19corvette91

Concerning my 1991 Corvette-

I recently added 1 5/8th Primary long tube headers and true dual 3" exhaust into flowmaster 40 series (no cats and no crossover pipe)

Now, once my L98 goes into closed loop, I get a very bad jerkiness/ hesitation on brisk acceleration (half to three quarter throttle). It is not present at WOT.

I drove the car around for a while with the O2 unplugged and the problem was not present. So I added a heated O2 sensor. Unfortunately, this did not solve the problem.

Long block and TPI manifold are 100% stock. Besides exhaust, only mods are 24 lb injectors, K&N Filter, EGR and Air pump delete. Never had drivability (spelling?) issues before the headers and exhaust.

Any ideas?

QCVette

To me it sounds like it is too lean. That is common when installing headers.

When it goes into closed loop it can not adjust far enough to get the right mixture. In open loop it is generally a little richer and might work better. Disconnecting the O2 forces it into open loop.

It sounds like you might need a tune. That should help both performance and mileage.

On my '87 I put on a better exhaust, air foil, and 1.6 rockers. I had quite similar results. I put in an SLP chip and it ran good again.

That is only my guess, perhaps others with more experience can chime in.

Good luck.

Kevova

Scanner?

19corvette91

Have not had a chance to get a scanner on it yet. I do have one. Hopefully before I put her to bed for winter.

I was under the assumption that headers did not usually necessitate a tune. However, I may very well need one!

Kevova

Most common problem is o2 sensor does not get hot enough to go into closed loop. Heated O2 should have corrected that. You need to see what O2 is doing INT/BLN will show that. It could be conquensidental ... Maybe a tune ....make sure wires are tight on plugs,no loose connections and headers aren't burning anything.

JimLentz

If there is any kind of exhaust leak upstream of the O2 sensor it can cause it to sense a false lean condition in closed loop and go overly rich.

aklim

Let's start with the basics. Is the timing correct at 6 degrees? Is the fuel pressure good and does it hold pressure after shut down? What is the fuel pressure like at WOT? How is the spark quality? Are the wires leaking? Are you SURE you got the firing order right? Sure, you switched from 22 to 24pph injectors but nothing jumps out at me that could be the cause of the issue. Get a scanner. Let's also see if you have any air leaks.

19corvette91

Timing is at 6 degrees. Plugs and wires are brand new. Firing order is correct.

Have not checked fuel pressure yet. Will do so and throw a spark tester on it when I get around to scanning it.

Only symptoms are jerkiness/ hesitation/ stumble at half to 3/4 throttle when in closed loop. Symptom not present when I disconnect O2 sensor. Runs fine under all other conditions with O2 connected. WOT with O2 connected is fine.

19corvette91

Appreciate the help, btw.

aklim

You don't have the problem at WOT because O2 sensor readings are not considered in those conditions. Also without the O2 sensor, it goes into "limp home" mode and reads off the program. What I am curious about is what happens when you hit the throttle. Could the pump be keeping up? Is there a leak in the diaphragm where the sudden increase in pressure shows up this way?