348 Cat sounds like 'Poof' 'poof' 'pop'

Last month I replaced an O2 sensor on the 5-8 side of my '94 348 Spider. I'd been getting a 1211 code.

Well, to be fair, I'd been getting the code on the 1-4 side so I had swapped the O2 sensors from side to side. Then the 1211 code moved to the 5-8 side in synch with the O2 swap, so I replaced that O2 sensor on the 5-8 side.

Did the full A/F ECU re-init when the car was cold.

And all was well.

But...

If I turn off the stereo, turn off the A/C, and listen intently with the car running at idle, I can barely hear a "poof" "poof" and then slightly later a single "poof" audible from the 5-8 cat.

These noises are timed about 3 tenths of a second apart for the first two, then perhaps half to three quarters of a second later the third, then they may not reappear for 5 to 10 to 20 more seconds (but same general pattern roughly when they do reappear).

So I'm thinking that I've got a miss. Perhaps this set off the O2 in the first place (if the noises are on the 1-4 side, then they must not even be making the db's of the faint 5-8 side).

I checked the fuel pressure regulators for good vaccum connections, then I checked the MAFs (383 ohms and 384 ohms respectively). I pulled the spark plugs on the 1-4 side to check for anything blithering obvious, and they all appeared pretty much alike and didn't look to my inexperienced eye to be fouled or have the gap off. Then I cleaned them with starter fluid, let them air dry for a bit, and re-installed them.

So correct me if I'm off base but I'm thinking that I need to pull and check the 5-8 spark plugs next.

After that, do a resistance check on the spark plug wires.

Anything else to check (and how) would be appreciated. Coils? Throttle position sensors? Coolant temp sensors?

Anything particular on the spark plugs that might yield a clue (color, carbon buildup, something that I don't know about)?

Thanks for your time and potential input.

 

If it matters, the idle is rock steady at 1000 RPMs. Also, I hear the cat sounds both when the car is stone cold as well as when fully warmed up (oil and water).

Revs smoothly and freely. But the pops are detectable at idle.

 

Thanks. I could *easily* see a wiring harness issue causing a crank or other sensor signal to fail or to be scrambled periodically. I may have to go that route, too.

For the moment, however, I'm going to concentrate on checking the basics such as spark plug wire integrity, coils firing properly, etc.

The sound from the cat/cats is so low that you have to strain to hear it, so that hints to my inexperience that I've got a weak spark rather than no spark at all at times.

But, it could be no spark at all at times, and it could be from a wiring harness fault as your guy found out on your 348. I'll add that to my list of things to check.

Thanks again.

 

Spark plug wires could easily be part of the cause here. I pulled the 5-8 plugs and wires this afternoon.

#8 wire measured 6.27kohms - spark plug had a black tip on top/external
#7 = 6.07kohms - white/gray tip *this resistance measurement surprises me
#6 = 6.24kohms - light black tip
#5 = 6.26kohms - light gray tip

Cleaned the plugs, applied dielectric grease, reinstalled plugs and wires. Started her up cold and the noises were fewer than before, but still there (and still pretty faint).

The noises are so faint that I could almost kid myself that they aren't there, except that you can sort of feel them and the idle has now started varying just a bit from 1000rpms to 1050 rpms and back, slowly.

 

Took her out for a drive tonight. She starts right up and runs seemingly fine during normal driving, but there is a noticeable hesitation when I punch the throttle (say, driving at a steady 3000 rpm and then deciding to floor it).

The hesitation is more severe with the A/C off. Less dramatic with the A/C on.

In Neutral, punching it from idle revs it freely. In gear at any low rpm, punching it has an initial lag/hesitation.

Idle is steady again, when cold and when hot, right at 1000 rpms.

The exhaust noise almost sounds like a faint burble.

None of the spark plugs had any spotting on the porcelan, so I don't believe that I'm getting any pre-detonation in the cylinders.

I'm leaning toward the sound being strictly inside the cats.

 

Bruce, I agree with what you say, but help me think this through, as I'm a bit confused about why I'd see hesitation on acceleration from say, 3000 rpms, if the problem was isolated to the idle air passages.

I'm thinking that Tasty348 and Ernie are on to something about the forward two crank sensors/harness (or rear cam phase sensor in back), or, that I've got one or more bad spark plug wires.

My idle only really wavered once. Except for that one time, it's been rock steady, but the hesitation on acceleration is repeatable, as is the exhaust burbling noise.

I'm going to check the spark on all 8 cylinders with my spark indicator tool, and I've broken out my automotive stethoscope to confirm that the burbling is emitting from the left cat or both cats.

