96' Honda Accord P0420 -- Leaking exhaust manifold?
 

Back in May, 2011 we took off the exhaust manifold to replace the gasket. We also replaced the o-ring.

We also took it off to replace one of the broken off studs. My dad drilled it out and supposedly "re-tapped" it. Well, the bolt fell out, so much for that.

I probably should have had it professionally done(too late for that, only constructive feedback please) And I see black soot around the broken stud we drilled out. That's got to be tripping the P0420 code. Which comes on between 1 -5 days after I clear the code.

I know for a fact the upper stream 02 sensor works because it fell out numerous times while driving and my car screeched to a brisk jog! It could possibly be the downstream 0^2 sensor, but I want to fix this before this May when I have to have my car inspected again.

Has anyone sealed a leaking upper exhaust manifold before?

How did your father do the repair the first time?

He drilled out the bolt into the block and re-tapped it at 5/16''.

(if only someone would have uttered the words stud extractor...)

You might want to look into a helicoil. 5/16 might be the right thread pitch to install the helicoil and match the original thread pitch.

The helicoil should fix the leak when done right thay are great. Now on your code P0402 that is a bad cat the ECU looks at the O2 senser in the cat and can tell when the cat is not working I just rep;aced mine with a NAPA unit fit great and so far so good stay away from one size fits all Get the leak fixed first becous nothing will be right when it is leaking

Sorry I got it wrong, the bolt is actually still in there. Aluminum block...steel stud. I'm not sure even if a professional could save it. They'd probably recommend replacing the block. Okay, that's happening.

To test how bad the leak was, I had my dad put a rag on the exhaust pipe and I opened the hood and I saw some stuff fizzling out where the gasket was black(on the upper exhaust manifold). It's actually didn't fizzle where the bolt was missing, it fizzled some towards the center and I torqued those bolts down a little bit.

Code situation: the code come backs every 2-5 days, so I think the catalytic converter is still hanging on a bit. I could try to take it off myself, it's actually the bolt on but the bolts are rusty to kingdom come. But I'll probably have a muffler shop put a new one on.

Do the exhaust leak first. O2 sensors actually measure oxygen DIFFERENCE outside vs. inside. Exhaust blowing onto the outside of the sensor will throw off the measurement.

Exhaust blowing onto the front O2 sensor will throw off the fuel mixture. That in turn can lead to exhaust composition that can't be properly handled by the cat. That in turn gets measured by the rear O2 sensor & it throws P0420.

Well, I'm trying to trouble shoot what shot my catalytic converter or what's taking it out.

It doesn't look rotted out, it doesn't rattle and there's no sign of rotten eggs.

Could it be the spark plug wires? How do I test these?

The scenario I said about exaust leaks, doesn't destroy the cat. It simply runs the engine at the wrong mixture, so the rear O2 sensor THINKS the cat isn't working.

Don't be too tough on your Dad. I had a broken manifold stud too, and no amount of left-hand drills/stud extractors/cuss words would get it out. I ended up drilling and tapping an 8mm hole and putting a new stud in...slightly off kilter, but it worked.

5/16" is slightly smaller than 8mm, so there's a possibility that you can re-tap the hole for an 8mm stud. Otherwise, Helicoil. $23 for the kit at Amazon and, depending on which stud, it should be pretty easy to do.

Well, I guess it's getting the helicoil, what's a fair price for a shop to do this job?

It's too risky for me to do myself.

I re-tightened the bolts where the leak was coming through, and I cleaned it best I could with some 409 and a toothbrush. I'm monitoring to see if the carbon starts building up again. I'm monitoring the leak where the one bolt is missing, I don't see any carbon coming through yet. In fact, when my dad put the rag on the tailpipe I only saw stuff fizzling from the area where the studs weren't broken. We'll have to do this again and see if anything is coming through.

A new gasket was installed, right? Put a straight edge on the manifold after you take it off to check for flatness.

I cleaned off the carbon that was coming out of the manifold, tightened the bolts and I don't see anymore carbon coming out yet. It's been a little over a week and I still see the silver.

I got a replacement 02 sensor for the upstream and I installed it today. I'm not sure if the code clears itself, so I went ahead and cleared it manually from the ECU.

