TBI tune after installing headers and dual exhaust

[Bryce Dean]

Both of my exhaust manifolds were cracked so I replaced them with a set of Headman long tube headers with 2.5" dual exhaust. I did not install cats but I did put mufflers on as I can't stand straight pipes. This is on an 89 4wd v1500 Suburban. The headers didn't have a bung for the o2 sensor, so it currently rolls around in my console.

Prior to changing the exhaust, I averaged 15-19 mpg on the highway.....now I'm only getting about 11. Every thing other than the exhaust is the same.....do I need to re-tune or adjust my timing? Other than a slight increase in power there seems to be no change in how it runs.

 

[Crispy]

Have an exhaust shop weld in the bungs for the o2 sensors unless you can weld it in yourself. Pre Cat o2s are important to an EFI system.

 

[Bryce Dean]

It had single exhaust before,so only 1 sensor. I didn't put cats back on either since I don't have emissions where I live. Do I need a sensor for both sides and cats?

 

[RoryH19]

I surprised it runs very well. The O2s are needed to air fuel mixture on fuel injected cars.

 

[RoryH19]

It had single exhaust before,so only 1 sensor. I didn't put cats back on either since I don't have emissions where I live. Do I need a sensor for both sides and cats?

TBI systems are not as advanced as new cars that have pre and post O2 sensors.
I'd just go with 1.
Cats are not needed but I run them. I try to be eco friendly and cut down on the exhaust smell.

 

[Bryce Dean]

Like I said, the only difference is in my fuel mileage

 

[skysurfer]

Like I said, the only difference is in my fuel mileage

Well, the new fuel economy bothers you enough that you started this thread so go with the above recommendations.

 

[Catbox]

Put the O2 sensor back into the system and the computer will probably reward you with better MPG.

 

[Bryce Dean]

Thanks for the advice, I'll install it as soon as I can.

 

[bucket]

It's been stated several times already, but here it is again to emphasize, lol. The O2 sensor is one of the most important sensors that the ECM uses to control the engine. Put it back in and your mpg will return.

When the new bung gets welded in, it needs to be oriented so that condensation in the pipe won't sit in the tip of the sensor.

 

[Yapay]

Both of my exhaust manifolds were cracked so I replaced them with a set of Headman long tube headers with 2.5" dual exhaust. I did not install cats but I did put mufflers on as I can't stand straight pipes. This is on an 89 4wd v1500 Suburban. The headers didn't have a bung for the o2 sensor, so it currently rolls around in my console.

Prior to changing the exhaust, I averaged 15-19 mpg on the highway.....now I'm only getting about 11. Every thing other than the exhaust is the same.....do I need to re-tune or adjust my timing? Other than a slight increase in power there seems to be no change in how it runs.

Did you do anything to the throttle body? you might need to adjust your idle. I agree that putting the o2 sensor back in should help as well.

 

[JoeR Jr]

It had single exhaust before,so only 1 sensor. I didn't put cats back on either since I don't have emissions where I live. Do I need a sensor for both sides and cats?

Ideally you could put your O2 in back of a crossover pipe, but only if it was close to the engine. A good compromise if to put it in either header, but definitely put it back in.
No, you don't need the Cat for it to work.

 

[F-64]

Your truck is running in open loop all of the time now. It has no idea if it is running truly rich or lean. It's guessing at the mixture using whatever sensors it has left and it isn't going to guess lean. It's going to run rich to protect the engine. You may want to check your plugs for possible fouling as well.

A note, try to get the O2 bung close to the same distance away from the engine as the old bung. Your sensor does not have a heater built in, it's only a single wire sensor, and is required to reach a certain temperature before it starts to work. If it's too far away, it may not come up to temp and work effectively.
Also listen to Bucket concerning clocking the bung so condensation doesn't pool on the sensor. That will destroy the sensor.

 

[75gmck25]

Everything that Broussard said is very important.
- An EFI system cannot function correctly without an O2 sensor because the computer needs the feedback to know how to keep the mixture correct. Gas mileage will really suck in open loop mode.
- O2 bung needs to be oriented so that moisture drips off and does not collect on the O2 sensor. Straight up is best, but rotated 30-40 degrees down gives more clearance to screw in the sensor.
- O2 sensors must be hot to function correctly. The stock TBI sensor is apparently a one-wire sensor that depends on engine heat to operate, so it must be close to or in the header collector. I'm not sure, but you may be able to replace it with a heated sensor (very easy to find), which would not depend on engine heat.

When I added an O2 sensor I bought about an 18" section of muffler pipe, and then had a shop weld in the bung and expand the ends of the pipe to fit over my exhaust pipes. Then I cut out a section of exaust and inserted the new pipe to replace it. That allowed me to pick where to install it, and get it oriented properly. I was using a Bosch four-wire sensor, so exhaust heat for the sensor was not critical.

Bruce

 

[mikedgarage]

Everything above is correct. Just know that with headers there is no way your stock O2 sensor will ever reach the proper temperature, it needs the mass of the cast iron. The thin tubes can't hold the heat. https://harristuning.com/Tbi/ has a lot of great information about changing to a heated O2 sensor and other TBI specific information.