So...

I finally got my '99 4.2 put back together....I bought it totally pulled apart with a bad head gasket diagnosis.

I replaced the all the gaskets, new plugs, oil pump, CPS sensor, and cleaned a LOT of carbon off the top of the pistons.

It runs very smooth, idles perfect, and is super quiet. The weird thing is I have this strange stumble or misfire once I hit about 2k RPM that continues through about 3k RPM. Seems worse when it's under a load...like a hill. No codes are being thrown...but I can feel it. It's like a jerk that continues through the RPM range. I can feel it when I rev the engine in neutral also, but not as obvious....more of a harmonic vibration. I've got a hard hot start condition going on also. Starts perfect after a couple of turns when cold....once hot...I will need to crank for what seems like forever...but probably 8-10 seconds.

A little back story...I started out with the Autolite 5144's...but after doing some research, I switched to the Motorcraft finewires...VAST improvement. I also bought a new coil pack thinking that might fix the issue....but it didn't. I have the Autolite wires. Fuel filter has been changed.

The only thing I didn't really do anything with other than new o rings was the injectors. Think that might be the issue? Vac lines seem good...I checked and double checked. What else should I look for?

 

Checked fuel pressure yet? Some vacuum leaks are a b!t$h to find!! I would also look at the spark when it starts to miss/stumble. Your issue sounds a little bit like mine but I have a vibration all the time.

 

Yes...bought a fuel gauge and I'm holding steady at 32 at idle...goes up a bit when revving, and settles back down. Pulled the vac line off the regulator and fuel pressure shot up to 40...which I read is normal. No fuel smell coming from the regulator. I thought maybe it was the head temp sensor, but I did replace that when I did the head gasket. The only other temp related thing would be the sensor in the air intake.

This wknd, I cleaned the IAC...no help. Today, I ordered new injectors, and a regulator. I've read in several places one of the injectors could be leaking and causing the hard hot start problem, and the hesitation/bucking....so if it's not that...I'm not sure where to look at next.

 

Is there a check engine light on?

 

Check to see if U are holding coolant system pressure and not suffering from a slow PSI leak. Likely U have a problem with UR work and it is still malfing. If UR head gasket or intake is leaking PSI it will allow steam into the combustion cylinder causing a RPM misfire type of problem. That cylinders piston will look like it is steam cleaned over some time. This can lead to the dreaded hydro lock damaged Malf. U probably haven't got a secure seal somewhere on the head. Just a steady drip of a drop of water into the combustion can bring about the high rpm miss.

 

Well....FINALLY problem solved. It was the injectors. I replaced them this evening, and the stumble is completely gone. Hot start problem is fixed, and it runs great...so much more throttle response and torque.

On a side note...anybody looking to do this job should stay away from the Herko injectors on Ebay...at least my experience was bad with them. They sent me 2 bad injectors, so after all the work of putting them in it ran like crap and smoked like I've never seen. I pulled them out again, and I could completely blow through two of them. I will say though they did not give me any crap about returning them. I ordered some other OEM injectors from another guy on Ebay that were twice the price, but defiantly worth it.

 

Did you check Rock Auto prices on injectors? They have new OEM, new aftermarket, and remans.