High idle on cold start: vacuum leak or IAC setup?

[jangell]

Time for a new 3.0L/MegaSquirt/EDIS thread. First, I’m running the coil pack now instead of coil-on-plug. Redoing the wiring for the CoP didn’t fix anything, so I’m guessing a coil is bad or something. Since the coil pack works, I’m tuning with that for now. I’m going to go to a junkyard later and try to source a more easily mountable pack and some longer wires. I might switch back to CoP after I’ve run the coil pack for a while.

Anyway, the car now starts, but it shoots up to 2000 RPM and stays there. This is after I turned the throttle adjustment screw all the way to closed, too. However, when I used IAC Test Mode in TunerStudio to completely close the IAC, the car starts and idles at around 800 RPM when cold. It starts the lose RPM as it warms up, so I adjusted the throttle screw a bit until it stayed running, and adjusted the fuel table to keep it in the 14.7 AFR range for good measure. It seems to idle fairly well once warmed up.

My question is about this high idle on cold start with the IAC. It’s about 45 degrees ambient today, in case that’s relevant.

I’m thinking I just have the IAC set up wrong. But I want to make sure there isn’t a vacuum leak before I delve into that. Since closing the IAC seems to let the car idle properly, that implies there’s no leak, right? Otherwise the car would still be idling way too high, wouldn’t it? But it seems odd that I have had to open the throttle a little to keep it running as it got warmer, too — I thought I’d need to close it more, if anything.

I also have an analog vacuum gauge hooked up. With the engine up to temp it shows >30 inches Hg at idle (it maxes at 30), well outside the marked green zone of 17-22, but I really have no idea what that means. Some Google searches suggest that it should be idling in that green zone, so that implies a leak.

But TunerStudio shows a MAP of 40 kPa, which I think is OK?

I previously set the ignition timing as best I could, with a 1 mm gap between the VR sensor and missing tooth wheel, centered as accurately as possible six teeth before TDC. So I think the timing is right, anyway. At least the exhausts aren’t glowing.

I’ll attach a log and tube when I get home. It’ll be of a warm engine; I didn’t take one of the engine cold.

It’s looking like I’ll be able to drive this thing soon.

Thanks!

— Joe

[jangell]

Log and tune attached.

In a video I took a couple weeks ago of the engine idling at 2000 RPM, I can see the vacuum gauge showing in the 18-22 in Hg range for vacuum. With the IAC closed and throttle almost completely closed I'm maxing out the gauge at 30 in Hg at 800 RPM. I had assumed a vacuum leak (or at least a misconfigured IAC) since closing the IAC got the idle down, but 30 in Hg seems like a lot of vacuum at idle. I'll grab a video of the gauge while the engine is running and revving later today.

I adjusted the first four columns of the fuel table together to keep the AFR around 14.7, so there's a jump between the fourth and fifth column. Nothing is really tuned yet.

CurrentTune-Dec152018.msq
2018-12-15_12.29.19.msl

Thanks again

-- Joe

[FABombjoy]

Your combo of spark advance and idle steps is causing the runaway idle.

There are several means to solve this. I would first set up closed-loop idle control. "Warmup mode" is less sophisticated than the original idle control system. Then you can also involve spark control in the idle system for ultra-stable idle.

Setting up idle advance will help stabilize warm idle.

Add an idle zone to your spark table - all the same value so little shifts in rpm don't cause shifts in spark timing.

[jangell]

You have confirmed that it was my ignorance.

I took your advice and set up closed loop idle control, which was much simpler than I thought t would be. I closed the throttle all the way and am now relying on the closed loop to keep the idle at 790 RPM (+/-30 RPM). It’s working quite we so far.

For the advance table, I just set it so that all 9 cells (3x3 grid) around the cell it was idling at (14 degrees at 40 kPa at 790 RPM), but I can’t test that today because the starter doesn’t want to crank anymore. (Applying 12v to the solenoid wire doesn’t seem to do anything, and the battery isn’t discharged, so something’s up). I’ll have to check further once the engine cools down and I have more time, probably on Friday.

Also, I’m thinking my vacuum gauge is junk. I finally thought to look at it without the car on (or even plugged into the vacuum system) and it shows 12 in Hg vacuum instead of atmospheric pressure. So that was just a big red herring.

Thanks again!

— Joe

[DMC5180]

Sounds like you fried the contacts in the starter solenoid. Does the solenoid pull, click with 12v applied? If it does but the motor doesn’t spin that’s likely the problem.


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[jangell]

It had been intermittent the last few days of testing, but it seems to have finally just stopped working. I have a push button hooked up in the engine bay so I don't have to go into the cabin to crank the engine, but it's basically the same wires as used when turning the key. Fiddling with the wires seemed to help, but it was probably just coincidence. I thought it was the button, but replacing the button didn't help.

I finally tested the wiring with a PowerProbe; I'm seeing ground on the wire going to the solenoid, and applying 12v doesn't cause anything to happen to the starter.

Similarly, I get nothing when I push the button -- no sounds at all from the starter motor or the solenoid. It's as though I hadn't pushed the button.

Luckily, compatible starters are readily available. I'm off work on Thursday and Friday, so I'll go down on one of those days and see if I can figure out if it's a wiring problem or the solenoid itself is broken. At worst I'll pick up another one at NAPA or Autozone.

Thanks

-- Joe

[Josh]

unfortunate to hear that you went away from the CoP. I was fairly proud of that. 99% likely it was a bad coil.

I may have a bracket for the coil pack that i ran before. Mounted to the passenger side valve cover. Its in my junk pile somewhere.

[jangell]

I got three more CoP coils while I was at the junkyard, but at the moment I just want to get it running. Don't worry -- I plan on trying CoP again in the future. I like the idea of CoP more than coil packs. I do wish EDIS would fail in a more obvious way when a coil is bad.

I already have the coil pack mounting figured out, so no problems there. Thanks!

-- Joe

[opethmike]

You should hug the car.

[jangell]

Apparently hugging worked, and I was able to actually drive the car today. I mean, I just drove it around the block, but still, it drove.

I never figured out what was wrong with the starter. After removing the header and pulling the starter, I just tightened the wires and it ran without a problem. I think one of those wires was just loose.

The brakes aren't doing so good -- the car stops eventually, but I wouldn't want to trust them for real driving. My mechanic friend is going to help me bleed them this weekend, but I plan on putting delorean.co.uk's performance brakes on sooner than later (I've been wanting to get those for years).

I also noticed that the car wants to stall if I quickly take my foot off the accelerator. I tried adding more fuel to the leftmost column of the VE table, but it didn't help as much as I thought it might. I'm not sure if there's something specific to handle this case. Decel Fuel is at 100 still, so I'm not quite sure what's up there, but I'll keep tinkering.

Beyond that, I have an exhaust leak to fix (clamp is loose), and the TPS keeps seizing on me (I'll try adding a gasket to space it a bit), but otherwise things are looking good.

Thanks again to everyone for their assistance on this conversion over these past nearly five and a half years!

-- Joe