83 sc backfires won't run help please
 

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
http://forums.pelicanparts.com/uploa...1413638862.jpghttp://forums.pelicanparts.com/uploa...1413638941.jpghttp://forums.pelicanparts.com/uploa...1413639020.jpghttp://forums.pelicanparts.com/uploa...1413639080.jpg

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.

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.

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.

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

http://forums.pelicanparts.com/porsche-911-technical-forum/689763-blown-airbox-replace-prevent-while-im-there.html

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....

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.

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

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.

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.

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

It is a choice you have to decide........
 

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

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

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

Eloquently put Bob. ;)

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

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
http://forums.pelicanparts.com/uploa...1427670330.jpg
I checked the distributor for signs of arching, but it looks like new:
http://forums.pelicanparts.com/uploa...1427670618.jpg
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:
http://forums.pelicanparts.com/uploa...1427670758.jpg
the O-ring was not making a seal on this pipe:
http://forums.pelicanparts.com/uploa...1427670853.jpg

I will update the post next week.
Thanks
Roadster

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.

83 sc backfires won't run help please
 

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 . :)

http://tapatalk.imageshack.com/v2/15...30408885d6.jpg

http://tapatalk.imageshack.com/v2/15...bb7f6fa6f1.jpg

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.

At this point in time, don't worry about the one hole with lower compression. Pretty common if they sit for a while to have a bit of carbon trapped between the valve and seat.

When you've got things sorted, just drive it. Most likely, it will clean itself out with a good long run. Not drastically low anyway, should it decide to stay right there.

Regarding the airbox, whether you repair this one, or replace it, make sure it has a pop-off valve.

angela

Dennis, this is fantastic! What's your price point?

$20 shipped conus seems reasonable to me.

Update: New airbox installed. New connector installed from Timmy 2. Connector was perfect replacement. Took some cranking to get started with a few exhaust backfires. Adjusted timing and idle to spec. Added a quart of oil when warm with engine running and the idle changed with the oil cap off but kept running As part of the repair I tightened all head studs, adjusted the valves, and reset the lash from the 1.4 to the higher end of the scale 1.7. I also put a new relay at the brain

When warm it runs beautiful

When cold bucks and hesitates terribly

I could not get an O2 reading from the exhaust as the level was very low. Note that the PO had drilled out the plug.

So I am happy it fired up but disappointed in cold performance. It takes about 4 miles before it runs good again

I will retest my pressures and report. Help please.

Michael

Maybe not a cis issue. Could it be the coil as it backfires a lot now?

Update: checked WUR resistance. It was 26.4 ohms , outside temp 53 degrees. The frequency valve vibrates with fuel pump on and engine off. Checking fuse 18 looked old so I replaced, and do have lamp working over glove box. Replaced the relay at the control unit. Engine runs horrible now both cold and warm. When I get back this weekend I will test pressures again and go back to the beginning. Question, how do you check the lambda control Unit. What am I missing

Suggestion:
After checking all fuel pressures are in spec...
Check the "brain" board for solder cracks with really good magnification. Hi res camera blown up works well.

Do these tests.......
 

Michael,

Get the engine to operating temperature and hook up a dwell meter using the test port. And the check the OXS voltage. Two things you have to check:
1). That the OXS is transmitting voltage reading to the ECU. Use a volt meter.
2). That the ECU is in good working condition. I don't know how to test and evaluate an ECU. I have several good ECU units that I use for the test to compare with the questionable ECU.

Until you have confirmed that these two critical parts are good and working, troubleshooting your problem would be quite difficult.

Could you do this test for me? Please measure the WUR resistance (Ohms) when the engine has reached or close to operating temperature. Or after 10 and 15 mins. from a cold start. You have to unplug the WUR electrical connector to be able to measure the resistance. A 10 min. and 15 min. interval would be my interest. Thanks.

