Ok so heres my dilemna. I am having a problem that seems to be common yet noone has fixed it or gotten a response on how to fix it. I start my car up, cold it runs fine, crisp throttle response, shifts and all. FP reads 43 at idle with vac on. Now when it goes into closed loop (when O2s start to read) is when my problem starts. If i step on the gas it sputters trying to kickdown and has a hesitation. I checked FP with the car idling and when it does the sputter the pressure drops. It also drops when you rev it up. It has a brand new fuel pump on it. My injectors seem fine no leaks. Coil and ICM are perfect (tested). Anyone able to diagnose this and give me some pointers?

A little info on the car. 94 formula has impy head gaskets, 503 cam, 1.6rrs, mid lengths, hooker catback, intake (cone filter on end of stock bellow), NGK tr55 plugs, new wires.

Now one thing I have been thinking is the KM. it still has the stock tune and stock KM. if I upgrade to the LT4 KM will it maybe help this? I am also thinking opti. But I am not getting an SES light at all for the opti. Would it be beneficial just to start with a new cap and rotor?

Please help guys. I have missed every chance to take it to the track because of this. An i didn't put time and money in to get nowhere with it.

 

I had the same problem in my old 94 T/A, turned out it was the opti.

 

how far is your fuel pressure dropping to?

have you check the fuel filter?

and it could still be the opti, even though you aren't getting a code. It has actually been in my experience, a bad opti will NOT throw a code.

 

The pressure drops to ~35 psi. I have not changed the filter but will tomorrow. It may be partial cure to the fuel pressure drop but it isn't my problem of the car acting up once in closed loop.

 

get that taken care of. When your o2's start reading, your BLM's are most likely getting WAY off due to running a cam without a tune. madz28.com or pcmforless.com will get you taken care of. Also, get the lt4 KM, that will help with valvetrain noise.

 

I have a guy that is going to tune it but he is an hour away and I can't even get it that far. I will see if it will make it to him and get it tuned.

 

otherwise find somebody with a trailor to trailor it there and give them gas $$ and a little bit for their time

 

I am taking it to him this weekend since he sold me a 2400 stall for the car. He is going to do an initial log and then a basic tune for my needs and then we are going to rent the dyno at a local shop and tune it on the dyno.

 

Have you tried changing the O2 sensors... Mine did those sorts of things years ago and after changing the opti and other things it turned out to just be the sensors. Might be a cheaper and quicker fix, especially since it starts when closed loop kicks in.

 

a miner leak in the head gasket will cause same symptoms.mine did that after head gasket change once i did a retorque inever had that problem again.

 

I didn't see the tune part...

Get a tune.

 

I do need to retorque the heads thanks for reminding me. I am going to put new O2s in this week. I tried unplugging the O2s to see if it would do the same thing and it did. According to some posts I read that should mean the O2s are good. I am still replacing them though. I am also going to drop in an LT4 KM. hopefully a tune will help it out. The initial log should tell us what all is going on.

 

So, you unplugged the O2's and it still started acting bad after it warmed up? Then, that pretty much rules out the tune. By unplugging the O2's, you're locking the car in open loop. If it's running good cold (open loop) and then starts running badly (still in open loop), then it's probably not the tune. I'm pretty sure a fuel pressure of ~35 psi would cause sputtering at WOT. I'd start with that. When you put the pump in, did you make sure you got the supply tightly secured to the pump? I had issues with mine until I hose clamped it. Just an idea.

 

here are the vids. In the first one I showed the FP tester to show what it is doing.

The second vid shows the way it acts once it warms up.

 

The fuel pressure is acting correctly. When you're giving it more throttle (slowing climbing in rpm), it's pulling more vacuum, closing the FPR. When you snap the throttle open, sending the vacuum reading close to atmospheric, is when you'll see the ~43 psi reading. Everything looked good there. I would suggest getting Datamaster (or Freescan) and log the car to see what the sensors are doing and whether or not you're getting tons of KR.