Rough Idle - 86 f-150
 

Hey Guys -

After a lot of searching on here and trying several ideas, it's time for some expert help!

The truck is an '86 f-150 5.0 EFI AOD.
Symptom - Rough idle. Won't start hot unless throttle is held wide open. Starts fine when cold. Truck sat for 3 years, so fuel system was gummed up.

Truck is bone stock with the exception of headers.

What I've done so far...
Pulled injectors and cleaned
New fuel pressure regulator, new pumps (in tank and on frame)
Replaced broken vacuum lines
Replaced upper intake gasket
New PCV valve.
New IAC
New TPS
New EGR
Rebuilt distributor
Complete tune up - plugs, wires, cap, rotor, filters.

Timing is @ 10 degrees BTDC w/ spout removed.

Computer is code free! System pass in both KOEO & KOER tests. I'm stumped.

Any suggestions greatly appreciated! :-X0A6

Run a compression check or, preferably, a leak-down test. And, what does the spark look like - is it white or blue?

When you first turn on the key (not cranking) do you hear the fuel pump run (prime) for 30 seconds?
Forget that - I just noticed you said it starts fine when cold.
Can you hook up your scan tool and see what the computer thinks is your coolant temp? If you scanner doesn't show this, take a reading off the sensor with an Ohm meter. You could have a bad sensor but no codes set beacuse sensor has failed within the allowable range (meaning it is showing cold all the time).

Thanks for the responses.

Gary - The spark looks blue. Ignition components have been replaced including the plugs, wires, distributor, cap & rotor, and ignition module.

I don't have a compression tester, so I haven't had a chance to run the test yet. The motor has around 40k since it was rebuilt.

I did run codes again since the truck had the battery unplugged for a long time recently. Drove it quite a bit and it came up clean on KOEO, but had several codes on the KOER this time...

12 - cannot control RPM - high rpm check

13 - cannot control RPM - low rpm check

21 Engine coolant temp out of range (truck was completely warm when I ran this and has correct 195* thermostat)

41 - H02S (sensor) system lean (bank 1)

32 - EVP Sensor below threshold

25 - Knock sensor not detected (I double checked and it is plugged in)

Do y'all think the coolant & knock sensors are the main culprits here? I've replaced both several years ago, so it wouldn't surprise me if they were bad... The truck runs really strong and drives great, just idles rough and is hard to start once it's warmed up. Any suggestions appreciated. Thanks guys!

I don't think it would be the knock sensor as it is used to tell the computer when it is giving the engine too much spark lead under load. But, at idle and starting there won't be any knock.

But, the coolant sensor could well be the problem or part of the problem. If the temp sensor is telling the computer the engine is cold it will be giving it too much gas in the air/fuel mix to compensate, and that will make the idle poor. Similarly, if the engine is warm but the computer thinks it is cold it'll give it too much gas for starting, which is being overcome when you give it full throttle. I would change it out and see what happens.

I agree with countrybumkin and Gary - if I had that array of KOER codes I would start with replacing (or testing) the engine coolant temperature sensor.

In general, one should start with the earliest codes and work towards the later ones. For your codes 12 and 13 - did you press the accelerator pedal down at some point during the KOER test?

P.S. Here is a useful website on these matters, in case you didn't know of it: http://oldfuelinjection.com/

Alrighty. I went ahead and replaced the coolant temp sensor, left battery unhooked for 45 min, and test drove again. Still idling rough.

Ran tests again. KOEO is still showing system pass, no codes.

KOER is now only showing 2 codes -

41 - ho2s (sensor) system lean (bank 1) Is this the oxygen sensor?

32 - EVP sensor below threshold - Is this the EGR piston sensor?

I've replaced both of these in the distant past, so I guess these could be bad as well... Funny it seems to be running rich, at least the exhaust smells that way. It's not black, but there's black soot in the pipes. I've checked for vacuum leaks and replaced a couple of original hoses that were suspect, but nothing's changed.

Coolant sensor was obviously bad since the code is not popping up again. The old one was a cheap plastic piece from o'reilly. I bought the correct style brass one at autozone and that seemed to fix that issue. Now if I can just get past this last issue I'll be back to business!

I do believe that's the O2 sensor. But don't know about the other.

I'd just keep going down the list. HEGO is the oxygen sensor. EVP is one I was not aware of, but yes it is part of the EGR: Fuel Injection Technical Library » EGR Valve Position sensor (EVP)

Finally decided to buy a fuel pressure gauge. I suspected a leaking injector, but it holds pressure fine.

Key on engine off it's 35 psi. Book says 35-45

Engine idling with fpr plugged in - 30 psi. Book says 30-40.

Engine idling with fpr unplugged - 40 psi. Increase of 10 psi... the book says is acceptable.

The numbers seem to be on the low end of the acceptable ranges the book gives. If I shut it down, the pressure goes up to about 42 psi after about 15 minutes and stays there. Could this be an indication of a clogged fuel filter? :-huh

Probably not the fuel filter's fault, but if it hasn't been replaced in all of this it should be. I would guess that it is a worn fuel pump and a regulator that is at the bottom of the range. But, it is in the range so I wouldn't worry about it.

Ok, I've done some more work and got it running better, but the rough idle is still a problem. I put some new injectors in last weekend and it helped a lot, but after it warms up the idle quality gets rough.

I ran codes again and KOEO comes up clean.

KOER comes up with:
41 - System lean (bank 1)
13 - Cannot control RPM during self-test, Low RPM during self test
I've set idle correctly with TPS sensor at .99 v
25 - Dynamic Response Test - Knock sensor not detected
It's plugged in, I checked.

Any ideas? It drives great. Plenty of power like it's always had, just rough idle that seems to get worse the warmer the engine gets.