After Removing Catalytic Converter ...

[vcp]

The old catalytic converter on the '87 was cracked at the front intake, the heat shield had fallen off, it was rattling, and I was getting backfire downhill under load. So after reading some posts here, we just removed it and put in a straight piece of pipe.

I was hoping the car would be quieter with more power and maybe the backfire would abate or disappear. But none of those happened! In fact the muffler now sounds much louder when revving, my power seems good then poor (changes), and it backfires worse under load.

Got it on the hoist and we couldn't find any sign of leaks in the exhaust system. In fact, it sounds fine at idle, but when I throttle up it sounds like one of those cars where the guy has purposely put on a really loud muffler. (I don't know what does that - removing baffles??) One mechanic suggested some parts from the cc might have gotten 'sucked into' the muffler when the cc was cut off and removed? Is that even possible?

The old girl was zipping along really well after extensive work last year (head gasket, timing belt, spark plugs and wires, distributor, fuel pump & filter, etc. etc.) But now has gone south since removing the CC. (Still running, still love her, but she's hurting a little.)

Suspect the backfire is coming from an old worn carb, but maybe we also need to adjust timing for the cc being removed??

Whaddya think?

[teranfirbt]

The cat will cut noise a fair amount. I'd get a 2" high flow cat from a reputable brand (walker, dynonax, magnaflow) and get it installed. A high flow cat won't affect power, but will improve emissions significantly.
Go through the manual and perform all checks on the carb. Might need a carb rebuild, there are some rubber parts that wear out over time.
Set your base timing to 10 degrees before top dead center, the 3 A always runs better when you do that.

[lannvouivre]

The cat shouldn't do anything to your performance. It just muffles noise, reduces emissions, and maybe slows exhaust flow a small amount. Removing it entirely probably wouldn't be the best cure for backfiring since it also helps burn fuel leftovers. I'd definitely look at your carb.

[Petros]

your mechanic does not know what he is talking about, no matter the condition of the cat, it will not cause back firing. Personally I would not listen to anyone that ignorant about the car's systems (perhaps and very likely he is not familiar with the systems on a car this old).

Back firing it caused by too much fuel and air in the exhaust, that ignites in the exhaust system instead of in the combustion chamber. this can be caused by one or more cylinders not igniting (or irregular ignition, sending unburned mixture into the exhaust), too rich fuel mixture (check float level, the AAP leaking, stuck fuel metering valve in the carb), or even a too lean mixture (incomplete combustion in the cylinders so it burns in the exhaust when it accumulates there). You would check for vac leaks, or plugged up fuel jets in the carb to eliminate lean running condition.

Also, if the cam timing belt was installed a few teeth off it could cause back firing, so you may want to double check the cam timing marks line up correctly.

You do not need a catalytic converter to make it run well, it is purely there for emissions reasons. It also serves to reduced some exhaust noise. WE have two old cars (including my daily driver Tercel4wd) that runs fine without the cat (both of them disintegrated so I had a muffler shop put in a by-pass straight pipe). Both got better fuel economy after wards, but both had slightly louder exhaust. It is cheaper to replace the cat with another muffler, or you can find an aftermarket cat to replace it (or just live with the slightly louder exhaust noise).

Fix the cause of the back firing before you fix the exhaust system. It it is too rich you will just ruin the new cat (if you install one). So fix the engine/fuel system issue that is causing the back firing, than fix the exhaust system. We had to replace the exhaust sytem on my wife's Mazda when the timing belt was getting shredded (unknown to us), it skipped a few teeth on the cam cogs and caused a big back fire that literally blew up the mufflers (there are two, plus the cat which was already by passed). It almost sounded like a bomb went off under the car, it caused the roof to "clang" from the concussion, scared the sh*t out of my wife and I was going "what the hell was that!" My first thought was to check the timing belt, sure enough: I got to replace the timing belt, and the whole exhaust system. Fortunately the local pick-n-pull had the same model mazda with an almost new exhaust system, replaced it from the cat back for $16. lucked out.

So find and fix the cause of the back firing first, than deal with the exhaust system.

[dlb]

hi van! i hate to sound like a broken record but for the backfiring, i would check the ignition timing and verify that the vacuum advance is working properly -- make sure it has some counterclockwise spring-loaded play.

good luck!

