So, I bought an LB9, MM5 GTA. It had really rank gas in it. When I bought the car, the fuel lines were disconnected, and car would run on starting fluid. After changing the tank, and adding a new fuel pump, the car wouldn't run. Tried a noid light on the first injector, and saw no light. After trying both sides, I was dumbfounded. My brother, who was helping me, found an article (maybe on here) that said that ANY injector that is low on ohms will cause the whole side not to fire! We unplugged all the injectors, ohm'ed them all, and found one bad injector on each bank. I tried the noid light with them all disconnected, and lights on both banks! Cool!
Ordered upgraded injectors from Southbay, and she fired right up!
Moral of the story: if she won't start, or runs rough, make sure to ohm your injectors! I know it's probably been said here a million times...

 

It's true. I couldn't believe it myself when I first started with TPI. All the injectors on one side are connected in parallel to an ECM firing output. If one injector goes to too low ohms, the ECM cannot fire the entire side. I realize in decades-old hindsight what Sequential EFI means. TPI is not sequential in the individual cylinder sense. I am amazed that an engine can run perfectly well when fuel is injected in an "averaging" way over the four stroke cycle directly into each port. With a carburetor, I can see how each cylinder draws in its mix from its runner when its ready, but with TPI, the fuel must go into the runner before the cylinder is ready to take it in. But somehow it works, and works very well.


Here's a great little summary of fuel injection types: https://www.cars.com/articles/what-a...1420690418419/

 

It depends on the intake manifold...

TPI manifolds seem to do just fine with batch fire. However, a short runner manifolds like the Miniram isn't all that happy with batch fire injection at idle speeds as the log style plenum and short runners, as the front cylinders tend to rob air from the rear cylinders. And I wouldn't be surprised if some cylinders are able to pull some fuel out other cylinders as their shots of fuel languish for a bit behind closed intake valves.

GM took full advantage of sequential injection on the 94 and up LT1's as you can see how they trimmed the idle fueling quite a bit leaner on the rear cylinders due to lesser air they get.

Above idle speeds, the valves are opening and closing so fast, batch vs sequential becomes less of a difference...

 

Sorry, but this is NOT correct. A TPI ECM does not know anything about an injector's resistance, condition, etc.. All injectors are batch fired at the same time if connected regardless of their condition.

 

The ECM doesn't "know" if an injector is bad, but if the resistance of one injector is way too low, it will act like a dead short and become the path of least resistance compared to the other injectors in the parallel circuit. The shorted injector will draw a much higher current than normal and if that exceeds the amperage capacity of the injector driver in the ECM, the other good injectors will not have enough current to operate.

 

^^^ What GTA350 said. Now if the injector coil goes high resistance, your scenario is correct- only that injector won't fire.

 

This is also correct. That is why the other injectors work when the electrical plug for one injector is disconnected.

 

I used to think that (long ago) they were all fired at the same time. I based my opinion on the ECM print showing one driver for the injectors.

I was schooled however by a member that actually used 8 noid lights, and he said it was bank to bank.

Now I only have one noid light, so can not confirm his findings, but if you have two it would be easy enough to verify.

 

Quote:

The ECM doesn't "know" if an injector is bad, but if the resistance of one injector is way too low, it will act like a dead short and become the path of least resistance compared to the other injectors in the parallel circuit. The shorted injector will draw a much higher current than normal and if that exceeds the amperage capacity of the injector driver in the ECM, the other good injectors will not have enough current to operate.

Correct, thanks for your post and apologies. I misread the initial post.

Regarding MPFI (TPI) engines firing all at same time:
https://www.thirdgen.org/forums/tpi/...injectors.html
https://www.thirdgen.org/forums/tpi/...oes-batch.html
Also per a TGO post I can't find:
Some would say these systems fire 1/2 at a time (odd bank, then even bank, etc, four injectors at a time). This is not correct. The reason for this mis-information is that the injectors run on two separate circuits. The only reason for splitting these as was done was to distribute the electrical load so as to not overload one circuit.

If we can't believe RBob, who has probably forgotten more about this stuff than most of us know, that in batch fire they all fire at the same time regardless of whether in Single Fire or Double Fire mode, then . . . .

 

I'll find my old post...sit tight...as I said, I thought they all fired at the same time, but was shown I was incorrect.

 

Start at post #12, man that was a long time ago....

https://www.thirdgen.org/forums/tpi/...ual-plane.html