1995 K1500 TBI Surges Under Load after warmed up

[1998_K1500_Sub]

So I tried unplugging my Vacuum line running to the EGR, didn’t notice any difference. I live on base out here in Yuma and there’s an auto shop so I’m gonna see if one of the technicians can help me figure it out as well. I have a good amount of tools in my truck, but no OBD1 scanner or fuel pressure gauge, so I’m going to see if the auto shop has either of those.

You've got a 1995, yes?

IIRC EGR in 1995 had a fancy valve assembly controlled by the ECU. The connector was maybe 4 or 5 pins... at least, on the 1995 4.3L engines.

(edit) I checked RockAuto and I see what you mean, the GMT400 5.7L appears to use the old-style EGR valve / vacuum motor.

 

[Bookario11]

You've got a 1995, yes?

IIRC EGR in 1995 had a fancy valve assembly controlled by the ECU. The connector was maybe 4 or 5 pins... at least, on the 1995 4.3L engines.

(edit) I checked RockAuto and I see what you mean, the GMT400 5.7L appears to use the old-style EGR valve / vacuum motor.

I think you’re right, I believe it has a solenoid that provides vacuum to the EGR, but there are no electrical connectors going straight to the EGR. Should I try unplugging that?

 

[1998_K1500_Sub]

I think you’re right, I believe it has a solenoid that provides vacuum to the EGR, but there are no electrical connectors going straight to the EGR. Should I try unplugging that?

You could try, see what happens.

This still seems related to the ECU entering closed-loop mode (although I've assumed this, based on your description of the situation), and I'm mulling it over.

See, when the ECU enables the EGR it also reduces the fuel flow (injector on-time); simultaneously, if the EGR doesn't flow enough exhaust gas, there will be excess (and unconsumed) oxygen remaining after combustion and the O2 sensor will detect the lean condition (Code 44). Too, the engine performance will suffer at these times. This is why I'm focused on EGR.

What I would try is to disable the ECU from enabling EGR. The only way I can think to do this, with your ECU and EGR set-up, is to prevent the ECU from entering closed-loop mode.

There are a few ways of doing this, and probably the "best" way is to disconnect the O2 sensor.

In the later models, like my 1998, the ECU can tell if it's connected to the EGR and, if it believes it's disconnected, it will disable EGR.

IF YOU disconnect your EGR electrical connector, and drive the vehicle for long enough, the ECU will probably figure-out that the EGR's not working, set a code / SES and disable EGR operation. This would be ideal, in the short term, for you, if it worked; but I don't know how long you would need to drive it for this to happen. Once it happens thou, it would be obvious by the SES and / or set code.

 

[1998_K1500_Sub]

You might try to test the vacuum motor / EGR valve and make sure the valve moves with applied vacuum, and doesn't leak down ("holds vacuum") reasonably well.

You might also try to test the solenoid that applies vacuum, it may be f'd up.

That's such a crappy EGR system.

You would be better off if you could easily disable it, but that takes a "tune".

 

[95Noobie]

So I tried unplugging my Vacuum line running to the EGR, didn’t notice any difference. I live on base out here in Yuma and there’s an auto shop so I’m gonna see if one of the technicians can help me figure it out as well. I have a good amount of tools in my truck, but no OBD1 scanner or fuel pressure gauge, so I’m going to see if the auto shop has either of those.

Any updates? I have a 1995 305cc TBI and it was a bit tough tracking down the correct EGR valve. I had to find one that ran on negative pressure. The one I needed ended up being on amazon and their software said it would NOT work for a 1995 chevy truck like mine but it turns out it was the correct one. If you need a new EGR valve now or in the futrue I can share a link.