86 2.9L idling rough/small surging/high idle

[86RangerXL]

My 86 ranger with the 2.9 has been idling weird ever since I got it. Sometimes when I first start it up, it surges slightly, but goes away after I rev it for a second and doesn't happen when its warmed up. The bigger issue is the rough idle. It idles rough whether its warmed up or not, but seems to smooth out while driving. You can feel the whole truck shaking a little bit on random intervals while idling and can kinda hear it too. It idles really high to, to the point where it jumps a bit when I put it in gear.

I've heard that so many different things can cause idle problems and I can't just go throwing tons of parts at it, so I don't know what is causing this.

 

[franklin2]

I threw tons of parts at mine, and it always did it. It was a 86 2.9 also. Ran great except at idle. I finally figured out it was idling a little rich. No codes, and tried everything. The only thing I didn't try was the fuel injectors. This truck had sit around some before I got it, so maybe the injectors are sticking. I didn't realize the injectors could have a problem like that till I see on here several guys had problems and it ended up being bad injectors.

You should do all the normal things first though;

1. Clean the throttle body
2. Clean the idle air control valve
3. Temporarily block the EGR to make sure it's not leaking
4. Check for any other vacuum leaks.

 

[86RangerXL]

I threw tons of parts at mine, and it always did it. It was a 86 2.9 also. Ran great except at idle. I finally figured out it was idling a little rich. No codes, and tried everything. The only thing I didn't try was the fuel injectors. This truck had sit around some before I got it, so maybe the injectors are sticking. I didn't realize the injectors could have a problem like that till I see on here several guys had problems and it ended up being bad injectors.

You should do all the normal things first though;

1. Clean the throttle body
2. Clean the idle air control valve
3. Temporarily block the EGR to make sure it's not leaking
4. Check for any other vacuum leaks.

I cleaned the throttle body just a few weeks ago, the IAC is new, the EGR stuff is brand new, there may be a vacuum leak, but I can't seem to find any. I just tested the fuel injectors by ear and I can hear them all clicking consistently like they should except for cyl. 6 which I can't get to easily so I couldn't test it just by sound. Some people say the O2 sensor, the map sensor, or the TPS could cause it.

 

[franklin2]

A long time ago I had a SVO mustang and I took the injectors out and went to the Ford dealer and had them test them. He did right there when I was in the shop, they had a tester machine. It tested flow rate and he also said they check for proper spray pattern. If the spray pattern is not right or they shut off with a little dribble of gas, I think that could cause a rough rich idle.

That's the only thing I could come up with on mine. I was going to take them out and see if I could make my own tester with a old fuel pump but I never got around to it and it's not on the road now.

 

[JerrySab]

Not to pee on your wheaties, but I think these old critters have a pretty lousy idle even when they're in great shape.

THAT SAID. New injectors will definitely help if they've never been changed. I was recently struggling with the exact same symptoms. I was stumped so I threw in the towel and finally took it to a mechanic. They attached an oscilloscope to various parts and found:
- TPS would randomly send DRIVE (like, full 5v) signal to PCM when truck is just sitting there. idle would spike. and remain until TPS stopped malfunctioning.

..also,
- Diesel stop screw on throttle body dialed in too far. RonD frequently gives the advice to only have the diesel stop screw dialed in enough to keep the throttle plate from sticking in the bore. He's right.

- small vac leak at throttle body. Easy gasket swap.