1998 3.0 Misfire/High Idle/Stutter...tried everything...HELP!

[Ajent]

Long time lurker, first time poster here. I've learned a TON on here and was hoping to get a little help for my specific issue.

I've got a 5-speed manual 1998 Ford Ranger 3.0 4x4 that has been giving me fits. It started with an infamous high idle that I couldn't track down. Under load (and even directly after a dead cold start) the RPMs hang at around 2.5K and only goes down once the truck rolls to a complete stop. Here is what a buddy of mine and I have done so far:

• Unhooked the IAC and high idle condition stopped
• Replaced the IAC (Motorcraft)
• Tested the TPS voltages by back probing - All good. in spec and all smooth.
• Checked and cleaned the throttle body
• Clean out the air box and new filter
• Unhooked the speed sensor on the rear differential and the high idle stopped. Speedo in the cab reads normal when speed sensor is connected
• Cleaned the MAF and tested another truck's MAF...still no change
• New PCV valve and checked PCV hose
• No sign of vacuum leaks

Additionally, even before we started tracking down this high idle issue, I noticed a small stutter and a loss of power at highway speed. It has since developed into something bigger. The first time it happened it felt like I was running out of gas. Now it does this bucking/stuttering very often anytime the engine is under load and can even take on a vibrating quality with a major loss of power or cutting in and out of power as if half of the engine isn't firing. We pulled codes and got a P0304 cylinder #4 misfire. It doesn't always throw that code, but when things really get nasty it does. So, things we've tried are:

• New plugs, wires, and coil packs (after several hundred miles on the new plugs, we pulled and inspected them, all the same and clean)
• new fuel filter
• Unhooked the cat
• Visually checked the cam shaft position sensor to be sure it wasn't chewed up - all good.
• Got Bank 2, O2 sensor 1 codes so I replaced the upstream O2 sensors
• Visually inspected and cleaned PCM connector

Any ideas where to go next? Does anyone recognize these symptoms? Could these two problems be related? We are thinking of:

• back probing wires from the coil packs to the PCM
• probing the wires to injector 4
• checking compression

 

[Ajent]

Long time lurker, first time poster here. I've learned a TON on here and was hoping to get a little help for my specific issue.

I've got a 5-speed manual 1998 Ford Ranger 3.0 4x4 that has been giving me fits. It started with an infamous high idle that I couldn't track down. Under load (and even directly after a dead cold start) the RPMs hang at around 2.5K and only goes down once the truck rolls to a complete stop. Here is what a buddy of mine and I have done so far:

• Unhooked the IAC and high idle condition stopped
• Replaced the IAC (Motorcraft)
• Tested the TPS voltages by back probing - All good. in spec and all smooth.
• Checked and cleaned the throttle body
• Clean out the air box and new filter
• Unhooked the speed sensor on the rear differential and the high idle stopped. Speedo in the cab reads normal when speed sensor is connected
• Cleaned the MAF and tested another truck's MAF...still no change
• New PCV valve and checked PCV hose
• No sign of vacuum leaks

Additionally, even before we started tracking down this high idle issue, I noticed a small stutter and a loss of power at highway speed. It has since developed into something bigger. The first time it happened it felt like I was running out of gas. Now it does this bucking/stuttering very often anytime the engine is under load and can even take on a vibrating quality with a major loss of power or cutting in and out of power as if half of the engine isn't firing. We pulled codes and got a P0304 cylinder #4 misfire. It doesn't always throw that code, but when things really get nasty it does. So, things we've tried are:

• New plugs, wires, and coil packs (after several hundred miles on the new plugs, we pulled and inspected them, all the same and clean)
• new fuel filter
• Unhooked the cat
• Visually checked the cam shaft position sensor to be sure it wasn't chewed up - all good.
• Got Bank 2, O2 sensor 1 codes so I replaced the upstream O2 sensors
• Visually inspected and cleaned PCM connector

Any ideas where to go next? Does anyone recognize these symptoms? Could these two problems be related? We are thinking of:

• back probing wires from the coil packs to the PCM
• probing the wires to injector 4
• checking compression

UPDATE: After tearing into the engine to get at the injectors to inspect, clean, and swap them to see if the problem jumped to another cylinder, we got everything back together only to discover the new plug wires we put on had breaks in them and that spark was leaking out all over the place. Not sure if it was poor quality wires or if we routed them too close to the exhaust manifold and they got toasted. NEW new plug wires are now installed and the truck is no longer bucking, and misfiring. Just goes to show that it is super important to double check that what you THINK you know is what you ACTUALLY know and to ensure newly installed parts are working as intended. I read somewhere on here that new parts are untested parts. Let that be a lesson to me in future work.

Now about that high idle???

 

[franklin2]

Try wiggling the engine temp sensor wires and see if the idle goes up or down. Maybe the computer thinks the engine is cold when it really isn't?