83 sc backfires won't run help please

[roadster49]

I finished my complete engine rebuild a month ago. Started up adjusted timing and she ran nicely. After the break in drives per Wayne's book she started to miss under load around 2000 rpm but ran great above and below that. I thought maybe old gas from sitting during rebuild so put in sea foam, a full tank of premium and she seemed to run better but still some missing. Parked it for one week and today when I tried to start there was a massive backfire that knocked the air cleaner off. Only a few backfires before.

So now it will turnover and fire/backfire and twice the engine revved high without my foot on the gas. Like a demon was back there.

Notes:
SSI installed but no O2 sensor PO did not have it connected. I was going to put one in but The SSIs bung was not standard so I plugged it. At idle she searches so I assumed it is a little rich.

All new wires, cap and rotor. Original coil, cleaned up the distributor advance mechanism.

One issue is the 12 pin connector has a broken end for the brown wire. I could not find a replacement so I pushed the pin in to the receiver. I rechecked this also.

I have the CIS gauges but have not used them as I am new to this system. Need to learn. And will do later today.

Your help is very much appreciated.

I will post some pictures

[mreid]

Sounds like you blew up the air box. Pull it and check it out. Massive backfires never have a good outcome on a cis car. Big enough and even a popoff valve won't help.

[74-911]

Originally Posted by mreid

Sounds like you blew up the air box. Pull it and check it out. Massive backfires never have a good outcome on a cis car. Big enough and even a popoff valve won't help.

My experience exactly... after a massive backfire, car would run but just as OP describes.

Air box was split apart at the seam even though a pop-off valve was installed.

[roadster49]

You are correct. Air box is blown. Found five screws still looking

So what caused it? Can the old box be repaired? By whom? Alternatives?

Advice please

[74-911]

Originally Posted by roadster49

You are correct. Air box is blown. Found five screws still looking

So what caused it? Can the old box be repaired? By whom? Alternatives?

Advice please

What caused it? Several possible issues but CIS just does it, sometimes for no apparent reason.

Three options:
1. Try to repair. Depends a lot on how it blew apart but some have had success with expoxy, etc. Search function should find some threads on how others have done it.
2. Put a "want to buy an airbox" in the Parts for Sale forum and take your chances somewhat.
3. Buy a new one. I took this option as I plan to keep the '74 for many more years. Pricey but I felt it was the best option in the long run.

[mreid]

Pelican sells a new one, but it's almost $600. You can take your chances on a used one or buy new, but if you don't know what caused the backfire you could blow up the new one.

[74-911]

This is one of the better threads re: blown air boxes and replacing:

Blown airbox - replace, prevent and while I'm in there

As to the cost of a new air box, I usually just order all my 911 parts straight from Pelican but on some of the more expensive parts I will shop around....

[Bob Kontak]

Originally Posted by roadster49

Can the old box be repaired?

One school of thought may be to remove the air box and JB Weld the snot out of it.

I say this not because it is the correct course of action but if coupled with study, an epoxied box serves as a real nice drone to makes further mistakes on while you work the bugs out, especially if you have more time than money.

CIS takes time to absorb. Some grunt work on the old one may be good insurance until you are a CIS whiz. Then spring for a brand new one.

[roadster49]

I think it good advice to attempt a reseal of the box and determine reason for backfire.

Is there a trouble shooting process for CIS issues? Backfires?

The engine ran ok except for a miss at 2 k rpms

From the threads sounds like I need to drop the engine again to get to the CIS.

Does it make sense to go with PMOs at this point? Or ?.?.

Do you think the not having an O2 sensor could have caused this?

Would it make sense to see a Porsche shop?

The car is a challenge but a great experience.

Thanks for all your help. This is a great community

Michael

[Reiver]

The only time I've had a back fire, after miles of great performance on a refreshed 930-10 ('83 euro CIS), was due to an overly lean issue.
I put a bung on my SSI and run an AFR gauge in place of the clock.
Generally, once set, it remains pretty close to initial setting but I have seen it lean out over time.
The gauge lets me catch that and I can adjust mixture anytime...now I know where the op sweet spot is you always know what is being fed.
I also change, tweak, the setting for hot summer/ winter. Beats the hell out of the clock info.

[Bob Kontak]

Originally Posted by roadster49

From the threads sounds like I need to drop the engine again to get to the CIS.

For the love of God, Man. Hell no.

Stuff the PMO idea deep in the closet until you understand the system and get it running right.

I don't know your skill set, but I have two Charles Probst Bosch fuel injection books. I can send you one.

Call me tomorrow. PM'ing you my phone number.

[roadster49]

PMO is now in closet.....moment of frustration after the rebuild effort!
Book would be greatly appreciated!

