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mgatlag · Joined: 02 Aug 2007 Posts: 647 Location: Avon, IN

Well I first posted this on Pelican but wanted to bring it back over here.

My exhaust pops when I downshift sometimes at higher revs. I just put a stock muffler back on last week. The cat has been removed since I've had the car and a resonator is in place as it sits now.

I had quite a few responses from Pelican and some of them were possible TPS, leaky injector, ignition problems (cap, rotor, wires, ets..) and a few others.

I do have an aftermarket header on it and wondered if more air is moving than the regular stock one. I just know that I've went through too many mufflers in the last couple years and the guys at the shop I take it to are probably getting tired of rewelding it for me. They seem to sort of blow out, either right before or right after the muffler after a couple months of driving.

Any other ideas of what to check? The other thing I've noticed is that when I first start the car it idles perfect at about 950rpms but as soon as I drive it (even just down the block), it idles at about 1400 and stays there until I shut down the car and restart it. Don't know if it's a separate issue or not?
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Michael
'77 1/2 924 N/A- 5 speed Audi box
'04 Ford Taurus - gone!!
'92 Jeep Cherokee Laredo - gone!!

Porsche... better than tea with Miss McGill
(Slap Shot)

morghen · Joined: 21 Jan 2005 Posts: 9095 Location: Romania

decel valve?
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924RACR · Posted: Fri Aug 27, 2010 8:50 pm Post subject:

I'm guessing you haven't adjusted the mixture since you changed in the header and resonator? It will run leaner, which means hotter, which will (or can) lead to popping on decel from high revs.

Probably need to richen up the mixture - I'd start with about 5 degrees (clockwise) on the mix screw.

We have this on the racecar just changing from the quiet to the loud pipe - drop one muffler at the tailpipe and replace a mandrel bend with a crimped bend!
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
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Joes924Racer · Posted: Sat Aug 28, 2010 12:50 am Post subject:

Yup its in the mix.
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Smoothie · Posted: Sat Aug 28, 2010 11:50 am Post subject:

Well, I'm sure you'll be pleased to hear...your problem can also be caused by what might seem to be the complete opposite of what Vaughn suggested.
That is a too rich mix on decelleration - something that the decel valve (aka- "anti-backfire" valve) is supposed to prevent.
With CIS, when the throttle is shut too quickly, the air supply is cut off suddenly, but fuel continues to flow for a moment. This causes a spike in A:F richness, and raw fuel flows into the exhaust. Then, assuming things are hot enough there, the fuel ignites and you get the popping exhaust sound + damage.
The decel valve prevents it by allowing a governed amount of air to bypass the throttle when the throttle is shut suddenly - thus some air is present to mix with the fuel and prevent the mixture spike.. It in effect causes every release of the throttle to "feel" like a smooth, gradual closing instead of a sudden shut.
Long story short...decel valve. Find it (assuming the '77.5 got one), and check it..

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Min · Posted: Sat Aug 28, 2010 12:09 pm Post subject:

Ok, I think its best to actually understand whats going on before making any decisions. First thing you need to know is, what is that popping noise? It is unburnt fuel in your exhaust, igniting due to hot exhaust pipe. For ignition to occur, you need air and fuel. What people generally find is that when they upgrade to a 'free flowing' exhaust, they get popping because the exhaust pipe is no longer full of exhaust smoke, and fresh air is flowing up the pipe from the muffler.

I first started researching this issue when I was tuning my EFI system and noticed popping on deceleration. Your solutions are simple, richen the mixture enough that you flood the exhaust with fuel and thus eliminate any fresh air. Lean out the mixture so that there is no fuel in the exhaust stream to ignite. Or reduce the flow of your exhaust system to prevent fresh air from flowing back into it. In my case I just tuned that area of my fuel map really lean.

Min
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dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

If your WUR has the vacuum diaphram at the bottom, check it to insure it holds a vacuum and is not leaking. During closed throttle deceleration, manifold vacuum should cause this diaphram to pull down and increase control pressure, which also leans the mixture.

