braapin127

My 98 civics idle keeps fluctuating ridiculously quickly. Roughy 4 fluctuations every second.I've bled the coolant, cleaned the IACV, and sprayed carb cleaner around all gaskets , vacuum lines. Nothing. When the car throws a code its a p1508. Ideas?

Miguel613ekb18c

Bad IACV if cleaning it doesn't fix it

TomCat39

code 1508 appears to be IACV malfunction. As the poster above me said, since you cleaned it and still have problems your IACV needs replacing.

braapin127

Guess I'll give that a shot then . Thanks

braapin127

before i replace anything i just wanted to post this video. this is how my car idles. this is not my video, however the car running in this video runs identical to mine. throwing p0135, and p1508 codes. video:

braapin127

Replaced IAC with no change to running

TomCat39

By the service manual, first is IACV which you've ruled out, so next is Idle Speed adjustment, then throttle cable, and last being Throttle Body.

That's what's listed as most probable causes and since you ruled out vacuum leaks already.....

If this is when cold the fast idle can be affected by the IAT sensor circuit.

Does it settle out once the car warms up?

And how is the condition of your air filter?

braapin127

Air filter is an older spectre cone filter. And no it always idles like that hahahha

TomCat39

Being you threw the code 1508 here is the chart for that:
So since you've replaced the IACV itself, that leaves the circuit and the Engine Control Module as the next likely culprits to throw that code.

Former User

It's uncommon for IACVs to fail. P1508 is an ELECTRICAL problem in the IACV circuit. Ohm and voltage test the IACV wires.

TomCat39

And here is the flowchart for that:

TomCat39

My copy of the manual doesn't look as nice as Ron's unfortunately.

Former User

Haha...I'm pretty sure we have the same manual.

braapin127

The bad idle stopped itself... I was driving and it was bad, shut off the car, went into the store, came out and it started running good. Went back to the shop and reset the cel and haven't pulled a p1508 since the other day. Now I'm throwing a p0505 after 12 miles roughly after clearing the code. I took the whole throttle body off and cleaned any carbon out.Aso tested the tps voltage. Voltage is correct. Put it all together and still throwing p0505