have a 98 f150 4.6 was running fine then all of a sudden engine started miss firring, if pass side coil pack disconnected while engine is running there is no difference in way engine runs,tried new plugs,wires and 1 new coil wich was swapped from side to side,tried new (used) pcm still no difference swapped injectors (screens fine) sublet to ford garage did cyld balance test 3 cyld showed average of 13% off all on the same pass side coil bank i have a hard time believing valves on 3 cyld. went bad all at once, removed pass side valve cover checked timming marks seemed fine. replaced fuel filter old one seemed fine.if rpms brought up seems slightly better. after engine started miss firring drove home 150 miles with loaded horse trailer have spark at all plug wires.
i am a fixed ops manager at a saturn dealership and my techs are scratching there heads... they think internal im not buying it....any sugestions?
thanks bob

 

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Just Added Note Compression Test Was Performed And All Cylds Were At 150 Psi No 1 Cyld Was Off By More Than A Few Psi!

 

Check the crank sensor wires and connector. see if the wires are chafed anywhere and the connector makes good contact. If you can, use an ocilliscope on the crank sensor.

 

I Will Check I Doubt That Could Cause Engine Miss On 3 Cylds Only with 1 Coil Pack, Would Effect All Cylds due to not signaling correctly or would cause cutting out or no start.

 

Are they the same cyl. when doing a power balance? Also, can you check fuel trim?

 

Cyld # 1/5/6 Are Cylds Miss Firring No Way To Check Fuel Trim...when Rpms Up Still Miss Fires But About 20% Better.

 

Has anyone double checked the wires to make shure they are correct? I've had them screwed up before.

 

have double checked firring order remeber was running fine then all of a sudden came into this!

 

Shouldn't the truck be throwing a code?

 

did have cyld 1 &3 when we 1st looked at it reset codes and they never returned just continued to run rough/miss. we had just put new exahst manifolds on it due to power loss from leaks (warped & pitted) and was told truck never ran so good as it had after that and ran great for 2 months then this happened, had had numerous 0-2 codes checked all data and tsb`s on causes there but none this time exahst leak would trigger those.

 

Have you checked any vacume hoses? Thats what happened to my fathers truck. It was a vacume hose to the throttle body.

 

My fathers 97 had two 90 degree hoses to the throttle body that were cracked. He spent hundreds of dollars to fix and to no avail. Replaced the hoses any it fixed the problem.

 

thanks for your post shampboys i will check but seems that would set a light for sure or create a high idle, im suspecting a wirring issue and was hoping someone in here had seen a simular case. im sure my techs have checked hoses, im having him traqce wirring harness for injectors and check for a open today.
will get back to you all later
thanks

 

No problem. I know that we had the truck to the dealer ship and they could not find the problem. Also there was no light and no high idle. We replaced the front hose and then the problem came back several weeks later. Found that there was another hose on the back of the T/B and replaced it with no further problems.