Fuel Pressure Test
Thread Starter
Burning Brakes
Joined: Apr 2005
Posts: 973
Likes: 0
From: Chico California
Fuel Pressure Test
I want to do a fuel pressure test on my 92 LT1 and I've never done it before. I'm having trouble attaching the hose fitting to the Schrader valve.
I assume the valve is the one in the top center of the photo (just to confirm)?
The two black fittings to the right of the valve are in the way. Can I just pull them straight back to remove them? I can't fit the hose fitting to the valve as they are in the way.
IMG]
[/IMG]
I assume the valve is the one in the top center of the photo (just to confirm)?
The two black fittings to the right of the valve are in the way. Can I just pull them straight back to remove them? I can't fit the hose fitting to the valve as they are in the way.
IMG]
[/IMG]
Thread Starter
Burning Brakes
Joined: Apr 2005
Posts: 973
Likes: 0
From: Chico California
My car suddely dies.
http://forums.corvetteforum.com/show....php?t=1921805
I got the line attached to the Schrader valve. No gas came out. I then attached the pressure gauge to the line and had my wife turn on the ignition but not start.
Zero pressure reading with ignition on. (I had previously removed and then replaced the gas cap to relieve pressure.)
The car was started and the pressure went to 38 and held steady at idle. Increased RPM and no change in the reading, still at 38.
Engine off then ignition on and the pressure instantly hit 48 and slowly and then more repaidly returned to zero.
I started the motor and let it idle, pressure at 38. It ran for about 5 minutes and started to die. Before it died, the pressure shot up to 56, held for a moment and then slowly went to zero.
I waited for a few minutes, started motor and same scenerio.
When the motor is running, it sounds normal and runs normal - until it dies.
Looking for comments, suggestions any help appreicated.
http://forums.corvetteforum.com/show....php?t=1921805
I got the line attached to the Schrader valve. No gas came out. I then attached the pressure gauge to the line and had my wife turn on the ignition but not start.
Zero pressure reading with ignition on. (I had previously removed and then replaced the gas cap to relieve pressure.)
The car was started and the pressure went to 38 and held steady at idle. Increased RPM and no change in the reading, still at 38.
Engine off then ignition on and the pressure instantly hit 48 and slowly and then more repaidly returned to zero.
I started the motor and let it idle, pressure at 38. It ran for about 5 minutes and started to die. Before it died, the pressure shot up to 56, held for a moment and then slowly went to zero.
I waited for a few minutes, started motor and same scenerio.
When the motor is running, it sounds normal and runs normal - until it dies.
Looking for comments, suggestions any help appreicated.
Last edited by deerra; Feb 1, 2008 at 06:36 PM. Reason: Added additional comments.
Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
A related issue...when was the last time the fuel filter was changed. This will have a very small impact on pressure.
I think you are having fuel pump issues. There is a check valve in the pump that prevents the fuel from flowing back to the tank (on the feed line). This could cause the rapid drop to zero. There are lines running from the tank to the front end. If you pull out the rubber dam around the gas cap you will see them. Do a advanced search or check you FSM to see which ones to pinch off to test check valve etc.
If these test okay then I would look toward a leaking injector. Pull the plugs after the FP has dropped to zero and see if any are cover with fuel.
I think you are having fuel pump issues. There is a check valve in the pump that prevents the fuel from flowing back to the tank (on the feed line). This could cause the rapid drop to zero. There are lines running from the tank to the front end. If you pull out the rubber dam around the gas cap you will see them. Do a advanced search or check you FSM to see which ones to pinch off to test check valve etc.
If these test okay then I would look toward a leaking injector. Pull the plugs after the FP has dropped to zero and see if any are cover with fuel.
Thread Starter
Burning Brakes
Joined: Apr 2005
Posts: 973
Likes: 0
From: Chico California
I started the motor and let it idle, pressure at 38. It ran for about 5 minutes, ran rough for maybe one or two seconds and then died. Before it died, the pressure shot up to 56, held for a moment and then slowly went to zero.
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Usually the pressure will drop to 0 psi or close to it and the engine will die if it's a fuel delivery problem. I've heard and seen stranger things. Have you checked for a leaky FPR diaphram? Checked the oil for gas?
