I have a 1989 F-150, 302ci, 5-speed, with 193,000 miles. I am going to list what is occuring now not knowing if they are all related or not. I am suspicious they are related because the symptoms appeared almost simultaneously.

1. The check engine light is still on and my code reader tells me code 31. Looking in my book, code 31 is for the EGR, EVP, and the EVR. The EGR and the EVP have been replaced. The EVR is not available so it is still stock.
2. The engine will only run off of the front tank. Switching to the rear tank kills the engine even with a full tank.
3. the engine surges between idle and about 2400 rpm when throttle is applied. This occurs from a start and also when in motion such as changing gears. I can stop the surging by getting off of the throttle. Once the rpm is to about 2400 rpm, the surging stops irreguardless of the throttle position.

Any suggestions to get rid of the check engine light and/or the surging will be greatly appreciated. I beleive to fix the rear tank issue means that I need to replace both of the low pressure pumps within the tanks.

 

Your IAC, iidle air control, might be bad or clogged. Try cleaning the throttle body with generous amounts of choke/brake clean when the engine is warm (and off). The iac is on the side of the throttle body but the holes to it are before and after the throttle plate. Make sure you douse those holes good. Then make sure you get the back side of the throttle plate good. Then fire it up(it might start hard due to the "extra fuel" in there) and give it a few more squirts while running so the snot gets sucked away from the throttle plate. Hold at 2200 rpms for a minute to get all the junk worked out of the exhaust. Check it out. If its still the same the IAC might be bad, OR...

You might have a vacuum leak. To search for a vacuum leak I use choke/brake clean and shoot hoses and intake gaskets and listen for a slight bogg in the engine. This means the engine has temporarily recieved excess "fuel"(<---brake clean) it doesn't know what to do with so boggs down for a second or two. To me its a good way to pinpoint a nagging vacuum leak.

If you go to Sears or Wal-Mart you can get your own code reader for less than 40 bucks. Also you might want to get a Haynes manual for your truck, it will help when you start narrowing things down.

 

I doubt that it is a vacuum leak or an iac problem, those problems only show up at idle. There are a few different things that cause suging. Could be an 02 sensor stuck rich, a partially stuck open EGR, or a fuel problem such as a clogged filter or a bad injector.

-Jon

 

I tend to agree . given the other problems he's listed with the tank i would be inclined to believe it is a fuel filter / regulator problem . the IAC being bad is easy to check . just unplug it when the motor is running . if the motor dies it is bad. I had a simular problem about a year ago when i joined this site . at an idle tho , a constant surging but above 2000 rpm under a load it was fine . ended up being several issues . TPS, MAP ,IAC ,EGR ,vacuum leaks, TAB and EVP.

 

Damn you guys just jump all over me I was just thinking of the age (and/or neglect) and the high rpm of the idle when it surges. As he mentioned on #3.
But you guys are making sense too. Its hard to figure shat out on a key board It seems everyone here has had a bad EGR and a vacuum leak.





except me

 

changing the plugs, plug wires, rotor, and rotor cap made the problem go away!!