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1994 XL running troubles and bad mpg


Well-Known Member
May 8, 2004
Reaction score
City, State
Millbrook, Illinois
Year, Model & Trim Level
2001 Explorer Sport Trac
hello all~

I have a 1994 Explorer XL, 4.0L OHV two wheel drive and 5 speed manual, 137000 miles.

The problems I've been having lately are spurratic idle. Sometimes it likes to run at about 1000 rpm, sometimes it drops down to like 550 and sounds like I have a big cam, other times it sits perfectly at 700. It has it's biggest running rough problems when starting it up warm. Most of the junk happens once it's warm. At times I can hear a spitting sound coming from the exhaust, which sounds like a misfire.
Also I am getting horrible gas milage. I have feathered it this entire fill up, and taken it on the highway at a steady 55mph... and I get 12 mpg. 12!!!! with a 5 speed 4X2. I'm thinking it's my MAF, TPS or my ECU, but I'm at a loss with no CEL. Next week I'm going to take it to my auto class and see if there are any sleeper codes that aren't tripping my CEL.

here's what I've done

plugs and wires
egr valve, tube, solenoid
o2 sensors
lower intake gaskets/valve cover gaskets
fuel injectors
coolant temp sensor
air intake temp sensor
fuel pressure regulator
fuel pump (pressure is a solid 35 psi)

I have also tried cleaning the MAF with electronics cleaner. No luck!! This is getting very very old. Any ideas would be welcomed


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I know a body can get anxious for a response. I was waiting to see what codes were pulled next week. If you don't want to wait until you get to shop class, you can pull the codes anywhere if you can find a short piece of jumper wire (I once pulled codes on my dad's '95 with a piece of bailing wire we found in a gas station parking lot). Instructions can be had all over the net: try www.dalidesign.com/hbook/eectest.html (my favorite, but I like to use a voltmeter instead of the CEL to output codes) or pulling codes at www.fordfuelinjection.com You could also wait, but I don't feel like I would have anything useful to contribute (beyond blind guesses) until I see what trouble codes come up, or if everything passes.

One clarification: "[fuel] pressure is a solid 35 psi" Under what conditions: key on engine off? idle? with or without vacuum applied to the FPR?

35 psi at idle. when I shut it off it takes a while to go down. all new injectors. The problem is there is no check engine light. Oh well... I'm at the end of ther ope with this truck anyhoo. Thanks, Sorry I just have been battling this crap for months

"35 psi at idle." -- I'm going to assume that that also means with engine vacuum applied to the FPR, rather than having the vacuum disconnected and plugged. Fuel pressure can be tested either way.
"I just have been battling this crap for months" -- I've been kind of following your saga, and it seems you've just had a troubleshooting nightmare with this issue. I don't remember if you definitively decided there was or wasn't a vacuum leak somewhere, but I do remember that you've been trying to figure this out for a while.

yeah... it's a royal pain in the butt. I'll let you know later if I pull any codes. Thanks for your help. The only vacuum leak as of right now is on my cruise control, but that is only when the CC is ON.... when it's off there is no more leakin. ::sigh!::


Peace out, gotta go to bed for school tomorrow. I'll update soon

OKay, success! I did the check engine light test and I came up with these codes





Could any of these cause my idle woes? Thanks all!
it's about 4:45 here my time, I won't be back until about 11 tonight (babysitting for $$$) but if anyone could tell me wtf these codes mean, it would help a lot


Any and/or all of them could be causing your idle woes.

1st thing to do, is to identify which type of code each is. According to my code lists, all of them could either be KOEO or continuous memory, and the 336 could be a KOER code (assuming you performed the KOER test).

After identifying which type of code each is, then we'll start by resolving the KOEO codes.

Ok well they are I suppose the continuous memory codes. I just did the setup where I read what's in the computer's memory, I grounded the grey single wire test lead and turned the key on to get the light to flash. I have not done the KOER tests yet, as I haven't had time (school) or money (gas) because it usually takes several times for it to work right. GRR! :) Anyway tonight I just made fifty bucks babysittin, haha

That particular test is called the KOEO test, and two types of codes are output during the KOEO test: KOEO codes and memory codes (separated by a single flash). so the question for you is: Which of those codes came out before the single flash (KOEO codes) and which came out after the single flash (memory codes). Your diagnosis of KOEO codes is different than for memory codes.

the 122 and 336 came out after the single blink. Thank you for the information! The other two came before.

So, that makes a KOEO 542 and 543 and CM 122 and 336.
KOEO 542 and 543 both point to problems in the fuel pump circuit. And, yet, the engine runs (interesting). Does the fuel pump run consistently or sporadically? What kind of fuel pressure do you have? There could also be a broken wire in the fuel pump monitor circuit and the fuel pump circuit itself is just fine.
CM 122 means that the computer has seen bad data from the TPS at some point in the past. Since there isn't an equivalent KOEO code, the fault only shows up when the engine is running, and could be intermittent. This is a good candidate for your symptoms.
CM 336 means the EGR sensor signal was out of range. Again, there's no accompanying KOEO code, so the fault only shows up when the engine is running and could be intermittent. It also is a good candidate for rough idle.

