Erratic Idle, possible interference? (with video).
 

Hey guys, if anyone has a clue what might be going on with my LT-8, I'm all ears to suggestions.

What seems to be happening to me is some sort of ignition interference. Randomly, the idle will start to jump around. It could happen 10 minutes into a drive, an hour into a drive, or as seen in this video a minute or two after it's warmed up. It seems to me too have gotten less intense when I switched to some Magnecor 10mm wires.

It worries me to drive the car and have this happen when I'm driving aggressively.

It seems to go away for awhile, sometimes briefly, if I turn the ignition off, then back on. Obviously this isn't optimal when driving down the highway though. Anyway, here's the video, anyone have any ideas?

You can see the idle jump to say 1500, then the RPMs almost go into a seizure, and the car usually dies. When driving the RPMs can jump as much as 2000 to 4000 RPMs.

https://www.youtube.com/watch?v=wafJ...e_gdata_player

The way the tach is jumping you def have some noise interference. Try reshielding the Cas wires. And see if that helps. Also check to make sure none of the wiring on the Cas is dry and brittle.

Thanks for the quick response

I was thinking about shielding those wires, but what should I use.

Could it be from them being so close to the alternator and spark plug wires?

How far back from the CAS should I shield them? To the firewall, or all the way back to the ECU?

And should I try shielding the alternator wire as well?

I'll check the wiring on the CAS, but its the ECU wiring from the plug all the way back so I can't imaging it is dry or brittle. I guess if my shielding doesn't work my next step would be to try a new CAS?

The wires should be shielded all the way back to the ecu. It should be grounded also, only at the ecu. You can you a tinned copper braid for shielding it.

That seems like a pain in the ass with the braid, since the harness is already terminated, and to try nd find the stuff. I'm wondering if there is something like a tape I can roll onto it.

Trying to find any EMI/EMF blocking tape on google is a nightmare.

Won't happen, going to just have to either reshield it or run a new shielded pair to it.

Alright, so I'll be ordering some of the tinned copper braid for it, but for now I noticed that the CAS is grounded to the frame by the battery. Its tough to remember how I did this, since I haven't really messed with the car in like 2 years, and i'm sure it was a couple years before that that I wired it up.

Could there be an issue with it being grounded there?

Again, thanks for all your insight.

Looking at it some more, it looks like the CAS wires are already shielded....

There is maybe 4 or 5 inches of bare wire, do you think that is enough to cause an issue?

If it helps I can post a picture.

No, the Microtech loom is shielded from the factory, but in events like these is never hurts to reshield it and check the Cas. Just for shits hot swap the Cas and see of that helps before re looming the harness

Now I'm starting to wonder if the CAS is possibly wired wrong, so I attached a picture.

As you can see in the picture, the two white wires coming from the CAS are grounded. The green and red coming from the CAS go to the green and red on the harness. It looks like there is a yellow and a blue coming from the LT8 harness as well. I had them wrapped in the harness, but it looks like according to the wiring diagram they should be grounded?

http://www.microtech-efi.com/downloads/17.pdf

If they should be grounded, what place would be best? The battery, the body, the engine block?

The Cas grounds should be connected to those two wires. If you check them with a ohm meter they should already be grounded at the ecu end. I will put money that if you hook those two wires in the bundle up to the Cas signal grounds your noise problem will go away.

I will give it a whirl.

The diagram is kinda confusing though, it doesn't show the blue or yellow wires, and it shows the other two going straight to ground.

I guess theres only one way to find out...

If you ohm them out they will be already tied to ground. It shows the blue and yellow wires being the sensor grounds. The red and green are the pos+ signal out of the reluctance sensors.

Thanks again for all your help, I can't wait to get some free time to test this out.

Got it all wired up yesterday and ran it for a few minutes, and didn't see any problems.

Got it down off the jack stands, started it again today seemed to be acting up again. Its really starting to bum me out. The banging in the video is my air filter hitting the fender well.


I can post a picture of the redone wiring as well.

I should also mention I think it was fine until It started to warm up.

It's possible then that there could be a problem with the unit itself. Did you try changing the Cas out!

The unit itself, you mean the CAS or the LT-8?

Also, do you think if the CAS was bad would it show OK on the sensor check screen?

Im pretty sure I remember it showing Home and Ref being OK.

Heres a photo of the wiring.

have you tried to put a bit more fuel in at the piont were it drops?

I don't think its a tuning issue. The car was tuned by Steve Kan, and there are times where it will sit there and idle at 800rpm rock solid for 5 minutes.

They way the tach is bouncing its def a signal issue. Try swapping out cas and see if it changed.

Im going to try and dig up another CAS.

In the meantime, looking at all these CAS and shielding threads, they all say the shielding should be grounded at the ECU. Is there a ground wire coming off the CAS wires at the ECU I possibly didn't ground? Im just curious as I have my ECU in the stock location and don't want to go tearing up the interior if I don't have to.

If you have the stock Microtech harness then the shield is tied into ground at the ECU end. Make sure not to ground the CAS end!

One thing you can do if you don't have another CAS is to loosen up the reluctor pickups and move them closer to the wheels. Use a piece of paper between the reluctor and wheel to make sure they aren't going to touch. Some CASs are pretty loose, some are pretty tight. Moving the reluctor closer will generate a stronger signal when CAS speed is low (as during idle).

Also, have you had your alternator tested? The rectifier may be failing, spewing out noise. If you have a scope then you can scope the alternator B terminal while the idle issue is happening and look for ripple. An AC voltmeter may also help.

I haven't had the alternator tested. I suppose they could do that at most parts stores? The alternator is the one from the -RE, and I switched changed the harness over to mate up with the FC harness. If it comes down to I may have to go back to a stock alternator.

I'll try to adjust the reluctor pickups, but I've had this happen to me on the highway so I'm skeptical it will work.