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Exhaust for Dummies/Fuel Leakage

Spiderman

Well-known member
Joined
Jan 14, 2002
Messages
139
Location
Grand Haven, MI, USA
Corvette
Gorgeous White '91 Coupe
I'm curious about the layout of the stock exhaust system on my '91.
1.OK, so on the front Y-pipe there are two units, I'm guessing these are the "pre-cats"?, and then in the tunnel below the driveshaft is the main "cat" ? which connects in front of the rear Y-pipe. When it comes time to replace my system I would prefer to know want I need to replace and what not to. I would mainly like to know how it works.

2.I believe the "pre-cat on the passenger side is breaking up from fuel in the exhaust system. You can hear a subtle gargled vibration sound when throttle is added from idle. One of my injectors may be stuck open. I have a fuel smell coming from the back of the car. From time to time when the car has been sitting for a while, I get a puff of white-ish smoke coming out right on the start up. (I did add a bottle of fuel injection system cleaner about 500 miles ago, I don't know if thats contributing to the burn off) Once the engine is warm there's nothing. I heard that fuel will eat your "cats" up if it gets down there.(?) I brought up the fuel smell with my mechanic and he said the MAP sensor won't allow my car to run "rich" so I'm thinking it's a failing stuck-open injector. He said if the MAP sensor is bad the car would be missing. back-firing, and all around running like doggy doodoo. My engine could use a little tweaking but it runs fine.
They say knowledge is power and I'm feeling pretty weak. Please help :)

Spiderman :w
 

Ken

Gone but not forgotten
Joined
Jan 30, 2001
Messages
8,236
Location
Hermosa Beach, CA
Corvette
1987 Z51 Silver Coupe
Spiderman said:
... the front Y-pipe there are two units, I'm guessing these are the "pre-cats"? ... and then in the tunnel below the driveshaft is the main "cat" ?

Got that right. :cool

I heard that fuel will eat your "cats" up if it gets down there.(?)

Got that right too. :cool

If you want to dump the pre-cats you can, but you won't be legal. The EPA says that everything must be replaced in kind. ;)

If the cats break up and make noise, the car usually runs fine. But if the get quiet when you know they are breaking up (they've been making noise and then stops making noise), it means they have clogged the passage of the exhaust stream and are preventing the engine from exhaling. In that event, the car won't get out of its own way!

Good luck in finding the "gargled vibration sound" on your '91. :Roll

_ken :w
 

Spiderman

Well-known member
Joined
Jan 14, 2002
Messages
139
Location
Grand Haven, MI, USA
Corvette
Gorgeous White '91 Coupe
Thanks Ken.

Can anybody help me with the fuel smell and puff of smoke on the start-up?

P.S. Out of paranoia, I want to clarify that I was referring to myself as the Dummy in the title of this thread. Hope I didn't rub anybody the wrong way. :BOW

Spiderman :w
 

Ken

Gone but not forgotten
Joined
Jan 30, 2001
Messages
8,236
Location
Hermosa Beach, CA
Corvette
1987 Z51 Silver Coupe
Oh, I forgot about that. :eek:

The usual cause of the smoke upon start-up is due to leaking exhaust valve seals: a problem common to the small block Chevys for a while. They later changed the type of seal used in the application and the problem was eliminated.

It isn't necessary to remove the heads to replace the seals, because as we are all too familiar with the idea that once "We're in here this far, why not...?" :L

The fuel smell... I don't know what to tell you except to check each and every part of the system, including the charcoal canister. ;)

_ken :w
 

Redbob

Well-known member
Joined
Dec 5, 2001
Messages
239
Location
Cross Roads, Texas, U.S.
Corvette
'89 dk red coupe
Spiderman said:

2.<snip> I heard that fuel will eat your "cats" up if it gets down there.(?) I brought up the fuel smell with my mechanic and he said the MAP sensor won't allow my car to run "rich" so I'm thinking it's a failing stuck-open injector. <snip>
Spiderman :w

Unburned or excess fuel will destroy your cats, since what they're trying to do is to "burn" the unburned fuel, so they'll get so hot that the ceramic honeycomb "substrate" melts down and/or breaks up. When this mess gets stuck in your pipes, you've got the recipe for major power loss; if all you do is cruise around town, though, you might never notice the power loss until you try to blast onto an interstate.
Your mechanic is only partly right: if you have a leaky injector, there's no way the MAF sensor can compensate for that.

What you need is a fuel pressure gauge (many auto supply stores have them - don't pay more than maybe $45) and a copy of the procedure for checking for leaky injectors. Those instructions might well come with the gauge itself.
Good luck,
- R
 

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