[skydawg727]

My swapped 3.4L V6 is idling about 1200-1800 RPM after its been running for about 5 min. The throttle body and intake are original from 2.8L but problem recently started. I have replaced the IAC but no effect. I do not see any open vacuum ports or hoses breached, and the throttle valve fully closes and is not sticking at idle, and timing checked.

I do not think there are any air leaks around TBI gasket and injector spray pattern normal. I am getting normal engine temps on gauges (but I suspect there is a 2nd temp sensor for ECM) and no check engine light.

Any suggestions appreciated

 

[Ed Mc.]

Does this 3.4 have new cam and lifters? If so it needs to run faster anyway, for 20-30 minutes, to properly break-in the cam/lifters.

How to Break in a New Cam - Tech - Car Craft Magazine

Check out how you can break in your new camshaft.

www.motortrend.com

Beyond that, how about adjustment of minimum idle speed and then the T.P.S. voltage? If baseline hasn't been set, it's possible to get a high-idle issue. Check my previous posts for the IAC and Min Idle Air procedure by Jay Vessels.

If you haven't removed the idle stop screw plug on the TBI unit, the easiest way to do that without taking it off is to drill or punch a hole in the middle of the plug, then screw a tough screw (such as a deck screw) in the hole until the screw bottoms out against the head of the idle stop screw. Keep turning and the plug will be forced right out. Geoff had that good idea.

The below short procedure has instructions at the end to reset the IAC, maybe that will help it to "relearn" the proper idle speed. It's written for an S10, so the wire colors on the ZuZu ALDL connector will be different (should be white with blue stripes). The 2 pins right in one corner of the plug. Or, connect the 2 normally-disconnected white/blue wires in the center console.

The temp sensor for the ECM is the one sticking out of the intake manifold, behind the thermostat housing. Be sure its 2-wire connector is plugged in. Also make sure that big mess of ground wires over on the pssgr side (the one that terminates in one big lug) is connected to the engine block. It's a bunch of ECM etc grounds and things go rather screwy if the bundle isn't connected.

 

[skydawg727]

Thanks yet again.
I don’t think the set screw would do much as fast idle is a recent issue long after engine swap. I did the TBI calibration above without much affect. Again, engine idles about right 700 ish rpm but a bit rough, but still climbs to about 1800 rpm after engine warms up. Unplugging IAC brings it down to about 1000 -1200 RPM. It also indicating a fault code 32 which relates to EGR which I don’t think would cause idle issue, and no code for faulty temp sensor which would have perhaps explained the higher rpm once engine is warm.

I replaced TPS years ago when trouble shooting rough idle after swap but will check ref 5v is getting to it.

I will look up suggested Vassel post And give it a try.

any other suggestions welcomed.

 

[Ed Mc.]

ECS (Engine Coolant Sensor) temp vs resistance curve attached. These can fail or be failing, sometimes they won't set a code. I've had 2 different rigs where the ECS acted up and neither set a trouble code.

 

[Ed Mc.]

You might have to tie into the ECM with WINALDL and a USB interface cable (or of the more modern versions of diagnostic programs available) to see what's going on. There's something that's triggering the ECM to do the fast idle events; beyond basic adjustments and checks of the various sensors etc, you have to "get under the hood" so to speak.

WINALDL: WinALDL - 160 baud ALDL reader

Commercially available ALDL-to-USB interface cable: OBD Diagnostics, Inc. - OBD2 All-In-One Scan Tool w/ USB

Good D-I-Y info: https://www.doityourself.com/forum/...3803-live-engine-data-pc-less-than-100-a.html

ALDL Android interface stuff: 1320 Electronics LLC

 

[Ed Mc.]

BTW my 3.4 has thrown an EGR code on occasion for a long time. IDK if it's because the 2.8 ECM is expecting a different flow rate when it "tests" the EGR system by lifting the valve and sampling exhaust gasses via the O2 sensor (under certain conditions, while driving steady-state), or what.

I guess the only way to prove that would be to install the 3.1 ECM I have kicking around. But dangit, I've been saving that one for after the big EMP hits! 😹

My solution if the Check Engine light comes on while driving: Key OFF, Key back ON, light goes out, keep on driving. Works like a charm!!

 

[skydawg727]

I remember trying OBD 1 readers and scanners years back and most only gave DTCs and not graphic or live data needed to really trouble shoot. …I will revisit available scanners. thanks

engine idle RPM continues to increase as engine warms up but cannot figure out why ECM is commanding it.

today, i again looked for vacuum leaks and replaced the MAP sensor, as well as spray ether around the intake in effort to find an air leak, but still no culprit to high idle. My next step is to check temp sensor resistance which could explain the higher RPM as temp increases, but doubt even a bad sensor would cause ECM to command such a high RPM. After that I am out of suspects and likely drop it off at a shop but it’s hard finding a shop experienced in older engines now a days.

Suggestions always welcomed.

 

[Ed Mc.]

What happens if you disconnect the ESC (Electronic Spark Control) wire in the console? Maybe the ESC is jacking up timing and making the engine run faster. Not too likely, but need to eliminate all factors.

How low can you get idle speed with the IAC pintle driven in (as per the IAC and TPS procedure) and then IAC disconnected? Can you control it then with the idle stop screw? You should be able to achieve 750 rpm or less. I think the target idle speed for these with everything connected must be somewhere around 875 or so.

I see in your other post that just unplugging the IAC gets speed down to 1000 rpm or so. But that's not necessarily as low as it can go, if the IAC is partially open. Only when you do the procedure to drive it shut are you (potentially) are eliminating the IAC as a factor.

Beyond that, the only things affecting your idle speed should be throttle opening and base timing setting. Or a vacuum leak. See what manifold vacuum is after driving the pintle in.

BTW the ALDL program gives real-time data, not just a code reader. So you'll see all the ECM inputs just as the ECM does, plus fuel trim, IAC counts, etc.