HOW TO: Check regulated fuel pressure, pump max output, & leakdown
 

NOTE: These tests will also work with the CMFI, (1995 and older) systems. However, the specs are slightly different. They are listed at the bottom of the article.


If your vehicle has a driveability issue, the fuel delivery system should be the first thing on your list of things to check. Correct pressure, and the ability to hold the pressure, is critical on this engine. Testing fuel pressure and leakdown will tell you if there’s a problem with the fuel delivery system. Incomplete or incorrect testing will usually lead to a misdiagnosis which can be time consuming, frustrating, and expensive. The following is based on information from the GM “dealer issue” shop manual. It covers both the procedures and the specifications for correctly testing the fuel delivery system. It applies to all 1996 through 2005 (and a few late production CSFI equipped 1995's) 4.3L Central Sequential Fuel Injection, (CSFI) engines controlled by fully compliant On Board Diagnostics second generation, (OBDII). The 8th character in the VIN must be “W” or “X”. The intake plenum is black plastic and both the "W" and the "X" engines look like this:

http://i561.photobucket.com/albums/s...CSFIEngine.jpg



The “W’ 4.3L uses 6 fuel injectors centrally located inside the plenum. There is a small fuel line that runs from each injector to each intake port. There is a nozzle and a poppet valve in the end of each line.

The “X” 4.3L uses 6 mini injectors located at the intake ports. There are no poppet valves used on this fuel delivery system. The “W” engines can be easily upgraded to this system when/if there’s a problem with an injector or a poppet valve, or you just want better performance ;)

If the fuel pressure or volume is insufficient, or leakdown is excessive, you WILL experience driveability issues. Symptoms of fuel delivery problems include, but are not limited to: Excessive cranking hot and/or cold, fuel odor, cranks but will not start, starts and stalls, hesitation or stalling on acceleration, running rich, running lean, misfire, induction, (backfire through the intake), lack of power, and harsh transmission shift points. Fuel pressure, volume, and leakdown are not directly monitored by OBDII so it can’t set a Diagnostic Trouble Code, (DTC) in memory and illuminate the Service Engine Soon, (SES) light. However, malfunctions that occur in the fuel delivery system can affect other systems that CAN set a DTC. For example: If the fuel pressure or volume is low, the oxygen sensors could detect a lean fuel mixture and simultaneously set P0171 & P0174. Keep in mind that if the engine exhibits any of the above symptoms, there may or may not be any DTC’s in memory.



Fuel Pump Activation

When the ignition switch is initially turned to the RUN position, the Powertrain Control Module, (PCM) activates the fuel pump relay for approximately 2 to 3 seconds. This time frame is referred to as “pump prime”. If the PCM does not receive a signal from the crankshaft position sensor during this time, it deactivates the relay. Fuel pressure must be tested while the pump is running, and the engine is off. 1998 through 2005 have a pump prime terminal in the under hood fuse panel that can be used to activate the fuel pump. Applying battery voltage to the terminal bypasses the relay and activates the pump continuously.

Attachment 30967



Preparation



1) Confirm fuel pump activation. (If the pump runs when the ignition is initially turned to the RUN position, or bypassing the relay, move on to step 2.) At the fuel pump module electrical connector: The gray wire must show battery voltage when the fuel pump relay is activated. The black ground wire must show less than 5 ohms resistance to chassis ground. Checking these may require dropping the fuel tank to gain access to the connector on the fuel pump module.



2) Make sure the fuel filter is not clogged or restricted.



3) Check for and repair any kinks, restrictions, or external fuel leaks in the 3/8” fuel pressure line, and the 5/16” fuel return line between the fuel pump module and the connections at the fuel metering block.



4) Connect the fuel pressure tester to the service port in the 3/8” steel fuel pressure line located at the rear of the plenum, near the distributor. Activate the fuel pump and check for leaks at the service port connection, repair if/as necessary. Here’s a link to an adequate fuel pressure tester: https://actron.com/content/fuel-pressure-tester-kit it’s not “professional grade” test equipment, but for occasional DIY use, it should be fine.



Attachment 30968



Several things influence fuel pressure while the engine is running such as the fuel pressure regulator setting, ambient temperature, coolant temperature, percentage of throttle opening, altitude above sea level, overall engine condition and engine load. All of which have a direct affect on fuel injector pulse width, (the time the injectors are open), which directly affects fuel pressure. Due to these variables, GM does not publish any “engine running” fuel pressure specifications for this system. Fuel pressure and leakdown must be tested with the engine off. NOTE: Checking fuel pressure and leakdown at the service port is only a“quick check”. There are things that the “quick check” does NOT test for.

