CIS At Altitude
 

I'm doing an Angeles Crest run tomorrow, and I know my car doesn't like altitude. It won't idle and can have momentary lapses of power. As I get back down to sea level, all good.

I know nothing about CIS as I'm new to this car and this system.

Help?

What's the altitude change from where you live to Angeles Crest?

Sea level to 4500-5500'

The air density is going to be less, but other than pulling over and adjusting the idle screw when you get to altitude, I don’t know what else you could do to compensate. Especially on a temporary basis since you’ll be coming back down.

I’d pull over at a gas station, raise the idle and check your tire pressure.

You’ll need to dial the idle rpm back down of course when you get back to sea level.

PS - I’m assuming the car is in good tune. No vac leaks or other issues.

Your CIS-equipped engine will be fine ... nothing you need to worry about.

John

I drove a few weeks ago up to about 4400' and the car wasn't fine.

That is why I asked the board in the first place.

Maybe I'm missing something on the engine?

I know a few guys who run stock CIS-equipped mid-years and SCs up there with no issue.

I suspect it may be time to go through the CIS and rule out any other possible issues? Vacuum leak check and fuel pressures check as a starting point maybe?

I live at 3500’ and drive to 0’ and 6000’+ pretty regularly, no issues from the CIS

When my 74 was CIS I ran it up to Pikes Peak and it had no issues. That’s starting out at 7000 feet and going to 14,100. So you likely have some tuning issue.

Am agreeing with a tuning issue or vacuum leaks - I've had a couple CIS cars and haven't experienced any noticeable issues with elevation change below ~5000ft. (Added - None of my CIS cars had O2 sensors or altitude compensation.)

I'd start with checking that the pop-off valve seals (if you have one), then look at hoses, injector seals, air box seams, etc.

Another thing to check is to make sure your vacuum-actuated decel valve (if you have one) isn't open at idle at lower elevations and providing extra idle air (it shouldn't be - might need adjustment).

I've driven the stock '82 911 from 800 feet up over Angeles Crest to Wrightwood & back several times in the last 20 years. Never felt anything different on the butt dyno at altitude. Also up over Fresno to Yosemite Valley, prolly over the crest at 7000/8000 feet.

The later CIS has a lambda controller which can adjust to changing AFRs to some extent.

Perhaps that's the reason for the different observations.

For another point, I've driven mine from 2700' to over 8000' with no issues.
78 SC with no O2 sensor

I run to 5k often where I live no problem.

When you have your problem again be prepared to pull over, pull a few plugs, take pictures and post them here. When you say your car "doesn't like altitude" can you please describe that in more detail?

It might be a case of your car running rich at the high altitude. You could carry a 3mm long allen wrench along and see if a small tweak of the rich/lean screw will make a difference. If your car is running rich at sea level it will be way rich at the high level. Your other option would be to play with the idle air bleed. Good luck!

I'm not sure on why it behaved strange at altitude. The car runs perfectly otherwise.

Maybe check if the FP is on the rich end of the operating band.

I can't recommend any further troubleshooting without more input from you. I would not adjust the mixture without seeing the plugs. If you are not capable of doing basic maintenance steps I think you will need to take your car to a qualified shop. Maybe there is a member who is close to your location that can help you on site.

My 78 does that sometimes when I'm up there. It may be contributed to the weather at times.
I noticed it when I pulled over and the car just stalled.
My C.I.S. works fine too, but like I said I think the weather was a factor.:rolleyes: