Engines Missing....no CEL

[Kappa2006]

Hey guys, I just purchased a 99 GT and am having a problem. The car misses all the time at low rpms...anywhere from 1200-1800 rpms when you put any throttle into it. Then midrange rpms seem to be fine until you punch it. Then the car pulls hard but spiratically misses. It also misses very randomly in the mid rpm range with several throttle positions. Its not like the car falls flat on its face. just a quick miss. No codes are set, stored or pending. Already cleaned the maf sensor. Checked for vacuum leaks, none. O2 sensors are cycling properly. Changed fuel filter and did a fuel injection cleaning wtih BG commercial cleaner right through the fuel rail. Cleaned the throttle plate. Haven't checked plugs or coils yet becasue I figured if there was a badly worn one, it would happen constantly, and not just at low rpms or full throttle. Checked tps sensor, voltage readings are correct. and its not like theres a burnt contact on the sensor because it happens at all positions of the throttle. EGR valve is opening at proper times, and the IAC valve is opening correctly. (all checked with scan tool) Ok, now I reset the pcm after cleaning the maf sensor, and the car ran great. pulled hard, no misses at all. Then after driving I would say 100 miles (guess), it started to come back. slightly at first, but progressively getting worse.
I had a guy at work (technician) check out the scan tool data and he said it seemed to him that bank two is running slightly leaner than bank one. So I checked for leaks at the intake manifold, all vacuum hoses, and all gaskets with propane, no change in fuel trim or idle quality. Well I think thats all the info I have so far. I'm completely stumped. If anyone has some ideas or input, please share. Thanks for your time and help.

 

[COramprat]

Don't be too sure it isn't a plug. Pull and replace and see if that helps. Check the cops to make sure they are clean and dry.

 

[03ghoststang]

Don't be too sure it isn't a plug. Pull and replace and see if that helps. Check the cops to make sure they are clean and dry.

i agree just pull them out and see

 

[stroked4.6]

Its not severe enough to throw a code so I would double check the COP's and pull all the plugs. If you haven’t given the car a tune up you might as well change all the plugs and make sure they are properly gapped. If im not mistaken you should gap them to .054 (stock).

Just for the car to be running in tip top shape I would change the plugs, fuel filter, replace the pcv, and check you air filter. Put some Fuel Injector cleaner in and a good tank of gas take her out for a spin.

Hope this helps
John

 

[mr-mike]

had the same problem it was the cops & it wouldnt throw a code at first then after a while it sent a code ....it was the cop # 3 ....put a whole new set ..$200 from e-bay no problems since then ...the cops do go bad & have some type of resistor inside which not even the tech's at Ford could explain ? there is a way to actually test them with an ohm meter ....hope this helps