Pressure Readings At The Service Port

Activate the fuel pump to pressurize the system. The pressure indicated on the tester is either regulated fuel pressure, or maximum fuel pump output pressure, whichever is LOWER. Regulated fuel pressure must be 60psi to 66psi while the pump is running. If the pump can only build let’s say, 40psi, you won’t know what regulated pressure actually is. Going by this pressure reading alone, a fuel pressure regulator could easily be misdiagnosed as the problem. To avoid this, maximum fuel pump output pressure must be checked.



Leakdown Test

With the exception of the return lines, this will test the entire systems ability to hold fuel pressure, (fuel pump, lines, regulator, and injectors). Activate the fuel pump to pressurize the system, and then deactivate the pump. Fuel pressure must remain above 55psi for at least 10 minutes. If this test fails, you will need to perform the fuel pump maximim output pressure and leakdown test to determine if the cause is in the plenum, or in the fuel tank.



Fuel Pump Maximum Output Pressure And Leakdown Test

The pump must be capable of supplying sufficient pressure, and be able to hold it after the pump shuts off. This is most easily tested at the fuel filter outlet, (forward connection on the filter). It requires an adapter between the filter and the pressure tester. There is list below with the parts necessary to build one. All fuel pressure and flow must end at the tester, with no fuel allowed to the engine. Disconnect the fuel filter outlet line from the filter. Connect the adapter to the filter outlet and connect the pressure tester to the adapter. Activate the fuel pump. Pressure must be 73psi to 108psi. Deactivate the fuel pump. Pressure must remain above 55psi for at least 10 minutes. If this leakdown test fails, there is a leak inside the fuel tank. If leakdown is within specs on this test, and leakdown at the service port is excessive, there is a leak in the plenum.



Fuel Pressure Regulator



Attachment 30969

This is a “typical” illustration, the actual regulator on this engine is configured differently, but operates the same way.

When the fuel pump is activated, it delivers pressurized fuel from the pump to the regulator inlet port, (arrow on left). The injector and service port are teed into the line before the fuel reaches the regulator. Spring pressure on the bottom side of the diaphragm keeps the valve plate closed which allows the system to build pressure. The amount of spring pressure on the valve plate determines regulated fuel pressure. The higher the spring pressure, the higher the fuel pressure is. When fuel pressure on the top side of the diaphragm overcomes the spring pressure on the bottom side of the diaphragm, the valve plate opens and directs the excess pressurized fuel back to the fuel tank through the fuel return line, (arrow on top). When the valve plate is open, fuel pressure decreases until the spring pressure overcomes the fuel pressure on the top side of the diaphragm, and the cycle repeats. The port labeled “vacuum connection” does not have any hoses connected to it. It is exposed to manifold absolute pressure inside the plenum. When idling or decelerating, (low fuel demand) the port is exposed to high vacuum which pulls on the bottom side of the diaphragm decreasing spring tension on the valve plate. This causes fuel pressure to decrease. When cruising or accelerating, (high fuel demand) the port is exposed to low or zero vacuum which increases spring tension on the valve plate which causes fuel pressure to increase. The highest regulated pressure is obtained when the engine is off, (zero vacuum in the plenum) and the fuel pump is running. Some, not all, fuel pressure regulators have an adjustment screw, however; adjustment is done at the factory and tampering with it is not recommended. If it’s not regulating the fuel pressure within specs, replace it. The fuel pressure regulator is quite simple in both design and operation; as a result, it rarely fails. It is however, a mechanical part and can develop leaks both internally and externally.



Yup, it's a lot of reading, but reading is free. Parts that are replaced unnecessarily make the job more expensive. Proper diagnostics ensure the lowest possible cost to repair the problem.



Adapters for testing maximum fuel pump output pressure and leakdown:

If your fuel filter looks like this,


https://blazerforum.com/forum/attach...ine=1437909384


you will need these parts:

QTY 1 Dorman #800-121 or 800-182 fuel filter connector. Available at most auto parts stores

QTY 1 1/4” MFL X 3/8” hose barb adapter available at:

http://www.homebrewing.org/SS-14-MFL-x-38-barb_p_3521.html

or

http://www.grainger.com/product/THOGUS-Male-Flare-Adapter-4HFJ2?functionCode=P2IDP2PCP

QTY 18” (minimum) fuel injection rated fuel line. Available at most auto parts stores.

http://shop.advanceautoparts.com/p/h-b-d-industries-fuel-injection-hose-pi-504/5194660-P?navigationPath=L1*14932%7CL2*14985%7CL3*15352#fr agment-2

