My 94 Accord LX 4 cylinder was idling too low and would cause the car to vibrate enough when the A/C was on to cause interior noise from vibration.

It appeared the idle speed was a bit too low with the car tach showing under 750 rpm when idling with the A/C on. Even with everything off it still only idled below 750.

I decided to do a base idle speed adjustment and went through the steps to do so, setting the ignition timing per service manual and then unplugging the IACV to adjust the base idle speed. I wasn't clear on which connector on the throttle body was the IACV plug and apparently initially unplugged the FITV plug... which made the idle speed INCREASE??? I wasn't sure at the time what plug it was, but knew it couldn't be the proper IACV plug, so I then unplugged the connector at the very top of the throttle body and when the idle speed dropped dramatically, I thought I had unplugged the proper plug and proceed to adjust the idle speed screw. The idle was in the 425-475 rpm range with a rhythmic cycling up and down about once per second.

Backing out the adjustment screw, I could hear the bypass air hissing sound increase and the engine rpm started to increase somewhat. It reached a point where it would go no further (rpm, not the screw) and while it still wasn't quite as high as I intended to adjust it to, it was now cycling around 500-520 rpm (now using an external tach to adjust), so I decided to continue, turning the car off, replugging the connector and then plugging the backup fuse to reset the ECM.

When this was completed and the car had been restarted and warmed up, the idle indeed was much better and the vibration ended. It did seem the engine rpm at times was "lazy" to drop now to normal idle speeds, but if left a few seconds, it would return to a more acceptable 750-770 rpm range (not fluctuating, but dependent upon conditions like A/C on/off, lights, etc.

After doing more research it appears what I was unplugging was actually the MAP sensor on the top of the throttle body just above the opening to the air filter intake tube. I believe the IACV must be partially clogged, so I plan to remove the entire throttle body/IACV/FITV as per DIY/FAQ directions this weekend.

I just have a couple of questions before embarking on this effort.

1) With a warmed up engine, I would have expected unplugging the FITV would have no effect on idle speed, and certainly wouldn't cause idle speed to increase!
2) Is the dramatic drop in idle speed when unplugging the MAP sensor similar to what would be expected to occur when unplugging the IACV normal?

I've determined the proper IACV plug is on the firewall side of the throttle body and I've been unable to unplug it so far. I plan to remove the throttle cables and then the throttle body nuts to be able to access the plug and hot water hoses, clean the whole thing and then reset the speed/recheck spark advance, pull the backup fuse again and see what the result is then.

This is my first Honda to do this work on, so I'm wondering if it is safe to assume the idle speed set screw normally would allow enough bypass air flow to easily exceed the base idle speed spec of 500+50 rpm?

I've read descriptions of cleaning the FITV that describe adjusting the ring span nut to tighten down that opening until there is no more slack, some replies to that thread (Honda tech) suggest this should not be done, but many have replied they have done so and still appear to have normal fast idle function. What is the opinion here?

Is the dramatic drop in idle speed when unplugging the MAP sensor to be expected? I live at around 650-700' above sea level... If this has the same effect of disabling the IACV as unplugging the IACV itself, it is certainly much easier to get to! LOL...

 

You need to do this with the IAC unplugged. With it plugged in you are fighting it when you are adjusting the screw.

 

Also you should download the 94-97 Accord shop manual from Online Manuals post in DIY forum. This will allow you to recognize parts and their function.

good luck

 

Thanks guys,

I have downloaded the service manual and had a 3rd party maintenance manual in addition for years. I had referenced both, but it was still somewhat vague to me which plug was the proper one. Frankly I didn't even see the correct one initially since it is pretty obscured in it's location on this particular Honda model.

Regardless, I took everything apart yesterday and cleaned everything up. The IACV indeed had a lot of carbon residue on the screen, which I'm certain was causing it to be mostly plugged up. After cleaning everything and putting it back together, my problem now is that there is now too much leakage from the FITV. It was slight before cleaning, but now is excessive and even with the idle speed adjustment screw completely seated, the idle level is near 600 rpm.

I didn't have new gaskets or O-rings and as everyone warned, the throttle body gasket indeed broke when removing the throttle body. I used some gasket silicon to "repair" the old gasket when I reinstalled it. I plan to obtain all new O-rings and a new throttle body gasket and repeat the process.

But my concern is the FITV leakage. Could the bypass air be due to old O-rings or possibly the damaged throttle body gasket allowing air through an unintended path, or do I need to replace the FITV itself?

Not noticing the intake air temperature sensor plug was bundled with the IACV plug, I managed also to damage the sensor side plug, when removing the IACV/throttle body assembly. It still works, but the plug is not securely seated, so I plan to replace that sensor as well to prevent intermittent connectivity.

The car is still drivable, but the idle is still somewhat too low when the A/C is engaged. Overall it is better than when I started, but not quite correct yet. I'm sure the fast idle leakage is preventing proper operation of the entire subsystem. I don't mind replacing the FITV, but it is somewhat pricy and I'm just wondering if the opinion is that the O-rings/gasket may resolve the problem.

 

FITV may be bypassing air all the time. It's supposed to close when engine warms. See following video.


good luck

 

O.K., I replaced all O-rings and gaskets for the throttle body, IACV, FITV and replaced the IAT sensor...

I still seem to have significant air flow through the FITV (lower hole in the throttle body) even after the engine has warmed up completely... Indeed screwing in the idle speed screw with the IACV unplugged all the way still leaves the engine idling around 600 rpm... If I place my finger over that lower hole in the throttle body, I feel significant suction, but the engine doesn't really drop significantly in idle speed.

I after turning off the engine, resetting the ECU and then letting the engine come back up to temperature... it seemed the idle speed was back to being below what I would like to see it at (significant vibration at idle when A/C is on, brake lights on, headlamps on, etc.).

I decided to back the idle speed adjustment screw out 1/2 turn and see how the car behaves.

I like the way it drives now... the idle seems smoother than when this all began, but I'm still somewhat puzzled over how the FITV is supposed to work. It would seem that as hot water circulates through the unit, expanding the wax plug and putting pressure on that plastic ring with the tapered brass plug, it would shut off the air flow through that path. I've removed all "slack" by screwing in the plastic ring (with part at room temperature). The tapered brass plug does move in/out smoothly against the spring loaded part of the plug...

I don't understand where the air would get through the FITV when the tapered part is against that plastic ring if you remove all the "slack". The engine does idle faster when cold (around 1K on the car tach) and now seems to drop in speed faster than previously before removing the "slack"... The Honda documentation indicates the valve is not user adjustable, but is the "normal" operation that when it expands, it takes out all the "spring loaded" slack and forces a completely sealed situation?

I'm guessing I can just live with it as is, since it's better than before, but I'm guessing at this point if I want it to work as factory designed... I will have to replace the FITV? I just have this bad feeling that I'll spend the money on the valve unit and it will still be acting the same way. The only other thing I can think of is that perhaps there is some sort of buildup on the seat side of that plastic ring causing it to not seal properly against the tapered portion of the control valve.

Is there any risk in backing the plastic ring completely out to ensure the bottoms side is smooth and completely clean?

 

I would try to repair or replace the FITV. You should be able to find a used part easily and cheaply.

good luck