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Key "on" the ECM will energize the fuel pump relay for 2 seconds. This will power the fuel pump for 2 seconds which should pressurize the fuel rail around 40 psi. If it's not doing this, you might want to test the fuel pump relay.
After the engine is running the fuel pressure should hold steady and move slightly higher when revved, but come back to to what it was at idle. After the engine is shut off, the pressure should hold where it was for a length of time. If it drops quickly, you have a leak in the system. You will have to pinch off fuel lines to find where the leak is. There are about 8+ places fuel can leak from. My fuel pressure will drop 10 psi over about 40 minutes. I consider that normal.
After the engine is running the fuel pressure should hold steady and move slightly higher when revved, but come back to to what it was at idle. After the engine is shut off, the pressure should hold where it was for a length of time. If it drops quickly, you have a leak in the system. You will have to pinch off fuel lines to find where the leak is. There are about 8+ places fuel can leak from. My fuel pressure will drop 10 psi over about 40 minutes. I consider that normal.
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Thread Starter
Burning Brakes
Joined: Apr 2005
Posts: 973
Likes: 0
From: Chico California
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Pull the vac line off the FPR. If you see gas in the line, you have a leaky diaphram that needs replacing. This can dump fuel into the crankcase and mix with the oil. If your oil level is above normal and it smells heavy of fuel, do not run the engine until the diaphram is fixed.
Thread Starter
Burning Brakes
Joined: Apr 2005
Posts: 973
Likes: 0
From: Chico California
Pull the vac line off the FPR. If you see gas in the line, you have a leaky diaphram that needs replacing. This can dump fuel into the crankcase and mix with the oil. If your oil level is above normal and it smells heavy of fuel, do not run the engine until the diaphram is fixed.
Many thanks for the advice.
Thread Starter
Burning Brakes
Joined: Apr 2005
Posts: 973
Likes: 0
From: Chico California
Pull the vac line off the FPR. If you see gas in the line, you have a leaky diaphram that needs replacing. This can dump fuel into the crankcase and mix with the oil. If your oil level is above normal and it smells heavy of fuel, do not run the engine until the diaphram is fixed.
I checked the oil and the dipstick reads high and has a slight odor of gas. Fuel is leaking into the crankcase.
This indicates - what??
Bad injector(s)?
Bad fuel pressure regulator?
My next step(s) would be to...........?
Thread Starter
Burning Brakes
Joined: Apr 2005
Posts: 973
Likes: 0
From: Chico California
Pull the vac line off the FPR. If you see gas in the line, you have a leaky diaphram that needs replacing. This can dump fuel into the crankcase and mix with the oil. If your oil level is above normal and it smells heavy of fuel, do not run the engine until the diaphram is fixed.
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
You need to make sure that there is no raw fuel in the FPR vac line. If you are sure, you can test the injectors. Since you have a LT1, you caught a break. Take the fuel injector covers off, and remove the 4 bolts holding the fuel rail to the intake manifold. With little effort pull the injectors up one side at a time still connected to the fuel rail. There is enough slack in the fuel lines that you can hold the injectors above the intake manifold and have someone turn the key "on". You may also want to verify the fuel pump is priming for the 2 seconds the ECM allows. When pressure hits the fuel rail the injectors should not drip a single drop. If one or more do, you will need to replace them. You have 24 lb/hr injectors.
Since the 92 is still batch fire, you may want to check the injector coil resistance as well. This is done with a OHM meter. Take the injector connectors off each injector and touch the leads of the OHM meter to the two injector terminals on the injectors. The reading should be around 12.5 ohms on each. If one is way off, it could "short out" if you will...that whole bank and the injectors will not open enough to keep the engine running.
Since the 92 is still batch fire, you may want to check the injector coil resistance as well. This is done with a OHM meter. Take the injector connectors off each injector and touch the leads of the OHM meter to the two injector terminals on the injectors. The reading should be around 12.5 ohms on each. If one is way off, it could "short out" if you will...that whole bank and the injectors will not open enough to keep the engine running.
