'85 gt exhaust problem help!

[Captain5.0]

I have an '85 GT that usually doesn't sound like it's supposed to. When I drive the car for extended periods of time, sometimes it will come to life, and sound amazing. The performance is enhanced as well. Usually though, the car sounds muffled. I thought about it being the heat riser butterfly valve getting stuck closed, but haven't checked into it yet. I thought it might be a cylinder dropping out due to a fouled spark plug, but I have replaced all the plugs and test rode and it still runs muffled. I guess the only thing it could be would be the heat riser flapper getting stuck closed. I am going to take it out and pull the flapper, but before I do, I wanted to get some feedback. Thanks!

Captain5.0

[escogt]

Could it perhaps be a clogged catalytic converter?

[Captain5.0]

No cats, factory headers going to a catless Hpipe with flowmasters. It sounds amazing when it kicks in, but I'm just thinking it's that flapper shutting on me. Thanks.

[Captain5.0]

I'm probably going to go ahead and pull out that heat riser valve, and take out the internals, and fill the holes. That will let me know if thats the problem. I'll also add, before the new plugs were installed, the passenger side exhaust pipe would not have exhaust steam coming from it like the driver side. Once the plugs were installed, the passenger side tailpipe had steam coming from it. It's 40 degrees here in Louisiana.

[Captain5.0]

Maybe I will get lucky enough to grab JaCook's attention on this one...

[83gtstang]

Hello, so the problem is intermittent? Where in Louisiana are you?

[Captain5.0]

I'm near Monroe, LA.. I have her up on jackstands right now, and sprayed the stud bolts on the header collector with PB Blaster, letting it marinate.
Figured I would check if anyone posted yet. Yea it's intermittent, comes and goes in no certain order. I'm gonna break the nuts off, see if I can get to the heat riser valve, and gut it. Tonight. Will see if that corrects the problem, will let yall know. I see your from Shreveport. Awesome to see fellow North Louisiana folks with FourEyedPride!

[83gtstang]

Awesome, not too far, hope to see you around.

[Captain5.0]

I just got done taking loose the connection to the header on the driver's side. I found out when I unplug the vacuum source, the flapper valve stays open. Took a test drive, and the car is flat out amazing. It has even flow on both tailpipes, and has that awesome growl. Doesn't skip a beat. For anyone potentially facing this problem, Just check that flapper in the heat riser valve. It's likely stuck and restricting your exhaust flow. This car is fun to drive again. Thanks to those who replied. @83gtstang, I might be heading over to the casinos soon, if you see an '85 Bright Red GT Vert, then that's me!

[83gtstang]

Cool I was going to ask if it was vacuum operated. I took mine off long ago when I went true duals. Great news! I'll have to keep an eye out, for that nice vert of yours, not too many foureyed cars around here. World of wheels is coming to town at the end of this month, should stop by then.

[JACook]

Most likely the vacuum hoses were... modified by a PO that didn't understand how the system is supposed
to work. That, or the green thermal vacuum switch for the EGR system has failed. The valve is only ever
supposed to be closed when the engine is cold, to aid driveability during warm-up. But I'm guessing that's
probably not terribly important in Monroe, LA...

[Captain5.0]

Jeff, thanks for responding.. Yea, it looks like previous owner put the H pipe on, then left the heat riser in. Also the belt for the air pump is bypassed, It looks like that may be the source of vacuum for opening or closing that flap. Yea, we have had quite a mild winter as always, but it's starting to catch up and be on par. Thanks again sir, this website is awesome.

[JACook]

The vacuum for the heat riser valve comes from the EGR port on the carb, through a check valve that has
three vacuum ports. EGR vacuum is also used, once the engine warms up, to release the check valve. The
air pump system diverter valve is also controlled by EGR vacuum, but only when the engine is warmed up.

So if a PO disconnected the vacuum lines to the air pump system, and left the ends open, you might never
get enough EGR vacuum to release the check valve, but the vacuum would still be there to close the valve
when the engine is cold. This would also be true if the EGR valve is disconnected and the line is left open.