Rough Idle - Almost everything replaced - Puzzled mechanic!

[dwildey]

I have a 1990 Full-size with the 5.8 liter.

A few months ago, I had a problem where the idle became no longer smooth and would jump as much and drop as much as 400 RPM. Reading through the forums, I started with a basic tune-up and then checked and replaced parts as needed. Of course the computer was not showing any problems with KOEF and KOER.

Symptoms seem to be worse when its the engine is WARM and it is in gear idling. It will normally stall when I put it in reverse.

Here is what has been done so far and the mechanic isn't sure and I've given up on guessing:

•  
  
• New plugs
  
• New wires
  
• New Cap and Rotor
  
• New Idle Air Control Valve (Genuine Ford OEM)- read about some problems w/generic
  
• New Throttle Position Sensor (OEM replacement)
  
• New Coolant Temperature Sensor
  
• New O2 Sensor
  
• Replaced Manifold Absolute Pressure Sensor (with used)
  
• Adjusted Timing
  
• Checked for vacuum leaks
  

* One thing I did not have checked and my mechanic doesn't think it could be related was the Fuel Pressure. I am taking it in tomorrow to have it re-evaluated but I suggested he look at that.

And thing we should be looking for or ideas from experience?

Thanks in advance!

 

[50joe]

May want to pull the vacuum line off the fuel pressure regulator, and check for fuel in the line. If so, thats one way to see if the regulator is bad.

 

[humboldt]

hey i just went through this with my truck its the distribtor... no joke i replaced everything that was the problam

 

[BroncoJoe19]

a surging idle is very often a vacuum leak.

Checking for vacuum leaks.

Some of the sensors and some of the actuators either read the amount of vacuum, or use vacuum to move controls, therefore it is important to the proper functioning of the engine that your vacuum system is fully intact. Otherwise one may get codes falsely indicating that there is a problem with a controller/actuator, or a sensor when in reality it is caused by a lack of vacuum.

Look at your rubber vacuum hoses (they are about 1/4 - 3/8 inches in diameter, if they are dry rotted or cracked, replace them. IF they slip on/off of their connectors too easily, they may have stretched out a little bit, cut the ends off, and put them back on.

You may listen for a leak. Sometimes using a cut off piece of garden hose is a good aid to listen with. Sometimes a cardboard tube (like from gift wrapping paper) is helpful. Just remember that whenever you stick your head inside a running engine compartment, that you are not wearing any loose clothing or jewelry that can get caught.

Another suggestion is to use a spray can of carb cleaner and spray around the bottom of the intake manifold, and the base of the throttle body assembly. IF there is a leak, you should hear a change in your engine.

Keep a fire extinguisher handy, or use water instead of carb cleaner (it works, but not as well.) Be aware that if you use water and the wires get wet and it runs worse... your wires may be bad, and the water is allowing them to arc instead of jump the gap of the spark plug.

Another suggestion is: to take a propane torch, turn it on (but don't light it) and move it over the vacuum hoses and connections. The engine RPM's will increase if there is a leak and the propane gets sucked in.

Courtesy of TheOldWizard... Don't forget to remind folks to check the PCV valve, hose and grommet !

 

[shift1313]

My guess is your EGR valve. I would double check the stuff Joe mentioned but also take a look at the EGR circuit. The actuator may be working fine but the actual valve could be sticking(no code thrown). Its very common for the internals to get stuck from carbon deposits(since you are running exhaust back through). What could be happening is the vehicle runs fine when cold because the EGR is open and trying to heat up the mixture faster. Once the vehicle is warm and the computer things the valve is closed it changes the fuel mixture but its not right because the valve is still open. Your O2 sensor sees this and the ecu tries to compensate but since it still doesnt know the EGR is open it will fluctuate.

One of my trucks had a horrible time with this and I ended up pulling the vac line and capping it and just making sure the egr was closed to start and now the truck doesnt have idle issues.

 

[shift1313]

oh yeah one more thing. Does this only happen in gear or is it worse in N? What tranny does your truck have?