QTY 2 Fuel injection rated hose clamps. Available at most auto parts stores.

http://shop.advanceautoparts.com/p/autocraft-3-8-automotive-fuel-injection-hose-clamp-4-pack-ac52f16v/89016086-P?searchTerm=hose+clamps

Total cost is approximately $30



If your fuel filter looks like this:


http://i561.photobucket.com/albums/s...psautlvfbz.jpg




You will need these parts:


QTY 1 Watts LFA-116 1/4” FIP X 3/8” compression adapter. Available at Lowes or Home Depot

http://www.homedepot.com/p/Watts-3-8-in-x-1-4-in-Lead-Free-Brass-Compression-x-FIP-Coupling-LF-A116/202254848?keyword=lfa116

QTY 1 Watts LFA-80 1/4” MFL X 1/4” MIP adapter. Available at Lowes and Home Depot:

http://www.homedepot.com/p/Unbranded-1-4-in-x-1-4-in-Lead-Free-Brass-Flare-x-MIP-Half-Union-LFA-80/203468369?keyword=lfa80

QTY 1 Dorman 800-170 Fuel filter adapter line. Available at most auto parts stores:

http://shop.advanceautoparts.com/p/needa-parts-steel-fuel-line-repair-kit-3-8-x-4-with-16mm-clip-800-170/16140334-P?vehicleIdFromReq=40747&isAllVehicle=false&vehicl eIdSearch=40747&searchTerm=800-170&showTitle=

Total cost is approximately $10



1995 & older CMFI specs:
At the service port with the ignition ON, engine OFF, fuel pump running:
Regulated pressure must be 55psi to 61psi.
It must remain above 50psi for at least
10 minutes after the fuel pump shuts off.


At the fuel filter outlet with the ignition ON, engine OFF, fuel pump running:
Pressure must be ~70psi to 100psi.
It must remain above 50psi for at least
10 minutes after the fuel pump shuts off.

Captain! Will be the most used STICKY ever!

We can only hope so. The letters are worn off my keyboard ;)

Nice Job Captain Hook !

Thanks! If something is not explained clearly enough, let me know.

It's not....... :icon_denk::icon_lol: could u dumb it up some more for me???:icon_saywhat::icon_denk::icon_twisted:

What would you like me to change?

I love this write up. Much appreciated. Thanks Captain Hook!

I dunno......:icon_dunce::icon_slapcry:

I'm going to be the dumb one here. For the 1996 and late and SCFI 1995, I understand what you are doing but what is the point of the LFA-80 1/4” MFL X 1/4” MIP adapter? Does this thread match that of the fuel service port allowing the use of a fuel pressure gauge? I just didn't think something like a 1/4" flare would be the same.

The 800-170 connects to the fuel filter outlet. The LFA116 connects to the 800-170. The LFA80 connects to the LFA116. The fuel pressure tester connects to the LFA80.

The rest made sense, I just didn't think it would be that style of thread.

Yup, service port is 1/4" MIF, with a Schrader valve. Don't need a Schrader for the test adapter. Just need to wrap a rag around it when you disconnect it ;)

Great write up. Thanks for spending the time to inform us with this great information. I don't know what else you could have added. Thanks again.

Thanks, I hope it was helpful :)

what size is the flare wrench to remove the fuel filter Captain Hook??

16mm for the flare nut, 5/8" will fit, but it's tight. 20mm on the filter, 13/16" will fit, but it's sloppy.

Thank you so much. I guess 16mm flare wrench is a specialty wrench can't find one. Guess I'll try sears. Thanks again.

16mm crow foot flare nut wrench, (Sears has them) on a 3/8" ratchet works great.


http://www.craftsman.com/craftsman-1...1&blockType=G1

Miss_tcjj
 

My 2000 blazer zr2 cranks, but won't start. Narrowed down to fuel. Threw money at it when fuel pressure tested below 55 psi cranking. New pump didn't change a thing. Wasted time and money. Started thinking pressure regulator. While I was searching more about how to further diagnose that, I hooked up battery charger to maintain. Damn thing fired right up. Unhooked battery charger and it reverted back. My question is how is this possible? My battery tests at 13 volts unhooked.

All fuel pressure readings are taken with the engine off, not cranking. Low voltage, or poor ground to the fuel pump will cause fuel pressure to drop. Take all 4 readings as explained in post #1, post them in a new thread, and we will help you diagnose the problem.

Thanks Hook! Great write up. It'll save me a lot of time and money and it's such a simple test.

