Diagnosing Very Low Idle in 1994 Ford Ranger

Maintenance/Repairs
1

Car:
1994 Ford Ranger with a V6.

Issue:
At engine start it up it idles at 500 rpm or lower and kinda throbs, sometimes shutting off.

Action thus far…

Attempt 1:
I pulled the IAC and the part was very grungy so I replaced it thinking it was the culprit. Alas it was not the culprit. Although it SEEMS like engine does not die as readily. The problem persist.

Attempt 2:
I clean the throttle body and Mass air sensor, replacing the air filter.
Problem persists.

Removing the wiring connection to the MAS and it idles just fine. Bad MAS?

Attempt 3:
Replaced MAS with new part. Drove it around a little. Still idles very low RPM on start-up

Clues:

Idle in ‘P’ is better after driving the car long enough that it’s fully warm.

It doesn’t die when stopped or heated up in gear.

It throbs to the point of shutting off.

Thoughts:

Maybe I need to just drive the car some more so the computer can “learn” the new sensor.

Next up is to break out the Vacuum gauge and test for a vacuum leak.

This car is not ODB II and there is no check engine light.

It’s really annoying to throw parts at a problem. Now I’m thinking the MAS may have been fine and I just threw $90 down the drain.

2

I’d test the coolant temp sensor for the computer to see if it’s putting out the proper signal.

Tester

3

+1 to Tester’s post. That was my thought too.

You might also check for vacuum leaks. I see you’ve already thought of that… and the vacuum gage will tell you if you’re struggling against a valve problem.

What type of ignition system does it have? Slightly retarded ignition timing can cause this too.

Oh, and so can a carboned up EGR valve that doesn’t close. My impression is that you have knowledge and are just seeking ideas, so I’ll skip the dissertation on how this works. Try closing off the EGR line and see if the idle straightens out.

Post back. We do care.

4

It runs better w/the MAF disconnected? When the MAF is disconnected it runs in open-loop mode. That means it doesn’t do the normal thing of using the MAF and O2 sensor data to figure out how much fuel to inject. This, plus the fact that it runs better when warm makes me think you have some kind of fuel/air mixture problem. I don’t know what your problem is ,but a couple ideas I’ll pass on anyway

Some Ford owners here have had a common problem with similar symptom, with a part called the DPFE sensor. It’s something to do with the EGR function I think. If you get too much EGR at idle, it can definitely cause the engine to run roughly. There should be very little to no EGR at idle. That could adversely affect the mixture too. Suggest to do a little Googling on that part, see if it pertains to your truck.

W/a 1994 I don’t think it learns by doing. It won’t likely get any better than it is now by driving it around. For most cars anyway, I think that learning thing came later.

Just b/c the check engine light isn’t on doesn’t mean there are no error codes stored in ECM memory. It would be worth your time to check what’s stored there. On my Corolla’s OBD 1 system that’s fairly easy to do, it just blinks the stored codes out on the dash light. I’ve had stored codes blinked out w/no check engine light which were helpful in diagnosis.

There’s some possibility you have an ignition module problem. What’s the state of your ignition system? New plugs, wires, distributor cap, and ignition rotor?

Fuel pressure could cause this. Do you have the ability to check the fuel pressure? If not, at least check the vacuum line at the fuel pressure regulator for any signs of gasoline in it. There shouldn’t be any.

Be sure to use a vacuum pump to check for vacuum operated devices that may have sprung a diaphragm leak, esp the brake power booster.

Ignition timing problems could cause this. Have you check the ignition timing?

Compression problems are a possibility too.

Edit: One more thing, a problematic throttle position sensor could cause this.

5

Thanks all. I’ll break out the multimeter and test the EGR and the coolant temp sensor(s) and get back to you. But the fact that it idles fine when the engine warms up sound like a temp sensor.

6

Okay so volt tested the ECT and it works to spec. I tested the dpfe it also works to spec. Finally tested the EGR and the found that by applying vacuum there was no change in idle but when I applied pressure (using A lot of
lung power) the idle leveled off. I feel I have a conclusive evidence the EGR is stuck open. I’ll remove the part and soak it in some carb cleaner overnight. If that doesn’t work I’ll buy a new one. I post back if that does it.

7

The final problem was a cracked air intake after the MAS. It was messing up the fuel/air ratio and jacking up the idle.