O2 Sensor? 1989 C2500 5.7

[Triplesn8s]

Hi All,
My 89 C2500 is running silly rich. It smells bad and I get buildup on the plugs somewhat regularly fouling one.. My first guess is the ECU is in some kind of a limp mode as the PO apparently eliminated the O2 sensor. At least I think this truck had an O2 sensor, I am not entirely sure. I cannot find any wires cut or plugs dangling loose that would indicate it had a sensor that was removed. But I have no idea where in the harness I would look for said wire or the color of it. Or what the plug might look like. Can anyone send a photo of the wiring harness pointing to where the wire should be coming out so I can try and track it down? Assuming of course it is supposed to have an O2 sensor.. TIA Bill

 

[Schurkey]

Every computer-controlled engine since 1980 1/2 had at least one O2 sensor. Your truck had one, most-likely in the left exhaust manifold, towards the rear.

If there's no O2 sensor connected, the computer runs off of programming. With the sensor, it runs off of modified programming, using more-accurate fueling based on the O2 sensor feedback.

There's got to be an O2 sensor body screwed-into the manifold, or you'd have a huge exhaust leak. That, or someone has put a plug in it, or installed a manifold with no sensor hole. Either one would be crazy, since an actual O2 sensor for that vehicle is about the least-expensive you could hope for. My guess is that you'll find the remains of a sensor when you go looking for it. The one-wire connector is likely hanging down somewhere around the steering column/firewall area.

Running rich, long enough, will murder the catalytic converter. But yours--if original--is so ancient that it needs to be replaced anyway.

Plug fouling? Sounds like the engine has additional problems. Oil consumption, perhaps?

Verify all tune-up items and procedures: Cap, rotor, plug wires, spark plugs, PCV system, heated air intake system, EGR system, initial timing, electronic timing advance. In your case, I'd want a cranking compression test of all eight cylinders, and a leakdown test of any that are weak, compared to the cylinder with "best/highest" compression pressure.

Connect a scan tool, verify EVERY sensor and computer output. Also look at fuel trims.

 

[Triplesn8s]

Sound advice, thanks. I wanted to check the timing, but cannot find the marks anywhere. If it’s a bolt on degree scale It is long gone. As I can’t find it. I wonder how often the TBI injectors go bad. I have a new Holley tbi unit I bought at a swap meet 10+ years ago but have not been able verify it is right for the truck. the number on the box doesn’t cross anywhere. All I can do at the moment is query and investigate online. I had rotator cuff surgery Thursday so I won’t be lifting any hoods for a bit. Thanks for the checklist. I have done plugs, wires cap and rotor. Next up the O2 sensor and timing, if I can find the pointer.

 

[Triplesn8s]

BTW, what / how is fuel trim accomplished? Based on prior experiences I have to assume that everything that can be manually adjusted is misadjusted.

 

[Schurkey]

I wanted to check the timing, but cannot find the marks anywhere. If it’s a bolt on degree scale It is long gone. As I can’t find it.

If this is like my '88 K1500 5.7L, it's at 12:00 above the damper. There is a bigass hole in the accessory bracket that the timing light is pointed straight-down through. I need both knees on the radiator hold-down, one hand on a valve cover or the cowl, and the other pointing the timing light down through the hole in the bigass accessory bracket near the belt tensioner.

It's a real pain in the tuckus. It works, but it's not intuitive.

For folks with more tools than sense, using a magnetic timing indicator is ENORMOUSLY easier. The timing pointer has the tubular extension molded into it, that the "magic wand" fits into. Set the machine for a 9.5 degree offset, and read the timing and RPM on the display.

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I wonder how often the TBI injectors go bad.

Possible. Not common. VERY not common if you're buying Top Tier fuel, and/or using Chevron Techron Complete Fuel System Cleaner at each oil change.

I have a new Holley tbi unit I bought at a swap meet 10+ years ago but have not been able verify it is right for the truck. the number on the box doesn’t cross anywhere. All I can do at the moment is query and investigate online.

There's lots of Holley TBI part numbers. Some are replacements for OEM throttle bodies, others are aftermarket intended to be used with a Holley computer to run the injectors.

The direct replacement for a 5.7L '87--'91 (I think) is the 502-6.
www.summitracing.com/parts/hly-502-6

Holley 502-6 Holley OE Replacement Throttle Bodies | Summit Racing

Free Shipping - Holley OE Replacement Throttle Bodies with qualifying orders of $109. Shop Throttle Bodies at Summit Racing.

www.summitracing.com

Holley sells a thicker throttle body gasket with tapered throttle holes, so the big 2" throttle plates clear the smaller holes in a stock intake manifold. This is the rebuild kit which includes the gasket, the gasket is (or was) available separately.
www.summitracing.com/parts/hly-503-5

Holley 503-5 Holley Throttle Body Rebuild Kits | Summit Racing

Free Shipping - Holley Throttle Body Rebuild Kits with qualifying orders of $109. Shop Throttle Body Rebuild Kits at Summit Racing.

www.summitracing.com

I had rotator cuff surgery Thursday so I won’t be lifting any hoods for a bit. Thanks for the checklist. I have done plugs, wires cap and rotor. Next up the O2 sensor and timing, if I can find the pointer.

BTW, what / how is fuel trim accomplished? Based on prior experiences I have to assume that everything that can be manually adjusted is misadjusted.

Fuel trim is done by the computer, continually, based on O2 sensor feedback. Not something done by the owner/operator.

You'd check it to see that it's near the baseline. The farther away from baseline, the more likely there's a fuel system problem.

Misfire screws up the O2 sensor, produces false "lean" signal. That, in turn, causes the computer to go into "rich command" where the fuel trim numbers increase. Or the O2 sensor can be faulty, the computer doesn't get accurate rich/lean indication...so it does what it's told and goes overly-rich or overly-lean based on faulty information.

 

[Russ B]

I have a 1989 K1500 5.7. Presume it is same setup as yours. Color of wire to O2S is purple. The O2S wire is black. The connector is about 5" from the vacuum hose that comes off the brake boost. It comes out of the big wiring harness at the firewall. Open that harness and you should find a loose purple wire. As for the missing O2S the sensor is located on the driver side exhaust Y pipe below the brake booster. Presuming that hole is missing, it was either plugged or previous owner removed the Y pipe. Not that expensive to just buy another Y exhaust pipe. As for the timing mark, it has to be there just tough to find and probably rusted. The crank pulley only has a small cut that needs to be painted so the timing light can illuminate it. The corresponding mark is a tab on the engine front timing cover. Best way to find is to get under engine, remove plastic cover and work from there.

 

[Triplesn8s]

Thanks for the update! I will try that. I have not found it yet. Could you please clarify, there are two wires going to the O2 sensor, one black and one purple? Do they both come out of the same spot in the harness? I need to takes some photos. I am 63 and recovering from shoulder surgery so clambering up into the engine compartment is a challenge. I literally crawl into the (oil sprayed) engine compartment from a ladder to do anything.

 

[Schurkey]

ONE conductor on the O2 sensor. There's a black wire coming out of the sensor, leads to a one-wire connector. The vehicle harness on the other side of the connector is purple.

In other words, the purple wire plugs into the black wire several inches away from the actual sensor.