ChefYota4x4

iTrader: (5)

IACV is the Idle Air Control Valve.. It is supposed to idle up the motor when warming up from cold. As the vehicle reaches, I believe, 86* temperature, it begins to close, slowing the idle down, until it reaches operating temp.(or a lil before), where the truck should idle right at 750RPM.

If you're getting better mileage with the TPS disconnected, that's news to me. I've read, most often, that while the idle will increase and basically blanket any EFI issues, .... it will SUCK gas, when disconnected, because the motor is coming out of 'Closed Loop' and running in a default mode. The TPS IS IMPORTANT, and you should not operate your truck for any period of time without it connected. There are tests for both the wiring connector(voltage, etc) TO the TPS and TPS resistance readings as well. Go to 4crawler.com and look at Rogers write up on the TPS. He's an electrical engineer... and I can't imagine him telling you it's "Ok to run it without the TPS"... while, at the same time, he's managed to disconnect other things like the VSV(the Vacuum switching valve on the top/back of the valve cover that regulates vacuum to the Fuel Pressure Regulator. According to him, you can run the FPR directly off the Plenum. He used the open port to run a vacuum gauge in his dash.

The EGR system is also important for emissions, and can dramatically effect performance, mileage, etc. However, like mentioned, the coke can test is quick and easy to eliminate that as a factor(if you don't want to go buy a vacuum pump and gauge for about 35$ total for both?). When Operating Temp, hold idle at 2000rpm and apply 10inches of vacuum to the top of EGR(vacuum hose that you've removed, plugged). The idle should drop, dramatically. If so, you can pretty much KNOW that your EGR is operating like it's supposed to. When you apply the vacuum, you're actually forcing the valve in the EGR back open.... causing that hot air to recirculate back into the Plenum.... which will basically stall the motor(if you don't have it at 2000rpm.... thus, you raise the rpm's for that test). There is another test for the BVSV, the valve right above the Coolant Temp sensor and Cold Start Injector time switch. Basically, car off, engine fully warm, you pump vacuum into the bottom line of the BVSV, with a vacuum gauge on the top line. If it reads vacuum when engine is warm, it's working. If it reads vacuum when the engine is cold, it's not working and needs to be replaced.(I'll have to look it up later for you, to be sure it's in that order, lol.... Been a few months).

There is also a 'basic' test for the Fuel Pressure regulator. 2 actually. One requires a vacuum pump, again... and the other just requires you first listening to it with a long screwdriver or stethoscope. It should be clicking away. Then, remove the vacuum line to it and plug that line..the idle should go up when you're idling at 750rpm. If you're testing it at racing(2500rpm or so), you would hook the vacuum pump to the FPR and force 10 inches of vacuum into it(VSV line plugged after removing from FRP....it's a vacuum leak, ya know?)... when you force vacuum into it at 2500 rpm, the engine should drop in idle very noticeably. That's because the regulator serves in keeping the fuel pressure at the right level, lower, when idling. When you pull the FPR out of the equation, you're allowing FULL fuel pressure all the time to the injectors... When higher RPM occurs, the VSV is supposed to shut off that valve by stopping the vacuum pull to the FPR... thus, allowing more fuel pressure so that it can get enough to run at higher rpms.

You should invest in a multi-meter, check EVERY sensor according to what you find on here, or in an FSM or on 4crawlers website(he's been VERY helpful to me). You need to KONW THAT YOU KNOW that your TPS and AFM are working properly. If the TPS is set incorrectly, I would imagine it could dramatically effect your mileage. If the AFM reads out of specs dramatically?...same thing! Another crucial item to have is an operational Check Engine light. Really, it's important, and besides, ...you CAN NOT pass smog without a working one. It's a 20 minute fix with a junk yard bulb and a screwdriver. If a working bulb doesn't fix it, obviously you have voltage issues in the harness somewhere. The FSM has several tests for verifying/ruling out/testing the CEL circuit, including many at the ECU. They're all easy tests with a Multi-Meter.

I do not recommend removing the Idle up Valve on the Plenum, either. It assists in idling up the motor when in turns, and this is especially important when off roading. Maybe he meant 'just to check for issues'???

These motors were meant to run for a quarter million miles without issue, .....with EVERYTHING hooked up the way they had it at the factory. If you choose to run things without certain items hooked up, ..DEFINITELY check with a Guru like Roger(4Crawler) before doing so.

Best wishes, wish I could be of more help, but I"m learning as well and I just am plum out of time for the moment.

Mark

T_F_E

I get over 25mpg on the highway every time. 4x4, manual hubs, stock size tires, 5 speed. I shift at 2500 rpm, maybe I just drive diesel's to much.