LT1 High Idle

[ReidB]

Hello.
OK, so I'm a noob from the land of carbs. As some of you know, I have a 94 CAD LT1 transplanted into a 95 K2500. Nothing fancy, just trying to sort out everything.
Having issues with trying to get an idle below 1500.
Over the weekend I found a vacuum leak, cleaned the throttle body and IAC, replaced the TPS. Put it back together, and wala... now it freaking idles even faster.
I'm using EEhack and am still fairly clueless. I tried to attach my last run but it won't let me. If someone could take a look at it and give their thoughts, I would be forever grateful.
Thank you!

[91ss]

Having those spark plug wires so close to the ECM wiring is not helping. Probably getting a good bit of inductive coupling to the low voltage signal wires running to the ecm.

[ReidB]

Good point. Thanks.

[91ss]

Good point. Thanks.

Take a look at the vacuum elbow at the TB on the passenger side. It's the one below the bigger line that goes to the valve cover. A common one to get brittle. Just noticed mine last weekend. The good fix is to use an old spark plug boot off the original 7mm wires. Which i just threw out a couple months ago... :( As a temp, i just coated with some black rtv.

[ReidB]

Yep, that elbow was split. Already plugged up. I think it may be the iac but Im getting tired of throwing parts at it without being sure. $$ I can get idle down by partially covering the triangular air opening between the throttle blades with my thumb.
Anyone know how to check the iac? It ohms out ok but it would be nice to be able to power it open and closed and check the plunger depth vs the bore depth. Seems like 5v on rt pins and lt pins would do it but not sure. Thanks.

[ReidB]

Yep, that elbow was split. Already plugged up. I think it may be the iac but Im getting tired of throwing parts at it without being sure. $$ I can get idle down by partially covering the triangular air opening between the throttle blades with my thumb.
Anyone know how to check the iac? It ohms out ok but it would be nice to be able to power it open and closed and check the plunger depth vs the bore depth. Seems like 5v on rt pins and lt pins would do it but not sure. Thanks.

Here's a screenshot of the data. Thank you for looking.

[91ss]

How long was it running? Can't make out the time. Temps look cold. The IAC steps at 127 is making it idle high.

[steveo]

the IAC is jacked wide open but you're above target idle, probably because your tps needs to be 0% at idle (it isn't trying to control idle at all), your TPS is 1.6%. close your throttle blades all of the way so they're almost stuck in the bore.

this may be due to a wiring fault or interference, , meaning TPS voltage may be unstable?

the LT1 ecm usually finds 0% tps by minimum voltage without issues, so unhook the battery for a bit to reset the ECM memory. then start it while giving it no throttle. it should self-calibrate.

Anyone know how to check the iac? It ohms out ok but it would be nice to be able to power it open and closed and check the plunger depth vs the bore depth.

yes, eehack 'control' section, check idle 'steps', then idle 'override', you can move the pintle that way.

[steveo]

another thing for that tps reading, without the engine running, move the throttle from closed to wide open a few times really slowly while logging, then graph 'throttle position'. it should be smooth with no dropouts or spikes, and return back to exactly zero every time. i designed eehack to be stupid easy to diagnose this kind of stuff with. you can also graph raw tps voltage instead of percentage by selecting view level 'extended' and looking for TPSVOLTS.

.. graphing stuff usually finds the cause of problems for me

[ReidB]

It was cold. Ran a few min. Didn't get any better as it warmed up. I forced closed loop it started to settle down then the injector pw hit 9.5 and had to shut it down.

[ReidB]

The tps is an oreilly pos. Got it to learn 0 but it only reads 77% @ wot. Taking it back tomorrow.

[ReidB]

the IAC is jacked wide open but you're above target idle, probably because your tps needs to be 0% at idle (it isn't trying to control idle at all), your TPS is 1.6%. close your throttle blades all of the way so they're almost stuck in the bore.

this may be due to a wiring fault or interference, , meaning TPS voltage may be unstable?

the LT1 ecm usually finds 0% tps by minimum voltage without issues, so unhook the battery for a bit to reset the ECM memory. then start it while giving it no throttle. it should self-calibrate.



yes, eehack 'control' section, check idle 'steps', then idle 'override', you can move the pintle that way.

Makes sense that it doesn't want to idle with the 1.5% tps. The blades close fully. They held carb cleaner on benchtop.
Graphing tps voltage is awesome. Didn't know it could. Multimeter swept 0.56 to 3v. Seems like it should hit 5v at wot.
Stupid easy is what I need.Great program/tool.

[ReidB]

the IAC is jacked wide open but you're above target idle, probably because your tps needs to be 0% at idle (it isn't trying to control idle at all), your TPS is 1.6%. close your throttle blades all of the way so they're almost stuck in the bore.

this may be due to a wiring fault or interference, , meaning TPS voltage may be unstable?

the LT1 ecm usually finds 0% tps by minimum voltage without issues, so unhook the battery for a bit to reset the ECM memory. then start it while giving it no throttle. it should self-calibrate.



yes, eehack 'control' section, check idle 'steps', then idle 'override', you can move the pintle that way.

I'll have to separate the control wiring from plug wires like 91ss suggested.
Need to reset tps and see if i can get it to settle down. I couldn't get pintle to control before i found vacuum leak.
Found out hard way do not share coolant temp signal with anything. It shares ground with iat and tps. Causes voltages to float freely.
Thanks for input! Have some troubleshooting to do.

[steveo]

you need to be careful about grounding sensors in a custom harness. when something connects to an ecm directly for its ground, you need to use that as your ground, as they're specially filtered for noise. don't ground anything else to them or you might fry the ecm from the load, and don't splice those and ground them to chassis.

this definitely could be your problem.

[89S10_Project]

The tps is an oreilly pos. Got it to learn 0 but it only reads 77% @ wot. Taking it back tomorrow.

Just so we know, none of the parts places manufacture parts- very likely the part you buy in an oreilly box is the same you buy in a Napa box, etc.

Further, are you sure you're not getting only 77% of mechanical travel in your linkage? Have an assistant push the accelerator pedal to the floor. Then try to manually move the linkage at the engine. Any additional motion bears investigation (floor mats, poor linkage design, etc).

I only key in on this because you did say it was a transplant.