1995 K2500 Chev Trk - 5.7L In & Out Of Closed Loop - FIXED!

[Tim Martin]

This one is really biting me.

The owner dropped this 1995 K2500 Chevy Pk Tk with a TBI 5.7L, auto trans, 89,000 miles on the clock. The original issue is that the engine hardly runs once it is started, has no power going down the road. Another shop replaced the plugs, cap, rotor, O2 Sensor, fuel filter and ignition wires. No change. So that shop recommended it be brought to my shop for diagnosis. I began the diagnosis with a scan tool. No trouble codes and fuel pressure at 13 psi. Well, I found a faulty EGR Vacuum Solenoid was stuck open which allowed the EGR valve to open with the slightest throttle opening. The engine then had a stable idle and ran beautifully till the computer system went into closed loop.

All of a sudden the Block Learn went to 90 and stayed there, as it does that, the engine wants to shut off due to the computer taking away fuel injector on time. However the O2 sensor readings were hovering at .75 - .90volt with little movement. The injector duty cycle is at 1.2 with absolutely no change even though the PCM is commanding a change and the engine hardly runs. Stroking the throttle will usually keep the engine running however it now runs like a lean engine with tremendous heat out the tailpipe but the Coolant temperature at 194. Map Sensor reading is 1.1 volt at idle and 20 inches on the gauge. Meanwhile, the Loop Status is going from "Closed" loop to "Open" loop and cycling. Even holding the engine rpms at 2500 the loop status constantly cycles between closed and open. So, I came to the conclusion we got us a bad PCM and I changed it with no change in the problem. So, I installed a new O2 Sensor with no change. I have a sensor simulator and I hooked that up to the O2 Sensor wiring and cycled the voltage rapidly and the engine runs beautiful. Hook the O2 Sensor back up and we got a crappy running engine. It can hardly get out of its own way going down the road as it wants to shut off.

What am I missing? With the exception of the EGR Solenoid, I have had little success on this one. I am embarrased to admit that this one is getting me down.

[brianp87]

Sounds like it could be a ground issue.

[ricmorin]

Well, let's think about this for a minute. What is added to the equation when the vehicle goes into closed loop? Oxygen sensor. It's already using MAP and CTS to a degree. Is this a one wire O2 or a heated style?

[Tim Martin]

Single wire O2 Sensor - it's got a 4L80E Transmission. Others use a three wire heated sensor.

Regarding the ground issue. I took the grounds off the front of the engine, ya know the ones there by the thermostat housing and sanded them to ensure a good contact but that did not help. Is there other grounds on this application?

[brianp87]

So the vehicle has no power in closed loop. hmm What is the short term fuel trim when it runs bad?

[brianp87]

Also jsut for giggles did you check the timing and the distributor. Seems like an off the wall shot but i do alot of distributors on these and find alot of these with the timing way off.

[brianp87]

Also jsut for giggles did you check the timing and the distributor. Seems like an off the wall shot but i do alot of distributors on these and find alot of these with the timing way off.

Also exh back pressure when u say it runs ok with o2 off is it out or unplugged? Meaning is it letting the back pressure release were with it in it isnt.

[Tim Martin]

Ah, you guys are thinking of newer vehicles. This one measures fuel trim by the block learn number which stays pegged at 90 which means it is seeing a rich mixture and is subtracting fuel. As soon as it goes into closed loop the block learn number goes to 90. Rather strange. it did that with the old as well as the new computer.

No restricted exhaust. I checked that by using a vacuum gauge and slowly accelerating the engine rpms. At 750 rpm I got 20 inches a vacuum. At 4000 rpm I got 21 inches a vacuum. Restricted exhaust? Rather doubtful.

I understand the distributer issue but I am wondering whether a slightly off timed engine will take it in and out a closed loop.

[brianp87]

Whats it do with the egr disconnected? What is the o2 voltage when acting up?

[Tim Martin]

EGR disconnected? No difference. O2 voltage when acting up? Fluctuating between .15 and .9 until Block learn gets to 90 and then O2 voltage stays at about .75 then will go into open loop. Strange indeed.

[ricmorin]

I think the reason it kicks out of closed loop is because it's reached the limit of adjustment. It sounds like the O2 thinks it's rich and keeps leaning it out to the max, when the vehicle isn't really rich at all.

[Tim Martin]

I think the reason it kicks out of closed loop is because it's reached the limit of adjustment. It sounds like the O2 thinks it's rich and keeps leaning it out to the max, when the vehicle isn't really rich at all.

I agree with you. An the solution or fix for that is ????????

[ricmorin]

Well it's hard to say from here, but the O2 is on my suspect list. What brand did you use? Do you trust it? Anything that might introduce fuel or fool the sensor? Are both injectors firing? Use a timing light to freeze the spray pattern. It's a weird one for sure.

[Tim Martin]

It's a weird one for sure.

Ya gotta be kidding!

I couldn't get a Delco O2 Sensor so I used a Bosch. I have usually had good luck with them. The scope pattern is precisely the same with the Bosch as with the one I took out. I am somewhat forced to conclude that the Sensor may not be the problem and yet with my sensor simulator the engine runs just fine, so I admit that I do wonder......

[brianp87]

try a new sensor.

[Tim Martin]

try a new sensor.

What brand would you go with?

Another thing. Is it safe to assume that the O2 Sensor is grounded in the exhaust system? Is there any way to check that?

[brianp87]

I use bosch all the time wiht no issues. I assume its grounded? Ive never ran into that issue. Im not to good on older stuff.

[steven kiser]

looking at this post all looks great. points to a bad 02 sensor. now, what if this person filled the tank with the incorrect fuel? i've had a few that made no sense and either it went away after the owner filled the tank again or after i drained it. a vac leak would read lean force rich, opposite of what's going on. the sensors appear to be reading correctly and wiring must be intact because simulator is making the truck run well. I'd lean towards a contamination issue with the fuel or incorrect mix for the truck. i see a few each year as the seasons change. people will dump the left over snow blower fuel into their gas tanks. they know that their lawn mower won't run on 2 cycle but are shocked when their vehicle craps out. i learned a long time ago to press people about doing this. some will deny it but i will work on my seasonal experience.

[Tim Martin]

Well what I might try is a ac delco O2 sensor from the dealer and see if that has any change on this one.

[jbadenoch]

testing the 02 sensor for ground to the exhaust, use a test light and touch the body of 02 sensor with other end on 12v pos