Fuel pump/pressure info

It shouldn't be that confusing....

When the pump turns off  (However that relates to the engine stopping) the regulator should trap pressure in the fuel rail. The injectors won't be firing, and the cold start injector better not be on. Therfore, ideally, the pressure will be maintained.

For example, for my Jimmy with a similar setup this is what they say:

Note the fuel pressure with the pump RUNNING, it should be 41-47 psi (284-325 kPa). When the pump stops pressure may vary slightly, then should hold steady. If not, refer to the accompanying fuel system diagnosis charts.

I'll check this out soon, because I want to hook up a gauge to check my fuel pressure and then I can get some numbers for you.

Chay

I thought I'd better take advantage of the fact that it wasn't raining today, so...

Fuel pressure with key on/engine not running is 41 psi.

With the engine running at idle it's 33 psi.

With the system primed to 41 psi and key off/engine not running, the pressure slowly drops to 30 psi within 30 minutes.

Is this an indication of something (injectors, regulator, etc) leaking which needs attention, or is this within tolerances?

Thanks Chay. Not that I know much about EFI, but I suspected the fuel pressure leakdown test results were probably acceptable. Nice to get some confirmation though.

Well, I still need to figure out why I'm only getting 14 mpg. The GT actually runs great, but this gas mileage sucks (literally).

The next thing I'll try is to replace the EGR tube and put back the stock ECM chip. I've had a custom "EGR delete" chip installed since last summer when I discovered the EGR tube was cracked and I blocked it off.

I also need to fix the "tick, tick, tick..." front exhaust manifold leak as I hear this can fool the O2 sensor and create a rich condition. I'm NOT looking forward to THAT job! (as I know at least one bolt was broken off in the head by the PO)

Maybe so, but it's a helluva lot easier to put the EGR system back together and swap back the stock chip than doing the other task.

Just adding a bit, I didn't totally read the entire thread as I only have a few mins.

        However a stuck injector is not the only thing that will cause fuel pressure to drop.   I have 6 BRAND NEW GM INJECTORS in my motor, I got them in a trade, and the person had paid $2400 for the injectors.  

When I shut the key off, my fuel pressure drops to Zero in less than 2 seconds.    The fuel pressure regulators can leak, rather badly.  At speed with the engine running, the fuel pump can compensate for the fuel leaked past the internal valve, however when shut off the pressure bleeds to zero in less than a few seconds.  (Remember this is fluid pressure vs air pressure and fluid pressure requires much less to bring it from 40psi to 0psi.)  Ah hell that entire last statement did not come out right, but hopefully you get what I was trying to say.   One of these days I am going to pop the regulator off and replace it.  However for the time being it still holds pressure at or above 40psi, with the vacuum removed from the MAP, the TPS held Wide open, and the idle slightly over 1500.  (Car See's no vacuum at MAP, Wide Open on TPS, it thinks you are gunning and runs the injectors at Max Programed Duty Cycle) 

Patrick wrote: Capt Fiero wrote: However for the time being it still holds pressure at or above 40psi, with the vacuum removed from the MAP, the TPS held Wide open, and the idle slightly over 1500.  Is this how you're currently driving the car?  And your gas mileage isn't terrible?

If you mean with the pressure at 40-50psi then yes.  If you are asking do I leave the MAP unhooked and the TPS wired open.  No not a chance, most engines would drop dead and not run for more than a few seconds like that.    I only unhook the MAP and Hold the TPS to trick the car into maxing out the injectors, so I can see if the fuel pressure remains stable when the injectors are allowed to run as fast as the ECM will let them go simulating a WOT run.    Once I verify the fuel pressure stays up, I hook the map back up, release the TPS and let the idle come back down.    

      As for fuel mileage, the car did great on its first fill up in BC all the way down to Olympia, and did pretty good until I had done 2 or 3 full tanks of Oregon fuel through it.      I normally get 25-28mpg out of it if I drive it sane, and even on the Arlington Run where I had the trailer fully loaded, driving it like a MAD MAN, I think I got 24mpg out of the car.     

Dr.Fiero wrote: Its your gas bill. Do whatever ya like.

Thanks, I plan to.

Look John, it’s all about eliminating variables. Something is totally messing with the gas mileage on the car.

This GT already had the exhaust manifold leak when I bought it last spring. The leak isn't that bad (the "tick, tick, tick..." actually almost disappears when the engine is hot) and it's gotten no worse in the time I've had the car.

I didn’t drive it much before I blocked off the cracked EGR tube and installed one of your "EGR delete" chips, but I recall I had gotten 230 to 250 KM per tankful at least once prior to the chip change. Ever since then I’ve been sucking fumes to get 200 KM per tankful if I’m lucky (and that includes situations where most of the driving was cruising on the highway).

