obd1 = obd FUN... OR, why I am mad at my truck right now

[curtis73]

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

114

Out of range voltage air change temperature sensor (ACT)

Normal operating temp not reached

116

Out of range Engine coolant temperature sensor (ECT)

Normal operating temp not reached

335	Electronic pressure transducer (EPT)/differential pressure feedback exhaust (DPFE) sensor

522	Neutral/drive switch (NDS)

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.

117

Low voltage Engine coolant temperature sensor (ECT)

Normal operating temp not reached

643 (doesn't exist)

332	EGR valve not opening

343 (doesn't exist)

176	Lambda sensor (left) too lean

543	Fuel pump (FP) circuit failure

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. Recent fuel filter, fuel pump relay, and reman injectors 5000 miles ago. No raw fuel in the FPR hose.
Can you folks make sense of my EEC-IV alphabet soup?

 

[RonD]

I would first pull out the computer and check the circuit board

There are 3 capacitors that will leak/fail after 20+ years, and cause running issues and odd codes

I just had to do my 1994 computer, $5 in parts and good as new
I would replace them regarless

It may not be the computer, but if it is you will be chasing your tail trying to diagnose codes that are not real

My repair thread here: https://www.therangerstation.com/forums/index.php?threads/odd-issue-with-my-1994.187821/

 

[curtis73]

I will look into it. So far I've tried checking grounds and adding an additional one (jumper cables from the block to the battery) but no change.

Tried whacking the EGR too in case it was coked up. No change, although that doesn't prove anything. The exhaust was recently off, so I have confidence I can get the flange bolts off between the Y and the cat. Looking at the catalyst inside won't show me how well it's working, but I should be able to see a clog or a shattered honeycomb if that's the problem. I also tested resistance at the CTS and got logical numbers, but is there only one, or are there two... one for the computer and one for the gauge?

 

[waterran]

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

114

Out of range voltage air change temperature sensor (ACT)

Normal operating temp not reached

116

Out of range Engine coolant temperature sensor (ECT)

Normal operating temp not reached

335	Electronic pressure transducer (EPT)/differential pressure feedback exhaust (DPFE) sensor

522	Neutral/drive switch (NDS)

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.

117

Low voltage Engine coolant temperature sensor (ECT)

Normal operating temp not reached

643 (doesn't exist)

332	EGR valve not opening

343 (doesn't exist)

176	Lambda sensor (left) too lean

543	Fuel pump (FP) circuit failure

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. Recent fuel filter, fuel pump relay, and reman injectors 5000 miles ago. No raw fuel in the FPR hose.
Can you folks make sense of my EEC-IV alphabet soup?

What is eng. mileage? Check for a vac. leak. Check compression. Have you disconnected the battery cable to reset codes? Block the EGR vac. line & see what happens. You state 5 times per second? That's puzzling. An A/C clutch can't cycle that fast, neither can the throttle. A vacuum leak can cause most of the DTC's you list. Also, if it's a SOHC engine, use a search engine for common issues with that Mazda engine. If it's a OHV Ford engine, check internet for "repair issues for 4.0 OHV ford engine." An intake coolant leak can set a multitude of DTC's.

 

[DEADEYE]

I don't know about you but I believe if I allowed myself, I would have body slammed my truck without even breaking a sweat.

 

[Ramcharger90]

I will look into it. So far I've tried checking grounds and adding an additional one (jumper cables from the block to the battery) but no change.

Tried whacking the EGR too in case it was coked up. No change, although that doesn't prove anything. The exhaust was recently off, so I have confidence I can get the flange bolts off between the Y and the cat. Looking at the catalyst inside won't show me how well it's working, but I should be able to see a clog or a shattered honeycomb if that's the problem. I also tested resistance at the CTS and got logical numbers, but is there only one, or are there two... one for the computer and one for the gauge?

I forget the name of the tool.

But you drill a hole before the cat test then drill another hole after then test the flow again it comes with a chart with what pressure to look for.

 

[rubydist]

For reasons I have never fully understood, in my experience, any Ford odb1 vehicle with a code for ACT (intake air temp) would never run right until that code was fixed. Somehow having the pcm think the ambient temp was way off seemed to mess up the whole program. So, my advice is to figure that one out as soon as possible.

 

[gaz]

rubydist,

Great suggestion!

 

[RonD]

For reasons I have never fully understood, in my experience, any Ford odb1 vehicle with a code for ACT (intake air temp) would never run right until that code was fixed. Somehow having the pcm think the ambient temp was way off seemed to mess up the whole program. So, my advice is to figure that one out as soon as possible.

Gasoline needs a 14.7:1 air:fuel mix ratio
This is a WEIGHT RATIO, not volume

14.7 POUNDS of air to 1 POUND of gasoline
14.7grams of air to 1 gram of gasoline

Gasoline's weight doesn't change much when cold or warm
Air's weight changes A LOT when cold or warm

"hot air rises"(hot air balloon), that's because warmer air weights alot less than cooler air, so it "floats" up on the heavier colder air

Cold Air intake..............why is it better?
Because cooler air is denser and heavier...............see ratio, 14.7grams of air to 1 gram of gasoline
So if you have heavier air you can add more gasoline and get more power
Engines are "peppier" in the winter months because of the colder air

This is why the air temp matters to the computer, and why it will effect engine operation if its off by too much

 

[rubydist]

Yeah, I know all that RonD, I am an engineer by background. Air density changes roughly 1% for 5*F change in temp. That doesn't explain to me why an OBD1 engine won't run worth a crap when the air temp sensor is reading wrong. The engine will run with a/f anywhere from ~11:1 to 18:1, and while it won't run right, it will certainly idle somewhat decently. Some of the Ford OBD1 engines won't even stay running when the air temp sensor is out of range. Unless the software code doesn't have a default temp when the sensor is out of range and the computer really believes the air temperature is infinity, that density change does not explain why it won't run.

btw, engines are not "peppier" in the winter, because winter blend gas actually has less energy density, which explains part of why your gas mileage is always worse in the winter.

 

[curtis73]

What is eng. mileage? Check for a vac. leak. Check compression. Have you disconnected the battery cable to reset codes? Block the EGR vac. line & see what happens. You state 5 times per second? That's puzzling. An A/C clutch can't cycle that fast, neither can the throttle. A vacuum leak can cause most of the DTC's you list. Also, if it's a SOHC engine, use a search engine for common issues with that Mazda engine. If it's a OHV Ford engine, check internet for "repair issues for 4.0 OHV ford engine." An intake coolant leak can set a multitude of DTC's.

Mileage is 112k. Compression was perfect in May when it was checked. No coolant leaks. No vacuum leaks detectable and no other symptoms of vacuum leaks. I've already determined that it's not an A/C issue, I was just using that as an example so you knew what it felt like.

 

[curtis73]

I forget the name of the tool.

But you drill a hole before the cat test then drill another hole after then test the flow again it comes with a chart with what pressure to look for.

It's actually pretty easy to do using the existing EGR system. I just stuffed a vacuum/pressure gauge on the upstream reference tube of the EGR and revved. Got 2-3 psi. I never understood referencing after the cat. Unless you have a significant blockage in the muffler, the pressure after the cat should be almost undetectable.