I have been battling a miss for quite some time and I think I may have stumbled upon the problem but not sure where to go from here. How i came to this conclusion was just process of elimination. Using a temp gun cylinder #1 is running around 190 degrees at idle while shooting the header tube. Off idle temp starts to rise. I took the fuel rail off and left 1 and 3 hooked up then cranked the engine. Injector 3 fires 4 times for every one on injector 1. I swapped 1 with seven knowing it to be good and no change. I ohmed the injectors and they were around 13 ohms. Then I checked for 12 volts with key on and it was there also. I moved on to the control wire from the ECM and checked for resistance but found no problems there. It only seems to be missing at idle and low RPMs creating a rich condition to 3,5,7. The miss is not as noticeable at high RPMs. They are stock 24lb injectors and i am running a delteq setup with a Fastchip tune. Could it be an ECM fault? I am out of ideas so hope someone can help!

 

What year is your car. '94 and above is sequential fire Vs batch for '93. If sequential I'm thinking of a couple of things. Could be a bad injector driver in the PCM not providing a good ground signal to the #1 . Or the OPTI low resolution pulse is a different duration for each cylinder. This tells the PCM what cylinder is firing, wonder if something is messed up at that end. Bad or distorted low res signal causing PCM to not see #1 position all the time. Delteq monitors the opti signals, but should not load them down going to the PCM. Also at high RPM PCM switches to batch fire and problem may clear a little. Nothing easy here, if you had a oscilloscope and know how to use one you could scope the #1 injector driver and also look at the low resolution pulses. Otherwise swap PCM with a friend to eliminate that.. Can you ask delteq if they have seen this before ?

 

It is a 95 TA. Ed at Fastchip also said it could be the driver and he can verify if it is before i have the pcm re flashed. There has been some upgrades that require a different tune. The opti is a new Napa unit so i hope thats not the problem. I also have a new opti connector.

 

If the injector driver was bad would you have to replace the pcm? Wouldn't a low resolution signal problem effect more than one cylinder?

 

Yes PCM would need replacement.. The injector driver is an integral part of the PCM circuit board, '94 and '95 are the same part number they run $50-75.00 at junk yards or you can get a used tuned one from a tuner for about 150.00. -A low res problem would have to be something like dirt or rust on the slotted steel wheel that spins thru the optical pickup. There are 360 equal width slots on the high resolution pulses per cam revolution. And eight different width slots identifying each cyl on the low res. The PCM only picks up low res failures if there are no low pulses for 720 high signals. So a crappy #1 pulse would not throw errors. Since your OPTI is new I doubt you have a rusted or dirty problem.. PCM looks like the culprit.. If you swap injector wires between #1 and #3 does #3 then fail to pick ?

 

The low res slot widths aren't all different. #1, #4, #6 and #7 are all 2* wide. They alternate in the firing order with the variable width slots.

#1 (2*) - #8 (7*) - #4 (2*) - #3 (12*) - #6 (2*) - #5 (17*) - #7 (2*) - #2 (22*)

If the slot for #1 was blocked, it probably wouldn't be firing at all... not just 1 out of every 4 times.

 

It sounds like the PCM is the problem after all. Bob I did not switch injector wires but I did switch injectors 1 and 7 with no change.

 

Fred, thanks for the clarification. Still trying to learn this stuff. Does that mean the PCM actually counts number of high res pulses within the low pulse duration to keep track of cam position. With that info seems they could have done a lot better than waiting for 720 high pulses w/o a low to throw a 16 or 40 lows w/o a high to throw a 36. It's a shame they did not add more detailed testing or reporting to make a deteriorating or intermittent OPTI failure easier to pinpoint. Not stopping the engine, but at least recording a missing pulse for diagnostic purposes. (just venting)

 

The low res pulse pattern is adequate to time the injectors and ignition. It doesn't need the high res to function. The idea behind adding the 360 slots around the ourside of the wheel (which provides 720 "on + off" events = equals knowing where the crank is within 1-degree) is to know where the cam/crank is BETWEEN the low res pulses. That allows the PCM to adjust the timing to reflect that fact that the engine may be increasing or decreasing RPM. That minor tweeking substantially reduced "spark scatter".... deviations from the desired timing. It was a major advantage of the system, and was credited for the low emissions, ability to run healthy timing advance, and improvements in fuel mileage.

Very interesting SAE paper written in 1992 regarding the LT1 engine. It also touts the advantage of reverse flow cooling system, including the automatic evacuation of air from the cooling system. Unfortunately, the evacuation system was only used on the Corvettes, and the F-Body and B-Body was left with the poor-boy, screwed up air bleeders.

 

The low res pulse pattern is adequate to time the injectors and ignition. It doesn't need the high res to function. The idea behind adding the 360 slots around the ourside of the wheel (which provides 720 "on + off" events = equals knowing where the crank is within 1-degree) is to know where the cam/crank is BETWEEN the low res pulses. That allows the PCM to adjust the timing to reflect that fact that the engine may be increasing or decreasing RPM. That minor tweeking substantially reduced "spark scatter".... deviations from the desired timing. It was a major advantage of the system, and was credited for the low emissions, ability to run healthy timing advance, and improvements in fuel mileage.

Very interesting SAE paper written in 1992 regarding the LT1 engine. It also touts the advantage of reverse flow cooling system, including the automatic evacuation of air from the cooling system. Unfortunately, the evacuation system was only used on the Corvettes, and the F-Body and B-Body was left with the poor-boy, screwed up air bleeders.