My front "Y" is now 2.5" w/o cats. The main cat is now a hi-flow w/air tube. Now I'm getting a lot of popping (explosions) in the exhaust during warm-up. But, after a few minutes, it stops.

I remember reading that the air pump only runs for about 5 min and responses to another post suggest that oxygen from the air pump is lighting off unburnt fumes in the main cat. I would like to test that theory.

After looking around the air pump, I couldn't find any connections that would control on/off operation. Did I miss the connector or does it really shut off with closed/open loop?

I'd really like to temporarily disable the pump w/o cutting the air tube in order to see if the problem goes away. How do I do that? (A search did not provide the answer).

Thanks!
gp

 

'89 pumps air to the headers on startup - switches to the CAT after warm up and keeps pumping it there unless something screwsup - At that point it all goes overboard, just to the right and in front of the waterpump. Flow is controlled by a switching valve driven (grounded) by the ECM. Power is from the Gages Fuse. There are no trouble codes, so to disable it, you can disconnect the harnesses at the valve (between the a/c compressor and water pump). That will have leave it shooting all of it's air through the overflow and though there is a muffler on the pipe, you'll problably still get a kinda of uncool thumpa-thumpa noise at the front of the engine. The real solution is to remove it all together. If you do, might want to switch to a heated O2, as that extra air to the headers gets it up to operating temp and into closed loop. You might also, depending on where you live, fail emissions, so you might want to keep it handy.

 

the pump doesn't physically go off... the air flow only shuts down.

remove the pump, block off the exhaust ports, and be done with it.

 

Only 2 posts and I have a contradiction. I don't know whether the pump switches from header to cat, cat to header, or off (after hitting closed loop). If it pumps into the exhaust manifold, can't see why that wouldn't continue after warm-up. O.K. it heats up the CAT, but isn't it just adding air to the manifold to complete the burn?

I've also read that the pump creates zero drag, so why go to the trouble of removing it? The bypass bracket is kinda expensive and I was thinking it makes the belt/alternator setup weaker.

 

Sure it doesn't go from the CAT to the headers? I thought this puppy helped to warm the CAT, so why blow into it after warmup?

Are you saying there's 3 possible outlets to the pump (header, cat, and overflow)? And, if I can figure out where the harness hits the valve, disconnecting it will cause all the air to overflow out its own little exhaust with mini muffler? And, no air will enter the engine/exhaust system.

Edit:
Just got back from the garage. Found the black diverter box (next to the A/C) that switches air between the manifolds and the CAT. Since the wires don't unplug easily, I disconnected the input. By experimenting with disconnecting/reconnecting the input/output air hoses on the divertor, I should be able to indentify if the air pump is causing the popping during warmup.

 

Air - even today where an electric motor is used in place of an accessory driven pump - is added to the headers to warm up the O2 and the exhaust stream which helps light off the CAT. Can't keep it there forever - it's not measured by the air metering device (MAF) and the O2 will interpret that extra air as a lean condition fattening up the pulse width to the point that the engine runs super rich (and rather poorly) - SO it switches the flow to the CAT prior to Closed Loop. Modern CATS don't need a continous stream of air to do their thing, so anything after the LT motors shut it off altogether. Ours are a tad old tech, so the air is redirected to the CAT to complete oxidation. Overboard is simply a place to send the air if something is amiss; ie, a trouble code. If you're having problems, the first thing to do is find out where the air is going once it warmed up - remove the hose and see what, if anything is blowing through it. Or, if you think air is still getting into the headers after warmup, clamp off the hoses. Also, be sure to make sure the header check valves are working. Air should only flow in the direction of the headers. If one is broken, it's going to suck in additional air regardless of what the pump is doing.

 

SunCr, your advice was correct.

I ended up disconnecting the intake hose to the air router. I also disconnected the output to the CAT. I connected these directly so that air is only pumped to the CAT.

This stopped the nasty backfiring/popping during warm-up. Now this question is what to do long-term. I assume the problem is not an indication of something serious since it only started after removing the front cats. Furthermore, I am still trying to decide if I want to put bullet cats back in the front.

The exhaust shop chopped up my original front "Y" even though I asked for it to be saved. (Thought I might need it for resale and/or if inspections required reinstallation. Currently inspections aren't an issue). Without the old "Y", I don't have the option of seeing how much it quiets the new system. (FYI, EVERYTHING is new from the manifolds back and I suspect the sound is similar to muffler elims WITH both sets of original cats). Grrrrrrrrr!