Then the next step, if the problem isn't found in the spark plug wires, is to get under the car like ernie says and peel back the wires to the crank sensors. Also, if the burbling is just coming from one cat, I'll swap the cam sensors from side to side.

Thanks for the tips, guys! Keep 'em coming.

 

Oh, I'm also going to check the air filter and airbox. Might as well hit all of the basics.

 

OK, that makes sense. Thanks. So can you walk me through removing the TB's again?

Here are pics of the 5-8 side TB. The yellow paint is on the top black TPS screws, but not on the idle air bypass screw.

Any special order that I need to start removing things to help avoid killing my tb seal?

Can you give me a step-by-step breakdown, please?
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OK, finished watching some football, got the kids in bed, and figured that I'd start on the tb cleaning per Bruce's great write up.

But first I wanted to check the airfilter (clean) and break out the stethoscope and spark indicator.

The stethoscope didn't help this time, though. The normal mechanical engine noises are just too loud through the steth to hear the exhaust burbling. Maybe there's a secret to hearing such noises that I don't know. Wouldn't surprise me.

For about 25 seconds I put the spark indicator on the wire for #1 cyl: great spark. Repeated for #2 cyl, lousy...only an occassional spark. Not good.

#3 cyl, great spark. #4 cyl, lousy. Hardly a spark at all.

#5, great. #6, great. #7, great. #8, lousy. It's like the #2 wire above.

#4 was by far the worst of the bunch.

So I'm still going to follow Bruce's great step-by-step tb-cleaning write-up when I get back on Monday, but now I'm wondering if I shouldn't be doing other things first.

I guess that I should swap the coils from side to side to see if I get 2 bad sparks on the 5-8 bank and 1 bad spark on the 1-4 bank (i.e. problems switch sides with the coil switch).

Am I correct in thinking that a crank or phase sensor problem would *not* cause the spark problem on just 2, 4, and 8?

An overall rich mixture wouldn't do that either, yes?

And just to confirm, lack of fuel from bad injectors wouldn't cause the above, right?

Should I be looking for *anything* besides bad plug wires and bad coils at this point?

Thanks for all the great comments, and for your time.

 

Oh, for what it's worth, here's my brilliant (pun intended) idea for new spark plug wires (just in time for cars to go to coil on plug so as to not need said idea):

Add a simple LED and circuit to the top of the spark plug wire cap. My spark indicator tool is tiny, was made years ago, and can be purchased retail for about $9.

People, these Ferrari spark plug wires cost more than $150 each OEM! You could have spark indicators added to plug wires for less than $3 each wholesale to a manufacturer. 2%. We pay more than that in sales tax.

We ought to be able to look into our engine bays and see leds flashing on every cylinder, telling us that we've got a great spark there 24/7.

 

Update

I swapped my spark plug wires from side to side. This moved the wire (among others) for cyl #1, which had a spark, to cyl #8, which didn't.

Fired up the 348 and hit each wire with my spark indicator tool.

Once again, cyls #2, 4, 6, and 8 had no spark.

My even numbered cylinders, two on each side of my engine, have no spark, as if I'm in a "limp home" or "economy cruise" setting.

Now correct me if I'm wrong, but doesn't the 348 have a limp home mode, but that this Ferrari mode runs all 4 cylinders on **ONE SIDE** of the engine, not 2 cylinders on each side, right?!

I haven't swapped the coils from side to side, yet.

Is there a common ground for the even numbered cylinders? Is there a limp home mode that I don't know about?

Or do I have two bad coils?

Any other tests that I should do prior to just ordering new coils?

 

Update

I swapped the coils from side to side. During the swap it was obvious that the back metal plates of both coils were rusted, so I cleaned them thoroughly.

Started up my 348 and checked the spark on all wires. Spark was now good on all except #8 and #4, which had almost no spark (intermittently would spark a single time then be quiet again for a minute or two).

 

Yes. It's gone from a subtle "poof" "poof" "poof" sound to more like:

MMMMMMMMMMMMMMMMMMMMMMMMkaMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMka

at idle.

So where before you'd hear a light series of 3 non-normal idle sounds, now you hear 1 in between a long set of normal idle sounds.

 

I don't know if each coil "back" has any function (ground or heat sink), but I figured that there was no harm in cleaning rust off and having shiny metallic back plates again.

I've got the power transistors to the coils off right now; not a spec of rust on them as if they are made of a non-rusting alloy (what a concept!), but...the metal electronic pins/connections are really, really dirty.

So I'll clean them up and swap them from side to side. The cleaning might help me, for one thing, and for another, swapping them from side to side might move the "no spark" condition from #4 and #8 over to #1 and #5 (if so, then I've got bad power transistors).