[Ed Mc.]

Are you getting a signal at the O2 sensor with the coax disconnected?

How about at the ECM end, with the O2 sensor end connected, but with the coax disconnected at the ECM.

Keep in mind that the shield is just grounded at the ECM end for EMI purposes; It's not carrying any signal.

The O2 sensor is grounded electrically via the exhaust system, thru the chassis/body/engine ground.

I wonder if it might help to ground the shield at the O2 sensor end, to an exhaust bracket or something similar that has direct electrical contact with the grounded side of the sensor.

Doing the heated sensor mod does help, but only if you're getting a good signal to the ECM to begin with. And the engine is definitely gonna run poorly without the O2 sensor signal; it'll be in Open Loop all the time.

HTH.........ed

 

[Ed Mc.]

That's a nice diagram, I'm saving it! The voltage output of the O2 sensor should vary within a specified range. You can test the O2 sensor out of the system, with a blowtorch and digital voltmeter. Check YouTube etc for that info.

I found this site with really good info on sensors, including how to to test with a digital voltmeter with the sensor installed:

https://www.carparts.com/blog/how-to-test-an-o2-sensor/

If you test the sensor out-of-the-system, then test by itself in the system, then test with the sensor-end-only wiring connected, you'll have a pretty good idea of what a "good" signal looks like.

If you end up getting the right signal from the ECM-end of the wire, then it should work when you connect it.

BTW, have you checked for trouble codes logged in the ECM? Because it should be throwing at least Code 13 for the O2 sensor circuit.

If there are no codes, maybe there's something wrong with the ECM. It certainly should throw a code with no O2 sensor signal, normal temps, and stuck in Open Loop.

Isuzu OBD / OBD2 Codes – TroubleCodes.net

www.troublecodes.net

Another thought, maybe there's a issue with the coax. You can get shielded automotive sensor wire that might work better, hard to say. I saw a lot of that on eBay.

 

[Ed Mc.]

Get that bulb reinstalled and if you're not getting the ECM to flash the light when you jumper the ALDL plug (or plug the 2 white-with-blue-stripe wires together), the ECM is probably shot. It should at least flash "12" which indicates the diagnostic system is working. Probably gonna flash "13" after that. And a code for EGR I bet!

I expect you'll find that the more "stuff" has been removed, the worse the engine will run. You can delete the smog pump without too much issues, you just have to use a shorter belt and a tensioner meant for a '91 3.1 ZuZu pickup.

But sensors actually attached to the engine, yeah they all should be there.

 

[Ed Mc.]

Yeah, these run quite cool on the temp gauge even with a 195F 'stat in there.

I've been running mine on very cold mornings and the iron block and heads take a long time to warm up. And it's quite cranky until it gets up to operating temp. Pretty sure it's not going well into Closed Loop because of the cold. I blocked off part of the grille's airflow with cardboard zip-tied to the "egg crates" and it made a big difference. Heater puts out a ton more heat, too.

Gonna do some research on grille covers and see if I can find something nicer for next year. For now, the cardboard stays on until it warms up! As an added benefit, I got 15mpg even on short trips taking my Daughter to work in the snow, in 4WD and sub-freezing temps. So very happy with that.

BTW the heated O2 sensor mod would likely help the engine go into Closed Loop faster and stay there better. I have mine installed and power wire run, but still need to tie-into a source of switched power. Geoff has a good writeup on that on his Trooper website, check it out here:

How To | Isuzu Trooper

www.geoffinbc.com

Glad you got to the root of the problem, now onto more fun things, eh?

 

[Ed Mc.]

I was working on the above reply when you posted the latest. Another problem that causes fuel restrictions on these is a clogged fuel filter. So if you haven't done that yet, something to consider. It's also possible you're losing fuel pressure when underway. Only way to check that would be to have a long hose attached to the gauge and run it out to the windshield area so you can see it. You could tie it to a wiper arm. Or pull the hood, then you'll be able to see the test gauge when driving.

If your fuel pressure isn't dropping when you drive down the road, it's probably something else.

You could also have a restriction in your exhaust system, that'll cause poor performance. You can check for excessive backpressure in the exhaust by pulling the O2 sensor and installing a pressure gauge.

If you search the InterWebs for "how to check for a clogged catalytic converter" you'll get tons of hits, this is one of 'em:

So, there's a couple things to look at. You may get to the point where you'll need to use the WinALDL program, which is free but requires a patch cord (available online) and a laptop with either USB or Serial port. If you're good with electrical stuff the WinALDL site has some excellent instructions for a D-I-Y serial patch cord that'll be very cheap to build.

The patch cord ties into the ALDL plug and the program allows you to collect stationary operating data or dynamic, on-the-road real-time info. Which will tell you a ton. But try the easy stuff first. WinALDL - 160 baud ALDL reader

If you disable the EGR, you're gonna get a Check Engine light. The ECM cycles the EGR periodically and expects to "see", via the O2 sensor, the change in mixture when EGR is active. If it doesn't like what it sees, it'll set a trouble code. But I don't think that's causing your running problems. The EGR is mostly for emissions, as it cycles some exhaust with unburnt hydrocarbons back into the engine to be burned again. That also reduces combustion temps which reduces NOx emissions, but it still ain't gonna make the engine do what yours is doing.

You can make the fuel pump run without the engine running by energizing the "Fuel Pump ALCL Connector". It's a disconnected wire up around the fuse box, black with red stripe IIRC.

If you power that wire up with +12VDC, the pump will run no matter what the rest of the system is doing. With the fuel filter disconnected, you could hook up a long fuel hose to the output of the pump, then run fuel into a container and see if it's pumping a lot or a little. I'm sure there's must be a capacity test for that, but I don't have the info. No doubt someone else here does.

Fuel system controls wiring diagram attached. Hope this helps..........ed

 

[Ed Mc.]

Here's a fuel system troubleshooting block diagram which might also help............ed

 

[Ed Mc.]

Venezuela! Man that must be scary living in a country with such turmoil. Must also be hard to find parts that you need.

And yeah, I don't imagine you're having any issues with freezing temps of 14.8F like it was here last week! We could use some sunshine up here in the PNW. It's a "balmy" 34.4F as I write this.

G'luck with the troubleshooting and Stay Safe............ed