intermittent check engine light with no codes.

[Flatpipe]

My 95 Gt is throwing an intermittent CEL. It does not have a code. I have had several different shops scan this car, with the light on and when the light is off. I have played the guessing game and replaced O2 sensors, EGR valve,throttle position sensor, air idle control sensor, coolant temp sensor, distributor, coil, fuel pump, and engine management control solenoid (ccrm).
I've heard of a loose ground wire at the fuse block might cause this. Does anyone have any insight to what the answer could be?

 

[General karthief]

Does the car run ok?
do you have any modifications?

 

[Flatpipe]

I have a ty cold air intake, but it uses all the stock sensors and is sealed very well. I thought there might be a vacuum leak so I went over that and made sure. I have a MSD 6A box. And some suspension mods. Other than that the car is stock. When the light is on it goes into a limp mod and the engine timing goes off. Loses power and all the valves rattle under acceleration. When the light is off, it runs great.

 

[General karthief]

Remove the msd box and try it

 

[Flatpipe]

It has been happening longer than the MSD box has been on there.

 

[General karthief]

Adding stuff like the msd box when your already having problems makes finding and fixing the problem harder.

 

[Flatpipe]

This car has had multiple problems. I have fixed most of them. The MSD box was a left over from my old hot rod. I tested it out and made sure it was working properly before i installed it and it has not changed the intermittent CEL. Everything seems to run fine most of the time, just every now and then the CEL comes on and the car acts up.

 

[jozsefsz]

Where were they attempting to scan your codes? Most shops see the OBD2 connection under the dash and use that, which isn't connected to anything (it was used on the v6's). The proper place to scan the codes is using the OBD1 connection under the hood (by the passenger strut tower) using an OBD1 scan tool.

If your CEL is coming on, it's throwing a code, or storing a code. It sounds to me like you've probably got a lean condition - valves don't rattle with changed timing, but engine knock definitely happens when you've gone lean. Check your fuel pressure when this happens, as you've replaced the parts that commonly cause it - o2 sensors and fuel pump. Could also be MAF related but getting to your codes is the only way to know.

 

[Flatpipe]

We are scanning at the OBD1 plug under the hood. I am wondering if the throttle body is leaking air at the shaft. When the CEL is on, and the rattling is going on, some extra gas pedal gets the car to down shift and it goes away. The rattling sounds like the engine is bogging down. I ordered a new OBD1 scan tool and ^" cable. I'll rig up away to run it inside the car so I can scan it when the light is on.