'91 VR4 Front Bank Running Lean, Need Help Determining Why!

[Jstarr427]

Hey guys.

Recently I put an engine in my 91 VR4. All went well, Has about 1700 miles on it no issues. In the past couple weeks I've been having issues with a check engine light coming on at steady throttle cruise. Code is #39 I beleive. It's for Oxygen sensor, front bank. Even though I had replaced both sensors when the engine was out with brand new ones, I called and got a warranty on the sensor. put the new one in, a week later same issue.

I've checked the following:
-All pinouts according to service manual, wiring checks good back to the ECU.
-Voltage coming out of 02 sensor is low.. like .2v. It will respond and kick up when I rev the engine, but at idle it settles back down.
-Pulled front plugs, all are white indicating a lean condition
-Fuel pressure is 36psi at hot idle with vacuum line connected.
-If it matters, I did the vac reduction, so I have zero solenoids on the firewall. -No EGR, but did keep evap as per the mspec diagram.
-Voltage at fuel pump is 11.0v at idle, decreases slightly, maybe .1v when racing the engine. The pump is not hotwired but I did bypass the relay as per stealth316

Other mods include Open filter, evo MR BOV (recirculated), 3sx preturbo intake pipes, MBC @14psi, gutted precats.

Any thoughts? only the front bank is doing it, 02 voltages for the rear bank look perfectly normal.

Thanks!


>>>>> The attached photo is of the log graph for the front 02. the top graph in orange is RPM. the bottom most horizontal line represents 1800 RPM. Bottom is voltage (obviously). I find the behavior of this sensor really strange, and don't see why it's running lean on just the one bank, given that It ran for 5000 miles before and about 1400 after the engine with no issues.

[vr4tune]

http://www.3sgto.org/f13/running-ric...ease-8574.html

[Jstarr427]

Honestly I had never considered this as an option. the manifold WAS cracked but I had it professionally welded. I also know the car is actually running lean on this bank due to spark plug condition. If I had an exhaust leak causing a lean reading, wouldn't the ecu add fuel, creating an actual rich condition?

[vr4tune]

Honestly I had never considered this as an option. the manifold WAS cracked but I had it professionally welded. I also know the car is actually running lean on this bank due to spark plug condition. If I had an exhaust leak causing a lean reading, wouldn't the ecu add fuel, creating an actual rich condition?

Yes it would! The bank would in reality would run rich but the o2 sensor would think its lean with a cracked header. Did you reset your ecu after welding the crack up? Because the long term fuel trim could take some time to self adjust. Also you may have a bad o2 sensor too. If a car runs rich too long a period then it can damage the o2 sensor as well as the cat.

[CoopKill]

Honestly I had never considered this as an option. the manifold WAS cracked but I had it professionally welded. I also know the car is actually running lean on this bank due to spark plug condition. If I had an exhaust leak causing a lean reading, wouldn't the ecu add fuel, creating an actual rich condition?

You are correct. Are all of the plugs showing lean? If not it may be an injector issue.

[vr4tune]

Hey guys.

Recently I put an engine in my 91 VR4. All went well, Has about 1700 miles on it no issues. In the past couple weeks I've been having issues with a check engine light coming on at steady throttle cruise. Code is #39 I beleive. It's for Oxygen sensor, front bank. Even though I had replaced both sensors when the engine was out with brand new ones, I called and got a warranty on the sensor. put the new one in, a week later same issue.

I've checked the following:
-All pinouts according to service manual, wiring checks good back to the ECU.
-Voltage coming out of 02 sensor is low.. like .2v. It will respond and kick up when I rev the engine, but at idle it settles back down.
-Pulled front plugs, all are white indicating a lean condition
-Fuel pressure is 36psi at hot idle with vacuum line connected.
-If it matters, I did the vac reduction, so I have zero solenoids on the firewall. -No EGR, but did keep evap as per the mspec diagram.
-Voltage at fuel pump is 11.0v at idle, decreases slightly, maybe .1v when racing the engine. The pump is not hotwired but I did bypass the relay as per stealth316

Other mods include Open filter, evo MR BOV (recirculated), 3sx preturbo intake pipes, MBC @14psi, gutted precats.

Any thoughts? only the front bank is doing it, 02 voltages for the rear bank look perfectly normal.

Thanks!


>>>>> The attached photo is of the log graph for the front 02. the top graph in orange is RPM. the bottom most horizontal line represents 1800 RPM. Bottom is voltage (obviously). I find the behavior of this sensor really strange, and don't see why it's running lean on just the one bank, given that It ran for 5000 miles before and about 1400 after the engine with no issues.

You are correct. Are all of the plugs showing lean? If not it may be an injector issue.

I thought the same thing but all are lean. I think it's ecu related at this point.

[Jstarr427]

The manifold was welded while the engine was out, so things have definitely reset computer wise since then.

I have pulled all 3 front plugs, and the condition of the plugs (white) does back up what the 02 sensor is telling me; which is the bank is running lean. I had put 2 new Denso 02's in when the engine went in the car, and then got a warranty replacement on the front, thinking it was the 02 sensor... turns out I guess it's not.

That's what I'm leaning towards at this point. The ECU was previously rebuilt at some point in 2005 according to the sticker, but I'm thinking now that somehow the injector drivers for the front bank are damaged, causing this issue.

on a side note I will be around here much more often... You guys have been more helpful than others on just the 'other' site.. I've barely gotten any answers there.

[vr4tune]

All the veterans and smart people moved over here. 3si has a lot more green members.

[Jstarr427]

All the veterans and smart people moved over here. 3si has a lot more green members.

I definitely see that now. I joined when the split first happened but wasn't around much because most of the content was still on 3si. I see that's changed now.

Anyway, I've made arrangements to try another ECU this coming week, so hopefully that will solve my issue! *crosses fingers*

[Keyan]

check the voltage going TO the sensor, if thats low, the output will be low.