Possible Causes for a Stutter from 2,200 - 3,000 RPM

[Basin Motorsports]

I just completed my swap over to my current project and I seem to have a stutter from 2,200 - 3,000 only. The car has 30lb injectors + Pro-M calibrated MAF which the previous owner added to the donor car. Stock a9L ECU, EGR system (all emissions) eliminated, and 100% OEM fuel system beyond the injectors.

The car runs fine in any condition from idle - 2,200 rpm and then 3,000 - 5,200rpm. It just has a small window of issue. It will fight through it, but you just can't pass quickly. It's only when it's warm (in closed loop mode) and runs through the gears in warm-up condidtion (before the car gets up to temp).

My thoughts are currently:
- injectors + MAF are too big and car struggles with too much fuel when going from a steady cruise to sudden acceleration
- someone suggested a poor ground for the distributor, and I'm lost where I could add a ground. The harness has been modified, and all grounds are better than OEM condition.

Anyone have thoughts?

I haven't checked codes on the car, so I may try that soon. It's not just a plug in and watch the lights as my set-up barely resembles OEM condition.

[wraithracing]

Without more information, my guess is that the 30lb injectors are just too big. Unless you are running a lot more cubes than a normal 5.0 and/or a REALLY Healthy camshaft, the 30lb injectors are a lot IMHO.

Stock injectors were 19lbs, the 93 Cobra was 24lbs, so again unless you really need the additional fuel available from 30lbs injectors that would be my first guess. Obviously more information and any current codes and/or live data would be helpful to sort everything out. Good Luck!

[Ourobos]

Fairly stock engine? Without a tune, I'd go back to 19# for sure.

Other possibilities is a bad coil. MSD had a bad run of them that caused me a very similar issue. A stock coil solved it, and this was on a 700hp street car.

[gt pony]

I run 30lb. injectors on my MAF converted 5.0L without the issues you're having. It has TrickFlow HCI setup and also a 190lph fuel pump, plus a free flowing exhaust system. In other words it really breaths.

With that said my advise would be too run KOEO-KOER tests. The other members have given you sound advise. Additionally, all of my emission systems are intact, plus, I had to move some pins around in the ECM connector because I am running an A9P processor and the pin locations for the TAB/TAD were different from the 86 speed density processor.

I would pay particular attention to the 02 sensor(s) function at that RPM operation. My guess would be that either one of those is dropping out (not switching) causing that stumble you described.

One final bit of advise, try one thing at a time, like replacing the coil for instance, then move on to the next item if the problem isn't resolved. If you don't know were the problem is find out were it isn't. Good luck, and keep us informed.

[dynodon64]

Recalibrating a MAF to use larger injectors, is a very poor way to be able to run larger injectors. The correct way is to recalibrate the computer, via a chip in your case. There are many more settings that need to be changed to a program to use larger injectors than just a MAF sensor to match bigger injectors. That was just a work around in the day to get around the problem of not being able to recalibrate a stock EECIV computer, before the chips came out later.

Like others have already stated, injectors are too big for your stock set up.

[homer302]

Well it's fun to speculate and it seems everyone is leaning towards the same thing. It really would take less than an hour to swap the 19lb injectors back in and just see. And aside from your time, it's FREE! I believe a bad distributor ground would not only show up under such a fine window so I would rule that out pretty easily. So it's a good ground until 2,200 and then it's a terrible ground and then once you get to 3,000 all of a sudden it's an acceptable ground again?? Who suggested that?

[78futura]

Fairly stock engine? Without a tune, I'd go back to 19# for sure.

Other possibilities is a bad coil. MSD had a bad run of them that caused me a very similar issue. A stock coil solved it, and this was on a 700hp street car.

No not an MSD!!
Msd is for dragstrip only and less than 50 miles a year then you only need a new one every second year...
Junk!

[Basin Motorsports]

Thanks for all the responses. I agree that the injectors are too big, and it seems that's the general consensus also. When I bought the '92 coupe 3 years ago, they had it. When everything from there was swapped into the '80, they came with the package. I don't have 19lb injectors and stock MAF to put back in there, but I'll keep an eye out for a set this Winter. The car is pretty much done for the year unless I just cruise it to town for one more meet-up this weekend.

The MSD coil I have is what came from the '92. Seems to run fine, and wouldn't make sense to just have a stumble at a small window. It's a few years old, so probably not in the bad run mentioned.

I'll try to get the harness out for the tests. It could be bad O2 sensors as they were the ones from the '92 too. I just wanted to get the car put together with a baseline and then start to uncover the small issues as they arise. This is one of the small ones currently.

[Ourobos]

I had a MSD coil that had the stumble from 2k-3k.. So believe what you want.

[ironman]

I had spark plugs wires that produced a miss when under load.

[AldeanFan]

Don’t k overlook the simple things.
I had the same problem on one of my cars and found a burnt plug wire and a cracked plug last night.

New plugs and wires solved my problem


Sent from my iPhone using Tapatalk

[erratic50]

The simplest source of a stumble in a specific RPM range that I’ve found is the rotor and cap. I’ve had a poor quality distributor rotor cause this exact problem. Cured by installing a new Motorcraft

if the spring contact is mid-sharpen and the center electrode looks tore up its very likely the source.

[Basin Motorsports]

The simplest source of a stumble in a specific RPM range that I’ve found is the rotor and cap. I’ve had a poor quality distributor rotor cause this exact problem. Cured by installing a new Motorcraft

if the spring contact is mid-sharpen and the center electrode looks tore up its very likely the source.

Wires, cap, and rotor are brand new, but I will go check them again. Plugs have about 150 miles on them. I'll check them too. It's easy to check stuff I have to look for an issue. Besides, it's a good excuse to be out in the garage.