Would you only use the GM AC Spider injector or would say GB Rebuild for 1/3 of the cost work just as well?:icon_shrug:

I would definitely avoid rebuilt, reconditioned, and remanufactured units. If you have a 1996 or newer, go with the upgraded assembly. AC Delco 217-3028, Delphi FJ10565 and BWD 63885, are all good choices. Check Amazon, Ebay, and other online sources for the best pricing.

Okay thanks . Mine is a 95.. Other good ones?I've seen this link.
Poor Mans Repair Guide For Chevy,GMC Vortec Fuel Injector spider or CSFI System

The link refers to 1996-2005 CSFI systems. They are equipped with 6 injectors, 6 spider lines, and 6 poppet nozzles. Your 1995, (more than likely) has CMFI, which uses only one injector with 6 spider lines and poppet nozzles. The two systems are not interchangeable. Repairing your CMFI injector assembly by replacing the injector, fuel pressure regulator, or individual spider lines, is not cost effective. Stay away from used, rebuilt, or remanufactured units, they're likely to have the same problem you're trying to fix.

http://www.summitracing.com/parts/ad...er?prefilter=1 this a good one?

There ya go! That's it. Don't forget to replace the plenum gasket, the lines, (commonly called "nut kit"), and the electrical connector for the injector.

That sir was one of the most beautiful things that I have ever read!
Thanks!!

Thanks for supplying the information left out of the *insert favorite phrase here* Hayne's manual... we've spent over an hour looking for the 'fuel pump test plug'... and after another hour searching online stumbled onto your great post & find out that it's located as a plug port inside the fuse box on newer models!

Thanks again!

Pages are too rough for toilet paper, and not absorbent enough for bird cage or cat box liner... pretty much worthless.

You good Captain Hook.If I need a help I know where to go!

So what would cause a regulated 45 psi fuel pressure with koeo but a direct fuel pressure of over 60.

With the engine off, and the fuel pump activated: 45psi at the service port could indicate a faulty fuel pressure regulator, or something else. 60psi at the fuel filter outlet could indicate a faulty fuel pump, or something else.


We also need the leakdown pressure at the service port, and at the fuel filter outlet, to make an accurate diagnosis. The article explains how to test them, and why all of the readings are necessary. Without all 4 readings, your guess is a good as mine ;)

Nicely done, Sir.

I have my 1/4” MFL X 3/8” hose barb adapter, the hose and the clamps. As soon as my 1/4” MFL X 3/8” hose barb adapter I will grab some pictures of it all assembled and connected to the fuel filter.

Pictures as promised. If this is incorrect please feel free to remove this post.

Here are all the parts you will need.
https://blazerforum.com/forum/attach...ine=1460119372

Dorman #800-121 - $7.66
https://blazerforum.com/forum/attach...ine=1460119372

1/4” MFL X 3/8” hose barb adapter from Homebrewing.org - $6.08
https://blazerforum.com/forum/attach...ine=1460119372

18” (minimum) fuel injection rated fuel line - $7.99
https://blazerforum.com/forum/attach...ine=1460119372

Fuel Injection Hose Clamps - $2.99
https://blazerforum.com/forum/attach...ine=1460119372

Here are some pictures of the assembly. It is pretty straight forward so I assume I got it right

This end goes to the pressure tester
https://blazerforum.com/forum/attach...ine=1460119372

This end goes on the output of the fuel filter
https://blazerforum.com/forum/attach...ine=1460119372

Completed assembly
https://blazerforum.com/forum/attach...ine=1460119372

Perfect, thanks! I've got all the stuff to build one, but haven't got around to it yet. I built this one a few years ago and use it all the time. It connects inline at the fuel filter outlet. It's a bit more expensive, but it checks everything: regulated pressure, system leakdown, pump max output pressure & pump leakdown.
https://blazerforum.com/forum/attach...ine=1460308534


I've got another one that goes in the return line that checks for internal fuel pressure regulator leaks.

So since mine didn't work I ordered a pair of Dorman 800-086 (they come in pairs). You can also use a dorman 800-082, (straight) The 90 degree bend makes it much easier to hook up.

https://blazerforum.com/forum/attach...ine=1460836122

Same as the other. except this one fits my ACDelco fuel filter
https://blazerforum.com/forum/attach...ine=1460836122

Here it is on the filter. Make sure you pull the plastic clip out of the fitting if yours is still on the fuel filter.
https://blazerforum.com/forum/attach...ine=1460836122

Once you have everything hooked up it should look like this when connected to the pressure tester. Mine worked great
https://blazerforum.com/forum/attach...ine=1460836122

Thank you very much for this write-up!! I noticed there was no vacuum hose connected to my fuel pressure regulator so I became worried, but this answered that question! Thanks again, all your work is much appreciated!