Startup SNAP!

Superman · 12-18-2001, 02:26 PM

Please guess at what might be causing this noise. It happens only at startup, just as the engine first makes fire. It happens on about 10-20% of startups. It's a fairly loud SNAP!. Not unlike a very loud high-voltage snap, but could be gas escaping my broken heads stud area.

But, it never happens while the car is running. Only at startup. The starter nose gear and the ring gear looked good when the engine was out last.

rstoll · 12-18-2001, 03:13 PM

My '83 does the same thing about the same frequency. Have yet to identify the noise. Kind of sounds like a vent snapping shut on startup. Have not heard it in a while. But it always gets my attention when it happens.

Superman · 12-18-2001, 03:27 PM

Somebody out there knows what this is.

rcwaldo · 12-18-2001, 05:41 PM

when I had my 78SC about 6 years ago, it would occasionally do just that, kind of a loud snap. Like someone struck a cylinder cooling fin with a small ball peen hammer type of sound.

I always wondered if it was one of the heads "clapping" as a result of a broken head stud?

Never figured that one out....

Chris

Doug Zielke · 12-18-2001, 05:58 PM

Jim... bet the noise goes away after you rebuild the motor.

BlueSkyJaunte · 12-18-2001, 05:59 PM

Mine's more of a metallic clang, happens about 20 seconds after it fires up. I'm guessing it's the cat heating up and expanding, because it's from the left side.

I've been known to be wrong, however :/

Eric Coffey · 12-18-2001, 06:45 PM

Hmmmm, all the people reporting the same problem seam to own SC's. Could this be yet another weak link in the SCWDP armor?

Could it be CIS-specific problem? With the others reporting the same symptoms, someone here should have a clue (John Walker?). It wouldn't be something as simple as backfiring through the airbox would it?

Superman · 12-18-2001, 08:06 PM

No, I've heard the airbox pops and BANGs and this is not that. It is either a high voltage CDI event, or the spitting of compressed air past a head (before or after the exhaust valve). At least those are my two main suspicions. I also wonder if JW knows this one.

Keep reading, all you hovering Pelicanheads. Someone knows this.

Jorgeman · Jorgeman's Garage

You've described it perfectly as what sounds like a high voltage arc or discharge--one time--only on about every tenth start-up! My 86 Targa has been doing it since I got it eight years ago. I get the distinct impression it's coming from left (fuel filter area) rear. Have always figured it was CDI surge of some kind. No ill effects that I know of.

George 86T

beepbeep · 12-19-2001, 02:11 AM

I have it too....and i have a turbo...

CIS? Oh god do i hate CIS ..... :-E

rstoll · 12-19-2001, 03:37 AM

Quote:

Originally posted by BlueSkyJaunte
Mine's more of a metallic clang, happens about 20 seconds after it fires up. I'm guessing it's the cat heating up and expanding, because it's from the left side.

I've been known to be wrong, however :/

You may be on to something Blue, I haven't noticed the noise after the cat was gutted. Do any of you guys experiencing the noise have a test pipe?

Superman · 12-19-2001, 09:00 AM

My CAT is gutless and the SNAP happens just the same. It does not ever happen after startup. It happens at the instant the engine starts to fire. About one second or less, after engaging the starter. It's not a metallic clang. It is a loud SNAP!

Glasgow 911SC · Glasgow 911SC's Garage

Superman,
I get that same snap at startup (not every time though). It's one of those things where you can't really describe properly to your mechanic and never happens when he's there.

Superman · 12-19-2001, 03:21 PM

It's good to know that others have the same symptom. This means that someone knows the secret. I'm gonna bump this sucker until someone confesses.

Doug Zielke · 12-19-2001, 04:57 PM

This is the textbook definition of SNAP!...

WARNING: Do not try this at your home!

Jorgeman · Jorgeman's Garage

Doug---Yep!!! That's what I heard!!

George 86T

RoninLB · 12-20-2001, 08:38 PM

Jorgeman....great post. Now I'm getting some info. This is the best thriller all week and this place runs out of brains. Never happened to me but I suspect it is a "whole picture problem", not a specific component weakness. Maybe it's an old age CIS syndrome that doesen't have a name yet. I'm looking for an educated possibility. It would be interesting if the answer leads to weak/bad design ground some where.

beepbeep · 12-21-2001, 12:21 AM

I'm almost sure my "snap" is mechanical...it seems like something around/in the transmission "aligns" the moment the engine starts rotating...

speeder · 12-21-2001, 09:47 AM

My '82 does this also, it even did it in front of a good Porsche wrench once, I said, "what was that?" He said it was nothing, just the high-energy ignition firing-on or something, tell you the truth I can't really remember because I forgot about it when he said it was normal. I've definitely never heard it in any other car, seemed like he had heard it 1000 times.

I would describe sound as being like a relay the size of car battery.

ebradway · ebradway's Garage

My '86 also makes a 'snap' when it first fires. But I have a different theory. To me it sounds very mechanical and seemed to first start when my first fan got 'grooved' by the outer-pulley half. I have always figured it was the outer-pulley half 'snapping' back into place.

Dismal thought but easy to check. Pull your fan pulley off and check for 'the groove'. My new fan and rebuilt alternator are soon to be toast because of a sleezy mechanic.