Axios

My 1993 F150 4.9 has a slightly high idle when warm. From what I understand, warm idle should be at around 750RPM. Its idling around 950-980RPM according to a Bosch scan tool. During the KOER tests, only one trouble code is thrown: 411 Cannot Control RPM During KOER Low RPM Check. I do not HEAR a vac leak anywhere.

Today I rigged up a handheld propane torch with a length of vacuum line on it and a piece of brake line as a wand, and with the engine at warm idle i went around directing a stream at various parts. I heard a SLIGHT rom increase when I directed it at the EGR valve, specifically at the diaphragm side. I couldn't pinpoint it, but it seems that either the diaphragm is leaking vacuum into the intake, or the gasket is not sealed all the way. It was repeatable, but it was a VERY slight change. I could also watch the vac gauge that was hooked up increase 2 or 3 increments as the idle speed changed. It's definitely not pulling vacuum through the egr vac line at idle, I tested that as well.

At idle (remember 950-980prm) it looks as if its pulling 20hg of vac. This is likely due to the increased idle speed. I was unable to find any other vac leaks with the propane wand, so tomorrow I'm going to rig up a home made smoke tester to try to verify the leak.

I guess I could also block off the EGR ports to see if the idle drops to normal, but is there anything else I can do? The only other issue I have with the truck is a vibration at part throttle on the interstate or while going up hill. Once the truck kicks down a gear, the vibration goes away. This is as of yet undiagnosed. I have looked at motor, trans mount and replaced driveshaft u joints already. The fluid is darker than it should be, but not overly so.

sdmartin65

Sounds like you might just need to clean the EGR, a place to start at least. The KOER is actually a sequence of tests/steps performed by the driver and the truck, you really only use KOEO on these old trucks.

Axios

Cleaning the EGR wont do a thing, since the leak is external. I have cleaned the EGR and the passages in the intake in the last 2 months, anyways. I've never heard of the KOER tests being just a sequence. It sounds like you are implying that they don't give any real value. For example, the inability of the IAC to control low idle during the test indicates that there IS a vac leak somewhere, or the IAC valve is stuck partially open.

raski

Eliminate any vacuum leaks from brittle lines. I would give the throttle body a good cleaning. Removal may make it easier, just get a new gasket.Make sure there is no buildup keeping the butterfly from seating. Pull the IAC solenoid off, clean the pintle of any carbon chunks. Clean out the bore for the IAC pintle. Get rid of any accumulated carbon. I do this on my truck every 2-3 years. Also, DO NOT mess with the idle hard stop screw.

Axios

Yep, all of that had been done in the last 1000 miles. Definitely no messing with the idle screw lol.

sdmartin65

Seems like you've covered the basics, I'd suspect intake manifold gasket leak.

Steve83

The EEC manages the idle speed. All you have to do is the scheduled maintenance & any repairs. Don't look for problems where there aren't any.Scan tools are notoriously UNreliable on these old trucks. I recommend you return it for refund ASAP, and only use a jumper wire, as this caption explains:


(phone app link)Use a stethoscope.


(phone app link)That's neither diagnosis nor repair. Don't CREATE new problems - that won't help you find existing ones.

Axios

I didnt know you could hear vacuum leaks with a stethoscope. I have one in the garage I'll try out. As far as the sac tool, I have a cheap plug in scanner that just flashes a light, and I borrow the expensive one from the parts store. My obd2 cars I have bluetooth dongles for. They are as accurate as they need to be. Plus, I have a lot of experience with EECIV ford computers and sensors. Yes, the info is limited, but far from useless. The purpose of blocking off the EGR valve will tell me if it's that's the leak that's causing the high idle. That is very useful information imho. It definitely helps diagnose. I'm more concerned about the ongoing rumble/shudder under slight load that it has. Replacing the U joints had no effect, even though they were certainly on their way out. Any other things I should try?

Steve83

A paperclip is even-cheaper, and just as reliable.It doesn't matter how expensive it is - it's NOT reliable.OBD-II requires a scanner - that has nothing to do with EEC-IV.Your experience level won't make any scanner more-reliable.I don't think anyone suggested it was useless - but the info you get from an EEC-IV scanner is not RELIABLE.No, it won't. The EGR port is exposed to full manifold vacuum. If it was leaking, the truck would either idle VERY rough, or not at all. It wouldn't be slightly elevated. Blocking it off is a waste of time, effort, & money (to buy a new gasket, and possibly bolts).

Axios

I just... I'm sorry man. You seem to have a "rebuttal" to every single thing that is typed. I dont know why you assumed I mean that having experience with with EECIV based vehicles makes a scan tool any more accurate. The output codes are RELIABLE, its people's interpretation that is faulty. Sure, the EEVIV is limited In its ability to provide a wide range of info...I'll grant you that. And you yourself have on several occasions told people to block off the egr ports temporarily as part of troubleshooting, from what I can see by searching old threads. I just dont get it.