SC Backfires, runs rough, then won't start

[PetrolBlueSC]

I have a 78 911 that backfired when I started the car to come home from work yesterday. Immediately, the rpms dropped to around 1000 and the engine ran rough. I could rev the engine and everything would smooth out. Prior to the backfire, including the last three months that I have owned the car, the rpms at idle were 1800 +- 300. I was able to drive home (30 minutes). After leaving the car off for about 30 minutes, I tried to restart, but could not. The engine turns over easily, but it will not fire. Gas is about 3/4 full.

I checked the pop-off valve and it appeared to have moved up a bit (the adhesive was no longer holding). I pushed it back into place and then reseated the air filter. How is the pop-off valve supposed to work when the flapper that pops up is being pushed down by the air filter? The pleats in my air filter are pushed in where they sit over the pop-off valve. Also, I poked around looking for any obviously loose hose or wire but did not see any.

I purchased the car in May and have loved driving it this summer. It's my first Porsche and I can hardly believe I have the car I always loved sitting in my driveway. I love driving it despite the clunky shifting and droning exhaust.

The PO installed B&B headers and muffler around 2006. Not sure if there is an O2 sensor.

So far my searching here are Pelican suggests that I need to start investigating the fuel and ignition systems. But I was curious if there was any particular issue to check since it started with a backfire.

Thanks

[PetrolBlueSC]

Here are a couple of photos of the car.

[Jdub]

Our '78s don't use an air sensor. The popoff valve is not pressed shut by the air filter - it is oriented to allow the cap to open just enough to release pressure and prevent the airbox unseating after an explosion.

The first thing is to confirm you do not have a leak through the airbox. A bad seal at the base of the pop off must be investigated. But the real issue is have you blown the airbox? Try to get a fingernail under any horizontal surface screw - should be no room, no play. Perform a search on airbox and Zimmerman for a start.

Once you know the airbox is fully sealed, turn your attention to why the valve went off. If you do not know that the rotor, cap, and wires are in good shape, get these replaced. A fresh air and fuel filter as well, again if you do not know firsthand they are near-new. You are looking to baseline the car with parts that you'd replace in time anyways. With these parts new, turn to the CO2 setting, valves (tight = bad idle, etc.) with fresh 'plugs. A fully-charged battery is an easy fix and prevents drain on the system (esp. alt. diodes), so that as well. Again, you'd do this anyways and you learn so much. This process removes uncertainty.

Only now should you move to playing with more expensive analysis and parts-swapping. Good luck - it is not rocket science and you will learn a bunch.

EDIT: The dent in the air filter gives you an idea of how fast/hard that top comes up when you backfire.

[pete3799]

1800 RPM at idle is way to high unless your talking about cold start idle.

[1982911SCTarga]

Prior to the backfire, including the last three months that I have owned the car, the rpms at idle were 1800 +- 300.

An idle that high for so long suggests that the intake system is allowing too much false or unmetered air into the fuel injection system. You need to do some exploration to find out what's going on.

By the way, you've got a pair of valuable, unobtainium headlight washer nozzles on your bumper.

Brian

[vash]

rotate the pop off so the air cleaner doenst impede movement..hinge towards the front..i think.

big backfires will knock you air fuel mixture off. it did on mine. mix off will cause backfires too.

[PetrolBlueSC]

Thanks for all the great suggestions. I have the 101 Projects book, so I'll go find the CIS adjustment instructions and also start checking around for air leaks. I knew that the idle was high, but I was having too much fun driving it to stop and make repairs. I'll also take a look at the Pop-off valve position. I a minimum, I have to reglue it inplace.
Thanks

[JJ 911SC]

Spray some Quick-start around air connection, if the RPM goes up, you got air leak.

[OsoMoore]

I don't have much technical to add, but wow, the car is beautiful. I am happily married to my 79 SC... but WOW!

[Sirius Driver]

make sure the pop off valve hinge is facing rear(you) most people assume that since it looks like a little toilet it should face as if you were gonna piss in it. Secondly make sure you have a rubber seal that fits properly into the valve. No gasket seal and youll have leak. If the gasket doesnt fit or is not seated properly youll have a leak. Secondly, check the or replace the cap and rotor. If it develops moisture if will have similar effects. A cracked cap will do the same. Finally, wires could be in need of replacement or theyve become loose at the plugs. Check your ignition stuff in other words.

