Isuzu SUV Forum banner
1 - 6 of 13 Posts

· Registered
Joined
·
6 Posts
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.
 

· Registered
Joined
·
6 Posts
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.
 

Attachments

· Registered
Joined
·
6 Posts
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
 

· Registered
Joined
·
6 Posts
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!
 

· Registered
Joined
·
6 Posts
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.
 

· Registered
Joined
·
6 Posts
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
 
1 - 6 of 13 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top