Fuel Injector Expert Needed
 

Has anyone experienced the A/F in a cylinder to change from uniform (~12's) idle to WOT to a combination of rich (11's) at ~1500 to 3000 rpm and lean (13's) at 5000 - 5500 rpm while the other cylinders remain in the ~12's. In my case I have two such cylinders and the other six are still uniform in a MPFI engine.

If yes, what was the cause of the change in injector performance?

well... if you believe your data and by that i mean that you move the sensors around and make sure you aren't getting false readings, then an injector that is dirty will go lean at longer pulse widths or one that is going bad will do wird things like hang open or fail to pulse. so the short answer to your question is yes... a bad injector can do it. the test is easy... you hook up the injector to your scanner and do an ijector pulse test with a pressurized fuel rig that you knock together out of some bits of hose. or failing that you swap the injectors into different cylinders and see if the problem follows the injectors. its easy to do.

If you swap injectors, the easiest test, take a look at the input screen.. I have sprayed carb cleaner thru them backwards with 12v applied for a short time and washed out some junk.. Spray it on a white paper towel and you will see if there is anything there. If nothing shows up.. the injector may need cleaning or maybe replacing.
Was this a matched set when you started or just the stock injectors?

Six cylinders/injectors are in lock step. Two are not. The two go way richer than the others at lower rpm and way leaner at higher rpm and then start back at max rpm. These are stock injectors with lots of prior history of being recorded with performance then being parrallel to the other cylinders/injectors. I back flushed these two injectors with carb cleaner with the injectors electronically opened (no paper towel). I also switched their position/cylinder but have not yet retested for A/F profile. I have learned my leasons the hard way and these days the data is reliable. Thanks for the input. TBC.

Rage,
Are you running 8 WB 02 sensors? in the boat?

If you want to get down to it, Megasquirt has an injector testor kit for about $60. It only includes the electronics. I was going to build one, but havent been able to find any hardware for the glass tubes and such. With it you could look at spray patterns and balance between injectors.
I ened up buying a set of injectors from Trick Flow, they claim a 1% balance between them and up to 100psi operating PSI before they over heat and cause problems...

No just one WB 02 sensor (Innovate LM-1) that I move around to test all cylinders. I repeat the same gradually increasing rpm procedure from idle to WOT on each cylinder then compare the results. I record MAP as well and the results cluster nicely indicating reasonable repeatability. See attached. Cylinders #5 and #7 are the ones of concern.

I don't know the answer but what if your injectors are just fine and it is the madifold air flow to those cylinders. Interesting that it is cylinders 5 and 7, isn't that why they do a 5-7 cam switch on the BBC? Is there a wierd air flow problem with your intake at those rpms and with those particular cylinders. Neighboring cylinders steal air from those cylinders. Just a thought. Maybe time to dump that MPI intake. Maybe a holly 4 barrel type or equiv. with 2000cfm TB. Good luck

Thanks. I have been told that the 496 firing order is different from the BBC with the scavanging issue associated with carburated intakes running too lean on certain cylinder(s) which is a fuel starvation and not an air starvation issue. So the rich condition at the lower rpm's could be air starvation but the lean condition at high rpm's is a fuel shortage or poor atomization issue. Too early to contemplate panic moves. It is a Raylar intake manifold.

so u have 8 bungs on headers,,,what about electrical interference from the coils or computer

I think it is a 4/7 swap.

Good point. I route the o2 sensor cable such that ignition noise is not recorded with few exceptions and for those I can tell what is noise and what is the base signal. #7 does have some ignition noise at high rpm but the base o2 signal is evident and still way leaner than the others at that high rpm. The computer voltage is too low to influence the o2 signal.

recheck any sensors or vacuum hose,,,near 5 qnd 7 cyl,,,could you have a air leak????

Well I swapped the injectors and reran AFR tests on the four odd bank cylinders. Nothing has changed so it is not the injectors. Vacuum leak is unlikely and I am not sure if it would even have any effect on a MPFI setup like it would on a carburated or SPFI system where the fuel is mixed with the air before entering the intake manifold. I just do not know but would seem like not. I do have earlier AFR data on all eight cylinders where this latest lean #5 and #7 cylinder condition was not displayed but the ECU did not have the latest ECU reporgram. TBC

what if you sprayed carb cleaner around 5 or 7 cyl,,,while you do one of your test,,,,if you did have a leak wouldnt it drive the ratio rich,,,and maybe then you could say you dont have an air leak

Well sorry I can not be sraying anything going down the lake at WOT with the engine hatch closed. What are yiou thinking?

at dock ,two guys ,hatch open,,,do your test,,,,anyway you can ,,,just trying to drive the the 02 sensor rich to see if it is an air leak,,,,,,you wont need to look at the other cyl just those two,,,this can be done at the dock at idle

Ok, I get the picture.