98 Cobra engine runs rough when under load

[jimcobber]

Hi, all-

My 98 Cobra runs rough when "lugging" the engine. Say at 45 mph in 5 gear, you give it some throttle you can hear/feel the engine missing. If you don't lug, in a strong acceleration the engine is powerful but you can feel slight uneven acceleration skips. I have replaced the entire ignition system from coils to plugs. The Ford mechanic is clueless about the problem. I am wondering if perhaps the injectors need replacement? What are the symptoms of malfunctioning injectors? How hard are they to replace for a shade-tree mechanic such as myself?

Thank you-

Jim

 

[ForcFed93]

I had a similar issue. Turned out to be my FRPP plug wires even though they only had maybe 200 miles on them.

 

[1998SVTCOBRA]

Was going to ask, what brand coil packs, plugs, and wires did you go with?

 

[ZeroDCX]

^ Agreed. This is a common problem with our cars. Wires will arc with surrounding conductors and our motors are all aluminum.

 

[ZeroDCX]

Was going to ask, what brand coil packs, plugs, and wires did you go with?

Normally FRPP wires are the best along with stock motorcraft coil packs. Spark plugs depend on your setup. Autolite 764 for stock applications and NGK TR6 plugs for light to mild boost applications. But that's always subject to preference.

 

[jimcobber]

Screamin Deamon coil packs and FRPP wires. The Ford mechanic said that I had put the wrong plugs (not motorcraft) and the wrong gap. So he replaced them. The problem is slightly better, but by no means fixed. He test drove the car and declared it fixed, but no, it's not. Maybe he test drove someone else's car is all I can figure.

 

[ZeroDCX]

What mods on your car? Stock applications should see 0.054 spark plug gap. When did you last change your wireset? If you still have your previous wireset and it did not display these issues, then swap back to your previous wireset. In the even that the issue is gone you've isolated your problem.

These wiresets can arc where we cannot see, beneath the spark plug covers.

 

[cheaneyt]

Interested in this. My cobra does this at times.

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[jimcobber]

The car is bone-stock, engine has 210,000 miles on it and does not burn a drop of oil. Wires, coil packs, plugs were changed about 3 months ago. The car ran really bad before I replaced all this stuff. I chucked the old stuff all out. I take it the general consensus is this is an ignition problem, not a fuel delivery problem? I'm willing to purchase another wire set if there is a chance that it will fix the problem. What wires do you guys suggest?

 

[1998SVTCOBRA]

Ditch the screamin demon packs and get motorcrafts.

 

[jimcobber]

Have you had problems with the screamin demon packs?

 

[97snakebite]

Trying to accelerate hard and 5th gear at 45 mile-an-hour just will not work your way too low on RPM

 

[ZeroDCX]

Eventhough your problem sounds like the typical wireset issue, the best information that I can give is to say, "datalog and diagnose" before throwing money and parts at the car.

Check for stored or pending codes. Most likely P0300+ (Misfire codes) to isolate which cylinder is misfiring and try swapping wires with another bank or cylinder to see if it follows. This also holds true for swapping injectors.

You could look at STFT, LTFT, O2 Sensor Pid data on both banks to see what's going on when the car starts missing. Higher negative STFT values indicate improper fuel burn (running rich) so the ECM is commanding less fuel and higher positive values would be lean (ECM is commanding more fuel).

During the miss/hesitation, if you're running rich then I would look towards ignition. See if fuel trims are pegged high or low, maximum values are typically +/-25%. See if O2 Sensors on both banks are switching. Check MAF AD counts, voltage, tps voltage, alternator voltage output, etc etc etc.

There a lot of variables to account for really, but taking some time to diagnose is better than throwing money at the car and remaining with the same problem.

 

[jimcobber]

Eventhough your problem sounds like the typical wireset issue, the best information that I can give is to say, "datalog and diagnose" before throwing money and parts at the car.

Check for stored or pending codes. Most likely P0300+ (Misfire codes) to isolate which cylinder is misfiring and try swapping wires with another bank or cylinder to see if it follows. This also holds true for swapping injectors.

You could look at STFT, LTFT, O2 Sensor Pid data on both banks to see what's going on when the car starts missing. Higher negative STFT values indicate improper fuel burn (running rich) so the ECM is commanding less fuel and higher positive values would be lean (ECM is commanding more fuel).

During the miss/hesitation, if you're running rich then I would look towards ignition. See if fuel trims are pegged high or low, maximum values are typically +/-25%. See if O2 Sensors on both banks are switching. Check MAF AD counts, voltage, tps voltage, alternator voltage output, etc etc etc.

There a lot of variables to account for really, but taking some time to diagnose is better than throwing money at the car and remaining with the same problem.

Thanks very much for the useful post!



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[ForcFed93]

Thanks very much for the useful post!

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I went through weeks of diagnosis because I didn't believe my FRPP wires failed so easily. I was wrong and wasted a lot of time and money. Start with your plug wires then go from there. Just look at the reviews on LMR.

https://lmr.com/item/M12259C464/1996-98-Mustang-46L-4V-Blue-Plug-Wire-Set
https://lmr.com/item/M12259R464/1996-98-Mustang-46L-4V-Red-Plug-Wire-Set

All reviews in 2017 for both colors show misfiring. The only good reviews are from 2014.

 

[Tillerman77]

As information, OEM wires from Ford have a 2 year guarantee. They solved my misfire problem this Spring on my 97 Cobra. Perhaps not as sexy as the FRPP wires, but they seem to be more reliable.

 

[cheaneyt]

Gonna be checking into this. As i have the red ones. And still get random missfires. And cylinder 7 , with all new crap

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[nomoretickets]

Sounds very much like inadequate spark. Pull the plugs and double check the gap. It doesnt cost you money and crosses one potential cause off the list. Bad wires are a possibility as well, I think other posters have discussed that adequately. How old are your coil packs? I know you said you replaced the ignition stuff recently, were the coil packs part of that? If not, they might be a likely point of failure.

 

[jimcobber]

Sounds very much like inadequate spark. Pull the plugs and double check the gap. It doesnt cost you money and crosses one potential cause off the list. Bad wires are a possibility as well, I think other posters have discussed that adequately. How old are your coil packs? I know you said you replaced the ignition stuff recently, were the coil packs part of that? If not, they might be a likely point of failure.

I replaced the garbage wires and coil packs that I got from American Muscle with Motorcraft OEM parts and now the car runs like a Cadillac, er, a Lincoln. Totally got rid of the misfire problem.

By the way don’t ever buy anything from American Muscle all they sell is crap.


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[96GT226410]

Glad you got it figured out.