Clogged exhaust?

[chicklin]

'86 K10 Suburban, 350, 700R4, stock catalytic converter and muffler.

So, I've been getting the motor on my new/old Suburban tuned up and I think I've got it dialed in pretty close. I'm pretty sure I still have a vacuum leak somewhere b/c it idles high (i.e. I had to set it rich on the idle mixture to make it idle smooth which then made it idle high).

Anyway, the other issue I'm now having is that I seem to have lost all my power, particularly in the mid-to-upper RPMS ranges, like when I've driving on the interstate.

The engine runs smoothly now, no misses, vibrations or noises, but I've basically got to floor it to get it up to and maintain highway speeds. The timing is set to 8 btdc per the sticker, so I don't think that's the issue. Even if it was too far retarded, that would generally just affect acceleration, not holding it at speed, right?

The other thing I noticed is that, with the window down and giving it some gas, I can hear the exhaust "spitting". It kind of sounds like when you have a miss in the ignition, but the engine is not missing. Could this be from the exhaust being restricted, pressure builds up and it finally blows through and makes kind of a "pooof" noise?

If it is the cat, is there a home remedy for this? Once, years ago, I had a truck with a similar problem and drilled several holes in the cat. I can't remember if it helped or not, though

[old Rusty C10]

if the exhaust is plugged, it will prolly be in the cat or muffler,, cat most likely and when you rev up the motor it will glowred hot and you may smell rotten eggs.
You can get a replacement cat for about $100 or so universal job at the chain auto parts store.. as togetting it there if you cant move the truck much you can disconnect the cat from the rest of the exhaust and take a pipe and happer and break up the element in it and blow it out by racing the motor. You can then reconect the rest of the exhaust so you can get to the shop. It is illegal to drive like that, but it should be ok to get you to the shop

[chicklin]

Originally Posted by old Rusty C10

if the exhaust is plugged, it will prolly be in the cat or muffler,, cat most likely and when you rev up the motor it will glowred hot and you may smell rotten eggs.
You can get a replacement cat for about $100 or so universal job at the chain auto parts store.. as togetting it there if you cant move the truck much you can disconnect the cat from the rest of the exhaust and take a pipe and happer and break up the element in it and blow it out by racing the motor. You can then reconect the rest of the exhaust so you can get to the shop. It is illegal to drive like that, but it should be ok to get you to the shop

Well, it's drivable, so I don't think it's completely clogged. I will check to see if it looks hot after driving home. 40 minutes on the freeway at 70 mph ought to do it

[Damien]

Did you change the cap, rotor and wires when you did the tune up. When I got my '83 I had the same problem. On the freeway I couldn't get it above 55mph. This was after I put in new plugs. Changed the other stuff and it has run great since.

[chicklin]

Originally Posted by Damien

Did you change the cap, rotor and wires when you did the tune up. When I got my '83 I had the same problem. On the freeway I couldn't get it above 55mph. This was after I put in new plugs. Changed the other stuff and it has run great since.

Yep, cap, rotor, coil, wires, plugs, new carb, fuel filter and fuel pump.

One other thing I thought to check is the heat riser "flapper" on the passenger side exhaust manifold. Maybe it got stuck or something.

Can you think of anything else that is not exhaust-related? That's sure what it feels like but I could be convincing myself prematurely.

[Damien]

It could be a weak could, but I hate to suggest you throw money it it to try to fix it. You could unhook the exhaust at the manifolds and take it for a test drive or put a pressure gauge in the O2 sensor hole to check the back pressure. I think it should be 2 or 3 psi or so. Not really sure on that number though.

[chicklin]

Originally Posted by Damien

It could be a weak could, but I hate to suggest you throw money it it to try to fix it. You could unhook the exhaust at the manifolds and take it for a test drive or put a pressure gauge in the O2 sensor hole to check the back pressure. I think it should be 2 or 3 psi or so. Not really sure on that number though.

Well, I did a little troubleshooting at lunch today. First thing I noticed was that when I would goose the throttle in the parking lot, I now get a nice cloud of black soot shooting out of the tailpipe. The truck does not smoke (either black or white) upon startup or at any other time. So, I'm pretty sure it was soot not smoke.

I also notice that if I kick the cat or the muffler while running I get little puffs of soot. I then shut it down and kicked them a bunch with the engine off. When I fired it up, I got a huge cloud of black soot. Think I found the problem

Last week, I ran a bottle of Sea Foam through the intake. I wonder if that, in combination with an old catalytic converter, finally plugged things up enough to cause power loss.

Off to the muffler shop. Will this ever end?

[billnorman]

Lettuce know what works. Sounds like removing the cat to check it out would be smart. Does it have the huge round pan converter or the cylindrical one? The big round ones are worth some scratch at a junkyard.

[chicklin]

Originally Posted by billnorman

Lettuce know what works. Sounds like removing the cat to check it out would be smart. Does it have the huge round pan converter or the cylindrical one? The big round ones are worth some scratch at a junkyard.

It's the pancake style. It actually started to run a little better after I beat on both of those and ran it down the highway. It's still struggling and little and making that spitting noise, but I can at least hold it at 70 on the interstate.

Taking it to my local exhaust guy on Monday. I only paid $1,000 for the truck, so I'll probably just have them replaced. Might as well.

[chicklin]

Well, got the cat and muffler replaced. My exhaust guy said material was falling out of the cat when he took it off and the muffler had a cracked seam.

The power loss issue doesn't seem as bad, so maybe it's fixed. However, the "spitting" noise in the exhaust is still there. I'm beginning to thinking I may have a miss in the engine somewhere. I'll address it in my other thread about the engine running rough.

[old Rusty C10]

even though you have new wires make sure that they are not touching each other and they are not touching anything real hot like an exhaust manifold... that would cause it to arc

[chicklin]

Turns out it was an improperly adjusted choke. It was not opening fully which was causing symptoms very much like a clogged exhaust. I'm gonna pull and clean the plugs since it was running rich and missing for a couple of weeks.