I have been finishing up my 350 engine swap. When I completed it I just used the cop 350 chip to get me going and noticed that I had problems around 5,000 rpm with the engine breaking up, and losing power. I used my GM tech friend's dealership tech 1A scanner from vetronix to see what was going on but when I plugged it in and set it up for the road test, my problems above 5,000 WENT AWAY!! It pulled smooth up to 6,500 when the scanner was plugged in(albiet not very hard cause it was running lean) When I disconnected it, and did the same runs the problem reappeared above 5,000 rpm. I went ahead and finished the project with a custom chip from command performance here in denver which I needed to do anyway. The performance Improved dramatically but I still had the same problem with the new chip above 5,000 rpm. I wanted to check out what the new chip was doing so I borrowed the scanner again with the new chip installed and when I pluged it in and set for road test, the engine pulls strong to 6,500. But when I take the scanner off, It runs like s**** over 5,000. What could cause this? Does the scanner do anyting to correct problems with the chip/ECM? Thank you all for your thoughts.I really appreciate the help..........thanks bob ------------------ 91 camaro RS convert,GM crate 350 with LT1 cam,edelbrock 6085 heads,9.5:1 comp, edelbrock TBI intake bored to 2", 65# injectors,454 TB, Command perforance custom chip(still tunning) ,dual fan conversion, ultimate TBI mods,TBI spacer,SLP 1 5/8 headers, 3"cat back,Flowmaster, T-5, centerforce 2 clutch, 3.42 torsen posi,alston SFC's,Global West LCA relocation brackets, edelbrock STB

The scanner probably changes the ECM mode.

That is done with a resistor between the A and B terminals on the ALDL port.

Open - normal operation

10k - ALDL mode - This is probably what the scanner did. This mode disables knock retard on some ECM's and that can explain your problem.

3.9k - Backup mode - Should run like crap

short - diagnostic mode


Try to put a 10k resirtor over the A-B terminals. This will probably make the engine run like with the scanner.

Don't run like that for long because if you have a problem with engine knock you could damage your engine.

You could modify the scanner (disconnect cable to pin B on the ALDL connector). Then you could scan the car in normal mode.




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Jonas Bylund

Interesting, I thought the scanner was doing something but I don't think it is eliminating knock retard. I tested for knock on the scanner and it wasn't showing any Knock at the RPM's I checked.( maybe the scanner is incorrectly diagnosing this, I don't know) I am noticing a somewhat lean condition as indicated by the BL numbers at 5,000 plus RPM but I still don't get why having the scanner pluged in eliminates the problem.............bob

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91 camaro RS convert,GM crate 350 with LT1 cam,edelbrock 6085 heads,9.5:1 comp, edelbrock TBI intake bored to 2", 65# injectors,454 TB, Command perforance custom chip(still tunning) ,dual fan conversion,
ultimate TBI mods,TBI spacer,SLP 1 5/8 headers,
3"cat back,Flowmaster,
T-5, centerforce 2 clutch, 3.42 torsen posi,alston SFC's,Global West LCA relocation brackets, edelbrock STB

I tested the car again today, with same results. Over 5,000, it breaks up bad but with the scanner pluged in NORMAL OPEN
mode it runs just great.Clearly,it is notable lean above 5,000 so we are correcting the chip but no one can figure why it runs well with my scanner. I am checking with Vetronics to see if they have an idea.
We also seem to recall that in normal mode
the scanner creats a 3.9 ohm resistance in the a/b terminal. Anyone got any ideas?
I am gonna try the 10 ohm and 3.9 ohm resistors but I am not detecting any Knock on the scanner. At least I am in good company, cause the tuner dosn't know and either does my friend who has been a chevy tech for 30 years.............thanks......bob

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91 camaro RS convert,GM crate 350 with LT1 cam,edelbrock 6085 heads,9.5:1 comp, edelbrock TBI intake bored to 2", 65# injectors,454 TB, Command perforance custom chip(still tunning) ,dual fan conversion,
ultimate TBI mods,TBI spacer,SLP 1 5/8 headers,
3"cat back,Flowmaster,
T-5, centerforce 2 clutch, 3.42 torsen posi,alston SFC's,Global West LCA relocation brackets, edelbrock STB