So, backdrop on the car:

1988 SE, with an NA S5 shortblock, S5 manifold. When the S5 manifold was installed, I had the injectors cleaned, and all brand new seals installed. Engine is a new rebuild, around ~2500 miles on it. Currently, the VDI & AUX ports are disabled, as I'm in the process of building a system to actuate them, off a stock smog pump. S4 TB, w/ the throttlebody mod. Rats nest is removed, etc etc etc, the only emissions left on the car really is the intake air temp sensor, the O2 sensor, & the BAC valve.

Using the diagnostic code tester, I get a code for a bad O2 sensor. It's kind of an off & on code. I notice the idle gets really bouncy the instant the ECU detects a flawed O2 sensor.

I'm getting around... 200 or so to a tank, granny driving it. About 50% of this is highway. My questions are:

1) how much "leaner" will fixing the O2 sensor make it?

2) Are their things I could do to lean it out without getting into the ECU? Such as soldering in resistors across the rats nest solenoid plugs, to "trick" the ECU into thinking everything's working right?


As of now I've got a new O2 sensor ready to go in by the end of the weekend, once I get the chance to throw it in. (Not a hard job, but I don't have a jack/jackstands where I'm staying at for the weekend). The point of this thread is to see what kind of improvements I *should* be seeing once the O2 sensors bad, and if there is anything else that can be done. I'm planning to go with the Rtek ECU here after I save up my pennys, haha. But hopefully I can get some help up until then.

Thank you, & sorry for the noob-ish question!

 

woah u get 200 miles a tank...lucky u

 

Fix the VDI & 5th/6th Ports. Most city driving/highway driving is around or below the 3500rpm range, when the ports don't need to be open. Having them open full time will hurt fuel economy.

 

Oh, by not connected, I mean they're shut at the moment.

 

Replace the O2 sensor and verify that the harness is ok. It's a 1 wire Bosch sensor. You can get it at autozone for like $30

 

i get more like 300 miles to the tank in mine... stock s4 gxl... the s5 has a bigger gas tank, so it goes further

 

Ok, just replaced the O2 sensor, and I'm not getting the code anymore. I went with a 2 wire, & ran the white, ground wire, directly to ground, to make sure it's getting a good ground.

However, I've still got a bouncing idle. When I did the S5 manifold swap, I used the air temp sensor that was in the manifold, I pulled that today. It was covered in oil/grease, the sensing end. I put the one in the manifold from my old S4 dynamic chamber, which was alot cleaner. I blasted it with brake cleaner, and installed.

Prior to doing so, and ever since I got the S5 intake on, I had a constant blue-ish smoke on idle. I am running premix, so it's got to be from the excess fuel/2stroke oil being consumed. After changing air temp sensors, that smoke has since gone away.

I guess it's time to drive her, & see what the milage does. I'm going to search and see what I can come up with as to causes for the bouncy idle. TPS is properly set.

Thanks!

 

how the hell u get that mileage?

 

A lot of my buddies have Muscle cars and they get better fuel economy than I do. and I probably get about the same as you if not worse. And I have a friend with an RE swap who gets about 250kms on a tank.

 

i get 240 with city, and 280 highway driving, but i drive at like 80 mph on the highway. if i drove slower, i could def get better mileage.

 

I think the rtek is going to go in sooner than I originally thought, haha

 

my car is not broken? no tricks, i did verify that the o2 sensor works, and the emissions stuff works, and i did a tune up, but other than that i haven't touched it.

 

Alright,, update, I think it's fixed/closer to being resolved.

So I had that bouncing idle, still, after switching out the intake air temp sensor, and the O2 sensor. The TB I had on the car, I got in a batch of free parts I got from a member on here. Well, he did the TB mod on it (removal of the butterflies closest the passenger side). I got to thinking, that there's got to be something related with that TB, as it idled fine/got better MPG w/ the old TB, which I'd taken all the extra stuff off, besides bare necessities, but just wire-tied the 2 butterflies open, so it was always reversible. Anyway, did that, re-checked the TPS with the digital multimeter.... Bouncing idle is now gone.

When I kick on a load, AC + radio + highbeam lights, I get the bouncing idle again, which goes away the INSTANT I disconnect the TPS.

Are there any causes that make a TPS "flutter" like that, besides it being dirty/sticking on the inside?

 

Did you set the TPS to 1 volt or some other method?

 

Disconnected it, w/ the car off, & probed it w/ the multimeter. I set it to 1K ohm, w/ closed throttle, & made sure at WOT/ throttle all the way off the TPS, it went up to 6k ohms.

 

If you never tried the 1 volt method I would suggest you try this and see what happens. There is no point of no return when it comes to setting the TPS so you could always go back to the previous method. The FSM spec indicates the TPS should read 1 volt at the ECU. Since the TPS is very, very, very old the ohm stuff probably doesn't work as well but I might be wrong. Anyways, to set to one volt just thoroughly warm up the car such that you drive it for twenty minutes or so then back probe the Green/Red wire at the TPS plug with the red meter lead and the black meter lead to a ground such as the alternator or negative battery terminal and take a voltage reading with "key to on" and "all accesories off." If it doesn't read 1 volt adjust the screw so that it does read 1 volt.

 

i like the factory way too, the ECU tells you the TPS is right. the ohm meter isn't bad, but it does ignore the wiring harness, and any difference in ecu's

 

Thanks, I haven't heard of this method! I'll be doing that tomorrow, once I've got some light under the hood. Thank you man!

 

LOL your gas mileage is so bad you guys whats wrong with your cars? My na gets like 350+ miles to the tank on the freeway. Its A s5 and streetported. It use to get like 410 or something like that before the streetport and changing the rear end to a T2 unit. You should check all the sensors on the engine for proper operation. 200 miles to the tank? I get better than that with drifting involved.

I drive 2 hrs to drift then drift a half a day then 2 hrs back on the same tank. There's something wrong with your cars. Well besides the guy that created the thread at least he knows there's something wrong with his car.

 

My stock n/a s4 gets 170-180 miles for 1 tank. Wtf.

 

Another update, as of 10.19.10:

Getting another O2 sensor code. there must be something else causing it to run rich, which is in turn, VERY pre-maturely killing the O2 sensor. I checked for continuity between the ECU pin & the actual O2 sensor, no problems there. So in my mind, that's got to be an O2 sensor failure, again?

What are some possible other options causing it to run this heavily rich?

Keeping in mind that:
-S5 NA swap, with RB header to presilencer to borla catback
-S5 NA intake manifold
ALL S4 everything else, injectors etc. Injectors were just cleaned and installed with brand new seals and orings.
-emissions removed except for bare essentials (to make less of a head ache having an s5 block in s4 car, plus I'm in FL, so emission testing isn't of a concern here.)
-motor is NOT ported

Are there ways to lean it out without going the rtek route? I do plan on going here eventually, but it would be neat to get some better economy now, haha.

Many thanks for the feedback!

Landon

 

did you leave the switch in the bottom of the radiator? that thing turns closed loop on and off.

 

Well mine was rated for 17 street and 23 hwy as you can see lol.... I'm getting like 18 on average though

 

Does your car still have the Variable resistor? Some will say it only affects the fuel mixture at idle but if the resistor is leaned out and you take the car for a spin at full throttle the car will behave like its starved for fuel. This tells me that it affects the fuel mixture across the whole rpm range but moreso at idle.

 

stop messing with the stock **** and go stand alone