop light in first gear. I noticed that the engine seemed to be pulling much more strongly than normal, when I glanced down at the tach and it was reading 7500-8000 rpm.

    I think that a stock 1986 Civic Si would rev to those kinds of rpm, but I'm pretty sure that the power would have tailed off quite a bit by that point, but probably not if it was really reading 1000 (or more) rpm high.

  2. hosmer

    hosmer I made the millionth post

    #10...also called the fast idle valve


    Most likely you have an air bubble in the idle solenoid, or your throttle body is gummed and the throttle plate is not closing completely.

    When it is bouncing, try pushing the throttle valve more closed, and it that fixes it, you will need to clean the throttle body.

    If not, there is a small cap on the idle solenoid, with two phillips screws, you have to take it off to release the air that is trapped in there, and make sure that only coolant is in it.

    As far as the tach goes, the EW motors simply don't rev that high (they make their peak hp at about 5500rpm's)....could just be a corroded contact either at the distributor or more likely on the back of the cluster (you'll see a discoloration)
  3. ewgoforth

    ewgoforth New Member

    OK, another mechanic had pointed out the idle solenoid (without naming it) and suggested that I take the cap off it and make sure it was clean inside. Is there anything else I can do to make sure the air is bled out of it? From what you're saying, it sounds like coolant is supposed to be going through it. If I remove the cap should coolant come out? It didn't when I did it earlier.

    I'll make sure that I can't further shut the throttle valve when the engine is bouncing too. That should be pretty easy. Although with the amount of throttle body cleaner I've sprayed into, it's hard to imagine it could be gummed up.

    So there should be I assume some small wires from the distributer that end up eventually at the tach? I'll see if anything looks corroded.

  4. ewgoforth

    ewgoforth New Member

    One more question, I think I have an electronic tach somewhere in my shed that I can use to double-check the accuracy of the tach in the dash, but off the top of your head do you know what the tach of an 86 Civic Si should read at a particular speed in fifth gear?

  5. hosmer

    hosmer I made the millionth post

    I forget what the EW tranny gear speeds are....my last 2 CRX's have been swapped

    -loosen the cover on the idle solenoid while the cars running
  6. ewgoforth

    ewgoforth New Member

    With the engine running I tried pushing on both the small upper and larger lower butterflies in the intake and couldn't get the bouncing to stop, it stayed the same. I tried cracking the cover of the idle solenoid, I even took it off, but the engine continued to run the same. There's a lot of engine vacuum underneath the cover of the idle solenoid, I assume that's normal.

  7. hosmer

    hosmer I made the millionth post

    have you bled the coolant using the valve on the upper radiator hose?
  8. ewgoforth

    ewgoforth New Member

    Just tried that, no air came out that I could see, idle is the same as before.

  9. LS_VTEC

    LS_VTEC you get the BRAKE

    Remove and inspect the FITV. If the plunger is loose, try to press it in while turning it until it threads back in. Use a flat head to turn it in some more after that, than re-install and attach the two coolant lines back on. Now, for the tach, either the speed sensor is encountering resistance or the speed sensor is on its way out. As for the coolant, have it flushed and re-filled. When you bleed it afterwards, do so in the correct fashion, not by the valve, either. Run the engine hot with the cap off to positively bleed out the system. Top off your resevoir and your done.
  10. ewgoforth

    ewgoforth New Member

    I pulled off the FITV yesterday (is this equivalent to the IACV on later cars?), but the engine was still hot so I didn't disconnect the coolant lines. I sprayed carb cleaner into the two holes that fit against the throttle body. I didn't see the plunger you're talking about. I'll try it again with the engine cold, what I did didn't help.

  11. KHLow2008

    KHLow2008 New Member

    Have you tried checking for vacumn leakage? The vacumn leakage from manifold gasket, PCV valve on the valve cover, Brake Power Booster seal and any other vacumn hoses can also cause high and eractic idling.
  12. ewgoforth

    ewgoforth New Member

    I've finally had a chance to look back at this car.

    I've completely disassembled the fast idle valve but everything looks okay. I assume that the coolant in the bowl on the bottom gets hot as the engine warms up, but I don't understand how that connects to the chamber in the middle.

    Inside it there's a couple springs and a plunger in the middle and then a threaded plastic cap on top. Hopefully there's nothing that's supposed to be between the threaded plastic cap and the top spring. It flew apart when I unscrewed it. Nothing seems to be seized or particularly dirty in the chamber with the two springs. Not sure how the water heating in the bowl on the bottom could really cause it to move however.

    Is this the same as the idle solenoid that you mention?

  13. 86REXsi

    86REXsi New Member

    its probably a vacuum leaki had a bad brake booster seal
  14. ewgoforth

    ewgoforth New Member

    Again with the bouncing idle...


    Under the throttle body, there's sort of a little vacuum "manifold." There are 3 metal vacuum lines that point towards the front of the car. As you're facing them, looking toward the back of the car, the one that's on the left (actually on the right side of the car) is connected via a rubber vacuum hose to the distributor. The one in the middle isn't connected to anything, and doesn't have anything plugging it that I can see. The on on the right is connected via a rubber vacuum hose to the top of #15 in the attached picture.

    I'm wondering about the vacuum line in the middle, it doesn't really look right, but I can't see anything that it looks like it should be connected to. I tried plugging it with my finger while the car was running. The idle continued to bounce and I didn't feel any vacuum on my finger. I tried holding an unlit propane torch for a pretty good while, probably 30 or more seconds and I didn't see any change in the idle either. I tried slowly running the unlit propane torch along all the vacuum lines on the engine, looking for an idle leak, and didn't hear any idle change that way either.

    FWIW, the tach seems to work fine now...


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