obd1 = obd FUN. OR... why I hate my truck right now.

Help me dive into these codes before I start throwing parts at it.  94 Mazda B4000 (Ford Ranger).

Driving:  Part throttle cruise is surging almost like a misfire but not rough-running.  Almost as if you are cycling the AC compressor on and off rapidly or instantly going from 40-60% throttle about 5 times per second.   I can make it stop by stabbing the throttle to WOT for a second. (this one is pretty obvious once you look at the codes).

I have suspected that the catalyst may be partly clogged.  My main reason for thinking this is that it's has always seemed to not live up to its whopping 160-hp potential coupled with two other symptoms; a ridiculously quiet exhaust and the little reference hose keeps blowing off the EGR valve.

This brings me to the codes.

 Then, I get a random string of codes that A) don't make sense, and B) some don't exist in the EEC code database.

643 (doesn't exist)

 343 (doesn't exist)

I get the EGR code.  That would explain my first symptom above.  Question is (before I throw a part at it), could a clogged cat be forcing the valve open/closed with the excessive backpressure at the valve?

The claims that it's not getting up to operating temperature are BS.  195 stat that opens/closes, and my laser thermometer verified 198 at the thermostat neck.  That suggests a cascade of sensor failures all at once, which I don't think would likely happen.

The Neutral/Drive switch makes me chuckle because it's a manual transmission.

Fuel pressure is easy to test... provided there isn't an intermittent electrical failure that would lead to a false good reading.

Can you folks make sense of my EEC-IV alphabet soup?

A bunch of sensors all failing simultaneously doesn't make sense. Sounds like an electric issue, which leads me to: check the grounds. All of them. 

I would test/replace coolent temp sensor, that looks to be the root of your issues.

I have seen clogged cats cause lean/rich codes along with egr issues due to back pressure. 

Some of thoes codes are definitely dummy codes, especially nutral/drive code. 

Also why I love owning a obd1 scanner being able to see live data. 

barefootskater (Shaun) said:

A bunch of sensors all failing simultaneously doesn't make sense. Sounds like an electric issue, which leads me to: check the grounds. All of them. 

That would be my first thought, too.  Too many sensors reading the wrong thing at the same time.

And all of them could be causing to to change from one driving mode to another (fuel on/off kind of thing, or thinking it's shifting).

Some digging on the neutral safety switch code.... apparently it is a pretty useless code that comes on if you are KOEO but not in park/neutral.  Since I'm assuming that the clutch switch is the NSS for this computer, it set that code because I didn't have the clutch disengaged.

I suppose my next idea is to disconnect the exhaust in front of the cat and drive it with earplugs.

Unplug the oxygen sensor(s) and go for a drive.

Having had several mid-90s ford products including a 93 Ranger, I would definitely follow barefootskater and alfadriver's advice on electrical issues. These fords can have all sorts of issues because of poor grounds especially in the rustbelt.

Don't worry about the 545 fuel pump failure code, if you stall it that code sets.

Like the others said, take the exhaust lose and drive it, to check for bad cat.

Do you have a California model? Looks like only they got DPFE.

If one of the sensors that use 5 volt reference is shorted it will cause the other 5v reference ones to read wrong. Both o2, engine coolant, TPS, intake air temp, DPFE, and octane adjust, use the same ground. The idle air control, canister purge, egr vacuum regulator, and injectors use a different ground. Seeing how you don't have codes for any of those, they are not suspect.

I would clear the codes and disconnect one of the sensors at a time and see if it runs better. The engine coolant code could be a stuck open thermostat, won't reach operating temp.  

Fortunately, it's not a rustbelt truck.  It spent all but the last winter in FL.

Still, a set of 4-gauge jumper cables didn't do any good.  One cable I put from the alternator bolt to the negative, the other cable I put from the AC compressor bolt to the negative.

I did put a vacuum/pressure gauge on the reference port that the EGR uses to detect flow.  Revving the engine a bit (3000 rpm) showed a bit of pressure, but it was hard to say exactly how much.  My gauge's first real marking is 5psi and it didn't get that high... maybe 2-3 psi?

The way the EGR system works uses a restriction in the EGR tube.  There is a port above and below the restriction.  When EGR is not active, the pressures at those ports are the same.  When EGR is flowing, the upper reference tube is lower pressure since it's exposed to manifold vacuum.  It then modulates how much the vacuum controller sends to the actual EGR valve using that signal.  What I discovered was that as long as the upper port's absolute pressure is equal to or less than the lower port, no code.  If the upper port sees higher pressure than the lower port, OR if the absolute pressure at either one goes beyond the range of the sensor, you get the code. So, by checking pressure at the lower port I was hoping to find a smoking gun.  Higher pressure would point toward my catalyst clog theory, but I have no idea if 2-3 psi is considered high.  Seems like a properly flowing exhaust should show next to zero, right?

It's hard to find diagnostic procedures for this truck.  I guess maybe they all rotted away before the manuals made it to the intarwebs?

sergio said:

Don't worry about the 545 fuel pump failure code, if you stall it that code sets.

Like the others said, take the exhaust lose and drive it, to check for bad cat.

 

Do you have a California model? Looks like only they got DPFE.

If one of the sensors that use 5 volt reference is shorted it will cause the other 5v reference ones to read wrong. Both o2, engine coolant, TPS, intake air temp, DPFE, and octane adjust, use the same ground. The idle air control, canister purge, egr vacuum regulator, and injectors use a different ground. Seeing how you don't have codes for any of those, they are not suspect.

