95 misfire/studder under 15 psi, 4-5 krpm

mopar70

Registered User
Hi folks,
I've had my car less than 1 year, and I'm still working out some bugs. Earlier this year I had misfiring under boost (moderate boost). Replaced the plugs with Motorcraft double platinums, and I thought it was running very well after that. During this summer I notice that under full boost, when the engine is hot, from 4-5 krpm I get some type of subtle misfire, or stumble or interuption of power. It is not as clear as typical "bad plugs" type misfire on these cars. When the engine is cold, I do not notice any lag/misfire. The car will pin my stock boost gauge in this rpm range, I presume this means the car is making 15 psi or more boost.

The car has many bolt-ons (almost all of them I think), large MAF, FMIC, MPX blower, mid length headers, MP inlet, pulleys, high flow cats, 2.5 exhuast etc.....I was told the injectors are upsized, but I don't know to what. I was told the motor was built 4K miles ago - but have no data sheets. I do not know if the fuel pump has been upgraded to be appropriate for the modifications yet. I don't think it has any chip/tune. By the way, how does one figure out if it has a chip/tune? Is ther an obvous way to determine the fuel injector rating?

So anyway, this past week I bought autolite APP103 plugs, gapped them to 0.035 inch, and still the engine response is the same. I do not think it is a plug issue. The plug wires are the big blue live wire models. They appear in excellent condition, and I checked the fit on the plugs a few times.

I have a friend who owns a speed/tuning shop, and I plan to take it them to have them tune the car, and use their dyno. But thought I would see if anyone has any other advice, or has experienced very similar problems. Hard for me to imagine that the motor would run perfect with all of the bolt on parts but with no tuning done - anyone agree?
 
First things first, I hope you're only running 93 octane through the car. If not, that may be your problem

Assuming you are running 93, you are going to need to clarify what injectors and pump you are running. If you're running 42's you may be maxing them out, and if you're running the stock fuel pump you'll be asking for a blown motor.

To check if the car has a chip, you need to remove the computer (passenger side kick panel, not too bad to take out) and see if there is a chip plugged into the port on the side. If there is no chip, you're best bet is to get the QH chip and begin your tuning adventures
 
If you have larger injectors, but have not compensated for them in the tune, then your fuel trims are likely off, and that could be causing or at least contributing to your problem. If you have an MPX, you would definitely be maxing out stock injectors, so it could also be due to running lean due to running out of injector, or fuel pump if that is still the stock one. It could also be a vacuum/boost leak, or detonation/pinging, or even clogged cats could cause symptoms like that, and without seeing it in person, it is hard to say what the problem is. Either way, I would be babying the car, not going over 10psi and not going over 3-4K rpms until you figure out exactly what you have to work with and what the problem is. As for telling the size of the injectors, look at what color they are. If the tops are a dark blue color, those are the stock 94/95 36# injectors. Neon green would be 42#s, and I know the 80#s that I have in my car are all black with a slimmer body, but there are also 50# and 60#, and I don't know the visual differences with those off the top of my head. First thing I would suggest would be to get a wide-band O2 sensor and A/F ratio gauge, and see what the car is doing.
 
Thanks.

I am running 93 octane purchased in the commonwealth of massachusetts. I've been thinking about upping the octane to see if it runs better.

I am concerned about the fuel pump as well - I wonder if that is how the motor ended up needing a rebuild.

You all may also find it interesting that my fiend with the speed shop worked at Walbro fuel systems in Meriden CT for a few years. So he knows the pumps. When I had my last SC (2001-2004) I think he let me "borrow" one.
 
I suspect the knock sensor/timing as well.

I looked closely at the injectors, then looked at photos on SCPs website. I am certain they look slim and black, just like the 60# injectors that Bill has on his website. They are all black, and they do not have different diameter changes like the other injectors that are in his photos. So I am almost pretty confident I do have bigger injectors.

Also, the MAF is 76 mm C&L. Not sure on the throttle body size, but it appears larger in outer diameter than the MAF (I say this based on the fact that the coupling to MAF necks down to the MAF OD, but the coupling to the TB doesn't neck down. Basedon photos on the SCP website and the perfect flange matching to the MP inlet, I think I have an 85 mm TB.

I will proceed to see if I have an obvious chip in the computer, then I shall blame the chip/tune.

MadMike's comments about babying the car almost make me feel a little bad for taking the car out and doing a giant burn out tonight - but it was fun.
 
If your running 60# injectors and a 76mm MAF...that is your problem. There is no sample tube available to compensate for the 60# injectors. A tune and potentially a fuel pump will get you on your way, or at least give you a great start
 
Would it be logical to get a larger MAF and sample tube to pair with the larger injectors?

Thanks to all for chiming in. I enjoy this diagnosis by comittee/guess/forum.
 
Would it be logical to get a larger MAF and sample tube to pair with the larger injectors?

Thanks to all for chiming in. I enjoy this diagnosis by comittee/guess/forum.

Ideally, you tune the ECU to support the larger injectors directly. Or at worse, you scale it so that it can handle the injectors (if too large for the ECU's tables).

The sample tube is just a work around.

Think of it as putting larger tires on the right, and turning the steering wheel on the column so that when you steer left to compensate, it's dead center.

It's better to fix the alignment problem caused by the larger right tire, innit?

RwP
 
Would it be logical to get a larger MAF and sample tube to pair with the larger injectors?

Thanks to all for chiming in. I enjoy this diagnosis by comittee/guess/forum.

Once you get up to 60# you really need to have a custom tune file in order for the ECU to provide the correct amount of fuel throughout the RPM range
 
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