KOEO-111, KOER- Cyl. 8 and 538
 

My 94 Bronco is running rough on low idle: it runs ok at start, but as soon as it reaches normal temperature and RPM drops to low, the car will start shaking and RPM will oscillate between 100 to 1000 and the car may die sometimes. I replaced both TPS and IAC and the problem remains the same.

When I tried to scan for codes, I passed KOEO (111) and KOER gave me two codes: one said Cyl. 8 and the other 538.

I tried to look for the meanings and didn't find any solid answer.

Does this mean there is something wrong with cylinder 8? I will check it now.

Please let me know if you know the problem. Thanks!

I just replaced all 8 spark plugs and cleaned the distributor. Didn't find anything abnormal.

Now the car runs exactly as before: still rough low idle. I adjusted the stop screw to increase idle speed. What I found is if I adjusted it to barely above 1000 RPM, it will stay there and runs smooth. But if I press accelerator and suddenly release it, the RPM will drop below 1000 RPM and start oscillating again. So I have to accelerate and slowly release in order to keep it above 1000 RPM and idle there.

Anybody knows what is wrong?

Cyl 8 means you have an engine with 8 cylinders

Code 538: Insufficient RPM change during KOER dynamic response test/ Operator error.

That means you did not "goose" the throttle at the correct time or not for a long enough period.

Put it back. That screw is exactly that, it's a screw designed to prevent the throttle blades from closing too far.

A rolling idle like you described can be caused by several items.

1. Vacuum leaks
2. Bad IAC
3. Bad TPS

Try disconnecting the IAC. If the rolling idle stops you could have a sticking IAC or a bad TPS. If the engine dies or the idle RPM remains very low, but steady, you may have a bad TPS or vacuum leak.

It's a process of elimination.

Thanks, rla2005!

I did all those before I replaced IAC and TPS. I bought those OEM parts (Motorcraft) so I think they should be good.

I only noticed one thing on the IAC: the old one has a spring at the other end of the solenoid but the new one doesn't. Is this normal new design or somebody forgot to put it there when assembling?

I will recheck vacuum tomorrow. I used starting fluid last time and didn't find anything.

This car doesn't have a MAF or MAP sensor, right?

Thanks again!

A 1994 5.8L engine Bronco used Speed Density style EFI so it uses a MAP sensor.

I'll say it again; I am not a fan of the spray method to find vacuum leaks on a EFI engine. I've tried it many times and usually find very few leaks. I use a hand held vacuum pump/gauge to test each vacuum circuit.

As a pre-emptive strike I highly suggest to replace all of those brittle vinyl vacuum lines. Thoroughly inspect the vacuum reservoir on the passenger side fender inner fender. It's well known to rust through the bottom. Then there is a small plastic reservoir on the HVAC blower motor case. It is frequently overlooked. The throttlebody itself could be worn, which causes vacuum leaks and/or binding.

As far as new parts go, new does not necessarily mean good. I am not aware of any design changes on the IAC. Perhaps someone else can chime in.

Thanks, rla2005!

I first disconnected the power to IAC and found engine runs slow but steady. So there is some leak. I did some vacuum check and found some broken lines, one goes to the ac direction actuator and the other goes to the square plastic box (maybe the vacuum reservoir you mentioned, with some particles inside when I shake it). There was a hole on the box I think caused by contacting with the belt. I fixed all those but the engine runs exactly as before!

Now I think I may have found the problem: there is a vacuum hose connected to the throttle body and it goes to the plastic box through a valve. And I found there is no power to this valve which means it never opened. I tried to bypass it with a hose and found engine runs the same. I disconnected the other end on the box and found no change. So this means no air can go through that box. Is this normal?

Since that hole on the box has been there for a while, maybe contamination/dirt completely messed it up?

Please let me know. Thanks!

I found the official name for that box is "Vapor Canister" and that valve is called "Vapor Canister Purge Valve/Solenoid".
So air/vapor should be able to go through this canister, right? I found it's completely blocked because it chokes the engine: if I unplug the hose, engine runs fast, if I plug it back on, engine shakes and RPM oscillates.

I just found another problem: the fuel pump makes a whining noise when idle. Could this be the cause of the problem? Fuel pump can't provide steady fuel supply? I need to replace it anyway since the noise is not normal.

That CANP solenoid only opens at certain times to allow stored vapors from the canister (from the gas tank) to pull into the engine. Usually after the engine is warmed up and cruising conditions. I do not believe it is held open for a long period of time.

Since the port for this solenoid is before the throttle body blades there should be no "vacuum leak" effect. The main thing to keep in mind here, that solenoid is normally closed.

I just cannot see that having any effect on the idle quality. If the engine runs better with that port open maybe you should check the air filter to see if it is plugged up.

Some vehicles have noisy pumps but work fine otherwise. Before throwing parts at this, check the fuel pressure. Many part stores rent gauges for this purpose.

Thanks! I will replace fuel filter and check fuel pressure first. I never heard fuel pump noise before. There is one or two fuel pumps on this car?

The air filter (a K&N) is OK, but I cleaned and re-oiled it anyway.

One high pressure pump located inside the tank.

Thanks! I bought just the pump on eBay for only $30. I will wait until I check the pressure to decide what to do with it.

there is a very specific procedure regarding the idle set screw on the throttle body. you need to do a search to get these instructions and get that screw back to its stock position or you will continue to have issues for sure.

What is your timing set at with the engine to temp and the spout connector removed? for sure check the fuel pressure although the fact things run good when cold and then lousy when at temp makes me think you may have an exhaust leak or bad o2 sensor.

get the timing info and the fuel pressure first though.