Recently bought a 94 F-250. 5.8 stick.

When I bought it, the previous owner had the Intake Air Control Valve unplugged. The truck idled around the RPM you would expect, but it was a rough idle (especially when cold).

I plugged the IACV back in, and it would occasionally idle okay, but then randomly surge to a high idle for no reason.

Tried replacing the IACV, slight improvement maybe? Too early to tell.

Any ideas here? No Check Engine light. Would like to figure out how to approach this intelligently, I'm new to these trucks.

The only other note I have is that the EGR line coming up from the passenger side manifold leaks. There was a repair done in the past that is no longer holding, so it leaks exhaust before the EGR valve. I didn't think this would be related.

 

There can still be issues that would cause the KOEO tests to fail as well as stored codes with no active Check Engine Light. Pull the codes...

That leaking EGR tube will contribute to a lean condition under light throttle/cruise conditions. Replace it.

I also highly suggest you put the throttle stop screw back to the original position. It's not an idle adjustment, it's sole purpose in life is to prevent the throttle blade from closing too far.

 

Thanks.


I may pick up a patch kit for the EGR line temporarily just for testing purposes. Due to the previous patch, the EGR valve itself may need to be replaced in addition to the line. Plus, the passenger side exhaust manifold needs to be replaced too, so it may be easiest to knock out that whole job at once.


I'm wondering if it would be wise to first clear the codes by disconnecting the battery, drive around a while, and see what codes it throws. Just to erase anything from memory that may no longer be relevant (bad/unplugged IACV, etc).
Are auto parts store scanners able to scan the 94 trucks? Pre-OBD2.


The throttle stop screw is where it was when I bought the truck. Not sure if it was ever moved from the factory setting.

 

Check the codes first as a baseline, then clear. A bad IAC will not trigger a KOEO or CM code. Some places have accessto an OBD-I code reader, but they are becoming less common.

You can run your own KOEO tests and stored display with a paperclip: http://www.fuelinjectedford.com/page13.html

Your truck should display 3 digit codes: http://www.fuelinjectedford.com/page45.html



If the idle RPM is above 400-500 RPM with the IAC disconnected then someone has moved that stop screw or you have a vacuum leak.

 

Will do. And I just looked up the procedure to pull codes, seems fairly simple.


One note about the idle adjustment screw: I can back the screw off to the point that the throttle doesn't even rest on the screw, and it will stay idling (high or normal). Sticky throttle cable?


A pretty serious vacuum leak is fairly likely. I'll be picking up some carb cleaner to try to track it down tonight. Is a broken EGR line considered a vacuum leak (intermittent maybe)? Wasn't sure since the break is between the manifold and the valve.

 

Fairly straight forward process. I suggest to invest ~$20 in a code reader like the Innova 3145. Very handy to have in the toolbox.


The proper procedure is to set the screw to just barely crack the throttle blades open with the engine off. If the screw is set to this condition and the idle RPM is high you have a vacuum leak. As far the the throttle cable goes, pull on the throttle bracket to ensure the cable is fully released.


It's most likely a very small vacuum leak. Keep in mind there are various vacuum reservoirs and yards of covered vacuum line, the carb. spray method has very limited effectiveness IMHO. I use a small hand held vacuum pump to test each circuit.

The quick test is to remove all the vacuum lines from the tees/ports except for the MAP sensor. Cap off all of the ports. This includes the vacuum line to the brake booster. Start the engine, if the idle RPM remains high the problem is on the engine. If the idle RPM is low or the engine stalls the leak is exterior to the engine.

 

Thanks for the Part# on the code reader. I'll definitely get that.


I think my plan for tonight, weather dependent, is:
a) See if I can pull some codes
b) Throw some muffler repair stuff on that leaking EGR line, and determine if the valve itself can be saved to be transferred to a new line.
c) See what's up with the throttle, including the adjustment screw and the (sticking?) cable.
d) See if carb cleaner reveals any obvious vac leaks.


