So I've been slowly working/tuning up my car over the past

I see no way loss in power and less mpg are connected to how much fuel you have, aside from the added weight a full tank would bring. But that's would cause the opposite of what you're talking about. The fuel in the tank does cool the fuel pump, so theoretically if the pump gets warmer it could affect how fuel is getting delivered, but in practice that's not likely if even possible. I have never seen it.

Your computer controls how much fuel is being delivered, and if too much or too little is getting delivered it'd likely throw a fuel mixture too rich/lean code. It'd also likely throw a code if the fuel wasn't the correct pressure. You can get a fuel pressure gauge to check if you feel so inclined, usually there's a Schrader valve on the fuel rail normally. How are you calculating fuel economy?

Fix the vacuum leak. IACV can cause rough idle and stalling at stop. So can the EGR valve. Just cleaning an IACV won't necessarily make it work, sometimes they just need to be replaced. You'll be able to tell if it was the issue once you pull it.

It might just be that it's an old car. I've seen tons of older cars where the Tach will occassionally drop in the 400s after revving and coming to a halt.

Really, it's difficult to diagnose intermittent issues in person, let alone on the internet.

Take a methodical approach. Fix the vacuum leak first. Then see if or how much of an improvement that makes. Then come back to this board and we'll go from there.

I forgot to add and explanation of why I say fix the vacuum leak first. The oxygen sensors will sense the extra air that isn't counted by the MAF. This extra air is called unmetered air. The oxygen sensors will inform the computer that there is too much air in the air/fuel mixture to maintain something called stoichiometry. The computer will command the fuel injectors to open for a longer amount of time to bring the air/fuel ratio closer to stoichiometry. If the computer determines that the injectors can't be opened for any more time than they already are, then it will throw a lean code.

>>1193451
That shit is in your head my man. I ran some bad gas through my truck and swore to god I lost power and fucked something up. I bought an engine scanner and monitored performance for several weeks and compared it with stock information and others poster info. Nothing turned out to be amiss except my inability to accept the truck was fine.

Check your blinker fluid.

>>1193451
Check the pickup tube in the gas tank for holes or leaks. I had a truck that would stumble and struggle below a quarter of a tank and that was the cause.

File: sea foam not see foam.jpg (146KB, 640x640px) Image search: [Google]

146KB, 640x640px

sea foam, see foam.
The difference is important.

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47KB, 943x358px

>>1195754
And one more because I care.