Back a couple years ago, I replaced the intake manifold gaskets on my blazer. (Felcro improved type) I also at the time upgraded the injectors assy with the new style. Shortly thereafter I did a complete tune-up, cap, rotor, wires, plugs,etc… (Of course only AC or best equivalent) Compression tested ~180 across all cylinders. So everything seems fine. No engine light, no codes, stored or pending. (I had purchased a code reader that also displays live data). All indications are normal.
So here is my dilemma. I seemed to have lost some fuel efficiency. I used to get just around 20 MPG, varies in winter ,better in summer. But that I attributed to the injectors, thinking there may have been a design difference and getting MPG in the mid 18. Well I went ahead and replaced O2's (3)., pulled EGR and cleaned, replaced a couple vacuum hoses. The truck runs well, starts up fine, hot or cold… but here is the thing…
I have noticed at times, and more so recently, that I can randomly get a boost in HP and better gas mileage on a day to day drive. It feels like a flip of a switch, I’ll suddenly have increased power, and MPG will go up. But fade out and back to what I considered normal after a day or so.
It really feels good with the extra HP, but can’t figure out what is changing.
So what could be causing this random change in engine performance?

 

I'd start with your live data scanner. Compare it's readings when the "switch" is on and off.

 

Good idea. I normally don't have my scanner hooked up while driving, but i now have it hooked up during my commute. Been watching spark advance and map. O2s and fuel trims run a typical pattern.
Truck is currently running a little above average, and seems pretty peppy. But seems to fluctuate a little as I'm driving. I can't pin down anything on the live data yet. Still monitoring...
Question. What about egr? Am I getting gases that can cause fluctuations in power?

 

So I have been trying to understand the relationship between MAP and spark advance.
Sometimes I will see spark advance maintain between about 30 to33 degree range and kinda hold there when I let of the gas. But other times it may drop to a negative number and I will feel the drag on the motor which seems to bog me down on deceleration. Although spark goes back up when i get on the gas it can feel kinda sluggish.
Fuel trims are consistent +- 3.5 or so, left and right banks.
So what roll does MAP play in spark advance? and
How might an intermittent (small) vacuum leak affect MAP measurements and spark?

 

Try keeping track of the weather also. My truck seems to run better on cool/cold dry days

 

Thanks for the suggestion, Ive been trying to pick up on that,but it just doesn't follow a reasonable pattern. I sometimes feel it runs better on damp days or high humidity but still can't be sure.
Pulled the throttle body off and cleaned it up a few days ago. I had almost forgotten that I had removed the baffle the other year. A definite improvement on throttle response, but of course its all just subjective to how quick you like your truck to respond. I also noticed that he MAP seal was rather loose, being that it was pretty much just sitting in the hole. I used a piece of shrink tube over the seal to give a tight fit. Well I can say overall its running better, and has good peddle response. But gas mileage is still below 19 mpg.
Interestingly though the Idle has been getting pretty smooth (over the last month or so) and this morning, on my way to work, when I was stopped at a light, it was so smooth I would have thought it wasn't running. Go figure. Oh ya and it was cold this morning, below 32.
I'm still wondering though if the EGR valve could be causing some changes in how it runs. Like if it gets stuck (closed?) would it be running lean or having some other affect????
Is the EGR proportional or just open/closed?

 

Could be, 19 is above average for these trucks

 

I'm lucky to get 15-16mpg...

 

Oh no. But then I am running all stock equipment and tire sizes. This has been my daily driver since 2001 so I'm very attuned as to variances in performance. Still original engine with more than a few miles.
Performance has improved since i cleaned the TB and sealed up the MAP. Now I am feeling there are changes in brake boost and trying to find a correlation. Like if I come to a stop during my drive and pump the brake hard a couple times it almost feels like an improvement in engine performance and the brakes have better response.
We are getting into winter here so I will be about 1.5 mpg down, I like to start her up in the morning when cold.