Unpredictable Startup in 94 gt

[Purple95]

My friends stock 94 mustang gt starts and runs fine 75% of the time but sometimes when he goes to start the car it'll start up and and idle up and down between 250-1500 rpm about ten times and then completly die. During this period if the car is given gas it blows alot of black smoke out the tailpipes and smells very rich and the car is completly undriveable. If he lets the car sit for a couple of hours it'll start right up and run perfect. If the fan comes on before the startup while the key is in the on the position the car will not run right it will have the eratic idle. If the fan doesn't come on before startup the car starts right up and runs perfect. We have no idea what the problem may be and the ccrm is less than a year old so we dont think that's it and no codes came up. Any help with this problem would be much appreciated.

Should the fan come on before startup?

 

[HISSIN50]

Look for either an open ECT circuit or see if the EEC is trying to go into diagnostic mode.

 

[WhiteCobra95]

Check the codes. Sounds like the coolant temp sensor to me. The fan coming on is a good indicator because it's a fail-safe mode for a bad sensor. If it doesn't happen all of the time, then it's an intermittent fault which will be trickier to catch. I would suggest checking the wiring harness if possible; measure the continuity between the sensor connector and the ECU. You might have to rattle the harness a little to see if you can find a break or a short. The other option is to just replace the sensor and see what happens. Good luck!

 

[GTA_V6_Mustang]

Look for either an open ECT circuit or see if the EEC is trying to go into diagnostic mode.

i second that exact thing to do. While it could be an intermittent issue with the ECT, it also sounds like the problem I had with the EEC going into diagnostic mode, though after a minute the car was fine again and It wouldnt blow black smoke but would smell SUPER RICH!! What I found I was able to do. When the car exhibits the condition (in my case fan would cycle and solenoids click but ez way was CEL was off when u where going to start the car, no bulb check) And take the jumper wire short diagnostic input to output and start the car, if when u pull the wire the car clears up, thats it for sure.