Surging Idle

[Cole94]

'94 Explorer XLT 4x4

This truck has had a lot of recent issues that were caused ethanmol phase seperation
read more ethanol seperation here: by Phase Separation in Ethanol Blended Gasoline

During that adventure, the following was replaced. (ALL OEM)

• Egr valve replaced
• Egr tube replaced
• DPFE sensor replaced
• Spark plugs replaced
• Plug wires replaced
• Ignition coil replaced
• Checked the cats (clean)
• Fuel filter replaced
• Fuel pump replaced
• Fuel Pump Assembly replaced
• Checked fuel pressure (30psi idle)
• Checked Crankcase vent
• ALL (I think) vacuum leaks discovered and fixed.

I need to pull the #1 plug to see if the bad fuel fouled the spark plugs.

It runs fine with everything except idle. In gear, it will rock the truck hard, and lights will dim with every surge.
It surges ~100rpm. It is idling at about 700rpm.
The (stock) exhaust sounds like a hemi.
IAC checks out fine (I think).

Methinks that there is something very wrong with the mixture. Any ideas are much appreciated.

 

[Turdle]

Could be a slow acting --sticky IAC valve. You might try cleaning it, if this doesn't work, replacement.

when cleaning it make sure the plunger rod inside moves freely.

How to: - IAC valve cleaning thread (w/pics)

 

[Cole94]

Yeah, I thought the same, but when I start it, the rpms start high and slowly go down. Could it be that it is just going too far past the point of idle (if that makes sense.)

Also forgot to mention that it has hard warm starts.

 

[Turdle]

IAC = Idle Air Control

The iac valve will constantly be moving back and forth to maintain idle speed. If it is sticking the idle may surge out of control.

Slow warm start might be a ECT sensor symptom but I would address the IAC valve first since it is a common maintenance procedure. In other words, you should do this every once in a while. every other oil change would not hurt.

 

[MrQ]

Ugh, ethanol is some nasty stuff.

It is the killer of fuel pumps in these Explorers and I have dropped way too many tanks for it ever to be normal.

 

[Rhett]

Tell me about it, I've dropped three tanks in the last year (twice in my own truck) and like clockwork when a tank needs to be dropped it is full of gas. Now it's all ethanol-free gas and Motorcraft pumps for me.

Also check the TPS, i've seen them have inconsistent voltage throughout their range.
ECT sensor is good suggestion too, if the IAC and TPS check out ok.

Finally, I will mention the throttle set screw (the adjuster on the TB). I have seen those move over time, affecting your idle to the point where a hot start results in not enough air. A little blue locktite on the screw threads will solve that, once you have re-calibrated your idle.

 

[MrQ]

Ethanol free is expensive and hard to find around here. This year I have already done at least 3 fuel pumps as well. 1st gen, 2nd gen, and 3rd gen sport.

 

[FR-425]

The fuel system on a 94' is ethanol OK.

Your Engine Coolant Temp (ECT) is bad.

There are two temp sensors one for the gauge and one for the PCM. The one that provides info tho the PCM is bad.

Symptoms include: Long crank on hot start. BINGO! Hard warm start is the "key" piece of info on this one.

It's flooding the engine on hot start because the PCM is seeing a temp of -40 F

Check the wiring, it may be a short

 

[Cole94]

Thank you FR-425. Could that also cause the idle surging?

 

[FR-425]

Absolutely

 

[Cole94]

Thank you so much. I'll post back tonight with results if I can get around to it. That is going to be awesome if this is the issue.

 

[FR-425]

The sensors are both right at the thermostat housing.

The one toward the rear of the engine is the ECT.

It's the one with a pointed sensor tip.

 

[Cole94]

Alright, so before I replaced it, I took some pictures of my fancy computer readings. All of these were taken with a warm engine.
Idle

35 mph

45mph almost at WOT

Don't know what's up with #1 O2 sensor. It's brand new. It reads with some throttle.

ECT looks fine. Or does it? You guys are smarter than me with this

 

[Cole94]

I will get a pic of the screen when the engine cools down and see what the ECT reads.

 

[mgmgmg]

Did you replace EGR solenoid? I had same issues with surging. I see you replaced just DPFE and EGR value but not the EGR solenoid? The solenoid always over looked....

 

[Turdle]

triple check the spark plug wire placement on bank 2, driver side

 

[Cole94]

wire placement is correct based on this image.

Yall can see the images right?

 

[Cole94]

Would it help if I uploaded a video to YouTube of what it is doing and linked it on here?

 

[mgmgmg]

Yes on video! , does the surging happen on cold starts bad then better on warm starts. Also what Bosch model number is that obd-1-2 code scanner, can you do live data on it. Looking
For a better obd-1 scanner then my innova 3145

 

[Cole94]

It's a bosh 1300. It does do live data. Here's a link.
Bosch OBD 1300 | Automotive Pocket Scanner | Code Reader

I'll have to upload the video in the morning due to internet issues. Goodnight all.