I have an 89 F150 302. I recently replaced the cap, rotor, new motorcraft plugs properly gapped and motorcraft wires. I just replaced the plenum gasket new motorcraft IAC and thoroughly cleaned the plenum, throttle body and egr passages. It still idles rough like it did prior to replacing all this. Any advice would be great. This idle is driving me bonkers and I cant trace it to anything sensor related and there are no codes. My only other thinking is dirty injectors but how would I clean those? Would a fuel additive do the trick?

 

tried 87 octane today and it still does it...

Any advice would be awesome!

 

I have similar issues and did the same as you and it did nothing. Would love to find out the answer to this as well. Do you have stalling issues as well?

 

No it dont stall it idles right at 8-900rpm but its a small misfire so it seams. Before it acted like the TPS was faulty and it wouldnt hold a steady idle at 800 for anything.

 

8-900 seems a bit high, so check for vacuum leaks.

Stalling at idle might mean your EGR is faulty and the valve is opening when it shouldn't. A blocked EGR passage wouldn't have any effect on driveability AFAIK.

 

800-900 RPM in neutral or park is normal. Verify you have the wires hooked up in the correct firing order as well as routing. Crossfire is a VERY common issue.

From Steve83:1983 Ford Bronco Diagrams picture | SuperMotors.net

 

Incorrect firing order and wiring for my year...

Double checked everything an the wires are correct.

It is now idling very weird. Once warmed up and coming to a stop, when a put the truck in park it will idle up to 1500rpm and slowly go down to 800rpm an jump right back up to 1500rpm and do this for at least 3 times.

Would that be a faulty TPS? I have tried checking the leads in the back of the plug connector but get no reading.

 

The wiring diagram in the link is for '93 and older. The link under the photo is for '94 and newer. Your truck is an '89 the first link should be correct unless you have a later model 5.0L engine installed.

 

Check the PCV valve. The spring may be weak or it could be worn inside. It will also raise the idle a bit.

 

People have been hunting that wavering idle problem for years. For some people, replacing the TPS does it, for others, it's the MAP, ECT, O2, vac leak, exhaust leak, or IAC. Even things that test good sometimes do it.
The TPS leads are way in there, you may need longer probes. Check the voltage and the ohms, and read the page on Old Fuel Injection (formerly Ford Fuel Injection). The computer resets the baseline setting at each run, so there is a surprisingly large range of acceptable voltage values. If you post up your numbers people will start saying they are far too low or high, take it with a grain of salt and take another look at the voltage charts.
-Mike