Help! Fuel mileage issue...

[96WillowGrnXLT]

Yeah, that looks like what's on my Ex. Now that I think about it, I remember that sucking sound coming from roughly that area of the intake. It was getting lousy mileage before I cleaned it, but it's possible that this could be causing an issue as well. It appears to be functioning normally, though. I ran a KOER test on it yesterday and the idle changed as the scan tool was running. I also think the EGR is working correctly because at one point in the test, the engine started running a bit rougher, as though the EGR was being opened further than what it should be at the test RPM. I couldn't see the live data when that happened, so I don't know for sure if that was the case. Thought maybe of running a KOEO test to see if anything changes when it's not running, but I don't think it would.

One thing I have noticed is for some reason, I have a flashing indicator on the I/M monitor 'Cat'. I drove it for a good 20 miles yesterday and it never stopped flashing to indicate that the monitor is working correctly. I'm pretty sure it's related, but the only enhanced code I could pull out of it was P1000, which just tells me that all the modules haven't reported in yet. Anyone know how long the drive cycle has to be to bring that monitor in line?

When it's warmed up, it seems to idle the same as when cold. It has an intermittent and very minor stumble to the idle. I checked the engine RPM when it does it and it doesn't drop much more than 50 RPM when it happens. It just doesn't seem right - maybe it is no cause for concern, but I can definitely tell when it happens. It will idle smooth for a good while (maybe 30 seconds), then stumble just that little bit for maybe 5 seconds, then smooth back out. You can feel it in the vehicle and see it on the engine when it happens. It does it whether there's a load on the engine or not, though it's more noticeable when there's no load on it in Park. I originally thought it was maybe an EGR issue, especially when I got that P0401 code earlier. But now, that code went away on its own and EGR appears to be working correctly, yet the idle is still just not right. Maybe that will help a little, because it sure seems related...

 

[2000StreetRod]

EGR valve

The EGR valve should be closed when the engine is idling. See EGR System O&T

Few of the V6s idle smoothly compared to the V8s. You may want to perform the TPS Test Procedure followed by the Idle adjustment procedure

I'll have to research the answer to your question about the drive cycle.

 

[96WillowGrnXLT]

I found something interesting tonight that might help shed some light on what's going on, but I'm not sure...

I drove it tonight and was down to a quarter tank, so I filled up. I went 149 miles and put 12.928 gallons in for a whopping total of 11.52 mpg. Ugh! When I got home again, I thought I'd dig around and see if I could trace down where that sucking sound I explained earlier was coming from. Well, I found a new sound tonight that makes me curious, and partially leary. I had a rattling noise coming from the passenger rear of the engine. I thought at first it was maybe valvetrain noise, but it wasn't a tick at all, and since it's not an SOHC motor, I know it's not chain rattle. Sounded kinda like marbles rolling around on tile. I got to reading your post about EGR testing, SR2000, and saw that at least on the SOHC motors, there's a solenoid back there for the EGR. Do you know if the OHV motors had the same thing? I wonder if maybe that might be acting up or is loose internally if I have it. It was pretty dark and all I had was the underhood light, so I couldn't tell if there was something there or not. But it's just one more thing to deal with now...

 

[2000StreetRod]

Evr

According to the Rock Auto photo below your EGR Vacuum Regulator (EVR) looks just like the one on my SOHC.

My EVR is mounted to the upper intake manifold as shown below.

One way I check for vacuum leaks is with a length of hose about three feet long. I place one end to my ear and the other end to the area I want to probe. It is very possible that the vacuum line to or from the EVR could be leaking. But a hose vacuum leak should not sound like a rattle. The PCM controls the EVR with an On/Off rectangular waveform but EGR should be off when the engine is idling. I would investigate the source of the noise further.