popping when on the gas under load

[greensc]

I have a 1978 911SC and I've been having a problem for over two years that I have yet to figure out. It seems to be getting worse lately. I've been trying to watch this forum for probably answers, and so far only the "I absolutely flat give up" thread looked promissing.

Whenever I drive (car cold or warmed up, outside temp cold or hot) and I accellerate by doing anything other than feather the throttle, I get something like a popping sound (not really a backfire) from the engine. It happens more if the engine is under a load, so is of course worse accelerating up a hill, although I have been able to get it to happen by just reving the car out of gear lately. The car runs (I think) fine when I don't get into the accelerator. (Although, it's been this way for so long, I'm not really sure it's running all that great even when feathering the throttle) This is a daily driver and I have not used the car in events since the problem started.

I have done the following:
Replaced the vaccum hose connected to the distributor.
Replaced the fuel screen in the fuel tank.
Replaced the fuel filter and fuel accumulator.
Replaced air filter
Replaced spark plugs
New Cat.
Replaced the ignition wires (I went to a heavy duty cable with no braided shield)
New fuel pump (although I'm not sure it's exactly the right one)
Replaced injectors (although I didn't put new o-ring seals in)

Now... I can think of a couple things I could try, but I'm so limited by time (and money) that I've been trying to reason out the most likely solution first.

I think it could be:
1) The injectors can wobble slightly in their seats. O-rings gone bad? Could this allow excess air into the mixture, causing it to run lean? So new o-rings with (what?) type of lubricant?
2) When starting cold, the car does not seem to warm up correctly: it runs at about 2000rpm them drops suddenly to around idle within 20 seconds. Bad warmup regulator? Could this contribute to the big problem?
3) How important is it that brand new ignition cables don't get close to one another? I have lost the "spacers" and so my ignition cable management is not good.

I have not done the following yet:
1) Test timing advance
2) Test the coil.
3) Test Fuel Pump delivery and pressure
4) Test for vacuum leaks

Of course I'm looking for other probably causes.

Thanks for reading all the way to the end if this long post.

John Bahr

(78 SC)

[Emission]

I have no idea what your problem is... I just wanted to welcome you to the Board! :-)

(Hopefully, someone better skilled than I can help you out.)

[Jadams1]

How many miles on your engine. Have you verified you don't have a broken head stud? Lot's of info on this subject. Do a search for broken head stud symptoms.

[stealthn]

You didn't say you changed the Distributor cap & rotor and checked the timing?....

[Kevin Stewart]

sc's are pretyy bad about burning up caps and rotors, replace them and see what happens, cheap enough to try, Kevin

[john70t]

If the poppping is a too-lean mixture backfire you might be in for a new air-box (our host has the pop-off valve).
The o-rings (as well as distributor boot) are a sure source of vacuum leaks and definitly a cheap no-brainer. I've been told to lube with motor oil but the idea of petroleum/rubber mixing doesn't sound good.
A quick way to find vacuum leaks is with propane and listen for idle drop.

It could be backfiring out of the tailpipe-coil from an incomplete ignition. The coil should be tested under real world conditions -on a dyno. Checking the resistance doesn't show if it's going to put out good spark under load in a hot engine compartment. Same as the battery.

Definity do more checks before swapping parts, it'll get pricy real fast and the replacements may be of lesser quality than the original.

[JackT]

Check your Warm Up Regulator. I had what sounds like the same probelm and did all the things you did with no success. Machanic replaced the Warm Up Regulator and have had no problem since.

[stormcrow]

John,

I also have a 78 SC and had a similar problem while I was adjusting the fuel distributor sensor plate. What could be happening is that the sensor plate is more towards the high side on the fuel distributor.

As you may already know, there are slits in the fuel distributor, one for each injector. In the center of the fuel distributor is the plunger which controls the fuel to the slits.

When the engine is at idle, the plunger will allow enough fuel to go through the openings to keep the engine running The plunger is situated more towards the bottom of the fuel distributor at idle. As the rpm's increase, the sensor plate lifts which allows more fuel to enter into the injectors. This is when the plunger is moved towards the top of the fuel distributor.

What could be happening is that the plunger may be near the top end of the fuel distributor which would allow for a proper mixture at idel, but when you accelerate and rpm's go up, the plunger is raised more and fuel flow is then restricted.

Look at your sensor plate and make sure it is between the top/bottom of the vortex when at rest. If it isn't adjust is so it is (Haynes manual has the info on setting it). After you have adjusted the sensor plate to get it into the correct position, try to start the engine. If the engine starts but stumbles, then start adjusting your co2 screw by turning it a quarter turn clockwise. Each time you do this, try to start the engine.

If the engine starts and runs at high rpm, wait a few minutes to see if the auxillary air valve is open then wait for it to close. If the rpm remains high, then turn the co2 screw in the counter clockwise position quarter turn each time until the engine starts to stumble. This will put the plunger at the lowest position in the fuel distributor and you can use this as a starting point to adjust your idle and co2.