[Mark]

My '81 doesn't have a cat (that's the way it came from the factory). It has a resonator instead. My '84 wagon's cat was corroded with some holes and was pretty gummed up so I removed it about a month ago and replaced it with a straight pipe for now like you did. The exhaust was embarrassingly-loud. I felt like one of those types who puts an expensive muffler on their car to make it sound like it's rusted out (for some reason these people also have lots of companies' stickers on the back of their car to tell everyone they paid $800 for a special steering wheel or whatever). -Anyway, I put a tailpipe on the muffler that points down to the road. This cut down on the noise a bit, but it was still really loud. I then installed an old resonator I had laying around and now it's ok. It's still not as quiet as with a cat, but I'll leave it for now. I remember a few years ago, my muffler fell off and I didn't even notice the sound change much (my brother was in the back seat and was looking out the back window and saw it go bouncing down the logging road, otherwise I wouldn't have noticed). The cat really does absorb most of the noise from our 62hp engines.

[Mark]

Also, check to make sure no vacuum lines were disconnected. When I removed my cat on the wagon, I first removed the exhaust pipe where it connects to the manifold on the engine. By sticking my hands down in the engine bay to remove those bolts, I kept accidentally popping off a vacuum line that was always in the way of my arms.

[vcp]

Thanks for all the tips!

And the mystery deepens. Drove 120km (70 miles) roundtrip on a mountain highway yesterday. Speed up to about 100km/hr (60mph). No problems until near the end of the first 60km when noticed a couple small bucks while going up a long hill at about 90 km/h. Then nothing further. Until ....

Early on in the return trip it started bucking more and more while going uphill. Reminded me of the earlier problem I had which turned out to be a spark plug not firing.

Pulled over and did a quick check under the hood but all hoses seemed attached and couldn't find a switch that said "Click Here To Reset Engine To 27 Years Ago", so decided to try again.

And noticed the bucking only seem to happen when going uphill with the throttle pedal almost completely depressed (some big hills). So backed off on the pedal, never pushing it to max and ... no more bucking all the way home.

WTF? Is the new fuel pump faulty and messing with me? Or ...

(Gonna re-check all the hoses and the vacuum advance today.)

[irowiki]

Did you replace the fuel filter yet?

Replace it again if you did, replace it if you didn't! That is exactly what my 83 did with a plugged fuel filter. Took 5 filters before it calmed down.

[Mark]

By "backfiring" do you mean that "put-put-put" sound, or a loud bang like a gunshot? I wouldn't think that a bad fuel pump would cause your symptoms. If the pump wasn't supplying enough fuel to the carb bowl I think that at heavy load, the engine would use up the fuel faster than the pump could supply it and the engine would stall completely. Unless it's something to do with the clutch or transmission, my guess is that the timing is off or there's a bad spark plug or wire. I once replaced all 4 spark plugs on my wagon and afterwards during acceleration I would lose power. It turns out one of the plugs was defective. It wouldn't fire reliably at high rpms. I replaced the plugs again and everything was fine.

[vcp]

irowiki
No, haven't replaced it yet but have it here. Will try today or tomorrow. Good to heat it fixed your same problem!

Mark
Sorry, been meaning to clarify that - much more a putt-putt farting than actual backfire. Plugs and wires are new in the last year, but maybe try new plugs if the filter doesn't help.

Thanks guys!

[Mark]

I think a small amount of "putt-putting" is accepted in older cars because there might be more unburned fuel in the exhaust than in more modern cars. The catalytic converter is supposed to get rid of these hydrocarbons so if you take the cat out, there will be more unburned fuel (hydrocarbons) in the exhaust and there will be more putt-putting (the sound is from unburned fuel combusting in the exhaust). Some people like the putt-putting sound and buy special exhausts to make sure you can hear it. Maybe this preference came from muscle-car/hot rod cars that would often run rich for more power leaving some unburned fuel in the exhaust. Lots of motorcycle people like this sound too, maybe for the same reason (it implies a more powerful, high-performance engine). If you're hearing it a lot on the Tercel, not enough fuel is being completely burned in the cylinders. This could be due to incorrect spark timing, incorrect valve clearances, poor spark, too lean or rich fuel mixture (caused by vacuum leaks or buggered carb), low cylinder compression... I'm trying to think of any others... Oh, a leak high up in the exhaust could allow fresh air (oxygen) into the hot exhaust stream and allow the unburned hydrocarbons to combust easier.
As for the loud exhaust, I don't think there's much you can do except put in a new cat or a resonator. On mine I thought for a minute about wrapping the exhaust pipe with some kind of sound insulating wrap, but that stuff is expensive, difficult to install reliably and can lead to faster corrosion from trapped moisture.

[Petros]

fuel pump would not cause that, could be partically plugged fuel filter, or possible leaky fuel line up stream of the pump (causes it to suck air into the line, causes fuel starvation).

[vcp]

Thanks, guys. Got my fingers crossed it's the filter. Next will try the plugs.

[SynthDesign]

What does it idle at when it goes put put at a standstill w/o load on the trans?