Idle fluctuation only after coolant warms up (searched)

ASSISTANCE Stock Engine-Bay Components
1

I’m at my wits end here after multiple search and troubleshooting sessions, please advise if you have any ideas g2ic!

I’ll start with describing the situation:
Engine idles perfectly steady at 1000rpm until the coolant temperature warms up operating temperature. When it does, I get a constant fluctuation around 850-1050 rpm. This happens every single time. Since my exhaust is pretty free-flowing, it is quite annoying when stopped.

My setup/thing i’ve done:
-New felpro intake manifold gasket + checked torque on all IM studs after several heat cycles
-New TB gasket, cleaned TB
-New air filter
-New spark plugs and wires
-Relatively new fuel filter
-New Walbro fuel pump
-New gasket sealing leak in 4-2-1 header
-Recently did my timing belt and head gasket
-Have removed FITV with blockoff plate
-Have removed or blocked off all EGR related parts/ports
-Not running a PCV, have 3 -10AN lines to a catch can (2 from valve cover, 1 block port)
-Bled coolant of air bubbles
-Loosened throttle cable
-IACV thoroughly cleaned with carb cleaner
-Tested second IACV, no change
-Tested TB TPS w/ multimeter, its good
-Tried another intake air temp sensor, same prob
-Reset the ecu many times

I haven’t messed with the coolant temperature sensor; however, the coolant gauge functions like it should and there hasn’t been any overheating. Haven’t touched the ECU either, but I doubt that its gone bad. What is left?

2

I have read elsewhere that a faulty MAP sensor can cause this.

I was having something similar and swapped my MAP sensor. no more idle bounce.

realistically it could be any number of things. but it sounds like you have addressed most of those.

3

When the fluctuating first started I had done a bunch of things to my car at once. One of those things was switching TBs to one with a mounted MAP sensor, which came with a used sensor. That could very well be the problem! Thanks man, I’ll give that a try.

4

I highly suggest going to a pick n pull new map sensors at oreilly’s are like 400 bucks.

I have a couple of em laying around too if you have trouble sourcing one.

5

Thanks dude. Actually got on it right away this morning, found a cheap ebay seller with quick shipping. I’ll update the thread with the results, but I’ve done just about everything else so maybe this will be the fix that works!

6

fingers crossed for ya bud…

7

this is 90 or 91 then?
try disconnecting the o2 sensor. see if the problem persists

8

Its a 93, but it has the TB + IM of a gen3/obd2 integra. I haven’t tried that so I’ll give it a go tomorrow.

9

yea o2 can cause problems at operating temps… I was having issuews with the car not wanting to start at all tho when it was at operating temps with a bad o2.

I’ll defer to the guru since he’s here now :slight_smile:

10

When I was experimenting with intake setups (filter/velocity stack/cai/sri) my idle eventually started to fluctuate like this once warmed up. Its good to start with cleaning the iacv. Other then that, if everything is working properly I highly recommend disconnecting the negative battery terminal for about 15 minutes then re-install it, this resets the ecu. Then while the car is completely cold, start it up with no accessories on and let it run until completely warm, this will allow it to “re-learn” the idle. The key here is once its started, dont turn anything on and more importantly do not touch the gas, just let it warm-up by itself.

Everytime ive switched intake setups this fluctuation happens possibly due to the increase/decrease in air intake? Either way, this remedy always resolves it for me and is a great starting point before dumping money into more parts.

Hope you figure it out!

11

Also, make sure the coolant system is bled as this will cause the same issues.

(I now see you did most of what I mentioned) Just make sure you are letting the car warm-up on its own after resetting the ecu, dont hit the gas at all.

12

Disconnecting the O2 sensor cleared up almost all of the fluctuation! Thanks for that suggestion. It looks like anti-seize was not used with the last installation though, and its stuck in a bad way :(.

My setup runs a small 14lb battery so I end up disconnecting it a lot to bring it inside saving it from cold nights and parasitic draw. Unfortunately I think this means that the ecu doesn’t remember any of its learned settings for idle from o2 readings and such. I wonder if this is causing a problem with the idle as well…

13

New used O2 sensor is in (took a good bit of propane torch action). Idle fluctuation is way down to an acceptable level. Problem solved!

14

rad.

15

Don’t forget there are two o2 sensors, so if one was that bad, chances are so is the other.

16

Good looking out. I have an aftermarket 4-2-1 header with only one O2 location, so there was only one to replace luckily.

17

a secondary o2 sensor wasn’t implemented until 96 with obd2 mandate.
glad you got it all sorted. however, you were very lucky that it DID turn out to be the o2 sensor. disconnecting the o2 sensor, and having the problem not persist, only confirms that there is a problem somewhere in the fuel injection/feedback system. which could be any number of possibilities.
normally, if a technician found what what you did, since the o2 sensor is highest priority in the list of inputs for determining air/fuel ratio, the next step would be to scope the o2 sensor and check it’s range and response time.
fogging in propane checks the range. snap throttle will test cross counts/response.

doesn’tmatter. you got it sorted though. cheers bud