I can already tell it's improved my cars performance. The tachometer seems to move a lot steadier and I believe it's shifting sooner at lower rev's. Even if it doesn't clear the code I'm glad I replaced it, the other one had been in and out of the exhaust 5 or 6 times, and it was bound to get stripped after awhile.

Well, code came back today.

I'm thinking about sea foaming the car. If I put sea foam down the intake to the throttle body, will that clean out my catalytic converter?

The can of sea foam says it will clean out the cat, but after putting the whole can in my tank, as directed with proper ratio of 2oz to gallons, I don't see a difference yet. Since it's been affecting my gas mileage and I know for sure now the upstream 02 sensor is good, this must mean it's my catalytic converter getting stuffed up.

Maybe I could just take a hose and run it up my exhaust to where my cat is and apply the sea foam. Or perhaps I could park on a slope at the right angle and pour it down my exhaust pipe, let it sit there for a couple hours and see if my code gos away.

Question: If the cat is causing the code, and I fix it, will the code go away automatically after a few days?

You can clear the code by disconnecting the battery or pulling the ECU backup fuse. Then see if it comes back. The CEL will go away by itself, but the error is still stored.

Not sure if that's a good way, trying to pour it into the exhaust pipe. Besides, you won't get a hose directly through the mufflers.

Another can, sucked into a vacuum hose while the engine is running, is probably the way to get it into the cat.

http://static.ddmcdn.com/gif/muffler-cutopen.jpg

Looking at a diagram of the inside of a muffler, I see now why it can't work.

I used to have a giant hole in my exhaust pipe, I should have attempted this then! (hah).

<strike>Perhaps I could drill a small hole in my exhaust pipe, pour it in close to the catalytic converter?</strike>

I can't believe it took me this long to think of it, I added the sea foam into the port where my oxygen sensor was on the exhaust manifold. It smoked like the dickens going down the road, it was pretty funny watching the car behind me stay back about 50 feet.

I replaced the 02 sensor with a Bosch, not the OEM denso variety.

Do you think this why I still have the code?

I checked the manifold, straight as an arrow, no cracks or anything.


Tomorrow I'm going to replace all the spark plugs and see if that does anything. I know they are probably overdue.

Lots of people have reported trouble with Bosch spark plugs in Hondas, but I've used them without problem. I'll suggest getting NGK or Denso plugs, but not really sure if the O2 sensor would be a problem. Unless the store gave you the wrong Bosch one.

I replaced the spark plugs today. An exhilarating, yet at the same, terrifying experience worrying that the spark plug was going to break off or strip.

I replaced them with stock ones from Honda.

They looked normally worn(tannish coloration w/ slight oil deposit on the threads) and the gap was nearly down to the bottom. They were definitely in need of being replaced but show no signs of engine problems. I coated the new ones with anti-seize applied with a toothbrush.

My engine hasn't thrown a code yet, so I reckon I tightened them well enough into the block :)

I fueled up again today and checked my gas mileage. I don't know if this is a sign of that sea foam cleaning my fuel-injector or if the oxygen sensor is working, but my gas mileage is up to 24.6 MPG now. I'll keep monitoring it.

Well, I tested the downstream 02 sensor and it's well within range.

Looks like it's the cat getting clogged up. Can anyone recommend a good bolt-on aftermarket catalytic converter? I don't need anything that will last 15 years like a OEM Honda cat, but at least something that will last 4 or 5.

Turns out there's a missing bolt on my exhaust manifold, which is throwing off my air/fuel mixture. The gasket is marked with black soot in two spots.

What's a fair price for a mechanic to drill it out and replace the stud?? Should I replace the one stud or go ahead and replace all of them?

Most of the cost would be labor, IMO. I would assume one hour? You would have to call and get quotes to find out.

I am of the opinion if it ain't broke, don't fix it. You may want to put a bit of anti-seize on the threads to prevent the nut from rusting on the threads.

Hmm, which nut are you referring too?

The stud is broken off inside the head and none of it is protruding from the manifold. My dad cut if off and taped a new hole.

IIRC, there is a two-way threaded stud that goes into the cylinder head. The manifold goes over the studs, then a nut is what holds the exhaust manifold to the cylinder head. This is based on my 95 EX that I sold more than a year ago.

I'm not sure if your accord is an LX, DX, or EX. The LX/DX may be different.