Help-Violent backfire- now engine runs rough

[sailchef]

My car is a 1980 911 SC 120,000 miles on original engine that’s never been rebuilt.
Last winter I replaced the crankcase breather gasket, oil pressure switch, thermostat O-ring, three oil cooler seals and installed upgraded oil fed tensioners. I cleaned up and tightened all the other oil lines and put the engine back in the car without to much trouble. The car ran fine before the engine removal, and I was only “in there” to fix a couple pesky and embarrassing oil leaks. After the engine was back in, it fired right up and ran fine, except for the rpm’s hunting between 900 -1200 during warm-up. This was a new symptom and I attributed it to a possible leaky intake gasket that may have occurred during the engine re-install. On acceleration it ran strong, during deceleration there was a little pinging in the motor. I drove the car conservatively and at one point I added a half a can of Seafoam to the gas tank.

A couple of days ago I got in my car and when I turned the ignition switch it cranked for half a second and then it backfired very loud. It was an attention grabbing loud bang. Initial inspection showed that it blew the pop off valve right out of its seat. The pop off went back in snug enough for me to try and restart the car. After a lot of cranking and coaxing the engine started but is now running like crap. It’s pinging and running very rough. It sounds as though the timing is thrown off.

Could Seafoam cause a leaky injector that caused the backfire?
Could enough air leak in thru loose injectors to make the engine run rough.
Shouldn’t the pop off protect the air box from cracking?
Where should I start trouble shooting?

Any help would be appreciated. Thanks.

[slodave]

Have you double checked to see if the airbox didn't accidentally crack, in spite of the pop off valve.

[Paulporsche]

There are a lot of things that can cause a bang and blown airbox. Usually it is a lean condition. This can be caused by a lot of things, incl a too lean mix, an out of spec WUR, a bad AAR, decel valve or AAV, and vacuum leaks. You should first verify that your timing is correct and your ignition parts are all operating correctly. Check for vac leaks incl around the injectors. Make sure your mixture is set correctly. Make sure your control and fuel pressures are correct. And , provided your popoff valve is working correctly, epoxy it back in place. As Slodave said, it is possible that the box was or has been cracked even though you had the popoff valve.

[sailchef]

I checked inside the box and around the outside as far as I could see with the engine still in the car. It'll be a couple of weeks before I can pull the engine and look around closer.

[sailchef]

After the backfire I had the car running long enough to check timing. It had been set at 5 before TDC but when checked after the backfire it looked as though it had changed to 2 before TDC, but that was taken on an engine that wasn't warmed up. Would that little difference make the engine run that rough?

[911st]

I think some spray carb cleaner around the air box when running to see if there is any air leaks at the box, at the intake runners, and or at the injectors. If there is a leak the motor it should respond to the cleaner.

If that dose not work, check your control pressures and I guess the ignition advance for good mesure.

[john walker's workshop]

pull the air filter cover and see if any of the little screws inside are 1/2 way out or missing. check the ones around the outside too. that would mean the box has split.

[al lkosmal]

check your popoff valve again. This is a very likely culprit. I've had mine come loose or all the way off, after a backfire, just like yours. Will cause rough running or, with a big enough leak,....no running. You may need to rebond it in. Also check that it's seal(rubber gasket/o-ring is still sealing.
regards,
al

[Rick V]

Was your pop-off valve glued in to begin with or just stuffed in the hole. I suspect that if it was glued in properly and you had a strong enough backfire to knock it out, the box it toast. I would also think that if the backfire was strong enough to remove the valve, even if the valve was functioning properly and just set in the hole that the box is toast. I guess I am saying I believe your box is...................................cracked.

[Otto]

You bent the sensor plate/arm out of adjustment. You have to relocate the sensor plate and set the CO.

[sailchef]

All the screws inside the box seem fine, none look as though they've been pulled out at all. I've looked as deep inside as possible and can't locate any cracks. I cleaned out a small piece of gasket material that was laying up inside the air box just below the sensor plate.

The pop off was glued in but the epoxy had gotten very dry and brittle, I am putting that guy back in tonight.

when looking at the sensor plate from inside the box it seems ok and goes up and down freely without any hangup. It even sounds the same as before. At this point I dont have the experteise to mess with it as well as some of the other suggestions so I will look at things I'm comfortable with first.