Thread Starter
Burning Brakes
Joined: Apr 2005
Posts: 973
Likes: 0
From: Chico California
You need to make sure that there is no raw fuel in the FPR vac line. If you are sure, you can test the injectors. Since you have a LT1, you caught a break. Take the fuel injector covers off, and remove the 4 bolts holding the fuel rail to the intake manifold. With little effort pull the injectors up one side at a time still connected to the fuel rail. There is enough slack in the fuel lines that you can hold the injectors above the intake manifold and have someone turn the key "on". You may also want to verify the fuel pump is priming for the 2 seconds the ECM allows. When pressure hits the fuel rail the injectors should not drip a single drop. If one or more do, you will need to replace them. You have 24 lb/hr injectors.
Since the 92 is still batch fire, you may want to check the injector coil resistance as well. This is done with a OHM meter. Take the injector connectors off each injector and touch the leads of the OHM meter to the two injector terminals on the injectors. The reading should be around 12.5 ohms on each. If one is way off, it could "short out" if you will...that whole bank and the injectors will not open enough to keep the engine running.
Since the 92 is still batch fire, you may want to check the injector coil resistance as well. This is done with a OHM meter. Take the injector connectors off each injector and touch the leads of the OHM meter to the two injector terminals on the injectors. The reading should be around 12.5 ohms on each. If one is way off, it could "short out" if you will...that whole bank and the injectors will not open enough to keep the engine running.
My mechanical experience is limited but I'll give it a try.
Thread Starter
Burning Brakes
Joined: Apr 2005
Posts: 973
Likes: 0
From: Chico California
I pulled my codes (for the first time)
I pulled my codes and have a H16, H42, and H74 as noted on the speedmeter display. As to whether or not I did it correctly, I don't know. 
I mis-stated my fuel pressure readings. The gauge was not completely tight on the pressure hose. I fixed that and my reading is 38 to 42 PSI. It holds after the engine dies and over the course of around 3 hours, bleeds down to zero.
From what I can tell from the codes I have an ignition problem, an ignition module problem (possible ECM failure) and I don't know about the code 74.
Any help is appreicated.
I mis-stated my fuel pressure readings. The gauge was not completely tight on the pressure hose. I fixed that and my reading is 38 to 42 PSI. It holds after the engine dies and over the course of around 3 hours, bleeds down to zero.
From what I can tell from the codes I have an ignition problem, an ignition module problem (possible ECM failure) and I don't know about the code 74.
Any help is appreicated.
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Since these are history codes I would suggest to disconnect the neg cable on the battery for a minute and reconnect or pull the ECM fuse. This will clear the ECM's memory and you can start fresh. Start the car and see if any of these codes come back. Remember to write down what module these codes came from to help in the diagnosis (1,4,or 9).
The code 16 is not good and usually points to a dying opti spark.
These two are in module 4 (ECM)
Code 16...optispark low resolution (fuel)
Code 42...EST (knock sensor) grounded
This one is in module either module 1 or module 9
Code 74 module 1 (CCM)...LED display open
Code 74 module 9 (ABS/ASR)...Low voltage
The code 16 is a good indication as to why the engine dies. You will need to make sure what module these codes are in (it will tell you what module the code is in on the speedo display). You need a FSM to diagnose each code starting with the lowest code first. Sometimes multiple codes can be the result of one failure or wiring problem and fixing the lowest code first might remove another code. I could help but sold my 92 last month and gave the FSM with it.
http://www.c4guru.com/articles/CCMCodePull.pdf
The code 16 is not good and usually points to a dying opti spark.
These two are in module 4 (ECM)
Code 16...optispark low resolution (fuel)
Code 42...EST (knock sensor) grounded
This one is in module either module 1 or module 9
Code 74 module 1 (CCM)...LED display open
Code 74 module 9 (ABS/ASR)...Low voltage
The code 16 is a good indication as to why the engine dies. You will need to make sure what module these codes are in (it will tell you what module the code is in on the speedo display). You need a FSM to diagnose each code starting with the lowest code first. Sometimes multiple codes can be the result of one failure or wiring problem and fixing the lowest code first might remove another code. I could help but sold my 92 last month and gave the FSM with it.
http://www.c4guru.com/articles/CCMCodePull.pdf