If the engine stumbles after you made an adjustment to the sensor plate, and co2 adjusting screw, then your fuel distributor may be where your problem is. It may be causing a lean mixture by not allowing enough fuel to the fuel distributor.


Let me know if this works, or send me an e-mail.

Steve

"A Porsche does more then just go fast in a straight line"

[David]

I've been fighting a similar problem with a '78. I replaced the WUR, checked the injectors, replaced the o-rings, polished the fuel distributor piston, checked fuel delivery and pressure. None of these were the problem. I found if I manually lifted the fuel distributor flapper, the car accelerated fine so I figured it was a vacuum leak. I replaced the fuel injection runner gaskets and boots and in the process dropped a nut in the intake, doh!. I didn't realize the nut fell in the intake so I start the engine. After pulling the engine and replacing the valve, I found two broken studs on one side and I haven't checked the other side yet. Now I'm wondering if the broken studs are keeping the engine from pulling vacuum as the engine is rev'd.

I'm curious to find out what your problem is. Hopefully you will find it before I've got the engine back together and running, because I'm going at a very slow rate.

[stlrj]

Sounds like a vacuum leak from a possible cracked air box or hose.

Try spraying some carb cleaner around your air box while at idle and see if the rpms come up.

Cheers,

Joe

[Vipergrün]

I'd check for detonation, exhaust leaks, broken head stud(s). I'd check the timing ASAP and pull the lower valve covers and verify that head studs are not broken.

Good luck!

[Doug Zielke]

Yes....check for broken head studs!
The noise caused by a broken head stud sounds similar to other problems.

[greensc]

Thanks for all your ideas!

Some updates to my last post:
The distributor cap and rotor have not been replaced for a long time. I will do that very soon. And, looking closer at my records I made a mistake: I have not replaced the fuel pump since the problem appeared.

Well, the more I dig, the more interesting it gets.

I started with the carb cleaner test on the injectors, and all vacuum hoses I could shoot. No increase in idle. I shot some directly in the air intake to see what I should listen for.

I then remembered that there was a large connection on the air filter side of the air box that was not connected to anything, and I had plugged it up with tape. (See the first picture) But there was also a very small hose that had been capped with a screw. (This was connected to the tiny metal hose connection just peeking around the top of the air box in the first photo)

So I removed all the heater hoses to see what else I could find... and low and behold, I found 1 large hose laying down in front of the engine, connected to nothing, which fit the large hose opening in the air box, and a smaller hose that would fit the smaller metal hose connection, but seeming to go somewhere else. (See second photo) Both of these hoses come from an oval shapped canister, mounted horizontally, on the passenger side of the engine compartment, just in front of the oil filler cap. (See the third photo) What is this thing??? The smaller hose from this "thing" seems to go down out the front of the engine compartment... somewhere.

So... assuming everyone but me knows what this weird box is...

What "upgrade" could have been done to cause the mechanic (of the previous owner) to do this?

Will I do any harm/good by connecting the large hose back up to the airbox?

Where does/should the small hose from this "thing" go to?

I am going to check some other things tomorrow.

(BTW: The car really is that color... Closer to the green in the second photo actually! Yes... Original paint.)

[gblumenstock]

Its the charcol canister. It is part of the emissions system that captures fumes from the gas tank. The long hose you refer to gets clamped to the hole in the airbox that you have taped up. Some people remove the canister. In California, it is required to pass the visual portion of the smog test.

GB

[David Bahr]

bump

[john walker's workshop]

pull both lower rocker covers and see if all the head studs are still tight, or if there's one or two that fall out as the cover is removed. just pulled these dilivars out of an SC today. they look fairly new, but the engine sat for 10 years. they appear to have broken due to age only, not use. they're just not dependable.

[Jadams1]

Glad I went with steel ones per your advice. Thanks!

[greensc]

Ok... I'm convinced I should take off the covers and take a peek. I'll do it this weekend while I've got the oil drained to replace cam oil lines.

The thing is, I took the car to "a Porsche Dealership" in Denver and they put on a new valve cover gasket kit, knowning I was hearing this noise. Nothing fell out and you would think they would check the studs.

Is it going to be obvious what to try to tighten?

I'm really not a full wrencher yet...

Regarding the Charcoal Canister: Reconnecting the large charcoal canister hose got rid of the slight gas smell in the car after sitting overnight. And I always thought 911s were supposed to smell like gas and leather...

If I were to remove the canister, I would assume the small hose going to it would then connect to the connector behind the airbox that now is clamped off?

[greensc]

An update: I checked all the studs... None broken! Yeah!

Now on to the ignition system, and then the fuel system.

[greensc]

BTW: Am I correct in saying that there are 24 head studs in all: 12 per side - 6 accessible from the bottom covers and 6 accessible from the top covers (with 3 of the 6 on top having an allen head).

John