Engine Code 66/98 Erratic idle

So I have done research on this prior to posting this thread, but I’m not a wiring genius in any way shape or form and have been a little confused on following jrichkers checklist. Here’s what I have done to the car. GT40 upper lower intake, GT40 heads, 24lb injectors/BBK 24lb MAF, 70MM TB and spaced, CAI. I got the code not to long after I did the intake and injector swap. Since then, I have swapped the 24lb maf with another one and the code went away for a short period of time before returning. After doing some research, I figured that it could be a computer issue, and since I have one lying around I figured there wouldn’t be any issue in swapping it. So I swapped it, and the car ran great. Drove it around the block with a couple small pulls, and it didn’t have any missing or anything. That is a problem the car currently has, if I go WOT it’ll buck and stumble, but when daily driving it doesn’t mess up too much. Just idling and WOT. Hopefully you guys might have some ideas.
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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I would look at fuel in terms of having an issue with your current setup, vs fuel supply not being sufficient. Your gt40 combo is rather mild and and could easily run on 19# injectors and stock pump. With that said, I agree with running 24s in this case, but it should still run on 19s

With that said, you just need to get a guage on there and see what your pressure is at idle and at wot. Just need to cross it off the list.

This could also be ignition related woth the usual suspects of the TFI/PIP/Coil. In fact, with it generating a code pointing to erratic idle, I’d suspect ignition over fuel given the tach signal is directly tied to it.

Still…gotta go through the motions and test.

If you erase the codes, what do you get now?


Stolen from Jrichker

Code 98 - basic computer internal tests not passed before it started processing the request to dump codes. Do a Key On Engine Off test before starting the engine. Wait until you see the 11 flash before pressing any buttons if you have a hand operated scanner. The computer is operating in Limp mode, so fix any codes it dumps prior to doing anything else.

Code 66 MAF below minimum test voltage.
Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing +12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for computer power relay switched +12 volts. Check for 12 volts between the red and black wires on the MAF heater (usually pins A & B). while the connector is plugged into the MAF. This may require the use of a couple of safety pins to probe the MAF connector from the back side of it.


If you disconnect battery and hook back up and immediately dump codes, you get a code 98. Need a few engine cycles first.
 
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I would look at fuel in terms of having an issue with your current setup, vs fuel supply not being sufficient. Your gt40 combo is rather mild and and could easily run on 19# injectors and stock pump. With that said, I agree with running 24s in this case, but it should still run on 19s

With that said, you just need to get a guage on there and see what your pressure is at idle and at wot. Just need to cross it off the list.

This could also be ignition related woth the usual suspects of the TFI/PIP/Coil. In fact, with it generating a code pointing to erratic idle, I’d suspect ignition over fuel given the tach signal is directly tied to it.

Still…gotta go through the motions and test.

If you erase the codes, what do you get now?


Stolen from Jrichker




If you disconnect battery and hook back up and immediately dump codes, you get a code 98. Need a few engine cycles first.
Ahh ya I didn’t know that about the code 98 part. Suppose that makes sense. So I could cross that off the list of importance? It SHOULDNT be TFI as that is brand new and replaced, but we all know brand new parts can fail lol. I can toss a gauge on and see what the pressure is. I’ve got it in the garage now, I’ve done a wire tuck on the passenger side so I’m gonna pull off the fender liner and trace the wires to ensure nothing is broken or messed up. Every time I do something that may be a possible fix I erase the codes and run through it again. But even then, every time I disconnect the battery after doing something the check engine light comes back anyway.