techsupport

#1
techsupport
Honda-Tech Member
Thread Starter

Join Date: Jun 2016
Location: Portland, Oregon
Posts: 3
Default Another Surging Idle Question?
Hi, I have a 1993 Civic Hatchback. The idle surges once the car warms up. I have read lots of posts to find the fox for thos issue and nothing works. These are the thinks I have done so far:
1) Purged cooling system of air
2) Checked all the vacuum lines for leaks and sprayed carburetor cleaner all around.
3) Cleaned throttle body with carburetor cleaned
4) tested TSP (.5v closed and 4.5v fully open)
5) Replaced the IACV

There are no codes. The only time I get codes are when I disconnect a sensor. The throttle body is a Skunk 2 Alpha throttle body and an Edelbrock intake manifold. Any help will be greatly appreciated.

Former User

Clean and adjust FITV.

TomCat39

I suspect the aftermarket throttle body doesn't have FITV provisions, instead has a coolant pass through like the Skunk2 Alpha Throttle Body.

SnaZZyCivic

I remember mine having a surging problem a few years back and it being the FITV. But that was a cold start surge problem if I remember right.

Did the surge start after installing the aftermarket throttle body?

Former User

FITV surging issues generally occur after engine warms up.

mk378

Unplug the IACV. The rpm should drop to about 450, i.e. barely running. If it does not, you need to find where the extra air is getting in.

techsupport

My throttle body does not have an FITV, probably a coolant pass through like you mentioned. I have done all the trouble shooting I can think of. I have now adjust the idle to 1500 rpm's to eliminate the surging. I read somebody's post where they diagramed running an air line from the cold air intake directly to the intake manifold. The problem was not enough air getting through the 70mm port on the skunk throttle body. I am running out of solutions. I got a great deal on the car and it's my daily driver.

mk378

What happens when you unplug the IACV?

The air bypass screw on the throttle body should be set so the engine runs 400-450 rpm with the IACV unplugged. When a 2-wire IACV is unplugged, the spring inside pushes it fully closed. You can confirm that by feeling for air flowing into the IACV inlet hole before the throttle plate. There should be no air flow with the IACV unplugged.

The ECU will then command the IACV to admit additional air to get to the programmed idle which is 750 rpm at operating temperature. If the ECU thinks the engine is cold it will make it idle higher. Having a fast idle occur when warmed up would indicate a problem with the ECT sensor.

The spark timing is also involved in idle control. Set the timing with a light don't just guess.

techsupport

When I unplug the IACV, the surging stops. The engine runs rough, and the check engine light comes on but the surging idle problem goes away. I will be working on my car today and I will make sure the rpms are between 400-450 with the IACV unplugged. I will look up the specs and check the timing too.

So, if air runs through the IACV while it is unplugged, I may have a faulty ECT sensor?

mk378

If it idles very slow with IACV unplugged because no air is going through it, IACV is working like it should.

If the ECU commands the IACV up to a higher idle than it should when you plug it back in, this could mean the reading from the ECT is wrong, making ECU think the engine is cold when it is not.

chrysler kid

Seems like solid advice, replace the coolant temperature sensor on the thermostat elbow.

My suggestion is to also check for vacuum leaks especially at the throttle body gasket since you have replaced it with an aftermarket unit. But I think he may be onto something with the coolant temp sensor

TomCat39

Also keep in mind that this issue might not be able to be completely resolved.

The OBD1 ECU is expecting the IACV to compensate the FITV valve. Both let air pass by the butterfly valve of the throttle body.

OP's throttle body does not have an FITV nor can you add one. As such, the IACV is being forced to operate out of the programmed parameters Honda coded into the ECU.

I suspect you may be able to get it close but may never totally do away with the issue.

techsupport

Thank you for all your replies. I do think the coolant temperature sensor may be the issue. I bought a new one today but it is the wrong size so I need to get one that fits tomorrow. I unplugged the electrical cable to sensor and was surprised that the temperature guage acted the same as it did when the cables were hooked up. Maybe, I hope, that the actual engine temperature is registering the actual temperature of the engine when it's hot.

DumpdEJ6

There are several coolant temp sensors if I recall correctly, one feeds info to the gauge cluster and another sends info to the ECU. I want to say that the one next to the thermostat is the one that sends a signal to the gauge cluster and the one in the cylinder head is the one that goes to the ECU, but don't quote me on that, I might have them backwards. Either way, that's probably why the readout on your gauge didn't change when you unplugged only one.

mk378

There are two temperature sensors in the head. Both are located on the transmission end of the engine, underneath the distributor.

The small one with one wire drives the gauge on the dash. That is all that it does.

The larger one with two wires is connected to the ECU.

The sensor on the thermostat housing is a switch that turns the radiator fan on. That is all that it does. It doesn't affect idling.

techsupport

Once again, thank you. You saved me some time and frustration by telling me this. I didn't know there were 2 sensors.

techsupport

Still no change today. I replaced the coolant temperature sensor (right below the distributor) and the radiator fan switch (looks like a sensor, by the thermostat). No changes, engine still surging. Looked for vacuum leaks again, and purged the air from the cooling system. Running out of ideas. I am missing something, but don't know what. I can replace the throttle body and ECU, or take it to a Honda Performance mechani . I also cleaned out the EGR valve too. When I unplug the wires from the IACV there is a little suction coming from the air by pass hole in the throttle body that goes to the IACV. When I plug my finger in the hole, the surging stops. Why would air be sucking through this hole when the IACV is unplugged. It's brand new?

mk378

The IACV should spring shut when unplugged. Does your old one do the same thing?

Your intake system MUST be able to starve the engine for air sufficiently to get the rpm below normal idle with the IACV unplugged, before looking for computer-related causes. If the ECU (through the IACV) can't physically stop air from coming into the manifold, it will idle high and/or unevenly. Before thinking about anything else electric you need to be able to make the engine slow down to much less than normal idle, only by unplugging IACV.

techsupport

I am not sure if the old IACV let air through while the electrical cable wasn't attached. I didnt keep it, just thought it was bad so I replaced it. Probably the best thing I can do is to go through the timing and tune up specs of the engine before spending more money replacing parts that dont need to be replaced. I won't be able to do that until the week end, more than likely.

techsupport

techsupport

The link above is a video on my surging idle and my latest attempt to fix it.

tony_2018

If you changed those two sensors did you bleed the coolant system?

techsupport

Yes, I bled the air out of the cooling system. I think I got it all. I think I have bled it 3 times although, no changed. Gonna try the timing and will go through all the tine up and throttle body adjusting this weekend.

tony_2018

Rais the frontend of the car a lil bit and rebleed. I had a similar issue where it idled fine during start up but after driving around and stopping at a stop light it would hunt around. took me a while and I waited for the fan to come on during bleeding process. I let the fan kick on 3 times and kept refilling the radiator. I don't think I got everything but my idle has gotten better. My yellow Lisle funnel is coming in and I'm going to do it again today to ensure I got all of the bubbles out.

techsupport

I tried doing that prior to changing the sensors, but didn't do it this last bleed. I will raise the front and rebleed this weekend.