Low idle after new exhaust (2 Viewers)

[93jdm]

Hi all

1993 JDM with 1fz

I just built from scratch new exhaust from head back. New high flow catalytic converter installed.

I only put back the front O2 sensors. The O2 bungs that I built does keep the sensor a little higher up then before, so they are not in direct contact with flow. But looking at the original, they were already a little high to begin with. But, I was thinking the rear were only for emissions purposes. So I did not put them back...

But now I have low, maybe a bit rough, idle. Cold start does not rev up like it did. Just sits at 5-600ish rpm.

Runs fine when I get going, feels normal driving. Just, low rpm idle. Is there a "learning" process for the ECU? I am not sure if the ECU is that complicated...

Are the rear O2 absolutely required? Was I wrong to assume they were emissions only?

 

[93jdm]

Anybody?

 

[aclos3]

I am not familiar with OBD1 trucks, but there is definitely a bit of learning time on OBDII rigs when the computer is reset. If you drive it for a few days and don't have any improvement, you know something is probably wrong with that O2 sensor or its mounting depth.

You could try unplugging it completely and see if you notice any improvement. When I had problems with mine, we were getting bad readings from the O2 sensors --voltage is supposed to be between 0 and 1v or something like that. Unplugging the O2 sensor actually improved things quite a bit since I think the computer will revert back to some default fuel map. I didn't drive much without the O2 sensor and just waited until a new one arrived.

 

[dallen341]

If memory serves...US spec 93 has one O2 sensor for Cyl 1-3 and a second O2 for Cyl 4-6.

Assuming JDM = US Spec with respect to emissions controls and O2 sensors, sounds like feedback to ECM is cyls 1-3 only.

Having said that, your description of sensors sounds like front sensors (plural) and rear sensors (plural) which would lead m e to believe you have cyl 1-3 sensor in plave as well as 4-6 sensor in place. If I recall, rear or "post-cat" O2 sensors do not control air/fuel but merely report condition of the cat.

Can you clarify the O2 sensors in place?

 

[ppc]

If memory serves...US spec 93 has one O2 sensor for Cyl 1-3 and a second O2 for Cyl 4-6.

Assuming JDM = US Spec with respect to emissions controls and O2 sensors, sounds like feedback to ECM is cyls 1-3 only.

Having said that, your description of sensors sounds like front sensors (plural) and rear sensors (plural) which would lead m e to believe you have cyl 1-3 sensor in plave as well as 4-6 sensor in place. If I recall, rear or "post-cat" O2 sensors do not control air/fuel but merely report condition of the cat.

Can you clarify the O2 sensors in place?

I think your wrong on your assumptions. The US ODB1 (93-94) systems use one sensor as you say cyl 1-3 and cyl 4-6. Both provide feedback to the ECM. The ODB2 (95-97) uses the front and back design where the front monitors the exhaust from all cylinders for ECM fueling control and the rear sensors confirms proper operation of the catalytic converter.

The JDM market has a totally different ODB1 implementation using a total of four O2 sensors. Uncertain which years that applies to.

 

[93jdm]

If memory serves...US spec 93 has one O2 sensor for Cyl 1-3 and a second O2 for Cyl 4-6.

Assuming JDM = US Spec with respect to emissions controls and O2 sensors, sounds like feedback to ECM is cyls 1-3 only.

Having said that, your description of sensors sounds like front sensors (plural) and rear sensors (plural) which would lead m e to believe you have cyl 1-3 sensor in plave as well as 4-6 sensor in place. If I recall, rear or "post-cat" O2 sensors do not control air/fuel but merely report condition of the cat.

Can you clarify the O2 sensors in place?

I did not take pics but, there are two front O2 sensors, one for each of the downpipes. And same for rear, there are two post-cat.

 

[dallen341]

Makes sense now. JDM nothing like US spec OBD1

 

[93jdm]

The only difference really is that, I did not install rear O2s.
Took out the 30 year old cat and replaced with a new high flow unit.

 

[dallen341]

My only other comment is check that bank 1 O2 is in correct pipe and not actually monitoring bank 2. Doing so would cause fueling issues with improper feedback to ecm

 

[93jdm]

Installed exactly where they came from. Same bank, same plugs.

 

[dallen341]

I got nothing

 

[Kernal]

A good scan tool that can show real time O2 sensor data should tell you what they're doing?? Maybe check the simple things not related ot the recent work if you haven't already ie: a loose/cracked/not connected breather/PCV/Vacuum/air intake hose??