Is this engine sound normal?

[strother]

I've probably asked enough basic questions to brand me for life, but I've got another one. Before I start, I'll tell you that I've spent the past 1.5 hours reading archived posts that turn up when I search "load" and "noise"...

My engine makes a rattling noise briefly when I accelerate hard under load -- usually from about 2000-3000 rpms or so in 2nd gear when driving around the neighborhood. It isn't horribly loud, but it is an additional sound on top of the engine noise. The noise only lasts a second or so until the rpms pick up in the gear I've selected. This is going to make it sound worse than it really sounds, but it reminds me of the sound a bus makes on accelration.

Is the sound normal? After reading the archived posts, I'm paranoid about broken head studs, and worried about timing and mixture. You should know that I just had the engine tuned up, including valve adjusment. It was doing this before and after the tune up.

[midlife]

Pinging? These engines do not like heavy throttle at low RPM's.

[Jeff Alton]

Could be pinging, does it sound like you are shaking a tin can with small pieces of gravel in it? A broken head stud can sound more like a small hole in the exhuast. Does it go away after a certain RPM? Mixture might be a little lean. JW has some specs for setting it and timing that folks have raved about, but be darned if I can remember them. A quick search should find reference to the specs.

Jeff

[strother]

I'm going to listen to it more closely this morning, maybe even try digitally recording the sound to post here. I'm not sure that it can be described as pinging -- that makes me think of a slightly higher frequency sound. This sound is a little deeper, kind of garbled. It could very well be pinging, but I don't know....

The sound does go away after a certain RPM. I'd guess that once I get to 3000 or 3500 it goes away. And it only does it under heavy load, not every time I'm in that RPM range.

I can't believe that it would be a broken head stud, given all of the maintenance done on the engine -- they did a compression test, everything was OK. Adjusted the valves and whatnot... I'd like to think they'd have checked for broken head studs.

[cmonref]

Does it sound like dragging a chain over the corner of the workbench? That would be the cam timing chains flopping loose because of a bad tensioner. Serious damage and big dollars to fix if not attended to NOW.

[strother]

Originally posted by Brian K. Haggard
Does it sound like dragging a chain over the corner of the workbench? That would be the cam timing chains flopping loose because of a bad tensioner. Serious damage and big dollars to fix if not attended to NOW.

Yikes! Thankfully it does not sound like that. Out of curiousity, would that noise only happen under heavy load and last briefly? It seems like it would be a more persistent noise.

[strother]

And here's who we're talking about:

[cmonref]

Strother:



Probably not, but I have wondered what noise a "going bad" tensioner would produce. The key is "dragging chain" as an unusual noise that bears instant attention.

[IROC]

I think, on rare occasions, I have heard this same sound. It has concerned me also as I am running on the ragged edge of detonation with 10.3:1 pistons (in a 3.2) and 93 octane fuel.

I have probably heard it twice (once on the track at Daytona during the Rennsport lapping for the posers - how cool). I'm very familiar with the sound of pinging, though, and this doesn't sound like that, but it is a disconcerting sound.

Mike

[69911e]

If it only does it in 2nd gear it is not pinging. Cars tend to ping more in higher gears due to the higher load. I would look at the CV joints or transmission, or it may just be a non-drivetrain related rattle.

[Superman]

I suspect this is a normal transmission noise. Trannies make a gravelly sound under load at low engine speeds. If it does not sound scary....that is, if it is not the dreaded chain tensioner noise or something, then just relax.

[jmohn]

I think it is "pinging", I'm sure if I let my SC get down to 2K RPM in 2nd and then accelerate hard it will ping. Try a tankful of premium gas and see if you still get the sound when accelerating from 2K RPM in 2nd. If you don't hear it, it was ping (premium fuel doesn't pre-detonate as easily as regular). Of course, if you already use premium forget the above test. One final opinion, 2000 is pretty low rev's for 2nd gear (or any other), I generally try to stay above 2500, if I need to go slower I downshift.

Jerry M
'78 SC

[strother]

Originally posted by jmohn
... One final opinion, 2000 is pretty low rev's for 2nd gear (or any other), I generally try to stay above 2500, if I need to go slower I downshift. ...

I do avoid the problem that way, but I fear that downshifting into first while going 20 mph will bring on another 915 rebuild more quickly than I'd like.

Still haven't gone out to drive it and listen more closely... I'll leave for work in a minute and listen.

[jluetjen]

What sort of fuel are you running? If you're running a "regular" grade, try adding a full tank of the highest octane that you can find. If the rattling sound goes away, it was pinging and you need to tune-up the car and confirm that the ignition timing isn't advanced too far. Your car was designed to run on regular unleaded and if it's still pinging on Super you've got a serious problem. Running a bottle of techron through the fuel system will also help clean out the injectors some so if the pinging is due to a lean condition from clogged injectors, it might help.

If the problem doesn't go away with the high-octane fuel, I'd suspect one or both of the chain tensioners.

[Tim Hancock]

My 79 SC was making an alarming rattling under acceleration and I think I found the problem. When I applied pressure to the shifter I could make the sound go away. This made me think the tranny had issues, but I just removed the shifter and coupler as I am installing a new interior and I think a slightly worn coupler/bushing set-up is where the noise is coming from. I wont know for sure for another month or so but I when jiggling it around, it definitely looks to be the culprit.

[Superman]

Sure, if it is pinging then you need to solve the problem, by using higher octane gas (I only use premium in the engines I care about), or by retarding timing just a skosh. Is your timing advanced beyond factory spec? If not, then pinging is not likely the problem. Or at least, premium should clear it up. Unless there are atrochious carbon deposits on your piston tops and valve heads.

No, I still think the more likely thing is tranny noises. Let me ask this: The noise, is it gravelly and grinding but not snappy? Pinging usually has a sharp edge. A crackling quality. Snapping. Whereas tranny grinding is just rattly. Another question: Has your shift coupler been re-bushed. Factory shift coupler bushings have an oblong bore that helps prevent tranny noises from entering the cabin through the shift rod. Replacement bushings have a round bore that insulates poorly. My car has the round replacement bushings, and I get plenty of crunch, grinding, rattly tranny noises. Particularly at low rpms under load. The shift coupler is located under the plate on the tunnel between the feet of your rear seat passengers.

[steven polaski]

Might be the cat when that stuff inside breaks free it can make some rattleing noise under the right load

[strother]

I just drove it, and man, it is a subtle and faint noise. I CANNOT hear it with the windows down. Don't know if that means it isn't engine-related and more likely tranny-related. It actually occurs at an RPM range lower than I originally said... Probably below 2000 in any gear, and really briefly, with it lasting a little longer when I really get on the accelerator. It sounds kind of gravelly, but I wouldn't say it sounds like pinging.

[450knotOffice]

Sounds like the tranny to me. Mine makes the same sound you describe at low revs and high load. In fact, just about every manual tranny I've driven with sounds like that at low revs and high load. My 1998 F-150 pickup with a manual tranny sounded like that all the time.

If you're really concerned, have a local Porsche mechanic drive it and tell you what he thinks.

That's what I'd do.

-Scott

[flyingplatypus]

It sounds like the transmission is just whining at you. The 915 is not very happy when you try to push it at low rpms. Dont load the 915 unless you're at least past 2500. You shouldn't be under 2000 anyways.