I would clear the codes and disconnect one of the sensors at a time and see if it runs better. The engine coolant code could be a stuck open thermostat, won't reach operating temp.  

I never checked specifically if it was a 50-state designation, but I have heard that as well.... CA got the EGR system, and 92-95 only some 49-state trucks got them.  90% of the aftermarket exhaust stuff does not have any provision for EGR at all so it must not have been that common.  I couldn't find a single manifold that had EGR when I considerered replacing one instead of fixing the busted flange bolt, but alas... three drill bits later.

This diagram is pure gold.  Thank you.  I will trace through that and check for reference signals.  I suppose it would be easy to check them all at port #2 in the EEC check port and jiggle wires?

After a 250 mile trip today, my laser thermo indicated 198 at the thermostat housing and it did take me 10 minutes or so to find the thermo, so that sounds about right.  Coolant temp sensor gave logical resistance readings.... but are their two?  One for teh gauge and one for EEC?  Not sure I had the right one.

So far the only thing about the EGR I check was to pull the electrical connector off the DPFE which should default the pintle closed.  I did notice detonation at around 3/4 throttle which makes me think it was giving a spark advance expecting EGR gasses to be calming things down.

All I've really determined with any possible degree of certainty is that I think the EGR valve itself is opening and closing causing the sputter, but whatever is causing THAT is still a mystery.  Catalyst clog, faulty DPFE, reference wire, junk in the EGR tube, or a good old fashioned vacuum hose leaking are all possibilities on this ugly table.

I have actually considered getting an ECM for a non-EGR truck and just crimping that pipe shut and disconnecting the other crap.  I might lose an MPG and a half, but it would at least eliminate the problem.

Now on to re-order new front bearings.  The ones I put on with minimal confidence this spring are bad enough that my brake pads and calipers are currently setting my toe-out parameters.  the nut/spacer/retainer/nut arrangement procedure inspires no confidence.  I did it by the book and twice now they have backed off and destroyed the bearings.  Grrrr.  I have a feeling I'll be buying two new hub stubs as well.

Anyone want a nice, rust-free truck that keeps breaking dumb crap every week?

Possible air leak between the MAF and throttle body? If I remember right on the bottom of the intake hose/tube would rub on the radiator shroud.

Can you block the ERG valve with a piece of sheet metal or something? 

Good guess, but doubtful.  It's one long piece of aluminum tubing with silicone boots.  Couldn't find any compromised spots.

There are $8 aftermarket EGR block off plates on ebay.

What about dirt on the MAF wires? Does it have a K&N air filter that oil could have gotten on the wires? That would make it run crappy. At WOT it would use look up tables and run in open loop. The fuel pressure should be 35-38psi.

How about an intake manifold leak on the side the o2 is lean?

There is only one O2 sensor located in the collector for the Y-pipe.  I was pretty surprised to see that.

MAF wire cleaned 3 months ago, but that doesn't mean it isn't dirty.  Not sure how it would cause an "EGR not opening" code but worth a look. It is an aftermarket non-oiled filter, so no real chance that the filter "oiled" it.

How many psi should I be seeing at the EGR tube from the manifold?  I know there should be some, but I have no idea how many.

Curtis73 (Forum Supporter) said:

Some digging on the neutral safety switch code.... apparently it is a pretty useless code that comes on if you are KOEO but not in park/neutral.  Since I'm assuming that the clutch switch is the NSS for this computer, it set that code because I didn't have the clutch disengaged.

I suppose my next idea is to disconnect the exhaust in front of the cat and drive it with earplugs.

I actually blew off my exhaust on my 91 4.0. It wasn't terrible loud but sounded sort of cool really but it was after the cat.

Also I had a surging issue like you said and it ended up being a stupid problem....the plug on the MAF had come loose. I never checked codes though, light went off after that

Curtis73 (Forum Supporter) said:

How many psi should I be seeing at the EGR tube from the manifold?  I know there should be some, but I have no idea how many.

At idle?  Zero,or close to it.

We had almost this exact problem with a Mazda 6.

It ended up being the rearmost o2 sensor.  We unplugged it and the car was fine.  Bought a new one and the problem never came back.

It also had a ton of weird codes that didn't seem to make sense.

Still, a set of 4-gauge jumper cables didn't do any good.  One cable I put from the alternator bolt to the negative, the other cable I put from the AC compressor bolt to the negative.

That sounds like a potentially misleading ground check. In each case, you provided a strong ground from battery to engine block. But many sensors have discrete grounds that don't all go to the engine block. Some may tie to body and some may tie to chassis. When there's a bunch of codes all at once, I look for a bulk ground failure or a multi-pin connector dislodged.

Obd1 diagnosis procedure:

Unplug one sensor at a time until it runs better.

The multiple sensor thing sounds fishy but remember one short in a sensor can drag the whole 5v ref signal down.

Streetwiseguy said:

Curtis73 (Forum Supporter) said:

How many psi should I be seeing at the EGR tube from the manifold?  I know there should be some, but I have no idea how many.

At idle?  Zero,or close to it.

I'm getting nothing registered at idle, but if I blip the throttle to 3000 rpms I get 2-3 psi.  I'm going to try to rig up a longer hose and see what I get at WOT.