Mysterious Idle Issue

[1995 Pony GT]

Hello all,
I am new to the forums here but have an issue that I have not been able to find a solution to and have been looking far and wide on google and many forums so I decided to finally post. I bought a used 1995 mustang GT 2 weeks ago and had a 2006 GT before hand. I really love these cars however with this one I can not seem to get it to run properly. The guy I purchased it from said the car had been sitting for 6 months which isn't too long but also some things could happen in those months. Now to get down to the problem.

The problem was originally a rough idle where the car would feel as it was misfiring a lot and the whole car would shake. Also the idles would be sporadic ranging between 500-1200. I did some research and saw that the IAC, MAF or pretty much any other sensor leading to the throttle body would most likely be the culprit. That being said I cleaned the IAC (didn't fix it so later replaced the IAC), replaced the MAF, replaced the TPS, cleaned the heck out of the TB and upper intake manifold, replaced the upper intake manifold gasket, put new spark plugs and wires in and hoped the problem would be fixed. After all of that work I started her up and there was still a rough idle although she did sound much much better. I then read about possible vacuum leaks and although I could not hear any I replaced all the hoses going to the upper intake manifold. when I reset the ECU by disconnecting the battery the car idled very odd when starting up nearly stalling and then shooting up to 1500 RPMs and kept doing that until I drove down the street at which point it would only jump between 900-1100RPMs. I then replaced the distributor cap & rotor and the distributor coil pack since they all seemed slightly corroded. Now the car has problems starting cold where it will take 5-8 seconds of cranking for the car to turn on and after it turns on the car will idle between 600-1.2k RPMs until I drive down the road at which point it will idle at around 1k and will jump up or drop down very slightly almost to the point of not mentioning. Also the misfire feeling is nearly gone but I can still feel it slightly when I am at a stop light. Every once in a while when I cold start the car the volt gauge in the car will jump around some before evening out too. Another issue I have that is probably related to it all is that if I start at 0MPH and slowly let off the clutch without giving the car any gas it will go into gear and if I try to cruise at about 1k RPM down the road then the car will start to buck. The bucking will be every few seconds at first but if I do not give the car gas the time in-between each buck will decrease until it feels like you are trying to go off-roading(lol). Finally, I had the battery and alternator checked at the auto store the other day and the test said they were both fine however when we were testing the battery the guy asked me to turn my headlights on and the AC on max and when I did that the cars idle kept dropping to the point where it nearly stalled and then shot up to 1500-2k RPMs

Sorry for this super long post but I wanted to give as much info as I could. Any help would be greatly appreciated and I'm hoping the car isn't a lost cause.

 

[90lxwhite]

@jrichker

 

[toyman]

Two things. First, remove and clean the battery to engine and engine to chassis grounds. Second, pull the data trouble codes (DTCs). The diagnostic port for the 94-95 5 liter (GT) is in the engine bay, passenger side between the firewall and strut tower. This is also an OBDI system vehicle. Once the grounds are confirmed as good and any codes present are identified you should be able to narrow down the search. With particular reference to vacuum issues the only reliable test is a smoke test. There are too many hidden vacuum lines and connections that only a smoke test can locate a source.

 

[1995 Pony GT]

Alright, I will try that when I get home and will let you know how it works. I will try to pull any codes and see if there's one that is pending. A question I have also is that the guy at the local auto parts store said that if there are any pending codes on the computer then if they do not trigger the check engine light immediately they will trigger no matter what after 3 heat cycles (he said after the car has heated to normal operating temps and cooled off 3 times). Also this morning on the drive to work I noticed another noise since I had the window down and it kind of sounded like aluminum foil was grinding on the street. I was only ever able to notice the noise when I was making a turn and I was at the end part of letting the clutch out to put the car in gear which pretty much only happens at stop signs since I am usually done down shifting right before any other turn. Thanks again for the help thus far!

 

[toyman]

When you disconnect the grounds to clean them you will also erase all the codes and the KAM (Keep Alive Memory). So in that sense, the computer (ECU) will need a few cycles to relearn certain parameters. CELs are most often related to emission issues so there may be others that only pulling codes will identify. So give the ECU time to make these adjustments before pulling the DTCs.

 

[1995 Pony GT]

Ok so I cleaned the battery cables last night and it did help some but did not fix the issue. I just pulled the codes from the car (the car was not at operating temps but had only been sitting for 2 hours after it was last at operating temps) so Im not sure if that would make a difference. The codes I got were 12, 13, 14 and 31. Im not sure as to what the DC motor is and if the voltage to the EGR valve are low then does that mean the EGR valve is going/already bad and I need to replace it or that I need to replace some sort of wiring for it? As for code 12 would that mean I was sold a faulty IAC valve or could that be caused by a different problem? Finally, for code 14 Im not sure as to how to fix engine RPM signal fault or PIP circuit failure. Any and all help would be greatly appreciated guys!

 

[90lxwhite]

Ok so I cleaned the battery cables last night and it did help some but did not fix the issue. I just pulled the codes from the car (the car was not at operating temps but had only been sitting for 2 hours after it was last at operating temps) so Im not sure if that would make a difference. The codes I got were 12, 13, 14 and 31. Im not sure as to what the DC motor is and if the voltage to the EGR valve are low then does that mean the EGR valve is going/already bad and I need to replace it or that I need to replace some sort of wiring for it? As for code 12 would that mean I was sold a faulty IAC valve or could that be caused by a different problem? Finally, for code 14 Im not sure as to how to fix engine RPM signal fault or PIP circuit failure. Any and all help would be greatly appreciated guys!

You'll have to pull the distributor to replace the pip. It'd be easier to replace the distributor than to replace the pip only and reuse the stock distributor due to the fact that aftermarket units come w the pip pressed on. But some say that aftermarket distributors are cruddy compared to oem.