Fuel Injected motor...miss from 1-3K
 

My magnum motor, is acting like a it has a holley 750 with a 6.5PV and a big cam, but a 2.5PV in and instant fix...Truck is....stumbling bad, from off idle(idle is fine, till up around 3k....above that seems ok, smooths out. It almost seems like its under a rev limiter/two step when its doing it popping.

Oh yeah it does it under load and in nuetral.

To say its a challenge to stage it this way, is a understatement!

Was running fine 2 weeks ago, nothing changed. It did slow down 3 plus MPH and a bit of et also, so it is likely a issue across the board. Motor is fresh.

I have read the carb related 3k and above. Wasn't sure if anyone had such an issue with a fuel injected motor in the past. Its a fast system, a old system, with a msd digital 6.

Something may have become dislodged during 900 plus mile tow back to me. Suggestions?

Thanks

Eric

Re: Fuel Injected motor...miss from 1-3K
 

Check your data logger or hook up the laptop and see if the MAP signal looks right. MAP sensor may have come unplugged (forgot to get plugged in) or could be bad.

Re: Fuel Injected motor...miss from 1-3K
 

Agree with Tim, should be able to see something amiss datalogging it. What fuel strategy are you using Speed dendity...AlphaN? Had an issue similar with mine once, was poor connection at TPS, wasn't reading it correctly of course Accel enrichment did not work without seeing TPS increase. If you can datalog i you should be able to tell if it's in the fuel part of it.Joe

Re: Fuel Injected motor...miss from 1-3K
 

Last year at the norwalk D-3 race we had a turbo start go bad so we went to a 12v optima's. For the next year I could not stage the car or do a burn out because the engine acted like it wanted to die. It wad a huge distraction and I lost the couple of races I was at last year because I couldn't get the car to come up on the converter after I staged.

It ended up being an issue with the map lagging just an instant behind wot. Dad does all the f.I. stuff so as far as I know he just tweaked the map. Another thing that was going on the O2 sensor was going out on the collector so the fuel air reading was a little off.

Another issue we had a couple of years ago, dad disconnected the water sensor at the shop and had forgotten to hook it back up. I go to do my burnout and the car is acting like its on the 3 step. I launch the car same thing acting like its on the limiter. So I just shift it myself, made it back to the trailer and check the log. The water temp was reading zero. The fuel was just a "little" fat, couldn't make fire. Just hooked it back up and turned the temp off so it just logs and has nothing to do with the f/a.

This is with the old holly stuff and we do have f.a.s.t. for another combo. Joe buchanan is a great wealth of knowledge, maybe he give u a starting point.

Re: Fuel Injected motor...miss from 1-3K
 

Change from 16 volts to 12 volts you lean the engine. The injectors open slower.
Dyno it on 12 volts, put it in the car with 16 volts, you have the oposite problem. It will be richer. I would check my TPS, MAP and all grounds.

Re: Fuel Injected motor...miss from 1-3K
 

Never mind Joe beat me to it. Took me and hour to write that on my phone.

Re: Fuel Injected motor...miss from 1-3K
 

Eric, just for grins have you looked at any settings in your digital box, I'm not familiar with all the dig. 6 can do but I'll bet it has some retards and limiters that could becoming active.This could at least eliminate the Ign. system as being an issue, it's almost certain to be ign. or fuel, determining which one is a bit harder. Nice part is you do have some diagnostic tools to work with.As Ed mentioned, look carefully at grounds, sensor inputs and at any settings that may have gotten accidentally changed in both the fuel and ignition system. Quick checks would include putting timing lite on veh. to ensure that's OK, could be having triggering issues.I'm sure if you take it step by step you'll find the issue, just don't forget that it may not be simply a fueling issue. Joe

Re: Fuel Injected motor...miss from 1-3K
 

I was constantly trying to tune my LT-1 myself and really needed to get it tuned on a dyno......I was using an Innovate LM-1 to get as gar as I did....on my own....

One raceday I was making Fuel Map changes across the board.....Just put in a percentage change richer......I thought it was a relatively small change.....

Well on the next run....It idled fine....drove up to the lanes fine....went to do a burnout and when I stepped on the gas.....nothing....motor basically did not even have enough power to spin the tires in the water at part throttle..... I staged it anyway and made the run and was not really off by much from an earlier run....

Took the enrichment back out and it was fine after that.....

It was just too rich to begin with and the added fuel really killed it.......

Re: Fuel Injected motor...miss from 1-3K
 

Thanks for all the replies......I will be pulling data riding around the pits and doing a "fake" staging to simulate the issue....I will try it cold and warm, will also pull a data run with MAP sensor disconnected(did that in the trailer this morning, and it seemed to rev up smoother, not totally smooth, but better than it did with it in....Will be sending off logs to tuner...didn't find any loose sensors, will get a nice smooth low rpm tweak and that should fix the drama.....but staging it bumping it it at idle then on with 1400lb of brake pressure to hold it while it stumbles up through 3 to two step at 4K is a bit of a challenge.....driving it up the trailer is another one!

Thanks

Eric

Re: Fuel Injected motor...miss from 1-3K
 

Eric, it is a pain, but you won't get acurate O2 log data at low RPM & lower throttle angles with open headers. Make up a header extention about 3' long, preferably with like a 45 deg bend in the middle, for logging your idle, driving down the return road or pits, and bringing it up against the brake (no 2 step!) to see where you are. A little time spent there wil make it all a lot easier for you or whoever is trying to tune it for you. Big cams pull clean air in open headers (The exhaust actually pulses in & out) during the overlap cycle. That causes bogus lean indications from your wide band. That compunds the issue if your trying to run it in closed loop! A muffler or long enough extention will keep the clean air from reaching the O2 during those pulses.

Good luck.