LaVidaLoca

I noticed my fuel pressure slowly drops. Went from about 25# down to 20# in about 2 min. After 30-40 minutes it was almost to 0.

I have new O rings on all injectors and do not have any fuel odor. Car starts great under all conditions every time.

I don't recall seeing a specifictation for this. Is this normal or should I be looking for a leaky injector?

scorp508

Thats too fast. I'd be looking for....


a) Leaky injector.
b) Faulty fuel pressure regulator.
c) Faulty check valve in fuel pump.

cdnguyred88

:iagree: Way too fast for losing pressur.

offershore

I have no clue what the leak down should be, I thought that was pretty good though, what should it be?

LaVidaLoca

If its B or C is there any reason to worry about it?

JCAIRE2

What was the pressure with the engine running?

My pressure holds at 38Lbs for about 2hrs, then starts to SLOWLY drop. After leaving the gauge on for 5hrs, it was still at 32Lbs. :cheers:

LaVidaLoca

Pressure when running is about the same as yours. When I checked it I just switched the key on and then off to get some pressure up. Sounds like yours holds pretty well. Guess I need to do some further investigation. My main worry is that I am leaking gas into the cylinder. I'm pulling all the plugs tonight to see if any of them are wet.

scorp508

While running? Thats kinda low for an 88. I know '85 runs at a lower pressure, I might have the spec around here somewhere.

JCAIRE2

Really? I thought I was fine. Now I have to go dig up the service manual. :D

scorp508

Well actually hold on, if that is with the vacuum line connected it is about right. If it is with the line off its too low.

scorp508

JCAIRE2

Ok, that was with the vacuum connected. Disconnected, it was 44. Looking at the table you posted, it's funny that the regular (non-corvette) 350 has so much more pressure. Would that be for a TBI system?

LaVidaLoca

Status:

Shut off engine then clamped off both the fuel line and return line at the tank and still had a slow pressure drop. That should rule out the regulator and check valve right?

Then I pulled the cold start injector and there was no fuel there.

Next I pulled each spark plug. They all looked pretty much the same, nice tan color. No significant gas odor on any of them. No sign of any of them running rich. Is there any other way to isolate where the leak might be?

Engine starts/runs fine. Shouldn't it have some other symptom if an injector is leaking?

So on a scale of 1-10 is this a:
1. Don't worry about it now, take care of it when you get around to it.

or a:
5. Get some injectors on order and go ahead and get them taken care of when they arrive.

or a:
10. Don't even start the car again until the dropping fuel pressure is fixed.

:cheers:

RonsRed88

I copied this information off this site quite a while ago, it's probably still in the archives somewhere. Hope this helps

---------

Diagnosis should begin by performing the "Cold Start Valve" test listed in section 6E3 (Chart A9) of the appropriate Service Manual to determine if the cold start injector is functioning properly. If not, the appropriate repairs should be made.
If the cold start injector system is found to be functioning normally, the following procedure should be used to determine whether a fuel pump check valve, fuel pressure regulator, or leaking injectors are the cause of the long cranks.

1. Install a fuel pressure gage on the fuel rail Schrader valve, (such as that supplied in the injector plugging test kit).

2. Pressurize the system by cycling the key on for a few seconds.

3. Turn the key off, and time the pressure reduction on the gage. (Initial pressure should be approximately 40 pounds per square inch (PSI), note the time it takes for a pressure drop of twenty PSI to occur. If the time exceeds twenty minutes, check valve, regulator, and fuel injectors are free of excessive leakage and the Service Manual should be referred to for other causes of long cranks.)

4. If the system pressure is reduced by 20 PSI or more in 20 minutes or less, then one or more of the three components mentioned are suspect.

5. Turn the key on for a few seconds to re-pressurize the system and clamp the rubber portion of the supply line closed using a pair of vise grips and a shop towel (to protect the fuel line from damage by the vise grips.)

6. Turn the key off, and again note the time it takes for a reduction of twenty PSI. If the time is a lot longer than the time noted in step 3, then replacement of the fuel pump is indicated.

7. If the times are similar, then repeat steps 5 and 6, only this time clamp the return line instead of the supply line. Once again note the time it takes for a twenty-PSI pressure drop. If the time is a lot longer than previously, a faulty pressure regulator is indicated. If the time remains shorter than twenty minutes, leaking injectors are suspected. To confirm that there are leaking injectors, raise the fuel rail slightly out of the inlet manifold without disconnecting any fuel lines and place a small napkin or piece of paper under each injector. Pressurize the system by cycling the key on for a few seconds then turn the key off and allow to sit for about 10 minutes. If a leaking injector is indicated by one or more drops of fuel on any of the napkins, all eight injectors should be replaced with service injector, P/N 10108481.

LaVidaLoca

RonsRed88

Thanks for looking up the info. This was almost exactly what I did. The only thing I can't do is pull the rail up with the fuel lines connected. They are metal, bolted to the block, and wind thru the AC bracket.

The 20 min time was something I did not know. Is that a factory spec? Did not see it in the service manual. Makes me feel a little more comfortable.

Vette 94

After reading these post I got curious and checked my fuel pressure drop and it dropped very fast. I have a 94 with an aftermarket fuel pressure regulator. My pressure is set at 42# when at idle. When I cut off the switch it immediately dropped from 42 to 24. It then dropped 3 # per minute until it reached almost zero. It went from 24 to 3 in 10 minutes. The car only has 30000 miles so I would think the injectors should still be holding up OK. Will the aftermarket fuel pressure regulator drop faster than stock?

LaVidaLoca

Don't know but the check in the above post should let you know. I just pulled the rubber boot around the gas filler and put C-clamps on my fuel and return lines. That eliminates the check valve and regulator.

LaVidaLoca

Found some additional useful information in a book about Corvette fuel injection systems. The long term fuel trim is stored as "Block Learn" and its normal range is 115 to 141. The lower the number the more the ECM is trying to cut fuel (running too rich). The book goes on to say that below 115 is in a range that indicates a leaky injector(s) and a possible problem. Mine reads 118 on the scanner so I am near the limit of the rich end of the range.

Think this might be the final piece I needed to decide to replace the injectors. :yesnod:

There have been some really good responses on this thread. Thanks to all who particpated!!! :thumbs:

scorp508

It shouldn't change it one bit unless you ripped the diaphram inside or if it is old and brittle. That pressure is dropping way too quick.

frank moran

I have an 85 and the pressure drops like yours, before and after an inj. cleaning, I have had others that droped fast and they all run perfectly and start fast...I have even pulled the fuel reails and pressured the system, they have a very little bit of moisture/fuel on them and according to the guy in Mich who cleaned them claims they all pretty much have a speck of fuel on the tip?? to get a BL change it has to be asignificant amount of raw fuel and the plug would probeably show it, if it runs and starts fine , dont worry about it.??