78' SC "new" Idle problem

[specialk22]

After a Sunday afternoon drive in my 911 SC today I arrive home and my car will not hold an idle. It fluctuates up and down and will die. It also smells like its burning rich.
It was a hot one today over 100 degrees. i wonder if my car is trying to tell me that it dosnt like long drives on hot days. The car temp ran a little over 200 but never close to red.
Any idea what the problem might be? Thanks

[Joe Bob]

When mine did that, it was a leaky airbox.

[DaveHak]

I had the same problem on my 83 SC...It would idle weak, and die out..I was pretty sure it was a fuel problem...Took it to my mechanic and it turned out to be a problem with the air box flap.. That's the first thing I would check.
BTW, All 911's like long drives...What they really hate is short stop and go on hot days.
Good Luck!

[specialk22]

Thanks Joe Bob and Dave. I will check that out tomorrow. I have had the car for about a month. It has been running great, plenty of power and the idle was good most of the time and every once in a while would have a similar idle problem but would never die like it is doing now. It was really hot today. Hope I didnt cook anything. I just brought my 70 T home today from a suspension upgrade and a few other fix em ups, maybe she is jealous
Thanks Guys!

[aldente295]

My idle problems were/are caused by two things - a distributor that needed a rebuild, and a WUR that needed the same.

[ephotrod]

I'm having the same problem with my 1978 911 SC. I bought her several weeks ago at a fair price and had to drain the tank change filters and the usual bit. But once i got her going there will be times she idles perfectly but after i hit the go pedal she will oscillate and die. I'm knew the Porsche world so i will be looking at all advise closely and would appreciate if you all can expand on your descriptions of what to look at.
Josh

[ephotrod]

Does anyone have a diagram of the air flap. I checked my air filter box and its sealed well.
Josh

[specialk22]

Welcome ephothod and thanks to all for your assistance in trying to resolve this problem. whatever it is it is minor because the car is running great as long as you keep your foot on the gas which is what I prefer to do anyway. As for the distributor, cant be that as I have crankfire ignition. I started it this morning and it idled great... until it warmed up and then went back to the rough idle.
Didnt have time to check the air box. But will let all know how it worked out.
Take Care.

[ephotrod]

Special k I have the same problem with my car so instead of opening another thread i would like people to expand more on yours so i can learn and resolve my own issue.
Josh

[boyt911sc]

A simple air pressure test would tell you whether there is air leak/s in your system. Stop the quess-work and it will save you a lot of time and aggravation while performing CIS troubleshooting. A leaking air box is almost impossible to find the culprit without the use of pressurized gas.

These are some pictures of Dave Santana's engine during the start-up test. Had the fuel pressure test and air leak test done, hook up the battery and it started the very first time. I was not surprised because these are the routine procedures I do and the results are impressive.









Tony

[gtihop]

how did you pressurize the system? I've used a smoke machine and found some leaks but this way looks interesting.

[boyt911sc]

Originally Posted by gtihop

how did you pressurize the system? I've used a smoke machine and found some leaks but this way looks interesting.

gtihop,

If you have used a smoke machine, a low pressure supply air would work too. There are several points where you could inject the low pressure air like:

throttle body
brake booster vacuum line
pop-off valve
crank case breather
oil filler tube
exhaust tail pipe
air box
vacuum line from throttle body
and many more.

If you look closely in the last picture in the previous post, I have an air line (near exhaust) that is connected to the the brake vacuum line (not shown). This line is connected to the line with pressure gauge (see pic). All you need is a continuous supply/flow of pressurize air into the system and works great. This is more convenient than my home-made smoke machine.

There are a few things I like to check during CIS troubleshooting:
fuel pressures (control, system, and residual)
fuel flow (CSV & injectors, fp)
air/vacuum leak/s sources

Tony

[gtihop]

not to hijack this thread, but thanks for the details. I am currently trying to locate problems that I think may be related to air leaks.
fuel pressures, cp, and residual are all within the bentley spec. symptoms include poor cold running (very lean) . I had airbox leaks last year around the seam that I temporarily repaired, have to go and and start searching again. Maybe time for a new airbox if that is causing problems again.

[specialk22]

Whats up everybody! Looks like you guys have been busy troubleshooting.
Not sweating the "hijack of thread" stuff, All knowledge is good knowledge so have at it gentlemen.
I am still having the original problem with my SC that started this thread. I have inspected my airbox (with my untrained eye) and I changed my air cleaner, inspected the airbox flap etc. The problem is now intermittent. Most of the time it is there but sometimes it just goes away. Car runs great when your foot is in it (which is how it spends most of its time when I am behind the wheel) but at idle it is terrible. All suggestions welcomed. Thanks

[specialk22]

Just took a close look at my airbox flapper with the assistance of a mirror and it looks like it is burned semi-melted. The seal is good. could this be the culprit?