Idle and power issues on my '95 Cobra (codes inside)

[02 281 GT]

A couple of weeks ago my '95 Cobra started acting similarly to when the MSD coil went bad a few years ago: sputtering while driving and at idle accompanied by a general sense of the car being down on power while driving. It will run like this if it manages to stay running when I start it. On many occasions it will die almost immediately after starting.

I hadn't replaced regular tune-up items in a while, so I started with that (Motorcraft cap/rotor/spark plug wires and Autolite plugs). I also ordered a new fuel pump just in case. The fuel pump just arrived today. After buttoning everything up, it's running exactly the same as it did prior to all the new components.

After this, I broke out the code reader: KOEO codes are all related to missing emissions equipment. Multiple KOER tests netted a few different codes which include 157 (MAF voltage low/grounded), 159 (MAF voltage out of range), and 998 (vehicle running in failure mode).

I cleaned the MAF after which the car ran flawlessly for about a minute. I was able to get a good KOER test with only emissions-related codes. After that good run, it started running badly again immediately after I shut the car off and tried restarting it.

What I have done so far:

1. Replaced fuel pump
2. Replaced spark plugs, distributor cap, and rotor
3. Cleaned MAF several times
4. Pulled codes during KOER test: 157, 159, and 998
5. Looked for vacuum leaks (couldn't find anything obvious)
6. Checked battery voltage (14.0 when engine was idling more or less correctly)
7. Swapped in a new coil to see what would happen (nothing changed)

The items above did not change the way the car runs at all with the exception of the one good KOER test I got after cleaning the MAF the first time.

What I have not yet done:

1. Cleaned or checked IAC
2. Checked TPS voltage
3. Checked fuel pressure
4. Reset base idle

Here's a video of the car running to show the typical idle issues. It's about a minute long.


View: https://www.youtube.com/watch?v=RETMwYjwcPw


I'm going to order a fuel pressure tester tonight (probably).

Extra information for what it's worth:

1. There's a vague smell of fuel in the vacuum hose off the fuel pressure regulator (not really enough to make me think that's definitely the cause, but it is the original regulator)
2. Every sensor is original to the car, however, I didn't get a code for anything but the MAF

 

[General karthief]

Smell fuel, replace the fuel pump regulator.
JMO

 

[02 281 GT]

Smell fuel, replace the fuel pump regulator.
JMO

Now that I think of it, I had forgotten, but I did also get a code for HEGO system lean. I don't remember the specific number code, though. I'll go ahead and order a new regulator.

 

[Chythar]

If the fuel pressure regulator doesn't clear the MAF codes, gat a new MAF sensor as they can go bad. I tested mine by 'borrowing' an off-brand one from my local auto parts store. It fixed my poor running issue, so I bought a better one from rock Auto and returned the cheap one.

 

[02 281 GT]

If the fuel pressure regulator doesn't clear the MAF codes, gat a new MAF sensor as they can go bad. I tested mine by 'borrowing' an off-brand one from my local auto parts store. It fixed my poor running issue, so I bought a better one from rock Auto and returned the cheap one.

Thanks. That'll be my next step.

Which better brand did you end up going with?

 

[Noobz347]

7.) Dirty or defective MAF element: Clean or replace the MAF element. Disconnect the MAF and start the car. If the idle smooths out, then proceed from here. Keep in mind that this does not work on every car.

The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element, remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).

The MAF output varies with RPM which causes the airflow to increase or decease. The increase of air across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow. Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF and ground.

From this: https://www.stangnet.com/mustang-forums/threads/help-me-create-the-surging-idle-checklist.698148/

 

[Chythar]

Thanks. That'll be my next step.

Which better brand did you end up going with?

I picked up a DELPHI AF10352, back in October of 2017, and it's still running great.

 

[02 281 GT]

From this: https://www.stangnet.com/mustang-forums/threads/help-me-create-the-surging-idle-checklist.698148/

I originally didn't run that checklist because the idle didn't seem to be "surging" to me. But after reading JRichker's definition of "surging idle", I'll go ahead and run through that this weekend. Shouldn't be too difficult. I should have it fixed quick at this rate.

I picked up a DELPHI AF10352, back in October of 2017, and it's still running great.

I was thinking Delphi. Thanks.

 

[02 281 GT]

A new Motorcraft FPR is on the way with a scheduled delivery for Monday. I need to get to work tomorrow, so I did a quick run to the junkyard with the wife’s car and found a ‘95 E-150 that coincidentally uses the same FPR as a ‘95 Mustang. It was a newer aftermarket unit. Threw that in when I got home and it’s running flawlessly. Looks like I found the culprit. I’ll keep the Motorcraft one on hand as a spare. Thanks, everyone.