Transient stalling and lunging

[Ihatemyxploder]

Hey guys, My 94 explorer 4x4 has just started stalling without warning while driving once and a while. It has the 4.0L engine with auto trans with 116,00 miles. It also will lunge forward occasionally while at low speeds(almost hit the garage wall). The check engine light usually doesn't come on, but did once the other day and stayed on for a bit.

I recently(last month) changed the vacuum modulator on the trans, and reset the computer by unhooking the battery overnight. All the vacuum lines are on, and the PVC and EGR were replaced last fall. It runs just fine most of the time.
The truck has had check engine light problems for years that the dealer is unable to diagnose, so I'm reluctant to take it in just yet.
Any ideas?

 

[Positive Vibes]

Did you have any of these problems before you changed out the EGR and PVC? Did you change your IAC or clean your MAF. You can buy a cheap code reader for $20 at Walmart. This would help when your CEL comes on and you could then get the codes.

 

[marragtop]

I suggest you pull the codes. Even though the CEL isn't on, in can still throw a code. Your problem could be the throttle position sensor which will throw a code.

 

[Ihatemyxploder]

Did you have any of these problems before you changed out the EGR and PVC? Did you change your IAC or clean your MAF. You can buy a cheap code reader for $20 at Walmart. This would help when your CEL comes on and you could then get the codes.

No..the stalling and lunging problems started months(6 or so) afterwards. The CEL has come on and off for years now, I think mainly due to the ATF my truck has been burning for God knows how long. The trans module was the lastest repair. I didn't change the IAC, and didn't know you could clean the MAF(it's about 5 years old), but I think I'll take your guys advice about the code reader.
Thanks...I'll look at those next.

 

[Ihatemyxploder]

Update:
Cleaned the MAF and IAC, and it still idled poorly. Rented a code reader, and got the following:
Code 336 EGR PFE/PDFE out of range. voltage higher than expected.
Code 538 Insufficient RPM range.

Could a bad IAC cause the EGR code? It's a newer EGR valve.
The truck continues to attmpt to stall at low speed, it also tries to overcorrect the stalling and the vehicle lunges forward at lights.

 

[MrShorty]

Could a bad IAC cause the EGR code? It's a newer EGR valve.

Not likely.

538 is a KOER code, and almost always indicates that the user neglected to perform the dynamic response (aka goose) test when indicated during the KOER test. Not sure what kind of instructions came with your code reader, but would suggest you review other sources of EEC-IV self-test instructions (see my "notes on pulling EEC-IV codes" in EEC-IV forum).

Was the 336 a KOEO, KOER, or memory code? Note that there is more to the EGR system than just an EGR valve. You have a regulator/solenoid that controls how much vacuum is applied to the EGR valve, you have a DPFE sensor that tells the computer how much flow is through the valve, all part of a feedback loop that keeps the EGR flow at the desired level according to the PCM's programming. Of course, these components are interconnected by wires and vacuum hoses.

 

[Ihatemyxploder]

The 336 was KOER code.

The code reader I used plugged into the harness in the engine compartment and the only instructions were to pump the brake and turn the steering wheel while a 2 minute timer counted down. Someone else told me I was supposed to rev the engine also, but since the code reader only came with an 8 inch cord, I guess I would have missed that while in the drivers seat. Grr.

I'm starting to think something bad may have been in my gas. I filled up my tank the night before I went back to work, it ran like crapola on the way to work, but ran much better on the way home, and only had one minor hiccup today on the expressway. I'm hoping thats what it turns out to be, cause I have plans to sell this thing within a month, and didn't want to dump a bunch of money on it.
Thanks for the help guys.

 

[MrShorty]

Did it pass the KOEO test?

but since the code reader only came with an 8 inch cord, I guess I would have missed that while in the drivers seat. Grr.

I usually goose the throttle from under the hood. Just grab the throttle cable linkage and quickly open then close the throttle. Of course, doing from the driver's seat isn't a big deal even if the code reader doesn't reach. The codes (including the cue to goose the throttle) come up as flashes of the CEL, so you could take your cues from the check engine light rather than from the code reader.

 

[Ihatemyxploder]

Well I had to give back the reader, so that will have to wait I guess. The truck ran good for a couple days, then started it's problems again when the temperature went from the 40's to the 10's and 20's.
I also noticed that I'm getting quite a bit of moisture coming out of the exhaust, and I seem to need to fill the coolant more and more. I wonder if the motor has coolant leaking into somehow, and that's causing the problem.
I'm gonna tackle changing the plugs in the next few days, so maybe that will shed some light on whats going on.
I was hoping to sell the truck soon, but it looks like it's just gonna fall apart instead.