SOP is to resolve KOEO codes first, so start with the fuel pump codes. Check the fuel pressure and make sure the fuel pump is running steadily. Check the wiring carefully to make sure everything is intact (including the fuel pump monitor circuit).

Okay well Having recently gone through a new fuel injector fiasco (which ended up perfect) I had tested the fuel pressure. Was a rock solid and steady 35 psi at idle, went up a little when I revved it up. Tomorrow I am bringing it to my auto class, where we have a few hundred grand worth of scanners and testing equipment. :)
Anyone heard of "Technology Center of Dupage" (TCD) or "DAVEA"? hehe

My plans are to hook it up to the Snapon "odis" universal scanner and record my sensor voltage outputs. I'm going to look for any problems with the TPS voltage, which would be an excellent start point. Also I will monitor the O2 sensors (although they are new) etc etc.
We also have a recently aquired smoke machine. I'm going to blow some smoke through my vacuum system and see where it spits out, because I could have a leak. Anyone know of a good insertion point for the smoke?

Boggling as the hotter it gets the higher it idles. I'm contemplating the fan system but that has never given trouble before.

Pretty much many months ago I put on a cone air filter instead of my air box. It had a noticeable increase in zip and sound, and about a 150 rpm higher idle (which made sense due to the increaed air flow) but I lost cold start ease so I put the old intake back on, and it stayed idling fast and then higher, when I thought it would return to normal. (maybe I damaged my MAF?)


Okay another update. Took the truck to my auto class and ran a smoke machine test, found the vacuum line going to the canp solenoid to be leaking. Replaced the line and no ore leak. It idled great and low where it should be for about 5 minutes. Back to high idle again, and when it's hot idling it misfires. Here is what I found using the Modis Snapon Scanner

KOEO: 543, fuel pump circuit open- battery to eca (what does this mean?)
KOEO: 542 Fuel pump circuit open - eca to ground (again what does this mean)
KOEO: 336 exhaust pressure high or egr PFE signal too high (what does this mean?)

I ran a KOER test and no codes were present through the test, I repeated twice to confirm, no KOER codes.

Now for the detailed sensor outputs I recorded at idle

~average idle bounced between 870rpm and 970 rpm (seems too high for me, when it used to idle at 700)
~Reference signal voltage was 5.01 volts, dead on
~TPS 0.96V at idle and changed smoothly as increase in throttle
~ECT 90 degrees celcius
~ ignition timing at idle was 20 degrees advanced
~IAT/ACT voltage was 2.03 Volts
~IAC was 2.3 volts
~MAF was 0.69 volts at idle, and went up smoothly as I increased rpms
~DPFE was 0.69 volts at idle and didn't do much when I increased rpms
~02 sensor #1 went from 90mV to 800 mV average which seemed good
~02 sensor #2 went from 2mV to 735 mV average, which also seemed good

If anyone can make any sense of these it would rock. I'm starting to think my ECU might be acting weird when it heats up since all my sensors appeared to me fine. Thanks!


Following your saga.....sounds like the process of elimination. Where are you Shorty???

When you say you didn't get any KOER codes, does that mean you didn't get any codes at all? Not even a pass (111) code? Did you wait long enough? KOER test takes a couple of minutes to complete before the computer outputs codes?

KOEO 542 and 543 both mean that the computer can't see the fuel pump circuit doing anything. The fuel pump must be functioning or it wouldn't start. I expect there's a problem in the fuel pump monitor circuit, between the computer and the fuel pump circuit. Going to need to look over the fuel pump circuit carefully to find the cause of these codes.

KOEO 336: Question: previously, this was a memory code. Did you clear the previous CM code and now it's a KOEO code, or does the Snap-ON scanner not do a good job of distinguishing KOEO and CM codes? I kind of hate to harp on this issue, but it can be important to be know which type of code it is. I've never used a scanner to get codes, so I don't know how well they work, but it would help if we knew for sure which codes were KOEO and which were CM (I'm assuming that at least one has to be a CM code, because you didn't say that you got a 111 pass code for either test).
The code means that the computer saw an out of range value from the DPFE, either during the KOEO test (if a KOEO code) or at some point during normal operation (if a CM code).
I will preface this by saying, I don't know a lot of details about these EGR systems, as my '92 doesn't have an EGR system. My '87 BII has an EGR system, and I've been playing with it to figure out how it works. I think I understand basically how it works, but I lack many of the details.
The computer says that it says a steady 0.69 V from the DPFE, that doesn't vary with engine speed. What does does the DPFE output key on engine off? The scanner data is filtered through the computer. What is the DPFE's output if you monitor it with a DMM (before the signal enters the computer)? If you don't read the same voltage (or 5-0.69), then there could be something wrong in the computer that prevents it from accurately reading the DPFE.

I opologize I got the 111 code during the KOER test, indicating no faults detected. I'm starting to think my ECU is getting effected by heat, since it only has problems when the vehicle overall is very warm. In the winter when the engine is warm but it's cold outside, it ran fine. I think I want to pick up an ECU out of another explorer and experiment with that. Thanks for your help