Pre-CAT took out CAT!

Anything related to the stock drivetrain, engine, transmission, axles, wheels...

Moderator: F9K9

Pre-CAT took out CAT!

Postby Torskdoc » Thu Sep 24, 2015 9:03 pm

2 weeks ago past Saturday, I go over to my sisters for a family party. Drove the quad over, fairly easy as the wife gets carsick if i drive like I normally do. ( .none of us like puke in the car.....), and back home afterward with no problems. RPM never got above 3K and I didn't get 30% on the throttle. Ran good. I heard some rattling around for a couple seconds after the starter wound down, then it quit. I didn't think anything about it.

Fast forward to Tuesday. Go to bank and I pull out of community on the main road and goose it. BWAHHHHHH! 1/2 throttle, lots of noise, 0" (ZERO) inches of mercury on vacuum gauge, and absolutely CRAP for acceleration, and no rpm higher than 3000 and it had to be in neutral for that :?: . Like it had a Nimitz class carrier anchor attached to it, :evil: WTF???? Back pedal and ease into throttle BARELY and the vacuum cycled to 10" slowly and back to ZERO so fast I couldn't believe it. Turned around and made it home, barely.

Got wife to punch the throttle a couple times while I had my hand on exhaust tip. Very little if any change in pressure. Turn off and put it on jacks and pulled the front O2 sensors. Started up and it had a bunch of power but throwing O2 codes. Determined that precat and/or cat was bad.

Went down to exhaust shop in area and had the precat pulled. It was full of loose chunks :blink: , so he pulled the main cat. looked down the inlet and it was approx 85% CLOGGED! :!: Ok'ed a New 3way cat, precat pipe replacement and had it welded in, and it was like night and day. Plenty of power :mrgreen: , smoked the tires (cheap chinese crap from Firestone) and now I can drive @ 65 mph on highway at 15" instead of 13" of mercury on vacuum gauge. Now mileage is 24.8 on highway (2 tanks average) while feathering vacuum at 14-15" instead of 22-23mpg @ 12"-13". With it breathing correctly, Acceleration is now enough to hold me back in the seat with it floored, and it's now faster off the line than the 95 Jimmy. :wave: Passing acceleration is now VERY GOOD! 5K on tach doesn't feel as if it's hitting a wall anymore, like it used to. But I've got the ECM programmed to shut off ign at 5300. so no problem.

I pulled our 18ft Travel trailer (gross 4300#) Monday to state park 70 miles away, and it pulled it very nicely. Acceleration was a lot better than it had ever been and I could maintain 55-60 @ 10" (about 14-15mpg per trip computer) without it hunting 4-3-4-lockup-3-4-3-etc., until I get to about 90% on throttle. It's got the corvette servo's and a Wait 4 Me ECM set up for towing on Reg fuel, with the kickdown and lockout trip set @ 90%, and the up shifts set a bit harder on it so that may have something to do with it.(can't stand Automatics that feel like a 60's FORD-O-MATIC or C-6 slush-box). Normally when it goes into 4th and lockup it stays there until I tap the brakes, or shift manually into 3rd. Which is way better than it hunting like the 4L60E does normally. Tank mileage average was 14.5mpg, mainly flat roads with the one large hill (3miles 7% grade) , and a shorter one of a mile to the State park. I normally drive with the trailer @ no more than 55, unless I have a hill coming up then a mile or two before hand I'll speed up to get some more momentum. Now I don't need so much. Coolant temps were consistant with weather. 75-80*, 80% humidity. Coolant temp 180, trans temp 160 with it climbing to 180 on the hill. Stock Fan setup with the thicker core radiator. And the new cat. I ran the A/C for about a 10 miles, but it tanked the mpg and I was using enough throttle to cause 5" of mercury to maintain 55mph. But coolant and trans temps only went 5* higher. :thumb: :D :D :D

So, signs that your precat is going.

1.) rattling noise near front doors underneath for a few seconds after engine start. This is the chunks, pieces getting blown to back of can and lodging together, or traveling into the cat housing and breaking up clogging the main cat.

a.) RPM drops off at about 3K.
b.) lots of intake noise.
c.) almost immediate drop in manifold vacuum to ZERO (0") inches above idle, barely tapping throttle.
d.) feels like truck is dragging an anchor for a Nimitz Class Carrier.
e.) upshift has a momentary boost in power then it drops off almost immediately

3.) Troubleshooting
a. Hand in front of exhaust and feel for flow while engine being goosed. No change or very little indicates a clog.
b.) removing the O2 sensors results in an increase in power due to less back-pressure. Engine should run at 3K rpm ....@ 15"-16" vacuum in Neutral with sensors out.

As the precat is welded at the junction of the "Y" pipe with no pipe between it and the "Y", it's impossible to replace it without welding. unless you have a tig/mig welder, best to leave it to a shop. Also the pipe between the pre-cat and the cat has a dogleg in it so you can't just cut the pipe in the middle and look at both cat's. It's almost easier on all involved if you DO have a cat problem to just go ahead and replace both with a 3 way. Rock Auto has the complete "Y" pipe with both cats for about $240.00 so that's an option. If you straight pipe it you have to devise a way to fool the 3rd O2 sensor.

Well worth the $$$, and thehour to hour and a 1/2 to rip out the old, confirm diagnosis, and replace with new. New "Y" pipe would take a couple hours for a DIY, but it's still an option.

COST: As of 09-2015 prices in Baltimore area ran between $280.00 and $340.00 for a 3 way cat, welded in pipe, and labor to weld it all in.
2002 S-10 ZR-5 Quad-CAB
1995 GMC Jimmy 4Dr. SLT
User avatar
Crew Elite
Crew Elite
Posts: 171
Joined: Sun Oct 28, 2007 9:00 am
Location: Glen Burnie, Md

Re: Pre-CAT took out CAT!

Postby booth9c1ss » Sat Nov 05, 2016 8:42 am

I had replaced my main cat with a magnaflow, only to have the pre-cut kill it. Now I have two "ultra high flow" cats.

Post-Standard O2 sensor spaced out with a spark plug fouler (search the web) for about $5 to eliminate code.

Posts: 10
Joined: Mon Feb 27, 2012 12:40 pm

Return to Stock Mechanical / Drivetrain

Who is online

Users browsing this forum: No registered users and 2 guests