Vaxii

Hi,
A have a very nice Hotrod, with a 1996 LT1.
It's driving me crazy (-:
My problem is that it is surging and is running fat on idle.(Dark plugs and O2 sensors)
I have replaced nearly every parts, and still no luck.
Fuel pressure, compression, and smoke leak test is done. Everything ok.
New Sparkplugs, wire (original suppresion core), optispark, TPS, MAF, etc.
Nozzles are checked too.
There is no fault code readings. except transmission fault, due to seperate F.A.S.T TCU

When I put my Scantool on, I am not able to read G/S readings, it shows 0 volt at all times. Though the digital signal i yellow wire to ECU is ok.
Should it not be possible to read g/s on Scantool ?

When I measure connector to MAF, I have 12V, Ground and 5 Volt, on yellow wire.
When I use my scobe, MAF is working fine.
If I unplug MAF, there is no change in idle (still bad), and there are no fault code, to read on scan tool !!! Shoud the be one ?
Is it possible that it is a wrong tune to ECU ???
What I'am thinking is it possible to set/program ECU, so it is not possible to se G/S readings (no MAF sensor) or can it possible be programmed, so ECU can not read MAF readings ??? Maybe due to an older tune for an engine without MAF ???

Execpt for idle, is it running very strong, and good gas milage.

Short and long term fuel readings are one digits.

I have purchased engine used, and according to previous owner, it shoud be tuned for this application.

Engine facts:
Edelbrock heads and intake.
Hotcam 218 degree.
Accel 30 pound injectors
383 Cui.

Injuneer

When you unplug the MAF sensor the PCM defaults to speed-density. Hence, PCM is fueling based on calculated grams/sec (requires accurate MAP and CLT sensor readings), not the MAF value. That also requires the PCM program to have the correct numbers in the volumetric efficiency (VE) tables, and the correct displacement. Depends whether the tuner updated VE when he did the tune. Yes, there should be a fault code. But it is possible the tuner tuned it to run in speed-density all the time, not requiring the MAF reading. Shouldn't be necessary to do that with the small cam and the small difference between the Edelbrock LT1 heads/intake and the factory heads/intake.

When you hooked up the scope to the MAF signal wire, what was the frequency reading at idle? Should be about 2,500 Hz. What does "I am not able to read G/S readings, it shows 0 volt at all times. " What voltage are you referencing? The MAF signal is a variable frequency, and the PCM uses an internal calibration table to convert the frequency to grams/sec. No "volts" involved.

Are you saying the long term (LTFT) and short term (STFT) fuel trims are all within +/- less than 10% (single digit)? Generally, you want them within +/- 5%. Are they typically reading about the same on both banks in each fuel trim cell?

What device are you scanning with?

How old are the O2 sensors? Where are they located? Are they 4-wire heated sensors? Have you tried new AC Delco sensors? Any chance of an exhaust leak before the O2 sensors? That will cause the engine to run rich, as will misfires.

You indicated the fuel pressure is "ok".... what is the reading at idle, both with and without the vacuum compensation line attached to the fuel pressure regulator?

Vaxii

Hi again

Thanks for adequate answer.
I can not read frequency with my old type of scope.
I have measured fuel pressure today, it is 28 psi at idle and 38 psi during gas responce.
Of course the MAF readings should be in grams/sec not volt. (0 grams/sec reading at all time)
O2 sensors are new, as the rest of the engine only around 2000 miles in all.
Engine is made in USA, and as far as I can see, is ECU programmed by Street and Performance. (Decal on ECU)
Today I smoke testet intake and exhaust system.
I found to small needle holes in exhaust near O2 senser. I weldet it up, but it seem to worsen the problem.
Now I got high longterm readings on both banks during idle. (still only Idle problem)
I have only 2 pcs O2 sensors, and they are located right after shorty headers.
I also have another problem. When engine gets warm, throttle is not always closing 100%, it is like there is some vacuum, who is keeping it open. When I stop engine throttle opens and close 100%. Almost like there is vacuum, who keeps it open ?
I don't know if can have something to do with the other problem.
I have measured volt at TPS it is 0,67 volt. at idle.
I have posted a small video on Youtube, where readings can be seen.

Regards Claus

Injuneer

0.67 volts is typical for TPS volts closed throttle.

If you want a guide to interpreting scan data, there is a link to my LT1 scanning guide here:

https://www.camaroz28.com/forums/com...-guide-886891/

Vaxii

Thanks for trying to help me out Guys.
I think I have found the problem today.
My only 1 1/2 year old Mezeire waterpump is leaking a little bit.
Resulting in water into new optispark ignition.

I have now ordered new Summit optispark ignition, and Summit will send me a new Waterpump due to warranty.
Hopefully this will solve the problem.