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Discussion Starter · #1 ·
Hello!

I have a 1991 first gen Trooper LS, it has the 2.8 V6 GM engine with TBI. My dad bought this truck second hand many years ago, he passed away and now I´m taking care of it and trying to make it run as good and pretty as possible! This Trooper has always had terrible gas millage for many reasons, but right now I have a serious issue with the O2 sensor.

For some reason, the previous owners decided to yanked out the O2 sensor wire from the sensor to the engine bay. So I connected an old coaxial video wire from the sensor directly to the yellow-blue cable on port D7 of the ECU.

I made sure that the signal part of the coaxial wire goes to D7, and that the ground part was connected to the white-green wire on port D6. I obviously bought a brand new, single wire O2 sensor because the old one had been sitting there many years.

After I mounted everything and checked the connections, I ran a oscilloscope to see if the O2 sensor was working, but got nothing but an erratic signal, it never showed a sinusoidal pattern not after being at temperature (evidently) or revving the engine.

What could be happening here?

I´m looking forward to fixing this, all your help is much appreciated!

Kind regards,
- Daniel.
 

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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

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Discussion Starter · #3 ·
I haven´t tried the tests you mentioned and can´t do them now with an oscilloscope because I don´t have one. When I tested it was with a borrowed oscilloscope. Is there a way to test with a multimeter?

I have measured voltage at the ECM O2 port with the coax and O2 sensors unplugged and I´m getting a constant negative 0,33v. (so I think the ECU is sending voltage to the sensor) Is this normal?

Yes, I´m aware the coax coating is for EMI purposes only, no signal travels through it. I like your idea of grounding it at the O2 end, might improve signal delivery.

I once adapted a three wire O2 sensor. I got the voltage to the O2 heater through the cigarette lighter on the dashboard. That was the only time I got somewhat of sinusoidal signal. But my mechanic friends told me that the mod wasn´t good because the heater would always be on while the truck´s running and that it would fry the sensor.

Yeah, open loop runs very deficiently. I can´t step to much on the gas without loosing power.

Thank you for your reply Ed!

Attached is the ECU input port diagram I´m using.
 

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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:


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.


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.
 

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Discussion Starter · #5 ·
Wow! Nice insights and info! I´ll look it up and test again.

I suppose the check engine dash board light for diagnosing is working, I have never used it because the previous owners removed the bulb, I suppose they couldn´t make it turn off because the engine is missing the EGR electro valve and some other stuff. But I can use it to find codes, certainly the O2 one should come up.

A good coax wire would help too.

I´ll keep you posted if I manage to make some progress.

Thank you for your help, Ed.

:D
 

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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.
 

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Discussion Starter · #7 ·
Hi! Long time without replying! Happy New Year!

I found out why the oxygen sensor was not working. The issue was that the truck was running without a thermostat, so it never got to work temperature. Installed it and now the spark plugs come out light brown reddish, not sure if optimal color, but there not completely black like before.

I haven´t tested the O2 sensor with an oscilloscope yet, but I assume it´s working just by the difference in the spark plug colors.

Thank you for your help!
 

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Discussion Starter · #8 ·
Now the truck has a problem that´s driving me mad; I can´t step on the gas more than a certain percentage without the engine bogging down under load.

For example, when on the highway, if I need to go faster or go up a steep incline I give it more throttle, but at certain point, approximately 40% of the pedal the truck actually produces less power and slows down, I have to release the pedal so it accelerates.

Driving like this is very frustrating because I have to rev the engine more and can´t really use it at its full capacity.

Since I rebuilt the engine, replaced TPS, IAC, MAP, fuel pump, fuel injectors, TBI, detonation sensor, spark plug and wires, rotor, ignition coil, distributor, O2 sensor, ignition module, well basically everything related to engine performance. Calibrated the TPS to 0.42V. Fuel pressure is 12Psi.

During the issue the engine sound changes, it gets more horse. My hypothesis is that at certain throttle position the air/fuel mixture is offset, tending to a lean mixture because I see no black smoke whatsoever. After much reading and watching all the TBI related videos on YouTube I think it could be a clogged fuel delivery line.

If I´m correct, the fuel pump will not be able to deliver sufficient gas when needed, however it does maintain the fuel pressure in specs. I´ll dissemble the fuel lines and clean them tomorrow, I really hope I find a solution to this exasperating issue.

What do you think about a deleted EGR system and the issue mentioned above? The EGR is not working, so I deleted it.

Much thanks and kind regards,
-Daniel.
 

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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:


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

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

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Discussion Starter · #12 ·
Nice! That´s a lot of helpful info!

Glad to hear you managed to increase MPG with the carboards covers! I live in the tropics in Venezuela, so it doesn´t get so cold, despite that, the engine gets to operating temperature in approximately 10min idling parked.

Yeah, I though the EGR wasn´t the culprit, but had to check.

I forgot to mention that I installed a new fuel filter. I also thought about exhaust gas restrictions, I´ll investigate about that further, maybe I´ll drive around without the exhaust just to see what happens.

I did some research about the fuel pump delivery rates, it´s hard to find, only found on a forum that those types of low pressure pumps deliver 20 to 25 GPH. I´ll do the volumetric test you mentioned and compare. I usually energize the fuel pump by jumping it´s relay.

Thanks, ed.
-Daniel
 

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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
 
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