idle surge when hot.

[Sam Powell]

When the engine is hot, it develops a lean surge that sounds like a vacuum leak, but have not detected a leak so far. It has a complex idle control that could be overcompensating. Any ideas about where to look.

I am still out of internet at home. Can spend only limited time here at work on the forum. Thanks for any advice.

Sam

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A REAL professional is someone who knows when to hit it with a bigger hammer.

[Wizard]

If idle rpm is wandering all over and hard to control, it might be your ignition map in the area for idle with strong vacuum is too hilly. It need to be kind of plateu-like pattern. It is MUCH quicker to control rpm with ignition timing than fueling or throttle control to add to your knowledge fact.

Cheers, Wizard

[Sam Powell]

The computer uses ignition timing to try and keep the idle steady, so timing does go up and down a fair amount as the idle varies, but it is in response to the idle RPM change. I don't know if this is helping, or maybe going too far, and overshooting.

It does surging this only when very hot, as in after it has set for five minutes and restarted, or the ambient temp is very, very warm, as it has been here for the past week. This rpm wandering could be set off by the mixture getting too lean. I have not yet moved the ECT senor to the head. I will do this, as I am sure the head is varying in temp more than the coolant.

The AF ratio bounces around as well as the IAC is trying to also keep things in line RPM wise.
Sam

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A REAL professional is someone who knows when to hit it with a bigger hammer.

[Wizard]

Can you look at the timing curve map (it looks like a chart), and note the leftmost area, that area should be almost flat.

Also, try to chase all the air leaks. Absolutely have to.

Cheers, Wizard

[Sam Powell]

Wizard, I did what you said to do. The upper left hand corner of the timing map for about 12 cells is all set at 12 degrees. I got into the idle control panel and found the areas of control that determine how quickly, and how widely the corrections take place. I played with those until it seemed to be pretty stable, even when the ECT was 198 in last night's heat. The head was around 205 at that point, all just sitting in a garage idling, which would be the biggest challenge for the idle control, and the cooling system.

It seemed to be better behaved this morning, but the temp is much, much nicer. It was 64 degrees at 6 AM instead of 78. It got to 95 plus the last 4 days. Brutal. Thanks for the tips.
Sam

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A REAL professional is someone who knows when to hit it with a bigger hammer.

[Sam Powell]

I got this cleared up. I was trying to get it to run too lean. These are big injectors at 36lbs/hr, and are not as stable as smaller ones would be. Hence, a slight variation in injector duration means a bigger change than if the injectors were smaller. When running it too lean, if it fell off the sweet spot momentarily, the RPM would drop, the IAC would compensate, and bring it back up perhaps a little too far. It would get into a regular rythm of up and down.

I was trying to get it to idle at 14.5:1. I gave up and settled for between 12.5 and 13.5:1. It idles fine now. Maybe some day I can get it refined enough to get it to idle leaner, but not now.

Sam

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A REAL professional is someone who knows when to hit it with a bigger hammer.

[Joshua Skinner]

12.5-13.5:1 is a real decent idle mixture. Idle mix needs to be a bit rich as there isn't much mixture activity in the way of swirl and turbulence so you have to cover up the uneven mixture with extra fuel to get a good idle.

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Joshua Skinner
car nut, telecom tech, tool using mammal
'72 Dart Swinger

[Sam Powell]

Thanks Josh. I suspect if I went back to the extended tip plugs, which I took out to try and deal with the preignition, that I could get it to idle on a leaner mixture. But for now, I am happy. Another strategic error I have been making is jumping into economy tuning too early. This has got to wait until it is dialed in better, and all drivability issues are squared away.

Sam

_________________
A REAL professional is someone who knows when to hit it with a bigger hammer.