Hi. I have a 1994 F-150 5.0 V8 with an idling surge issue that I CANNOT figure out. The truck idled real rough until I took it in to have an O2 sensor replaced. That made things better, and I was advised to replace the TPS and IAC, which I did. I also cleaned buildup out of the throttle body.

The above made engine perform better. However, I'm having flucuation in my engine RPMs, which bounce around particularly at idle. I replaced spark plugs, plug wires, distributor cap and rotor with Motorcraft parts but that didn't fix the issue. I've unplugged the MAF sensor and EGR to test if that may identify cause but doing this didn't matter -- still have the same issue.

I don't hear a hissing like a vacuum leak and the engine itself runs smooth when given gas -- no knock whatsoever and no CEL since the O2 sensor was replaced. My tach runs similar to this (I'd say my flucuation is another 100-200 RPM higher or lower with slight vibration, it isn't violent shake by any means): https://youtu.be/PIks2fylTfQ. I'm at a complete loss for what could be going on ...

 

I had the same problem and put sea-foam in the carb.. spray a full can into it. Also a can into the gas tank and added a can of oil treatment to the crankcase seafoam also.. it smoked like a bitch for about 15 minutes but cleared the problem. I was thinking sticking valves or something...

 

Thanks. I've debated Seafoam but seen mixed reviews on whether or not it's good for a motor and actually does anything internally. I'll research it more based on your suggestion. Another suggestion I've gotten is possible intake vacuum leak...anyone have experience trying to diagnose and troubleshoot this? Can a basic DIYer do this or should I go to a shop? Haven't found much online so far.

 

Follow up. I sucked about half a can of Seafoam into vacuum line yesterday and that improved things. RPMs still fluctuate but have to study needle to notice is. Hopeful once this tank of gas with Gumout's Regane added works through things level out entirely. May Seafoam again in a few days.