Another 3.2 idle surge thread
 

Hi all!
I’m looking for your collective wisdom and ideas for yet another 3.2 fluxing idle issue.

1986 Carrera 3.2, US spec with cat removed (euro pre muffler). Top end rebuild recently by PO, included injector cleaning and rebalancing.

When I got the car last year, it ran well enough with a few gripes .. under full throttle I’d get an occasional miss, sometimes when revving it out I would get a backfire on lift and occasionally there was a strong fuel odor on cold start. The fuel odor bothered me .. so we begin..

First, I decided to remove the tamper seal on the idle mixture adjust screw (on the AFM) and turn the screw out 1/2 a turn. This did appear to fix the fuel odor.

When the weather got warm, the idle hunting started. I could usually settle the idle by using the clutch to lower the revs. I noticed the cylinder head temp sensor looked original, so I decided to try and replace that with an2 wire. This made the idle hunting worse .. now 100% of the time when the engine is warm.

Next .. I threw parts at it. I replaced vacuum lines, vent lines, oil tank cap, O2 sensor, ICV and then the DME. None of this helped.

I finally put a LAF gauge on using a tailpipe adapter. I could stabilize the idle by unhooking the ICV and setting it to ~60% closed. AF was rich .. about 12:1. Adjusting the idle mix screw waaaay too much out only got me to 13.25:1. This is with the o2 unhooked. Connecting the 02 brings the AF to 14.5:1 as expected. I put everything back together and went for a drive. The idle started hunting again after I backed out of the garage. The car runs like crap .. little power over ~30% throttle, you can feel the engine missing.

My guess is something is causing the engine to run too rich at idle .. the DME tries to open the ICV to compensate, causing the surge. I measured the resistance of the intake air sensor and cylinder head temp sensor while the engine was hot .. both measured as expected.

Next .. I’ve reset the idle mix screw (fingers crossed this addresses the low power issue) and set the idle adjustment screw to 1/2 turn out. I’m letting the car cool and then will toss in a set of new plugs I have on hand. My thought is a bad plug may cause a too rich condition? Maybe the old plugs will show something.

Ideas and thoughts? I’m about to the ‘give up’ point .. maybe take it to a shop or convert to a MAF!

Thank you!

800 idle speed thru '86. Are you there? Too high idle and the computer does it's adjustment thing via the ISV.

Thank you. Yes. For a hot second when I had the af leaned out far as I could, I jumpered the test port, disconnected the O2 and got the base idle to 800 with about one turn out on the idle adjust screw.

Once everything was hooked back up .. idle surge again.

I feel I need to set the base idle with the mixture at 14.5:1 … but I can’t get the base mixture to lean out enough. Huuummmmmmm

Here are two videos .. first idle set, mix adjustment max lean, everything unhooked. Second, mix max leaned, O2 and ICV reattached ..

https://www.icloud.com/sharedalbum/#B0mGGXqixG0EmhB

You should check the fuel pressure. I had the same problem due to a faulty fuel regulator.

you have most likely bad oxy sensor ..i`d disconnect it no need for it In Texas you have no emission problems....since you do not have CAT no need for it....

Ivan

95Tango -

I read your description, and it sounded like you were exactly where I was two years ago. I have an 86 3.2, US spec, recently rebuilt, and started having a surge problem at idle. Initially it was only when cold, and only after the first throttle input, but it started to get worse as time went on, and after a few weeks would continue even when fully warmed.

I had my injectors cleaned and rebuilt as part of the engine rebuild. Replaced the original one-wire engine temp sensor with a two-wire. Replaced the ICV. It had new plugs and wires as part of the rebuild, but I replaced them again anyway. Swapped in a known-good DME. Set and reset the idle. With and without the O2 sensor. With a new O2 sensor. Nothing seemed to work. My AFR was sub-12.1 and I couldn't seem to get the engine to lean out.

I went down the MAF route with Sal Carceller, and that definitely fixed the issue. That being said, as I was troubleshooting, I ran across two issues that I didn't address (had already ordered the MAF at that point).

1. Got under the car with it running and warm, and temperature checked each exhaust port. Two of them ran ~20-30F cooler than the others. Swapped injectors around, and the temperature change followed them. I had figured that, being cleaned/rebuilt, they were fine. Nope. Dumping a good bit of extra fuel that the DME wasn't expecting, but only really affected it at/near idle.

2. When I took my AFM off for the last time, I got curious and blew out the air bypass route with cleaner. The route the adjustment screw uses to introduce unmetered air. It was filthy. I mean filthy like someone had been spraying oil vapor up into it for 130,000 miles. Not sure how much that may have restricted my ability to alter the AFR, but I'm sure it didn't help, and I never thought to check it until after I'd given up.

Hope some of this helps.

- Chris

AFM air bypass path is definitely something to check as chrisk70 advises in #2 above. Also check the idle switch on the throttle body. There is an adjustment that may get out of whack and if that switch doesn't close when the throttle is not being depressed then I believe the DME doesn't know it's supposed to activate the idle circuit.

I had a problem with that switch that caused very similar symptoms as you describe. It's easy to check.

Bill

Another thing to check on high mileage intake is the o ring on the idle screw. And since it was rebuilt intake runner bolts can loosen.

