Alright, this is getting old.

I have a low throttle/ steady throttle stumble between idle and 3000 RPMs. these are the car's specs:

383 Stroker, GM847 cam, AFR 210 heads, 1.6RR/appropriate springs, Pacesetter longtubes/full 3" exhaust, 42PPH injectors, Full Dyno tune. A4 with a 3600 Stall.

Symptoms: On cold start, car runs great, is very smooth when holding throttle steady/accelerating from a stop. About 3 or 4 minutes in, the problems start. light throttle from a stop, the car hesitates, and then accellerates unevenly. It feels like a surge, like I'm pumping the throttle. while cruising steady, whether at 20MPH or at 65MPH (or any steady speed when rpms are below 3K) it surges and breaks up quite a bit. Give it some throttle, it immediately clears up. Car will pull HARD and SMOOTH at WOT all the way to the shift points without problem, even from a standstill.

I took it back to the tuner last night, thinking it was in the tune. Fuel trims are dead on. O2 sensors are switching fine, voltages are right. basically all sensors monitored with tunercats are correct. Air Fuel is right on the money, and even through the tuners could FEEL it doing the buck/surge as described above, they didnt see anything while monitoring that would explain it. The only strange thing they could find was that the MAP at 950 RPMS (idle) was sittinng at about 65 Kpa. This might be a bit high... I've always read that it should be about 45-50. Thats the only thing that they found that was even possibly abnormal. They tried adding/subtracting fuel down low, no effect. They tried adding/subtracting timing sown low, no effect. They Forced open loop, No effect. We unplugged the MAF, and while it started harder, it did not have an effect on the symptoms. They tried moving lockup around... no effect. Fuel pressure is correct as well. They are as baffled as I am, but they don;t see a single thing in teh tune that could cause the problems.

There are no arcing wires. All spark plug wires are OTVC MDS 8.5MM, and are still lookinng like new. I also have DEI heat sheilds on them for added insurance. Optispark is a known good AC Delco unit. I have swapped it out with my spare AC Delco unit (new) same symptoms. New OEM Coil. No vac leaks that ive detected. Plugs are new NGK coppers, and are gapped correctly. O2 Sensors are brand new Delphi sensors, and have been swapped a couple of times without effect. Since all the sensors are working right according to tunercats, I'm leaning towards possibly a ICM issue? Is it possible that could only rear its head at cruise and light throttle? ALSO. Onnce in a while, I have a hard Hot start issue. It will turn over GREAT, but does not wannt to catch. It takes a few times cranking it to get it to stumble to a start... and finally it runs right.

Sorry for the novel, but i wanted to put all the info in one spot so educated guesses could be made. Any ideas guys? This is pretty depressing since this is a new motor, new trans car... and Its not fun to drive.

 

81 views, not a single suggestion?

 

mine does the same thing! but i only have bolt ons I'm thinking cap and rotor

 

Bump. Someone has to have SOME idea of what I can try....

 

I might help us to help you if you told us the vehicle year.

Did you remove the screen from the MAF?

 

Sorry about that. Its a 1994 Camaro.

I did NOT de-screen the MAF. I do have some updates though:

I did the ICM spacer mod tonite, no efffect.

I UNPLUGGED both O2 sensors and took it for a drive. The car now runs glass smooth at light throttle, on the highway and on the streets at 25mph. It ran smooth right away, the car was already warm. It took a few minutes for the SES light to illuminate.

I'm guessing that the O2's are the problem? Please say I found it......

 

Nowhere have you mentioned scanning for codes. Some codes in OBD1.5 won't illuminate the SES light.

It could be bad O2 sensors, unplugging them keeps the PCM in open loop-and you said in forced open loop it ran fine.

I've had engines run better when the "correct" ACDelco spark plugs got installed. The 1994 LT1 calls for ACDelco #14 spark plugs. My '03 S10 would kill NGK plugs in days. ACDelco plugs run just great.

 

I did scan for codes, sorry. No codes were present until i started unplugging sensors.

The Tuner decided to force open look ,but he did not notice a change. When I disco'd the sensors, it was night and day. I've ran NGKs in this car before, and have never EVER had a single problem in all the years that I've been using them, even in turbo applications.

I pulled every one of them BTW, and they looked normal.

 

How old are the O2's?

 

The Divers side O2 is about 2 months old. The Passengers side O2 is about....8 months? not sure. I used proper extension harnesses on them, and the sensors are both Denso.

 

My car runs pig rich when it's cold. I had two new O2's which fouled out within six months. How fast were the O2's switching? They can still switch when bad, but are slower to do so.

 

I did not get to see the sensors switching. This was while driving the car, while the tuner sat in the passengers side seat with the laptop. I know they WERE switching, just not how fast. I've maintained from day one that this felt like a lazy O2... Ive had a similar issue with a 4 cyl turbo car that had a single O2... it would surge, you could watch the narrowband portion of the pillar gauge go rich, lean, rich, lean, every 2 or 3 seconds, not multiple times a second like it should.

 

my car does something similar either cold or warm i just took it to be cam surge, it clears up with just a tad bit more throttle and over 1500/2000 rpms. Looking forward to see if you find the problem though.

 

I should have my new AC Delco O2s today. I'll let everyone know.

BTW, Rock Auto has them for 30 something plus shipping each./ : With two day shipping to me, two cost me about 86 bucks.

 

Okay, so NEW AC Delco o2 sensors went in on Friday. Same issues occur when the sensors start switching. I DID notice that the stumble at idle and the stumble at low throttle around town at low speeds is a bit better. Not 100%, but better. The Highway stumble is still just as bad as always.

Next idea?