Brand New D15Z1 - Lean Burn Stutter / Hesitation

[94CivicVX]

Hi all, I am new to ECOmodder, but have been dealing with honda swaps for 10+ years. I have a 94 civic VX with a DX tranny. Sorry for the wordiness but its necessary

I am having the lean burn stutter/hesitation issue with. Once at operating temperature and cruising at light throttle the car struggles and hesitates, as soon as I give it gas it runs perfectly. It also runs perfectly when the brand new Honda O2 sensor is unplugged.

I have searched and have done a ton of troubleshooting - many people post this issue but I have spent hours combing through threads and I haven't seen a fix??!!! Most threads just end with the person having tried EVERYTHING with no solution posted.

Engine:

Brand new OEM D15z1 that had never been started. (Got it from a Technical College, it was donated from Honda in 1992. It sat on a engine stand it's entire life) It was completely brand new when I installed it and had never ran. It was only missing a distributor. (Of course I replaced front and rear main seals and head gasket and checked the valves/springs cylinder walls and everything just in case- It was perfect.)

My car was a 94 DX but I did an entire cabin harness swap from a VX. The D15z1 also had a brand new engine harness on it. So my swap was plug and play. The car started instantly after swapping the engine in and everything was great with no engine lights. But once at operating temp I get the hesitation while cruising as it goes in and out of lean burn. I will stress that it runs FLAWLESSLY with the 02 unplugged.

Things I have done/tried:

-Checked all wiring, no shorts/open wires
-Set proper base timing with timing light
-Brand new distributor, wires, NGK ZFR4F-11 plugs, fuel filter
-Changed the brand new injectors out for some other VX injectors (did nothing)
-Changed out the brand new O2 sensor out for an older oem O2 sensor (did nothing)
-Checked Voltage/Resistance on TPS, IAT and MAP sensor - all were good. Also changed those 3 sensors with others just in case (did nothing)
-Checked function of EGR, the ports were spotless as the intake mani and EGR valve are brand new.

One thing to note : I am using a VX exhaust manifold but I fabricated a catalytic converter bypass pipe and am using a DX downpipe. The DX cat is still in the OEM location under the car as well. I am concerned that perhaps it needs some backpressure at the O2 sensor to read correctly or something??!!

I'm totally stumped. Any insight at all would be greatly appreciated

[94CivicVX]

Anyone have any ideas? Would a PCV issue cause cruising hesitation in lean burn?? Or would a PCV problem just mainly affect the car at idle??

I have no clue where to go from here

[ksa8907]

problems start at operating temp? check your coolant temp sensor, does it get too hot? maybe a bad, incorrectly installed t-stat?

as far as i know, the vx uses a wideband sensor.
is the new o2 location farther from the head than stock?

[nbleak21]

Originally Posted by 94CivicVX

One thing to note : I fabricated a catalytic converter bypass pipe and am using a DX downpipe. I am concerned that perhaps it needs some backpressure at the O2 sensor to read correctly or something??!!

I'm totally stumped. Any insight at all would be greatly appreciated

I'm not 100% tracking what you are saying here, If it's that you are running a DX exhaust manifold to a cat-delete, then I'd say this has something to do with it!

[Ryland]

Non stock VX exhaust will screw up the o2 sensor because it tends to be over sized, it's not back pressure from small exhaust, it's the smooth exhaust pulses that a correctly tuned exhaust creates.

[94CivicVX]

Thanks for the replies, my thermostat is brand new OEM, but maybe I should check the Coolant Temp Sensor??

I suspected too large of an exhaust may be an issue, but hadn't seen any info about it. I did find two people in a post who claimed that it didn't matter what happened after the first O2 sensor, but I was skeptical.

Just to clarify: I'm running a VX exhaust manifold that connects to a short custom pipe that then runs into a DX down pipe and then through the OEM DX catalytic converter that is under the car and then out through the muffler.

Basically with the exception of the exhaust manifold and custom little pipe, the exhaust system is OEM DX.

I will work on installing the VX down pipe, in the mean time I'm open to any other suggestions

[Ryland]

Originally Posted by 94CivicVX

Basically with the exception of the exhaust manifold and custom little pipe, the exhaust system is OEM DX.

The VX exhaust is around 1/3 smaller then the DX, because the DX engine tends to run that much faster with the torque at higher RPM as well.
On my first VX I ruined an o2 sensor in less then a year by having a slightly over sized exhaust and it's something that I noticed on my current VX along with my brothers VX is that a large or open exhaust seems to lead to lean burn not working right and a shorter o2 sensor life.

[94CivicVX]

Thanks for the info, I'm going to order an OEM VX mid pipe - I already have the down pipe. I swear I've tried everything else. Now it's just a matter of finding a CAT somehow...sheeshh they're expensive.

When I get the correct exhaust on I'll let everyone know if the stutter went away. Stay tuned...

[Rusty94cx]

My cat has been hollowed out and my 02 is dead. But I have questioned the need for it on the hondatec board a couple of guys have vxs with no cats. So... Possibly they don't notice the hesitation or...
http://honda-tech.com/showthread.php...t=cat+hollowed

[94CivicVX]

After checking out that link in the above post, I'm not too sure that those who are posting are very familiar with the VX exhaust system. Who knows though, when you get an O2 sensor thats good let us know how lean burn works with your hollowed out cat...... my situation is still quite different than a hollowed out cat, I have custom little 3" wide straight pipe into a larger than OEM exhaust.

On top of my lean burn hesitation issue I noticed today that my shift light no longer comes on, but the bulb is still good super annoying, not sure why that would happen. No CEL on.

According to TomO (a moderator on ECO modders), "The upshift indicator (as Honda calls it) reads the MAP sensor, TPS sensor, and rpm signal to determine the best upshift timing."

I wonder if it's possible that my TPS has a bit of a light throttle dead spot, I tried a different MAP sensor and nothing changed.