Awesome recommendations y'all!

Update:
I cleaned the AFM, including the air bypass path (thank you chrisk70 and wjdunham). I pulled the cover on the AFM, and it really didn't look too bad (car has ~86k miles) .. but I went ahead with the wiper adjust procedure and very light cleaning of the contacts.
Put the car back together again .. new plugs in, found one plug wire had a loose connector .. fixed that, reset idle mix to 2 turns out .. idle adjust set at 1 turn out... time for another test drive
Running like crap .. it would not even rev over 4,000k .. felt like it was hitting a rev limiter. Doing searches I see this looks to be what happens if the crank speed sensor gap is not adjusted correctly .. odd, as I didn't touch that... then, I realized the car has run like this since I put in the rebuilt DME. This morning I swapped the rebuilt DME with my original .. car runs great again. Also, no idle surge .. but the weather is cooler, and I noticed the idle crept up to ~925 rpm once fully warmed up.

So .. my plan based on the help above..
1) check idle switch (it does click at the correct adjustment .. but I have not checked continuity)
2) Check AF with Wideband again .. adjust if possible .. reset base idle
3) Check fuel pressure

I'm also wondering of adjusting the idle mixture using a Wideband is OK. I may taking it to a local shop for a proper CO adjustment. Thoughts?

I have used a wb o2 sensor it is fiddly with adjusting idle, then Afm then back to idle then back to afm…
All with IAC bypassed. Took me about 45 min. Ended up tying a string to my hex key after dropping it on fan shroud and retrieving it🙄

If the chip has been replaced with a wong or a later chip idle may be 900.

Yes, with a non-OEM chip, and using the Porsche procedure for setting the idle, the idle typically "hangs" (high idle) on engine deceleration.

Just dealt with this today. Had missing at idle, afterfireing, and generally poor running. The connector for the AFM wasn’t getting good contact. The pins and socket wear over the years, when I pulled back the dust boot on the connector and used a sharp probe to push on the female receptacles, this fixed my whole poor running issue. As these cars age, stuff wears out from heat, and all the not good stuff that being old brings. A light push and put the dust cover back on and it runs like a top once more. In chasing every possibility in regards to fuel and intake signals, I did find that the throttle idle position switch was not as fresh as it could be, it was slightly out of resistance when checked. When I removed it while the engine was running the idle increased to about 3k, when plugged back in idle went to 750rpm. I imagine when the idle switch went to infinity the computer drove the ICV full open and the 3k idle. Plenty of witchcraft and sorcery in these electrical systems.

That is exactly what I do to test if the idle switch works, and yours seems OK.
Otherwise the idle speed wouldn't change when you unplug it.

Another thought, what position is the fuel quality switch in ?
see link below if your not familiar with it, just curious as I think you mentioned the rebuilt DME.

https://www.911chips.com/fuelsys.html

Hi all,
Thank you for the input so far! Still having this gremlin. I’ve been throwing parts at it .. here is where I am at.

Replaced:
ICV
Spark plugs
Oil tank vent lines and various vacuum lines
O2 sensor
Head temp sensor
Fuel filter
DME (Focus 9) & chip (have both a stock ‘89 tune and a SW chip)
Crank speed and position sensors, and re-adjusted gap
Fuel pressure regulator
AFM cleaned and re-tracked
Idle mix set to 14.5 a/f, AFM idle mix screw about 1 full turn out
Idle air by-pass screw all the way in
Disconnecting the idle switch causes idle to go to 2000 rpm .. reconnecting restores idle to 1,200
Engine has 87k miles and had a recent top end rebuild by PO
Injectors were cleaned and balanced
Valves adjusted by me about 5k ago

Current status:
Idle mix is too rich and can only be set properly with FQS set to 3
Idle is too high .. 1,200 when warm with the SW chip or 1,100 with occasional flux with stock ‘89 chip
Car runs great.. except at high RPM. 2nd gear full throttle will miss at 6k and not pull past 6,100 .. like a rev limiter
Throttle body seems to have slight resistance at full open

Thoughts ..
Rich condition at idle and high idle is too much fuel or incorrect air/sensor reading? What’s the connection to the high rpm issue

Latest ideas I have..
Check and clean throttle body .. adjust throttle plate if needed
Replace AFM with a known good
Check if distributor is installed correctly?

About the only thing you haven't replaced is the fuel pressure damper. Have you checked that it is OK by removing the vacuum line and checking for fuel seepage out the nipple? Could be that is causing fuel pressure issues if it's gone bad.

AFM needs to be set with a CO analyzer - anything else is guesswork.

The idle air bypass needs to flow air or you will be too rich. 2 to 3 turns out is the general ballpark.

You didn't state the status of the closed throttle switch when installed. Does it meet the specifications detailed in the manual regarding when it is supposed to be engaged/disengaged?

There should be no challenge to movement of the throttle system. If there is, something is out of alignment.

If you are rev limiting with both chip programs then it's likely not the chips causing the issue.

Previously, you stated you put the original DME in and it worked better. Did you take it out?

Went through similar fun hunt, FPR and Damper had the greatest impact. Both were allowing fuel to enter via the intakes, let to rich running and unburnt fuel smell during starts.