I noticed #4 injector seems more loose than the others. I've never pulled them before but I understand its easier than removing the spark plugs. I have the plastic fuel lines so I will be soaking the lock nut over night. fortunately I allready have the o-rings. I tend to buy parts ahead of time for projects that I want to explore and this is one of them. it looks like it might be a good time to look at them all and replace any other parts like the holders or the injectors themselves. I tend to work very slow, one reason being I will read twice, think about it three times and then try to get it right the first time, and the other reason being that work takes up a huge amount of my time.

I will report back and want to thank everyone for their help so far

[RussR]

Originally Posted by Otto

You bent the sensor plate/arm out of adjustment. You have to relocate the sensor plate and set the CO.

i'll bet this is what it is.

i've seen it happen on a few mercedes-benz's using the same injection system - really mangles the flap!

[sailchef]

Is the hole circled in red supposed to have a srew in it or is it part of the vacum system. I looked at it closer with a mirror and it doesnt seem to have a bottom or any threads on the inside

[Howard M]

Check the "front" (hidden) side of the pop-off valve glue joint with a dental mirror, to verify that there is a sufficient fillet of cement. Smear a thin coat of silicone on the rubber seat to help it seal and to prevent sticking. I blew mine valve out of the mounting hole several years ago. No problem since, but now I keep a large rubber plug in the glove compartment....just in case.

[pookie]

Is the pop off valve too high? If I remember correctly mine was set lower in the airbox than that.

Did you check dist cap and rotor? I had a backfire that cracked my dist rotor and it ran like el crapo.

[sailchef]

Its fixed!

After the backfire I had initially just pushed the pop off back into the hole without inspecting it. It felt tight but now I know it wasn't tight enough. Last night I cleaned the base of the valve real good and put a coating of epoxy around the neck and installed it. I let it sit overnight to cure. After work I came home went into the garage and turned the key. She fired right up and sounds as good as ever. no pinging, popping or rough running.

Its amazing how such a small leak could cause and engine to run so poorly.

Thanks for all the help.

[Paulporsche]

That hole you mentioned is just a drain.

You should now try to eliminate the cause of the popoff valve popping in the first place.

[sailchef]

Originally Posted by Paulporsche

That hole you mentioned is just a drain.

You should now try to eliminate the cause of the popoff valve popping in the first place.

You are right Paulporsche, Now that the motor is running as good as it was before, I've decided it's time to learn all about the CIS.

I've avoided it up to this point staying busy working on the mechanicals of the car. I've been reading about the CIS here in the forum for a while and I think I have a good idea of how it all works. I'm not ready to replace the heating element in the WUR but I know what everyone is talking about when they mention it. One step at a time, right?

Over the last four years of ownership iv'e put in a new CAT, stainless exhaust, heat exchangers, oil-fed tensioners, half shafts, rebuilt transmission, calipers, rotors, stainless brake lines, thermostat o-ring, breather gasket, oil pressure switch, oil cooler seals, shifter bushings and a bunch of stuff I've probably forgotten about. I still have a box full of parts " for a rainy day" that I've ordered but haven't gotten around to installing yet. The car was a mess when I bought it. Had no clue what a PPI was. There aren't too many Porsche for sale within a hundred miles of where I live. It was the first one I ever looked at. Saw it, wanted it, bought it. Paid way to much for it.

For now I'm going to take a break on replacing the easy stuff and pour myself into learning how to tune the CIS. I've ordered the CIS gauges, 3mm allen wrench and a couple sets of valve cover gaskets from PP. I've got all the books and with the info found in these forums I plan to learn all I can about the "mysterious but loveable" CIS.

Its probably time I post a pic as well. She's a daily driver, and thanks to the farm surrounding my property is always dusty.

[Paulporsche]

Congrats on some great work!

Check out Jim Williams' website on CIS. Souk has also posted here re CIS. You will find lots of info. Wayne's, Probst's and the Bentley Book are also helpful. You can also find via a Google search the Bosch CIS manual.

If the FI starts giving you trouble again, post a thread. Also if you've never had your mixture set you may want to do so to set a baseline.

[berettafan]

Sailchef howdy from a fellow delmarva resident!

Sounds like you might be a chef? Where at?