Misfire? +code 25/35 (Page 1/4)
A_Lonely_Potato SEP 19, 03:18 PM
Ello, a few questions for y'all. Since i've had this car, it has always had a high idle. 2.5k cold, sometimes down to 1.3/1.4k warm, but often 2k warm. When i force the IAC closed by these steps;

- Bring engine to operating temperature
- Shut down
- Enter diagnostic mode
- After a few seconds of diag mode, unplug IAC
- Start the car in diag mode

Then it will idle at right about 850 +/- 50rpm. This leads me to believe it is not a vacuum leak. It seems pretty rough there though, sends trembles through the whole car. This alongside the sound of the engine when idling at about 1.2k-1.5k(it was hunting a lot when i took this recording, at operating temp)



I will fully admit, I am not at all familiar with what a misfire sounds like. All i know is that to me, this does not sound right.

In this whole time of it idling high, it has never thrown a code 35, until a couple days ago. It has actually never thrown any codes, which did amuse me given that its never been quite right. Anyways, when i drove down to Boone NC, I wasnt using my car much while staying. One of the days that I did, my car threw a code 23. I assumed this was because the sensor got wet, it had been pouring rain for days, and my intake does not have the water box(P/O did that). The code went away and only showed back up intermittently when it was rainy. On the 700 mile drive back home, no codes came back. Then just a couple days ago, suddenly I have a code 25, and a code 35! Finally it recognizes that it's idling too high! It has been running a lot rougher since then, the code is intermittent. The car will start bucking when cruising at low speeds(15-30mph) but seems to run fine when given the beans.

My fuel pump won't prime anymore as well, that came out of nowhere. Gonna check the relay, if that's fine i'll check for signal coming from the ECM. I just replaced the ECM not long ago because of this issue, so if it went bad again its still under warranty. Means ive got a wiring issue somewhere though. Sigh, I need to swap this soon, seems this engine is the source of all the frustrations i've got with this car! Been working on getting the doors working on my S10 yesterday and today, should be wrapping that up today though, so i'll be able to focus on the car. Thanks in advance guys
Gall757 SEP 22, 05:17 PM
You still may have a vacuum leak. Forcing the IAC closed should stop the engine, so it is leaking somewhere.
A_Lonely_Potato OCT 04, 07:02 PM

quote
Originally posted by Gall757:

You still may have a vacuum leak. Forcing the IAC closed should stop the engine, so it is leaking somewhere.



I swear that the car previously would stall when i plugged the IAC port in my throttle body, but today it did not stall, it ran at about 1.2k. So i've got quite the vacuum leaks to find i now see.
Patrick OCT 04, 08:27 PM

quote
Originally posted by A_Lonely_Potato:

I swear that the car previously would stall when i plugged the IAC port in my throttle body, but today it did not stall, it ran at about 1.2k.



Now cover the entire throttle body opening with your hand while the engine is running.

- If the engine stalls, the idle stop screw has probably been messed with.

- If the engine still runs, check for obvious things like a cracked EGR tube... and check that the tube that goes from the intake manifold to the back of the TB is indeed inserted properly.
A_Lonely_Potato OCT 05, 02:16 PM

quote
Originally posted by Patrick:

Now cover the entire throttle body opening with your hand while the engine is running.

- If the engine stalls, the idle stop screw has probably been messed with.



If the engine stalls when i take away its only air source, the screw has been used? How does that work? Also, my idle screw has the plug covering it. I will still try this later.


quote

- If the engine still runs, check for obvious things like a cracked EGR tube... and check that the tube that goes from the intake manifold to the back of the TB is indeed inserted properly.



I don't have an EGR, but i'll check the port for the EGR tube on the manifold and the TB tube.
Patrick OCT 05, 02:55 PM

quote
Originally posted by A_Lonely_Potato:

If the engine stalls when i take away its only air source, the screw has been used?



Yes... IF the engine doesn't stall when just the IAC port is covered... and yours didn't.
A_Lonely_Potato OCT 05, 08:57 PM

quote
Originally posted by Patrick:

Yes... IF the engine doesn't stall when just the IAC port is covered... and yours didn't.



It took me a long time to figure out why this would tell me anything lol. Anyway, it does stall when i cover the TB intake. My idle screw is capped off, and doesnt appear to have ever been messed with though, so i'm not sure where to go from here...


(its all scraped up from me trying to see if it seemed factory)

[This message has been edited by A_Lonely_Potato (edited 10-05-2022).]

Patrick OCT 05, 09:43 PM

quote
Originally posted by A_Lonely_Potato:

My idle screw is capped off, and doesnt appear to have ever been messed with though, so i'm not sure where to go from here...



That makes two of us.

If the engine stalls when the entire TB is covered, that would seem to indicate that there are no vacuum leaks beyond the TB. But if just the IAC port is covered, and the engine remains idling at 1200 rpm, that would seem to indicate that the butterfly valve is open too far... which is usually the result of someone messing with the factory-set idle stop screw. But you say the plug covering the screw head is still in place. I suppose it is possible that someone could've reinstalled a plug after changing the idle stop setting, but that doesn't seem too likely.

Does there appear to be excessive play in the butterfly valve shaft? The air's got to be getting in somewhere!

[This message has been edited by Patrick (edited 10-05-2022).]

Hudini OCT 06, 09:16 AM
Have you done an idle relearn?

From avengador1: "From my factory service manual for my 87 V6: If idle is too high, stop engine. Ignition on. Ground diagnostic terminal. Wait a few seconds for the IAC to seat then disconnect the IAC. Start engine. If idle speed is above 800+/- 50 rpms, locate and correct vacuum leak."
Patrick OCT 06, 02:03 PM

quote
Originally posted by Hudini:

Have you done an idle relearn?



Wouldn't covering the IAC port completely eliminate the IAC valve from the equation?