Poco444

I recently bought a Airaid MIT to replace my dried out intake tube, My stock intake tube had the ISR mod. The FPR line was relocated to a vacuum port on the front side of the intake manifold. Drove that was for a few years without issue. When I put on the MIT I decided to use the vacuum port off the MIT tube. Capped off the one on the front side of the intake manifold or plenum or what ever its called. Took it for a drive an noticed a difference. Can't pinpoint the difference but my mileage seemed to improve. I think I understand how a FPR works, but if a vacuum source should have the proper amount of vacuum to operate correctly. My question is......Which vacuum port creates the most vacuum for either performance or improved fuel economy? Intake tube or manifold.

Oh by the way.....There were no performance differece between the stock intake tube with the ISR and Airaid MIT. I've noticed my AI temps have jumped slightly after install. That was my mini review if the Airaid MIT.

mikes19984x4

mileage "seemed" to improve??? by improve what do you mean, as i'm interested in swapping over my fpr line to the intake if a noticable increase in mpg can be obtained...

Poco444

Well that's what this thread is about. FPR returns unused fuel back to the tank. I achieves this by using a vacuum source. When at idle the fuel pump is still pumping alot of fuel so the FPR help to alleviate some of that unneeded fuel back to the tank. This is when it operates at it's fullest. However, when on the throttle more vacuum is crated and the FPR sucks back less fuel to allow the engine to get its fuel needed. I'm trying to see if anyone has tested the various vacuum ports to see which one draws the most.

To answer your question as to fuel economy.......My scanguage indicates a higher mpg and for a longer duration at a complete stop with engine running before it reads 00.0. This seems to let me know that I'm using less fuel.

Now I'm no rocket scientist but if these vacuum ports have different readings of vacuum or low pressure it seems to me we could switch things around to suit our performance or fuel economy needs.

4Crawler

iTrader: (3)

All about the FPR operation:
http://autoshop101.com/forms/h22.pdf

In short, the FPR needs some vacuum source on the intake manifold to work right. The FPR has a spring inside that sets the pressure and the vacuum pulls up on that spring lowering the pressure based upon engine load. High vacuum = low load = lower pressure = less fuel injected. Low vacuum = high load = higher pressure = more fiel injected.

Poco444

Ok now that we established that, my concern is different set of pressures from our various vacuum ports. If one port is drawing more vacuum at idle your vehicle would use more fuel. If that same vacuum line were to be placed at another port drawing less vacuum wouldn't it not use less fuel? I'm thinking aerodynamics here. I wouldn't want to assume that each vacuum port is drawing the same amount throughout out intake system. I would lean more towards slightly different pressures but possible enough to make a difference as far as the FPR is concerned.

4Crawler

iTrader: (3)

By "more vacuum" I assume you mean a higher vacuum as read on a gauge (as in 25"Hg is more than 20"Hg), so more vacuum = lower fuel pressure = less fuel sent through the injectors. Unless you have vacuum ports that lie along the sweep of the throttle plate, there should be the same vacuum in all the intake, unless there is some special restricted part of it or something (not familiar with the intake you mentioned). But if you have a big intake plenum with a throttle body at the front end and pistons drawing in a stream of air past the throttle plate resistance, all points in the intake plenum should share the same vacuum. Put a vacuum gauge on the ports in question and see if there is any difference.

And it is not like the FPR is making the all the control over the fuel/air mixture in the engine, it is just working with the ECU as part of the overall system. The ECU modulates the injectors based upon many factors including the O2 sensor in the exhaust. The FPR's job is basically to lower the fuel pressure at light engine loads so that for a given injector puilse less fuel goes into the engine. I would guess there is some minimum injector pulse width that exists and you would not want to be right down at that limit and have the engine running too rich, as there would be nothing the ECU could do to control the mixture. So by dropping fuel pressure when the engine does not need as much, it makes the ECU's job easier. Injectors flow less fuel at low pressure and more fuel at higher pressure.

Poco444

So 4Krawler, essentially you are saying that if there were to be a variance in atmospheric pressure withing the intake system the ECU would compensate for the difference. Is that correct?

4Crawler

iTrader: (3)

Essentially yes, the ECU "learns" what the injectors do as the engine runs and it makes a map of at given conditions, so much injector pulse = so much fuel.