I am having an issue that started about 4 months ago with the '67 with the 5.3 / 5 speed manual swap. The motor is a mildly built 5.3 with stock heads milled .050" and a TR220 cam. The issue is when I am coming to a stop and push the clutch in the engine does not want to idle and tries to die. It recovers about 25% of the time, but most of the time if I do not heel/toe it and give it some gas it dies. The issue mainly appears when I am using engine compression to slow down and put the clutch in for good when nearing a stop around 1500-2000 RPMs like you would normally drive. It doesn't matter if I am slowing down from 80MPH or simply cruising in a residential area in 2nd gear at 20MPH. If I put the clutch in at speed and do all of my slowing down with just brakes it is fine. I checked for all the obvious things, fuel pressure, spark, vacuum (sprayed carb cleaner everywhere possible) and nothing affected the issue. The issue does not change with engine temperature and the car runs completely normal otherwise. I had a tuner in the area check it out and he couldn't seem to narrow down what was causing it.

I moved to a new state about a month ago and talked with a very reputable tuner and he felt like it was likely a tuning issue. After two days on the dyno he gave up and said he's dealt with this issue many times before but has always been able to solve it. He says he can see no mechanical issues with the car and everything seems good. He data logged the issue a few times and the only weird thing he saw was that when the issue occurs the IAC is commanded to move by the ECU and it just doesn't. I swapped the entire stock throttle body with another known good one to rule out the IAC and it is still performing the same way. I verified the wiring to the IAC is good and there are no intermittent connections. This issue is becoming very frustrating and the car is borderline unsafe to drive. Any ideas?

 

Couple of things to add. I am running VSS. IAC count is in the 60s at idle. The tuner messed around with all of the buzz words I have seen in the tuning section, air follower, throttle delay, etc. I am not a tuner and have no ability to extract or check data besides a scanner so that it why I didn't post on the tuning forum.

 

Do a half turn on your throttle set screw. Unplug the TPS. Key on engine off for 60 seconds. Plug it back in. Start it up. Let it run for a good five minutes.

 

I'll give that a shot tonight. Thanks Darth.

 

I messed around with that tonight, opened the throttle blade up in five stages all the way to where the IAC couldn't control the idle anymore. Drove it in the same loop and situation each time and issue persisted with no discernable difference.

It will operate fine a handful of times when coming to a stop, then it will try to die every so often even if recreating the exact same scenario. Any other ideas?

 

Not without getting into the tune unfortunately. It took me months to get mine right when I had that problem. I was just hoping something simple like reducing your IAC counts would help.

 

I've used three different tuners on different cars in the past and my current tuner is by far the most knowledgeable and I trust his tuning completely. If he can't get it 100% with the tune I don't think it is a tuning issue. Is there any other hardware issue that I haven't checked that could cause this?

 

The usual stuff. Vacuum leaks, slow IAC, exhaust leaks, stuff you've already checked.

Ah! Faulty alternators have been known to cause idle issues.

Jake fusion chased this FOREVER on his car, and it ended up being a bad ECU I believe.

Edit - if you're confident in the tune that's good enough for me. My next diagnostic step with an unknown vehicle is all that was.

 

No worries at all, no offense taken. I would expect an alternator to cause consistent issues, not random events unless I am wrong? Car holds a steady 14.4V all the time, except when it it trying to die of course. Verified my vacuum again, 12 cold and almost 14 hot. It is a custom grind cam, but close to TR220, 220/218 on a 114. Seems a hair low on vacuum?

Every time he logged the event he noted that the computer commands the IAC to move and it just doesn't. Everything is clean and the IAC is from a known good TB. Should I just eat the $90 and put a new AC Delco IAC in? I only hesitate becausee I don't want to start chucking parts at it if it isn't an intelligent possibility.

 

Had similar issues on my car and a buddy's across the street. Told my tuner about my issues, he played with some airflow stuff. Helped but didn't solve the issue. He ended up using the factory throttle crack table. He said, some car need it, but most don't. Copied and paste that table into throttle cracker, and perfect now. Same thing with my buddy's car across the street. No more random shutting off coming to a stop.

 

IAC is a possibility. Where I'm stuck is you replaced the throttle and I thought I read the IAC with it. So I don't know if I would expect another new one to fix it. You could try it, but I don't have high confidence it'll help.

That cam is small enough to where it shouldn't be tune related. And like you said - it's random not consistent.

Have you checked for consistent fuel pressure? If not, that's one thing to verify. Also, I sorta randomly thought plugged cats.

 

Thanks for the help again guys. I remember the tuner talking about messing with the throttle crack table. I'll ask for more info on what he did.

I swapped the entire TB from a running 100% stock rig. What is odd is that after swapping it the issue shows up about 2X as much as before, but still random. That is what is making me suspect maybe the IAC, although it doesn't make 100% sense.

Would you expect more vacuum or is 12 cold and 14 hot what is normal from that small of a cam?

 

I've had times when you can't do enough with the follower and cracker. I ended up doubling the underspeed air and halving the overspeed air to fix it

 

I know he played around with the underspeed and overspeed air and it was inducing a lot of "cruise control" but it didn't mitigate the issue.

He even added in a 2 second delay on the throttle decay to try and see if that would totally eliminate it. When that delay happened it would idle fine, but sometimes the delay wouldn't happen and then it would die. That was the main frustration he had with everything he tried was that the car wouldn't always do what it was being commanded to do. Would that be a sign that the ECU could be the culprit?

 

It could be. The guy who kind of went through this nothing works no matter what thing was jakefusion. Might be worth PM him to ask.

 

That cam with a M5 transmission idle speed should idle fine with the stock tune settings. Do you know if the tuner loaded a manual transmission operating system or is he trying to use an auto OS? And a reputable tuner spent 2 days on a dyno troubleshooting an idle issue? That sends up red flags for me.

 

That is why I chose this cam, get the power and sound but maintain driveability. He loaded up an MT tune and is using my VSS. Had the tuner in my previous state spend a couple days on it also. The guy who tuned it most recently is THE guy in the area. Everybody with an LS has had him tune it, everything from 1,500HP twin 408 builds to stock. He was very frustrated with this issue and that is why he took so long exhausting all options.