Tony

Tony and Tim. Thanks will do Friday when I am back

Another day in analysis:

Checked WUR. 26.4 ohms. Outside temperature 49 degrees

Fuel pressure system - 6.6 bar
Fuel pressure controlled- 4.5 Bar
WUR connected - @ 2 minutes - 5.4 Bar
4 minutes - 5.5 Bar
5 minutes - 5.6 Bar
10 minutes- 5.6 Bar and WUR 9.4 ohms

Checked perma tune and all resistance levels were within spec
O2 sensor readings were 0 before start and was able to see exhaust readings fluctuate with engine running

Pulled #5 plug, it was light grey in color

Cleaned 14 pin connectors and perma tune connectors
Installed a spare coil
FV vibrates
It started, grumbled and backfired a flame out the exhaust.
As it warmed up still grumbled and rough with occasional backfire

Got it to run but shudders and misses bad on acceleration

Adjusted air and idle and was able to get it to run a bit better at low Rpms
Surges high low at idle
Runs strongest when warmed up and high Rpms
Overal poor drivability

So any next steps or recommendations welcome

You control pressure is absurdly high.

Hook up with Tony and send him your WUR. He will calibrate.

Your poor car is "out of gas" it's so lean.

Let me find the graph and post.

You system pressure is too high as well.

http://forums.pelicanparts.com/uploa...1429930091.jpg

Hi Bob,
Got a call into Tony
Hope that's it! It is driving me nuts

Missed your calls.....
 

Michael,

I was in the garage working all day testing and calibrating a dozen WUR's and was a very frustrating day. Got only three (3) WUR's to pass QC tests and stopped working pass midnight. I missed your phone calls and left a message.

Your fuel pressures are out of wack!!!!! How did you mess up the fuel pressures? Have you been tinkering the WUR and FD? Both your WUR and FD need to be calibrated if not rebuild. I am quite busy these days testing CIS components and finishing two (2) engine rebuilds side by side. But I will find some time to look at your WUR and FD.

Tony

This is for Euro/RoW....
 

Bob,

Michael's car is an '83 SC USA model and has a WUR-090. The control fuel pressure chart you provided is for the Euro or RoW cars with WUR-089 that is vacuum assisted unlike the WUR's used with the lambda type 911.

And I completely agree in your observation about the very lean condition. With those fuel pressure readings, I doubt if you could make the engine run well even after a very long warm up. Both the WUR and FD need to be calibrated for the engine to run smoothly and efficiently.

Tony

You are correct, as usual, Oh great WUR Guru. :) It was late and I was painting fenders all day. All I can think is it's a fume related mistake. (or beer?)

Another thing I was thinking is, can the high system pressure bump the control pressures up by default? Maybe something is incorrect upstream? Off spec fuel pump?

I just replaced my original fuel pump from my 81. Guess what? My front end damage messed with the gauge and I was only out of gas. My original fuel pump is old but functional and sitting in a box in Ohio still waiting to serve......

In the middle of the night..........
 

Michael,

I woke up in the middle of the night (actually very early morning) thinking about these data. These could not be right unless your pressure gauge is defective but I doubt it. Check your fuel line return. There is a flow restriction between the FD and the fuel tank. Keep us posted.

Tony

Tony,Bob,
I will recheck my procedure and numbers later today and post.
Michael

Tony,
I have my phone ringer on high so I don't miss your call

Tony,

I don't think this is the issue. If you had complete blockage, control pressure, at worst, would be equal to system pressure. That is not the case.

System pressure is too high.

All ears though. Just thinking, not arguing.

Here is the correct graph:

http://forums.pelicanparts.com/uploa...1429978648.jpg

Read my lips.............
 

Bob,

Did I say 'complete blockage'? I said flow restriction some where along the the return line (just kidding because I'm in a good mood right now and not grumpy). And I agree with you with all my heart that the system pressure is ridiculously too high. And you could be correct. The very high system fuel pressure could be caused by the actual primary valve setting or a flow restriction some where along the return line. The fact is we don't know. We need to test and confirm.

So I spoke to Michael about this problem and discussed the next step to diagnose the culprit/s. Let's wait for his test results and hopefully it would give us a better picture of the situation. My initial reaction was the gauge could be bad or defective and that's another thing we have to know.

An example of cold control pressure test reading @ 38°F (early this morning):
http://forums.pelicanparts.com/uploa...1429981939.jpg

And the system fuel pressure test reading:
http://forums.pelicanparts.com/uploa...1429982593.jpg

I have another gauge but using three (3) gauges in tandem is over-kill. One (1) good gauge is more than sufficient DIYers would need.

Tony