[boyt911sc]

Michael,

Since you have dropped the engine and did the engine rebuild before, getting that engine out would not be a big problem to you. Some people like to do the removal/installation of the CIS unit in situ. But I prefer to do the removal/installation of the CIS unit with the engine on my test stand. Then test run the engine before putting it back. This way, I don't have to worry if there is a leak somewhere. The pressure test for air/vacuum leak takes only several minutes. It is easy to find an air leak but very difficult to pin point a vacuum leak source.

Engine drop...........................4 hours.
Engine install.........................6 hours.
CIS unit removal....................2 hours.
CIS unit install.......................4 hours.
Total...................................16 hours.

To fix/repair/test air box..........8 hours.

So you have to decide which direction to take. It is a choice you have to make. But the bottom line is that the air box has to come out first.

Tony

[roadster49]

Thanks Tony,
I will drop the engine. I think the best way to make sure all is proper.

Do you think I should drain the gas tank and clean the screen?

Thanks again

[kodioneill]

Take the distributor cap off and check the internals just to be safe.

[Discseven]

Originally Posted by Bob Kontak

One school of thought may be to remove the air box and JB Weld the snot out of it.

Eloquently put Bob.

ROAD... how'bout some pics of air box?

[roadster49]

It finally warmed up to 23 degrees in Michigan so I started the task of replacing my air box.

I checked the CIS resistance and fuel pressures:
Temperature 23 degrees Fahrenheit
Resistance =26.5 ohms
Fuel Pressure harness unplugged = 34.5
Fuel Pressure harness Plugged = 60
Fuel pressure full = 98

I checked all the injectors and they all sprayed in a V pattern. They did weep a drop or two.
My spark plug wires are not shielded, but I have a new set that is so those will go on as well as the new air box.
I do not plan to put the air box relief valve in at this point.
All the plugs looked light brown

Compression:
1 = 160
2 =160
3 = 125/150/145 (3 attempts as it was out of line with the others)
4 = 165
5 = 165
6 = 165

I am a bit concerned about #3. I checked the valve adjustment, and it was fine. I torqued the case studs again and they moved a bit. I plan to do a leak-down next week.
I also plan to see if my cam valve lash is still in specifications.

Note the engine has about 300 miles after a rebuild and the re-ringed alusil cylinders

no blue smoke!

Discoveries:
when the engine backfired it blew the box apart at the seams. The metal runners look like a barbecue and there a lot of gas in the bottom of the air box. Would that happen when I was cranking the engine to check compression? Why did it not go into the intake runners?
I have a new cold start valve and will replace the old one

I checked the distributor for signs of arching, but it looks like new:

The previous owner had drilled out the adjustment screw that is supposed to be factory set. In addition the area was not sealed leading me to think it was unmetered air:

the O-ring was not making a seal on this pipe:


I will update the post next week.
Thanks
Roadster

[Reiver]

A guess.
On the 930-16 USA model the lamda controls the enrichment (on a 930-10 euro the wur does).
Yours, you stated, was unplugged so the box that controls the lamda is supposed to go to an auto default setting without input.
What was your AFR/CO levels as if too lean you can get a backfire on start...another item to check is your wur/pressure device.
The lambda control, wur, fuel head work together with the cold start device and throttle plate to deliver the proper fuel at the proper time...if any one is grossly out of spec you can have a bf.
All of these systems will kind of compensate for each other a bit depending upon how you have your CO/AFR set but only to a point.
Once your air box is airtight check your CO when you can run the car....has the wur ever been rebuilt? If you have a solid spray pattern your fuel head is prob ok.
You should see fuel in the intake area if you have been trying to unsuccessfully start the car...that's part of the cold start/diffuser function. Before the diffuser was put in place the cold start element just dribbled fuel into the airbox/runner area.
My 930-10 ran great even with a very out of spec wur....because I'd enriched the afr to compensate...if the afr got lean it would lightly backfire.
I had the wur rebuilt by Larry Fletcher at CISflowtech (guru and 1/2 with any bosch CIS element) and the car runs better but now modifies afr properly on demand...this is a euro but since your lambda is not functioning I suspect you may have a lean situation with a similar result.
Like I said...a guess.

[timmy2]

Need one of these for your lambda male pins?
I just started casting them. Couple of air bubbles in the first one, but the next ones will be better. Completely functional though...
Send me a PM if interested .

[Pazuzu]

Make sure you document and verify EVERY SINGLE SCREW in the blown airbox. My car decided to drop one of the screws that holds that metal spider in place into the combustion chamber, lead to an ugly piston.
Porsche put a lot of effort into making the airbox "safe" (all of the nuts and such are outside any possible path to the pistons) UNTIL they added that spider in 1980, when they put 3 small screws with no locking mechanism right in there.