Hey guys,
I just put a down pipe and cat back on a friends otherwise stock 93. It is using the stock main cat.
Since the install, it has started Kinda jerking under hard accellaration. It almost feels like the boost is going away for a split second. It's not violent, it's just a light jerk. Almost like a missfire. It does it a few quick times in a row. I think it does it in all gears as well. I'm not sure if it happens at a certin RPM or not.

The car has ~106k miles with a new engine about 6k miles ago. Turbo's are in very good shape.
She said that it didn't start right after the down pipe install but kind of gradually came on.

Thanks for any advice.

Mike

 

You better tell her to stop driving it.

Since you didn't give us a boost pattern, for all we know, she COULD be hitting fuel cut. This is the main reason why Pettit sells pills with their DP. To eliminate the possibility of hitting fuel cut.

It's really the only logical explanation as to why the car jerks after installing the DP. Unless, of course, at the exact same moment, something else failed within the control system.

 

No boost pattern given since there is no boost gauge installed? If so, need to install a boost gauge for the first step in diagnostics.

 

I think this is a problem with grounds. When you did the cat back did you put back the metel ground that was on there, try to add some grounds to your car and tell us whats happening.

 

Is it possible that the extra power being made is pushing something over the edge? I have the exact same problem that only started when I finally got my secondary to boost. Before, I had 5 psi or less above 4500, and never had this problem. I'm changing plugs, wires, and fuel filter this weekend to see if that changes anything.

Good luck,

 

This would be where I would start. It's easy to check. At the rearmost portion of the main cat there should be a grounding strap, make sure it's attached to the exhaust system. If it is attached, get a boost gauge hooked up and check the boost pattern, should be 10-8-10 (hits 8 at the crossover to the secondary turbo, about 4500 rpms @WOT, very briefly). While it doesn't really sound like fuel cut, it could be possible. I never hit fuel cut on mine until I added a high flow cat (same exhaust mods too, DP,CB). It could be just a matter of putting in new spark plugs too. Good luck.

Frank

 

Thanks for the replys.

She has a boost guage and says it's boosting about 11-8-11 .

I did put the ground strap back on the main cat.

When doe's fuel cut come in? I'm not familiar with 3rd gen fuel cut but I know that in a T2 it's like hitting a wall. The car just cuts out and won't go any faster.
This is nothing like that. It just jerks a little and keeps accellarating.

I also installed new plugs when I put the exhaust on.

I'm going to see her today. I will try to drive the car and get more info.

Thanks,
Mike

 

Here's the fuel cut chart:
1000rpm-11.8
1500rpm-11.8
2000rpm-11.8
2500rpm-11.8
3000rpm-12.2
3500rpm-12.4
4000rpm-12.8
4500rpm-13.3
5000rpm-12.2
5500rpm-11.3
6000rpm-11.1
6500rpm-10.7
7000rpm 10.7

So I guess it's ok to hit 11psi till you really get up there. so as long as you are not spiking during transition, you should be ok.

I agree with some of the other guys. Ground is the next thing to check. With the stock ecu, though, the 3000rpm hesitation, which feels like a bucking/jerking motion will most likely always be there. You can reduce it, but it will usually be there to some degree.

A good example is my ECU. I had the 3000rpm hesitation, but when I dumped the PFC in, it went bye bye. I forgot the exact reason to this, but I remember reading about it somewhere that the stock ecu actually has a problem internally with the grounds, and modifications usually makes the problem more noticable.

 

Thanks Flybye,
Thats very helpful info.

Mike