79 911 sc missing and popping through intake?

[ngc414]

I made a previous post about my car I just bought missing, popping and backfiring badly at idle. It seemed to run better at a higher rpm. I did a search and started ruling out the easy stuff. The first thing i did was pull the air filter and this is what I found. Looks like its popping back through the intake. Which would lead me to believe it could be a valve issue. Has any one seen this before? any suggestions?

[Alan L]

What is like like to start? Does it run better cold/hot? More clues.
Alan

[ngc414]

I just picked the car up yesterday, so I havnt drove it much. It starts right up every time, but the idle surges, misses and makes a loud popping noise. It seems to run better when you get it up in the rpm range. It runs the same if its hot or cold.

[Alan L]

Could be so many things, the list is long. But leaness seems a possibility.
There is a mixture adjust screw down a hole between the fuel head (all the fuel lines coming out) and the rubber air hat next to it. A long 3 mm allen key will get down there. Could try turning clockwise 1/4 turn (richen) and see if improves. If not , turn it back and scratch head more.
If it does improve , could be due to air leaks since seems better at revs (higher metered air flow). Need to find them - cracked air hoses, leaky injector seal etc etc.
Alan

[smestas]

Does the car have a pop-off valve? If not you might want to check to see if the airbox isn't cracked from all the backfiring. Just an idea.

[OldTee]

Either take it to an expert and leave the car and your credit card for a day or so OR... Buy a Bentley, a set of fuel tester gages, some tools and get started. You may have several problems but the first is to make sure the basics are OK. Fuel pressures first then start looking for air leaks. If I did not know the history I might go ahead and replace the accumulator and fuel filter for starters. (swapping parts to troubleshoot a CIS system can be very expensive. Ask me how I know.) Once that 79 settles down and starts running well you will be among the happiest of fellows.

[al lkosmal]

the hole burnt thru your filter looks like it could be roughly the same size and location as a popoff valve. Is it possible that your popoff valve is loose or off? If it's loose it will run badly, due to the vacuum leak. If it's off, it's unlikely that you could get it started, but...I'd check the popoff 1st, then cracked airbox, vacuum hoses, etc, for leaks.

regards,
al

[ngc414]

The car does have a pop off valve, I have not checked it yet. I will need to reasearch what a popoff valve does and what to look for, since I am not familiar with them. I did pull a plug before I came to work today. It was charred black and the gap looked too wide. didnt have time to check what the gap was. I picked up a set of plugs, fuel filter and some fuel stablizeron my way into the office . Thanks for the advise I will check the popoff valve, fuel pressure, plugs and look for air leaks tonight as well and see what exiting adventure it takes me on LOL.

[Porsche_monkey]

Sounds lean. You need fuel pressure gages now. Or borrow some.

[ngc414]

Originally Posted by Porsche_monkey

Sounds lean. You need fuel pressure gages now. Or borrow some.

where is a good place to tap in with a gauge?

[Porsche_monkey]

I think it's on top of the WUR.

Anticipating your next question, you should get a Bentley or similar manual. You will get all your questions answered here, simple or complicated, but a manual will be faster.

[ossiblue]

Originally Posted by ngc414

where is a good place to tap in with a gauge?

As stated above by Old Tree, start with fuel pressures. The gauge taps into the line from the fuel distributor to the warm up regulator (WUR). Do a search there are many threads and pictures regarding control pressures.
If you have lots of time, mechanical savvy, and written resources available, trouble shooting the CIS can be done at home.
If you don't feel up to it, I'd suggest you find a CIS Porsche expert that you trust to dial in your system. He will be familiar with what you describe and will be able to fix and dial in the CIS so you have a smooth running car and the knowledge that all those potential air leaks and CIS components are in good working order. Just don't be tempted to replace parts to try and fix it--paying an expert is less expensive in the long run. Just my thoughts.