I have a check engine light on for P0171 and P0174 (lean-both banks). The light comes and goes. This started happening after replacing my intake manifold gaskets and fuel pressure regulator(BWI? Advance Auto brand) and cleaning my throttle body. The blazer has always had a slight stumble to it, even when cruising on the highway, but after the gasket and FPR it now bogs on the first attempt at acceleration after it has set for awhile. I tested fuel pressure today and here's what I have:

Ignition On, pump on 59-60psi
Ignition On, pump off 55psi
Leak down after 10 minutes 50psi
Starting Cold 59psi
Cold Idle 52
Hot Idle 50
Accelerating/Decelerating Range 58-47psi
Shutting Blazer off, psi jumps to 59, then down to 52, then creeps up to 56 within one minute.

I've read quite a few threads and it appears I'm on the edge of a bad fuel pump. All numbers are from the Shrader valve. Fuel filter was replaced around one year ago and did not affect performance. I fogged the intake tract and only found an air leak where the black plastic "bonnet?" covers the throttle body. I have somewhat reduced that leak with gasket sealer but I need to come up with something better. The Blazer has always started immediately and still does.

Before I spend $300 on a fuel pump, has anyone else had my issues and fixed it with a fuel pump? Do my psi's sound bad/good? Any other relatively easy things to check before spending that kind of money?

PS. I don't have a scanner and have not mastered the multimeter. I did check the TPS and have the correct voltage. Also have a nice smooth increase in voltage when opening the throttle.

 

The fuel system diagnosis suggests to add a special tool to isolate the fuel pump or pinch the return line to test the pressure of the fuel pump. The fuel pressure tester I borrowed does not have that attachement. Any suggestions on how to isolate the fuel pump to test it?

 

i must watch this thread. 100% the same problem im having. took it to a shop and they found a rotted vac line and they fully cleaned my maf sensor. SES light has not come back on but if i let the truck sit for a while and i go to take off i can give it 50% gas and it barley accelerates or stumbles untill i slow down and try again then i got full power. my fuel pressure is around the same as yours. is your engine ticking or knocking at all on cold start?

 

I'm getting the same codes (171 and 174), but not the same symptoms. I just checked again because my SES light came back after a few days. Now in addition I have a 507 which is "Idle Air Control System RPM Higher Than Expected". Haven't searched much yet or investigated, but I'll be watching this thread also...

Edited to add that I replaced the IM gasket last July.

 

Your fuel pressure & leak down look to be within spec up until the engine is running. GM doesn't specify running fuel pressure, but my experience with these injection systems suggests that anything below ~50psi is too low to properly open the poppet nozzles.

But before we jump into that, have you ruled out any of the other possibilities that can cause a lean code? Vacuum leaks? Exhaust leaks?

 

No, no concerning engine noises. A little more engine vibration can be seen that what I remember from before, but otherwise sounds good.


When you say GM doesn't specify fuel pressure numbers, what about the fuel pressure range mentioned in the diagnostics for P0174? I attached it (or tried to). It seems to imply 55-60 is good, which agrees with your comments that my running PSI is too low.

As far as vacuum leaks, I did a ghetto smoke leak detection. I used a fog machine with a couple of reducers to attach to the brake booster nipple on the intake manifold. It showed a leak at the throttle body canopy attachment area, you know the black intake cap above the throttle body. It was a fairly good size leak so I put a bead of gasket maker on it just to close off that leak and I checked again. I could not find another leak. Granted that the fog machine only has a certain amount of pressure and forces air out instead of in, but it was worth a shot.

I have not noticed any exhaust leaks.

 

I was speaking of operating values as in driving down the road. There is just too many variables (engine load, throttle position, engine RPM, etc) to detail that kind of info.

A leak at the hat could cause a system wide lean condition as it would be allowing outside air into the intake manifold that wasn't detected by the MAF sensor.

How about exhaust leaks that could be allowing outside air into the exhaust affecting the O2 sensor readings?

 

Regarding the intake, the light has gone off since sealing that leak, but it use to go on and off before I sealed it. It also still bogs when backing out of my level driveway when you first start it up in the morning which is the main sign that the problem is there. I orginally thought that I had to be the cause of the problems since I just replaced the intake manifold gasket and fpr, but I wonder if the change in the fpr might have been just enough to show a weakness in the fuel pump.

Its overdue for its state inspection so I'm somewhat stuck unable to drive it much without risking a ticket and driving it enough to believe its good enough to pass inspection (it wont pass inspection with a check engine light on).

As far as the exhaust leak, the underbody and exhaust are in great shape in appearance and sound. It looks all original but still solid. The only place it could be leaking is at the manifold, but I do not hear or see any leaks there either.

By the way, I appreciate your help!

 

Something I just thought of. A friend of mine had a problem with the AIR system check valves which resulted in melted rubber components for the AIR system. He had a lean code that we ended up determining was caused by the leaking check valves and open system. You might want to check the lines from the AIR pump (under the radiator near the passenger side frame rail) to the check valves to make sure that they are all intact.

 

Quick update. Bought a scan tool. Plugged it in and got these readings:
(Figure I better write these down before I accidently erase them.)

P0171 & P0174 (Lean bank 1 & 2)

Fuel Sys 1 CL
Fuel Sys 2 CL
Calc Load (%) 3.9
ECT 194
STFT B1 1.5
LTFT B1 19.5 (research says >20 triggers DTC)
STFT B2 1.5
LTFT B2 18.7
MAP (inHG) 9.4
Eng RPM 979
MAF (lb/min) 0.944
TPS (%) 0

I also want to mention that according to my scan tool, the system checks for the cat and evap system have not completed. I had to unhook my battery last week to clear codes to pass inspection and haven't driven it alot so that makes sense. I belive I've read something about the drive cycle needs to occur first? My tool is suppose to be able to run a check on the evap system so I'll probably do that next. I have a lot to learn so I plan to experiment with the tool and get some live data, graph some activity and see what I can figure out.

Feel free to leave comments and advice. Plan to use this thread to update the quest to get my Blazer running decent. (updates will be sporatic)