What issues would exist if I left the air pump connected directly to the CAT? If I did this, would I want to remove the routing valve and/or plug the manifold inputs? Also, why does air injection into the manifold w/o the front cats cause such severe popping?

Gregg

 

the air injection has a tendency to burn exhaust valves...

so don't fret it.

the air pump has a valve array on the front of the motor near the AC compressor.

 

Never heard of an air pump burning exhaust valves - technology has been around since the '70's. Mix, during open loop operation isn't effected by the air. If it malfunctions during closed loop, it'll will go rich and generally it'll run so poorly (or generate a 44), that you'll want to fix it.

As posted earlier, no air to the headers delays closed loop operation and it may not get there at all, so without it, a heated O2 helps (even though current, heated systems still rely on an air pump).

As long as the header check valves are in good shape, they will seal off the headers - though at least to me, without the hoses, the look isn't the best. I would remove them to test - you should only be able to blow air through them in the direction of the header. Pretty easy to tell though, once you have it off, if it's broken.

Possibility exists that you don't have a pump malfunction. Was everything up to snuff before you changed the exhaust? Have you scanned it to verify the Coolant Temp Sensor reading?

 

How does this delay it? How can I tell if it's been extended significantly?

I don't think I have a pump malfunction. The problem did not exist before the exhaust mods. I have not scanned for temp, but the interior guages read normal.

-- and --

the question remains... Why does the popping occur when air is injected w/o cats? FYI: I am NOT the only person reporting this as a symptom after removing front cats.

gp

 

Unless there's a hole or leak, popping/backfiring is unburned fuel. Overly rich mixtures destroy the CAT - why did you replace it? You might want to look at Block Learn and see if it's rich/lean/normal during closed loop operation.

Dash readout isn't connected to the Coolant Temp Sensor which the ECM uses like a choke - The colder the temp reported, the more pulse width or fuel added by the ECM. If it's skewed, fuel delivery will be wrong, particularly during open loop - too rich or too lean. Might also be a faulty ECM. Quick test is to substitute another CTS (you don't have to install it - just plug it into the harness). If there's any difference on a cold engine, you probably need to replace it. Otherwise, a scan will tell you a lot.

 

I put side pipes on the car. Though they're designed to bolt directly to the stock CAT flange, my (slightly) used system was for an LT1 -- so the holes were mislocated. Given the choice between buying the correct flange @ $100 or putting a new, hiflo mid-cat in place of the 17yr-old, 52k mile factory original, I chose the later. (The LT1 version comes with a matching flanges and 3" connector so a slip-fit could easily be done. And, I could have cut the factory flange off the main convertor, but who wants to weld new stuff to old crap).

Since the system was apart, since no one speaks fondly of the stock converters, and since adding a new front "Y" would complete the exhaust tranformation, I decided to go for it. This was also considerably cheaper than headers which I'd also recently considered. With a new high-flow mid-cat (& no pre-cats), I hoped the car would breathe better, be just as clean (or cleaner) than a 17 yr-old system, and sound a little more aggressive.

There was nothing wrong before the swap. Since others have complained of popping after loosing the front cats, I'm not convinced I have an abnormality/malfunction. I do conceed that the factory may have set the mixture overly rich during warm-up, but why doesn't it pop with front cats mounted? That's just weird!

Nathan Plemons suggested an exhaust leak to another poster asking this question awhile back -- but there is no leak AND the symptom is only present when air is injected into the (factory) headers.

gp

 

For me - without knowing how the engine is supplying fuel, it's just a guess. I've never heard of the so called popping just because the front CATS have been removed. From what you've described, you've got a rich condition that was being masked by those precats. The stock fuel tables are good and if it were mine, I'd hook up a scanner to verify that everything was normal. Otherwise, there's a lot of folklore out there.

 

Because the exhaust pipes are light brown (on the inside) and because the mileage is normal, I wouldn't conclude there's a rich condition -- though it was my first guess as well. Certainly, if something is popping in the exhaust IT WOULD HAVE TO BE FUEL, right? Still, I'm trying to keep an open mind...

To a backyard mechanic (like me), it seems possible that valve springs and/or their closing rate could be affected by a (significant) change in back pressure. And/or maybe a leaky injector could create the fuel source necessary to pop without an overall rich condition?