This truck is not a daily driver. It sits the vast majority of the time, and I bought it cheap to basically put 500 miles per year or less on it. So I have the luxury of time & ability to tinker here.
Over the next few months I'd like to get it to be a truly solid work truck. Issue 1 is what we're talking about now, issue 2 is it needs ball joints, then 3 are some oil leaks.

 

Good plan

 

Minor change of plans due to getting tied up at work, but this is what I did:


Last Night:
- Patched the already-patched-but-broken EGR tube. Truck is way quieter, and seems to have made a small positive effect on how it runs.
- Repaired 2 small plastic vacuum lines that were broken. Noticed these visually, did not use carb cleaner or a gauge (have one arriving Saturday along with the code reader.
- Noticed that the plastic piece at the throttle-body end of the throttle cable is broken. It's binding on itself. Will replace.
- Unplugged the EGR valve while running just to confirm that the check engine light actually works.
- Drove around a bit with some stop and go. Initially, idle was good. By the end of the drive, its high.


This morning, Cold Start:
- Started up fine, idled perfectly before driving.
- Put it in first, get it out of the driveway, try to drive down the road and throttle cable is binding on itself. Give the throttle a good push to free it up.
- Get to next stop sign, and its idling high now. Rest of the ~25 minute drive, its idling between 1500-2200 RPM.


Got to first destination:
- Shut down truck for maybe 3-5 minutes. Unloaded crap.
- Started back up, warm start. Idles perfectly.
- Start driving, throttle cable doesn't bind.
- Drive for about 10 minutes to get to next destination, with some stop-and-go. No binding, perfect idle the whole drive.


Thoughts:
- Clearly there is an issue with the throttle cable, that could even become a safety issue. Will replace immediately.
- I'm wondering if the throttle cable issue is "confusing" the computer? The need to force it to unbind just after a cold start, and then maybe the plates aren't returning to the same position that they were at startup, is causing the Throttle Position Sensor reading to not match up to what the computer wants to see?
- There are probably still some vacuum leaks. All address those once I have the testing kit. Also, I'll replace the EGR tube, and I'll have to replace the valve itself since it is bonded to the broken tube with muffler repair wrap.

 

Did quite a bit of driving around today, and it seems to confirm this:

- Regardless of hot or cold start, if the throttle cable binds when you first take off, the truck will idle high throughout that drive. Even if I go under the hood and physically push the throttle to as closed as it will go, the idle will still be high. This is why I think that the cable issue is causing a screwy signal to the computer (maybe through the TPS), because the issue will not "reset" until you shut down and restart, regardless of what you try to mess with mechanically.
- If it does not bind on first take off, it will idle great.

I am getting some sort of air leak type noise from under the dash (?) that may be a vacuum leak. And the temporary repair (over another repair) on the EGR tube didn't hold, so those parts are on order.

Now I'm having fun finding the right throttle cable. Coming up with 4 different possible lengths for a 94 5.8

 

I believe you are on the right track. The ratchet algorithm (computer programming) constantly looking for the lowest signal value from the TPS. Based on your symptoms and observations I think this is a very large contributor to your issue.

 

I am encountering a similar high-idle issue, and trying to troubleshoot. No constant ChkEng light, but it did come on and go off during a recent drive. I want to see if any codes stored, but I cannot seem to find the OBD-I test connector, ANYWHERE!

My engine is 5.8. I'm led to believe I am to look for a connector like the attached image shows in someone else's engine compartment (labeled with STI, signal return, etc.). All I can seem to find is the light-gray rectangular connector shown in the other 2 images, and I found that connector behind the interior cover just forward of the passenger door.

 

A 1994 model year truck is OBD-I therefore the STO and STI test connectors are under the hood near the driver side hood hinge. It may be tucked into a plastic cover that is labeled EEC Test.

 

Your statement agrees with everything else I have read, but mine is no where to be found. The only additional info I can offer that may affect the placement is that my van is a hi-top conversion van with the auxilliary rear AC/heat, TV, mid-mount additional stereo, etc. Most other work I have done on the van has followed all other "regular" vans as far as placement of wires and components.

 

You have a van.......a different animal. The data link connectors are located near the driver side headlamp.

For reference: