Idle rising in park

[geneSHO98]

Had my mufflers deleted a couple days ago, and since then I haven’t noticed the occasional idle surge in Park that it used to have. Anyone have similar results?

 

[99sho-time]

deleting mufflers shouldn't do that imo. could try remove the negative terminal on the battery for a re learn.

 

[gamefanatic]

Well there are lots of reasons the idling might surge. Does it do it all the time or just sometimes? IAC could be opening too much or stuck throttle, vacuum leak.

 

[geneSHO98]

Let me rephrase this; the idle surge *stopped*, after the muffler delete. It has not occurred since, and was wondering if the same thing had been experienced by anyone else.

 

[SHOdded]

How much of a surge was it? Could it have been A/C compressor going on/off? But it sounds like the engine had a "breathing" problem, resolved by the muffler delete. Which in turn goes back to maintenance history.

 

[stephen newberg]

As pointed at above, its not likely that the mufflers really had anything to do with it, but rather than removing their back pressure helped for the time being reduce some other cause to where the surging stopped. So, you still have the original problem, just not that particular symptom. IAC is certainly the first place to look.

pax, smn

 

[E1]

So now the idle is smooth?

When I was designing the intake for the supercharger on SHO R T S,
(https://www.v8sho.com/SHO/Eaton M90 On SHO Motor.htm)
I did a bunch of research on intake runners and discovered that 'pulse' modulation variances caused by a bunch of things like - wide/narrow runners. Long/short runners. Cylinder scavenging and valve overlap - can cause reverse reverberations to shock through an intake system and wreak all kinds of unexpected havoc on smooth airflow.

That said, if there was some condition in your mufflers which put the exhaust out of 'tune', it could have indeed resulted in a mild back pulse upsetting the smooth flow of air through your motor.

If it is idling smooth now. I guess I would leave well enough alone.

 

[99sho-time]

So because we have a fancy intake its more sensitive to excessive backpressure? What about barely any back pressure.
I run federal cats (1 less substrate0 into a 20 inch glassback (i hate the sound) and no mufflers. also have a cone filter

 

[E1]

Not what I said. Actually, the butterflies are there to INCREASE air velocity at low RPM's to reduce backpressure reverberation issues. (Closed secondaries make all the air go down one tube instead of two = air volume consumed per revolution being equal, must be going faster down only 1)

OP maybe had a muffler with a baffle that was pulsating causing a reverb? It is funny how shockwaves can go backwards through an exhaust system sometimes. Think tuned equal length headers vs non-equal length.

 

[99sho-time]

Forgive me as this is over my head. Sounds like its more of a harmonic resonance issue not so much a this or that amount of air flow out the back.

 

[geneSHO98]

How much of a surge was it? Could it have been A/C compressor going on/off? But it sounds like the engine had a "breathing" problem, resolved by the muffler delete. Which in turn goes back to maintenance history.

Surge was random, didn’t always happen but would happen at least once a day. With the defrost on, with it off, didn’t matter. Can’t really comment on AC, my AC blows cold for a couple minutes, then the cold stops. Our summers here in western WA are usually mild enough to do without it.

As far as maintenance history, previous owner had it for about 2 years, replaced coolant lines and surge tank, spark plugs, power steering lines, front end components and transmission refresh. Gave me a folder with all the invoices, and I trust him as he also happens to be my father. Owner before him, after passing away, his widow had the car sitting for five years but claims to have faithfully started it every week as her husband had instructed her.

 

[geneSHO98]

So now the idle is smooth?

When I was designing the intake for the supercharger on SHO R T S,
(https://www.v8sho.com/SHO/Eaton M90 On SHO Motor.htm)
I did a bunch of research on intake runners and discovered that 'pulse' modulation variances caused by a bunch of things like - wide/narrow runners. Long/short runners. Cylinder scavenging and valve overlap - can cause reverse reverberations to shock through an intake system and wreak all kinds of unexpected havoc on smooth airflow.

That said, if there was some condition in your mufflers which put the exhaust out of 'tune', it could have indeed resulted in a mild back pulse upsetting the smooth flow of air through your motor.

If it is idling smooth now. I guess I would leave well enough alone.

Yes, idle is now smooth. Doesn’t rise in park, and I don’t think it surges in drive while idling either anymore. I only drive it one day a week, the other 6 I’m driving a FedEx truck. Will continue to monitor this Sunday.

 

[luigisho]

Maybe catalyst chunk made it's way down? Not sure how else a muffler would be a restriction. If it worked it worked. Hopefully this is the end of your issue.

 

[E1]

Maybe catalyst chunk made it's way down? Not sure how else a muffler would be a restriction. If it worked it worked. Hopefully this is the end of your issue.

I would doubt any real 'restriction' in a muffler.

What I was getting at is an internal issue on a muffler. Here is an example -

Our mufflers are a chambered design with perforated baffles between the chambers to meter out the airflow and smooth out the exhaust.
The baffles are welded into the muffler body. Perhaps a 20 yr old weld failed and a baffle was bouncing forward and backward in its chamber like a big speaker cone. This would cause internal shock waves by altering the cubic volume of that particular chamber - when the baffle moved backwards, the chamber volume would increase. When the baffle moved the other way, the chamber would be compressed making a change in backpressure.
When the muffler temp was just right and the idle speed was just so, and the moon and stars aligned, that baffle could have jumped back and forth like a speaker cone and put a harmonic pulse into the whole exhaust system.