My injectors ARE 17 yrs old. What usually goes bad with them? And, could valves be affected by the decrease in pressure? What's the check for these conditions?

Thanks
gp

BTW: If you think popping after removing CATS is folklore, look here.

Also, I don't have shop in K.C. I currently trust. Superior Chevy charges out the wazzoo and they add more on to that (compared to other shops). Also, they refused to paint my car -- because it was "too much work to paint a whole car". Finally, they took advantage of an older friend on a car trade. (They paid $3k LESS than wholesale on a 2yr old, extremely low mileage car and pressured her into believing it was a good deal).

Morse Chevy estimated a clutch slave cylinder change at $750+ because the transmission needed to be pulled. When I INSISTED that transmission doesn't require removal for this procedure and asked them to check the service manual, they said no.

McCarthy wouldn't bleed my clutch cylinder because they said it was too difficult. Also, their wheel alignment (after a repair) caused premature tire wear. (Subsequently, another tire shop corrected the camber alignment and it's been fine ever since).

There are only two Chevy shops left in K.C. and they are farther way. So "just getting it scanned" is a pain!

 

Glad I don't live there first you can ohm the injectors, you can also connect a noid light to see if their all pulsing, and you can check them for leaks using a fuel pressure guage. I don't think a small increase in back pressure is your problem-just my 2 cents.

Btw, Morley is helping me with a scanner setup, the better scanners are $300 plus, but you can buy a used laptop on Ebay for under $100,
get the Craig Moates scanning program FREE, and the power cord for about $5.-you can scan while you drive and have the benefit of a large screen.

 

No - I don't think backfiring is folklore - A blanket statement that the CAT is restrictive or poorly designed and some of the fixes or possible causes suggested are nothing more than wild guesses. If you want to findout what's going on, you need to see why the system is doing what it's doing. Scanners are relatively cheap. Trying to improve performance without a good base is a waste of money (as is replacing good parts because someone thinks that's what's wrong). Believe me, I've been there (particularly when scanners weren't so cheap). I also subscribe to the theory that the last thing I did is the reason the next thing is broken. Shop Manuals aren't that bad of an investment. Troubleshooting hints for a rich condition are found under troublecode 45 which is what you use if a scan shows a Block Learn of less than about 122. They're fairly inclusive: Fuel Pressure; Rich or Leaking Injector (Balance and Ohm test); Fuel in the Oil (change it); HEI Shielding (hookup an analog tach to the distributor and check it with the Dash Readout); Canister Purge (clamp off the line or plug the TB inlet with a vacuum cap); MAF - (difficult to pickup even with a scan - rule out other causes first); Fuel in the regulator hose; Intermittent TPS (use an analog voltimeter to make sure that the voltage is linear - a DVM will simply average it and you'll never see it), If the scan shows lean (Block Learn greater than about 132), use the 44 chart. An O2 that's being tricked into sending a signal that it's lean will make it rich. That's air or any contaminent not measured by the MAF getting pass the O2 sensor. Faulty air pump parts, exhaust, vacuum, plenum and intake leaks, water or coolant in the combustion chamber are all potential causes. Either condition can also be caused by worn valve train or engine components. Check compression (hot and cold), vacuum. Do know that most OEM emissions parts were pretty good - that's how the big boys avoided those nasty recalls.

 

Thanks for the tips. Have to say I'm a little overwhelmed at the possibilities. I did replace the O2 sensor and the canister may be suspect (long story). I'll check it next. Hopefully, I can find where it hooks to the TB.

 

I already have a Compaq laptop 1.8Ghz. How do I get the software, the cord, and figure out how to plug it in? This sounds cool!!!!!!

Edit:
I found out that an '89 vette has the earlier style code scanner hookup. OCD1 or some acronym like that. Can you hook a PC up to this primitive computer interface?

 

hook a PC up to this primitive computer interface?
OBD1 standard before OBDll
The right cable and software lets you see what the ECM is doing.

 

Hey Greg, take a look in the scan and tune section, seek out Morley, he's a neat guy and very helpful. The Craig Moates program can be used on either OBD1 or OBD2 I believe. I haven't got the cable connection for attaching the computer to the vette yet, actually I'm getting much needed help, cause I'm very illiterate in some areas
Kinda like a kid here, waiting to try it. You may just want to post in the scan and tune section, something like " anyone help me setup a scan program on my laptop" you'll get responses, bunch of nice guys.