I'm having the most baffling problem I've ever experienced on my 94 Formula summer car. I’ve been chasing this problem for about 2 months. I only have an AutoXray scanner so my info is not as detailed as it should be. Sorry.

When the car flips from Open Loop to Closed Loop it instantly starts bucking and spitting like it is misfiring horribly. It shows NO codes. The BLM on the right side maxes instantly at 160. And the left side drops to 108. No matter what throttle opening the right injectors then jump to about 2.5 to 3.0 mS more than the left side. The right side plugs are black rich. This holds whether the car is at idle or WOT and everything in-between.

These have been my problem-solving procedures:

• The plugs and wires were new, being replaced last fall. The Opti showed no codes. However, I still took the car to a shop and put it on an ignition scope. NO misfires, no ignition problems whatsoever. And besides it only happens in closed loop
• The O2 sensors were cycling rapidly and did not indicate any problems. I switched them side to side and the problems persisted. I swapped them with my known working new O2 sensors from my 94 Trans Am and the problems persisted. No exposed or fried O2 wires.
• I swapped the MAF and MAP with my 94 TA and the problems persisted.
• I swapped the PCM with my 94 TA and again with my spare PCM and the problems persisted.
• I have good fuel pressure without any spikes or drop-offs. The pressure does not bleed down rapidly when I shut the car off.
• I can’t locate any exhaust leaks. I can’t locate any vacuum leaks.
• I have good voltage from the alternator and battery.
• I don’t have any mechanical problems with the valvetrain or short-block. A leakdown test indicated all cylinders were between 1 and 4%. It only has 36,000 miles.

This is killing me. I want to take the car to cruises and do some racing at the strip. The car runs perfectly in those early minutes of Open Loop. Then watching the scanner screen and INSTANTLY as it flips to Closed Loop the problems begin. I’m stumped. I’m done. I’ve exhausted my knowledge. I need help.

I don’t get it. What am I missing?

 

Are there any mods on the engine, or is it basically stock?

 

when you swapped pcm's did you swap proms also or are they the same i ran into this once but it was with 89' formula tpi prom was no good replaced it car ran fine not sure if this is same cause but is same symtom

 

It's a heads/HOT cam/ported intake stock short-block with MAC headers. I've had 30,000 trouble-free miles on this combo since 1998. This is the worst diagnostic problem I've ever had and I've many computer-controlled cars. I thought my TPI cars were bad I am really upset with this car right now. I forgot to add that with the other PCMs they didn't do the 160/108 BLM, but they still had at least a 40 count plus spread, like 157 on the right and 111 on the left.

Could an EGR problem cause this?
Could a leaking and open check valve in the AIR system allow air into the header which would throw off the O2 sensor?

Thanks to anyone who can help me with this.

 

Any exhaust leaks upstream from the O2 sensor would cause this symptom. I just worked with a customer the other day that ran into this same problem, and it was because the EGR tube was open on the manifold (it was an LS6).

 

Vince, that is what I keep thinking also. But I can't see or hear any exhaust leaks. My next step may me to remove the header and see if the gasket is leaking on the bottom.

 

you can also use a smoke machine through exhaust to find any leaks there/alot of shops are getting them now

 

Smoke machine? I hadn't heard of that. How does it work? I am thinking exhaust leak.