Dennis
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mgatlag · Joined: 02 Aug 2007 Posts: 647 Location: Avon, IN

Ok well I'll try to adjust the mixture first. I do have the correct allen tool to adjust it. And how will I know if I have done it enough? I'd rather go the lean route and not flood the exhaust with fuel if I had the choice. Or does it really matter which route I go? Is one way better than the other?
And I've never known about any decel valve before, didn't know it had one. Guess I need to do some searching here on the site and do some learning.

On a side note....Blackberry phones do not respond well to water. I got thrown in a pool last night by the girl friend and the phone went swimming with me. So now it's in a container of rice, hopefully all the moisture gets out and it works again..

_________________
Michael
'77 1/2 924 N/A- 5 speed Audi box
'04 Ford Taurus - gone!!
'92 Jeep Cherokee Laredo - gone!!

Porsche... better than tea with Miss McGill
(Slap Shot)

mgatlag · Joined: 02 Aug 2007 Posts: 647 Location: Avon, IN

Ok duh... I do have a decel valve. But when I was looking at all the vacuum lines I noticed I have one that comes off a T from the large line off the brake booster that is open and nothing connected to it. I started the engine and put my finger over it and the engine slowed down.

But I wonder what is supposed to be connected to it? On that same large line is the T that goes to the bottom of the decel valve and that is connected. And the lines were all replaced last year with new. Guess I missed that open one. Oops...

I also noticed something else. On the big rubber boot that covers the fuel dist, there is a small hose that comes off the side of the boot and curls back towards the rear of the car. It has a large bolt in it and a clamp. Didn't see that in any of the diagrams. Wonder if that has anything to do with the AC being deleted many years ago....
_________________
Michael
'77 1/2 924 N/A- 5 speed Audi box
'04 Ford Taurus - gone!!
'92 Jeep Cherokee Laredo - gone!!

Porsche... better than tea with Miss McGill
(Slap Shot)

Grenadiers · Posted: Sun Aug 29, 2010 1:12 pm Post subject:

Not sure at all about your question, but I think we need a pic of the girlfriend. Stat.
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mgatlag · Joined: 02 Aug 2007 Posts: 647 Location: Avon, IN

_________________
Michael
'77 1/2 924 N/A- 5 speed Audi box
'04 Ford Taurus - gone!!
'92 Jeep Cherokee Laredo - gone!!

Porsche... better than tea with Miss McGill
(Slap Shot)

mgatlag · Joined: 02 Aug 2007 Posts: 647 Location: Avon, IN

_________________
Michael
'77 1/2 924 N/A- 5 speed Audi box
'04 Ford Taurus - gone!!
'92 Jeep Cherokee Laredo - gone!!

Porsche... better than tea with Miss McGill
(Slap Shot)

mgatlag · Joined: 02 Aug 2007 Posts: 647 Location: Avon, IN

Well I think that open vac line solved the popping exhaust. That seemed way to simple....Went for a drive and it didn't do it at all. I'll see how it does in the morning when I go to work.

I still need to figure out the idle thing though. Gotta figure out why it's at 950 when I start it and then jumps to about 1400 as soon as I drive it.
_________________
Michael
'77 1/2 924 N/A- 5 speed Audi box
'04 Ford Taurus - gone!!
'92 Jeep Cherokee Laredo - gone!!

Porsche... better than tea with Miss McGill
(Slap Shot)

924RACR · Posted: Sun Aug 29, 2010 8:40 pm Post subject:

Might be a sticking throttle. I have that in my 931, have to re-bush the throttle body someday... once I get it running again...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

Check timing also. Having the timing off will cause that popping sound. At least, it did on mine. Vacuum advance may not be working correctly, causing the idle to stay high. I'd check that before adjusting the fuel mixture.
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'80 924 n/a SOLD