Is it the EGR delete chip which is to blame for the crappy mileage? I don’t know.

Is it the front manifold exhaust leak which is to blame for the crappy mileage? I don’t know.

Or is it something else completely different which is to blame for the crappy mileage? I don’t know.

However, I most certainly DO know it’s a helluva lot easier to put a replacement EGR tube and stock chip back in the GT than drilling out broken manifold bolts in the head to fix the exhaust leak on the front bank.

If I do this and nothing changes, fine. You’d be right, and it wouldn’t take much for me to put it back the way it was if need be.

However, if I do this and all of a sudden the mileage improves, well...

CFoss wrote: I think one of the ways to check out what you have is to hook up Winaldl and check your BLMs. If the 02 is crapped out or the leak is causing a fake lean condition, your BLM will be way high, as the ecm is trying to compensate for a 'lean' condition.

Chay, in This thread  you commented on the "black magic" of interpreting WinALDL readings.  I still have that WinALDL file Here that you had looked at back in November. Yes, I should do another test, but nothing has changed in regards to my gas mileage since that WinALDL reading was taken four months ago (although the stock 195 degree thermostat was reinstalled shortly after those readings were taken).

CFoss wrote: Pull and check a few spark plugs...are they black? Check the exhaust pipe...is it black? These indicate a consistent rich condition. If the plugs are tan coloured I suspect the problem is a heavy right foot! Ok, you probably wouldn't go to this trouble if you didn't think of that already....I know that's my problem though.

The tail pipes are definitely black and sooty. Last weekend I pulled the #5 spark plug (yes, I will pull them all to have a better look), and the ceramic tip was absolutely white. I was surprised.

The weight of my right foot is not an issue here as driving "like an old lady" has resulted in the 14-15 mpg readings.

CFoss wrote: "Why is there never time to do it right the first time, but always time to do it again?"

Well, that statement only rings true if doing a repair the first time never results in resolution. My record is better than that. I'm sure yours is as well.

CFoss wrote: Sometimes doing the hard thing is the right thing.

And sometimes it's the simple things we overlook which are actually causing the problem.

Yes, I plan on doing the "hard thing" (fixing the exhaust leak) eventually (as I don't like the sound of the "tick" when the engine is cold), but as I reported previously, my gas mileage was up to 25% higher when I first got the GT last spring. At that time it already had the exhaust leak but the EGR had not yet been "deleted". Therefore, I'm not convinced at this point that the exhaust leak is responsible for the bulk of the problem. I'm not saying it's necessarily the "deleted" EGR which is responsible either, but it's a whole lot easier checking that first than pulling the head(s) off to drill out the broken exhaust manifold bolts.

This will all get sorted out eventually.

Patrick wrote: Chay, in This thread  you commented on the "black magic" of interpreting WinALDL readings.  I still have that WinALDL file Here that you had looked at back in November. Yes, I should do another test, but nothing has changed in regards to my gas mileage since that WinALDL reading was taken four months ago (although the stock 195 degree thermostat was reinstalled shortly after those readings were taken).

Chay, I uploaded a slightly newer WinALDL file Here which was recorded after I had reinstalled the 195 degree thermostat.

Not that I understand BLM very well, but it's interesting to note that of the 810 separate measurements taken during this particular reading, 436 are at 120, whereas 33 are at 127, and 8 are at 128 (the remainder are between 121 and 126). There are NO readings over 128.

What does that indicate regarding being rich and/or lean?

Bobz0r wrote: Sounds like quite the mystery. I'm interested in hearing more about this.

It is a mystery, Rob. Stay tuned!

If you have a bad injector, or 2, maybe on that one cylinder, the ECM can't tell if ONE injector or 2 injectors is bad, so it just cranks the fuel up the whizoo. That is the only way I could see having a spark plug bleach white and still black tail pipes.    Bleach white plugs are simtoms of running really LEAN and heating and burning off everything in the chambers.   However jet black tail pipes is a sign of Over Rich condition.

       Not to beat my head on a wall, but when was the last time a new O2 sensor was installed.   What Brand did you install?  If its anything but Delco, I would go back and get a Delco one.   My 4.9 running NGK O2 sensors ( 2 of them) the mileage was darn near Gallons per hour.  Idling in Traffic or WIDE OPEN 200kph on the highway and it was going through the same amount of fuel per hour.  It was horrid, 29mpg on the highway, 10mpg if I was lucky in town.   The NGK ones would not switch properly in the Caddy, as soon as I put the Delco ones in, that the cars software was designed to use, everything went perfect.     The caddy motor has one sensor per bank and can set an error code on what bank of injectors has an issue.  Still not as good as single O2 per cylinder, but still decent.