[LakeCleElum]

If it starts fine cold, but not hot, could be the accumulator...I have an '81 engine in my 73.5 and that was my issue....It's all good now...

[tcar]

Originally Posted by PetrolBlueSC

Thanks for all the great suggestions. I have the 101 Projects book, so I'll go find the CIS adjustment instructions and also start checking around for air leaks. I knew that the idle was high, but I was having too much fun driving it to stop and make repairs. I'll also take a look at the Pop-off valve position. I a minimum, I have to reglue it inplace.
Thanks

Get the Bentley SC manual too.

[SCadaddle]

Originally Posted by PetrolBlueSC

....I checked the pop-off valve and it appeared to have moved up a bit (the adhesive was no longer holding). I pushed it back into place and then reseated the air filter. How is the pop-off valve supposed to work when the flapper that pops up is being pushed down by the air filter? The pleats in my air filter are pushed in where they sit over the pop-off valve.....

Realize I'm a newbie to the 911SC, but I'm almost certain the other day when I replaced my stock air filter using one of Mahle filters, I removed the old one, put the new filter in the top of the airbox----the part that gets removed---with the pleats in the filter facing up i.e. inside the top of the airbox. Then I put that assembly down and onto the rest of the airbox and fastened with the straps.

Such a simple procedure, however, I searched this forum for the correct way to install an air filter and also checked the "101 projects" for the details to no avail.

[MotoSook]

Loose pop-off valve...first issue. Do not try to perform any other troubleshooting of the system until this is resolved. Visually inspect everything, yes, but do not make any operating adjustment.

Get a pot of coffee on Saturday morning and search here for all the CIS threads to familiarize yourself with how a CIS works.

[E Sully]

This is how the pop off valve should be mounted. Remove the valve and lightly sand the surfaces and epoxy back in place. Even with the pop off valve, a backfire could cause a crack in the air box or leak at one of the intake runner gaskets or rubber boots. Try as suggested earlier and spray a little starting fluid around the intake to see if the rpm changes. The metering plate might also need to be checked to make sure it is set correctly.
The 1800 rpm idle seems to indicate the cis already had an air leak or is was not properly adjusted. If you want to maintain the car yourself, a set of cis fuel gauges is needed. Porsche 911 & Turbo CIS Fuel Injection - Page 18
.CIS Primer for the Porsche 911

[tuchan]

Originally Posted by PetrolBlueSC

the pop-off valve and it appeared to have moved up a bit (the adhesive was no longer holding).

This needs to be fixed and re-seated before you do anything else.

I glued the o-ring in to the bottom of the housing so when it does "pop off" (excuse me) the o-ring does not go missing on me, plus the unit will close properly on its own.


Have you got a photo of the pop off valve you can show us as it is now?

[PetrolBlueSC]

After reading all the comments and measuring them against my limited abilities, I settled on resetting the pop-off valve (no brainer) and also a fuel filter replacement (not sure when it has last been done and it was cheap). I removed the pop-off valve, scrapped the old epoxy off and lightly sanded. I did the same around the hole in the air box. I mixed up a generous batch of epoxy and applied it to the pop-off valve. After pushing the pop-off valve into place, I used my epoxy mixing tool to push epoxy into all the nooks to make sure there were no leaks around the base of the pop-off valve. I also made sure that that hing was facing the rear of the car. While the epoxy was curing, I replaced the fuel filter. After puting everything back in place and cleaning up I attempted to restart the car. He (Wolfgang) started after turning over for only a couple of seconds.

What is interesting, is that right after startup (cold) the rpms at idle were still high (1800), but once it warmed up, it dropped to 950 +- 50 rpm. The previous owner had mentioned that he could never get the idle less than 1600 to 1800 rpms. I am begining to believe that there was a leak around the pop-off valve since it had been installed. I did not see much excess epoxy when I pulled the pop-off valve off, which makes me suspect that it was not fully sealed.

Thanks for everyone's suggestions. Getting you input and comments helped me solidfy my approach and prevented me from chasing my tail.

[Bob Kontak]

How sweet it is. Good job.

Lived in Anchorage five years down near Earthquake Park off Northern Lights.

[vash]

Nice!!!