5.0 stops lopeing

You may have a misfire or vacuum leak..more info needed.

The may be the is engine running lean causing a misfire at part throttle but it gets richer with more throttle or you may be running lean from a vacuum leak. Also it could be a bad TFI module causing a misfire. Did you change out the TFI?

Take both battery cables off for 30 mins ..then connect them start the car let it idle for two mins the drive it around and see if the problem is better. If it's the same you may have a fuel problem ..also you may want to check codes if this does not work.

I would check the codes to be sure..look for code 41 or 91 or 96 just to be sure before changing anything else.

Okay if it runs better now ..but still has the idle problem on cold start it could be a bad ECT or ACT sensor ..but until you can pull the code you never know. But i will tell you this a bad ECT will make your car run lean on cold start also how old is your fuel pump? and have you changed your fuel filter ? From what i can tell ..your car is running lean.. because when you push the throttle ..the a/f gauge should go to rich not lean.

See the "Surging Idle Checklist” for help with all your idle/stall problems. You can guess at the problem and throw parts at it, or you can use the checklist to help you find the problem quickly and inexpensively. The checklist is right here in the Stangnet 5.0 Tech forum and you don’t have navigate to some other unknown web site. It‘s free and doesn’t cost anything: at last count there were more than 103,000 visits and still climbing

The quick and easy way to dump the codes is in there too, and all you need to do it is a paper clip! The first two posts contain all the fixes & updates. At last count there were 24 possible causes and fixes for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.

fknitup 4x4 said:

Alright spent 5 hours trying to hook a scanner up won't do crap chased all wires back to computer all have continuity don't know why it won't read. When I try a koer it picks idle up that's all.its a snap-on scanner and tried a different scanner also. By the way replaced 02s sounds perfect a/f at last stoich bar at idle when I accelerate the a/f dissappears into lean wot ok still won't cold start dammit this efi stuff is really touchy

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It sounds like you have computer/wiring problems. Please post the model year of your car so I can post the correct test path for Computer will not dump codes.

Computer will not go into diagnostic mode on 86-90 model 5.0 Mustangs

Disconnect the battery positive terminal before making any resistance checks.
The voltage drop in the ground cable will cause incorrect resistance readings.

How it is supposed to work:
The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the computer into self test mode. If this ground is bad, none of the sensors mentioned will work properly. That will severely affect the car's performance. You will have hard starting, low power and drivability problems. Since it is a dedicated ground, it passes through the computer on its way to the computer main power ground that terminates at the battery pigtail ground. It should read less than 1.5 ohms when measured from anyplace on the engine harness with the battery pigtail ground as the other reference point for the ohmmeter probe.

What sometimes happens is that the test connector black/white wire gets jumpered to power which either burns up the wiring or burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60.

The STI (Self Test Input ) is jumpered to ground to put the computer into test mode. Jumpering it to power can produce unknown results, including damage to the computer. The ohm test simply verifies that there are no breaks in the wiring between the test connector and the computer input.

How to test the wiring :
With the power off, measure the resistance between the computer test ground (black/white wire) on the self test connector and battery ground. You should see less than 1.5 ohms.



If that check fails, remove the passenger side kick panel and disconnect the computer connector. There is a 10 MM bolt that holds it in place. Measure the resistance between the black/white wire and pin 46 on the computer wiring connector: it should be less than 1.5 ohms. More that 1.5 ohms is a wiring problem. If it reads 1.5 ohms or less, then the computer is suspect. On the computer, measure the resistance between pin 46 and pins 40 & 60: it should be less than 1.5 ohms. More that that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.

If the first ground check was good, there are other wires to check. Measure the resistance between the STI computer self test connector (red/white wire) and pin 48 on the computer main connector: it should be less than 1.5 ohms. More that 1.5 ohms is a wiring problem

The following is a view from the computer side of the computer wiring connector: it is for an A9L, A9P computer.




Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the black/white wire goes. Almost every sensor on the engine except the MAF is connected to it.



See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine

See the graphic for the 10 pin connector circuit layout.

fknitup 4x4 said:

Harness out of 88 speed density with mass air pigtail. If wires have continuity why won't it scan? Start up have to hold acc down starts in rich and slowly goes to last stoich 1st lean bar as I accelerate steadity it dissappers in lean till 4200 or so then starts moving around wot reads into rich

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What wideband are you using? Sounds like you have one that gives you a light show instead of a readout of afr or lambda.

If your resistance reads less than 1.5 ohms from sensor signal ground on any sensor(Black/white wire) to battery negative cable ,you are OK.

If you don't have less than 1.5 Ohms 1.5 ohms from sensor signal ground on any sensor (Black/white wire) to battery negative cable ,you have problems. Check the resistance from computer pins 40 or 60 to pin 46. You should see less that 1 ohm. If you don't, the computer has internal damage to the signal ground. See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.

What wiring harness got burned? The one on the passenger's side that contains O2 sensors and such? If you got some wires shorting out in that harness, you could have burnt your signal return pin 46 in the computer. This would probably account for why you couldn't get your codes to scan. Are you using a different computer then designed for your trans (auto vs manual computer)?

This is a great example of what to look for inside your computer.



Good luck with it.

Joe