My 94 6spd is getting just under 38 psi fuel pressure, car on, engine off. From what I've read it should be higher? Pressure drops very slowly (takes a couple hours) and stays around 30 while driving. pump, pressure reg. and filter are all new. Old reg. had fuel in the vacuum line. I guess my question is, is my new pump weak? Could this be contributing to a stumbling issue the car is having? It's a new acdelco pump. Thanks

 

How are your injectors? It sounds like you "may" have an injector issue. But I put a new reg on a C4 I had and it was bad out of the box.

 

A leaking injector is possible but would that keep the initial fuel pressure low when you turn the key? I would think it would just make the pressure drop sooner.

 

One of the "must have" books, IMO, for 1982 to 2001 is "Corvette Fuel Injection (and electronic engine management)" by Charles O Probst. (ISBN 0-8376-0861-9)

Cutting to the chase, fuel pressure, according to Probst, for the LT1/4 should be 41# (280kpa). A special note is included with regard to FP and the LT1/4 which says, "LT1 and LT4 specify a single valve (FP regulator) because these engines are sensitive to fuel pressure."**

**Chart Ch 7, pp 164: (Relative port fuel pressure with the engine off).

If your static FP is 38#, with engine off, it is only shy by 3 psi (according to Probst's chart for the LT1). For 3#, I'd not be too concerned; maybe try another FP gauge and compare. However, falling to around 30 while driving is "a bit" off (read: LOW).

The dynamic fuel pressure (drop) may be a source of the problem; i.e., 8# is about twice what I would expect (but I'll research a bit more for confirmation and get back). Holding static pressure (with switch off) indicates you don't have a leaking injector(s) and the check valve in the pump is working too. So, I'd be inclined to do the easy thing first, and change the fuel filter, and then go for a drive and see what the FP does. (Course clogged fuel lines would not be fixed by changing the filter, but far less likely to be restricting flow; just to keep in mind.) ALSO, I'd be interested to know what the pressure does at WOT above 4500 rpm or so (max fuel demand). IF FP drops into the mid 20s, then I'd say you could be having some "issues", not the least of which engine damage due knock or burnt valve(s) (ask me how I know...)

If the fuel filter change resulted in dynamic pressure remaining close (w/in a pound or so) of the static pressure (LT1/4) at WOT, and w/in 4#** of static reading at idle, I'd say you don't have a fuel delivery problem.

**Note: I'm extrapolating from other non-LT1/4 examples here - so anyone with exact dynamic FP figures for their LT1/4 with engine idling, PLEASE chime in!

So, the flip side is, if FP stabilizes with changing the filter, you have your answer - to the FP question at least. If that doesn't fix it, check for fuel in the vacuum line leading from the FP regulator (if you haven't already): gasoline in the vacuum tube = bad regulator. Change it and test again.

If you're sure the regulator is good, and the filter is not the issue, then the pump come under the microscope. So, changing the pump might be the practical thing to try - but understand that pumps can be bad right out of the box (has been my experience), or die shortly after putting it into service (called "infant mortality" in "mean time between failure" speak)

Well, let's hope you're right about a fuel delivery issue, and at this point it your problem is solved. But, if NOT solved, we have methodically approached the fuel delivery - up to the injectors at least - and can move on to something else (but FP sounds to me like the culprit - someone with FP readings at idle would be a nice piece of info to sum up this "chapter".

P.

 

This spring I replaced all my injectors and decided to replace the filter and regulator also. Everything was 17 years old so I figured it was time to replace them. The new regulator pressure was very low. Lower than the original, which I thought was too low to begin with. I bought a second regulator, different manufacture, and it too was low. Returned both units and bought an Aeromotive adjustable regulator and set it for max GM recommended pressure. The improvement in the way the car responds to throttle changes is impressive. Of course it cost three times as much but now I have perfect pressure and a car that has a little more pep and better throttle response so I figure I'm ahead at this point. Other then a complete engine upgrade (cam, heads, exhaust) there is very little you can do to get more out of these LT-1's. This simple upgrade made a very noticeable improvement to the overall enjoyment of the car.

 

Do not know if this would actually bring your pressure up, but buying the equipment yourself to hook directly to the fuel rail and inject from the pressurized can of injection cleaner with your fuel pump fuse pulled might help in the cleaning of the injectors.

If you don't want to go to expense of buying the equipment you could locate a garage that does this type of injector cleaning on a regular basis.

It is a very easy process to just do a hose connection to the injector rail where you most likely took a pressure reading.

The equipment I bought has a inline pressure gauge to read as well.

 

Paul Workman:
My 96' LT-1 is supposed to have a static pressure (ignition on, engine off) of 41 to 47 pounds according to the FSM. Both regulators that I bought had 38 pounds. My 17 year old original had 40 pounds. The new Aeromotive is set a 46. The manual states that you should have a 3 to 10 pound drop at idle (engine on, high vacuum) but this varies and should always read less than static reading.

 

Ah, thanks 4 the missing pieces. Then, if I understand it correctly, the drop from about 38/static to around 30/dynamic, the drop (at least) is about normal, but the static level is about 7-8# low (according to your FSM). That seems a bit on the low side. Still, in closed loop, the ECM may be adjusting, so that a lean condition in normal street or hwy cruising.

How-some-ever, at WOT (open loop) the O2s are not at play, and the ECM falls back on set PE values. I can see how a lean condition could occur, especially if the pressure was 7-8 pounds lower than desired. However, to the OP's initial question - stumbling - if it occurs at normal traffic/hwy driving, then those that suggested injectors and or spark plugs/wires/coils...may have their finger on it.

FSM values: Good to know. I'll make a note.

 

A direct quote from the FSM:

"A lean condition may result from the fuel pressure being below 284 kPa (41 psi). A lean condition may cause DTC P0131, DTC P0151, DTC P0171 or DTC P0174 to set. Driveability conditions associated with lean conditions can include hard starting (when engine is cold), hesitation, poor driveability, lack of power, surging, and misfiring."


To support this in real life, at 38 and 40 pounds my car was sluggish and had hesitation when changing throttle position. After bringing the pressure up to 46 pounds the car responds quickly and smoothly. When idling my pressure now only drops to 43 pounds. Once again let me express that these numbers are for a stock 96' LT-1 and other years could be totally different.

 

I appreciate all the responses. I should mention the plugs, wires and o2 sensors are new. So it sounds like 30psi running is low, haven't gotten a chance to check it at wot. Strange thing is I think it's running rich not lean, I smell fuel in the exhaust and get an occasional misfire. Mpg are terrible. I think I may have an exhaust/ vacuum like I haven't found yet. Just wanted to try and rule out the fuel system before moving on.

 

Pete919:
I don't have an FSM for a 94' so can't tell if 30 at idle is low or not. We basically have the same engine so one would think your and my numbers should be close. If they are then 30 pounds is way under my 43 when idling. Getting my numbers up made a big difference in the way my car runs. Both of my new regulators were 38 psi and some earlier C4's used those pressures. At 38 psi my car was sluggish and slow to respond. I can't imagine what 30 psi would be like. First figure out why it's low and if the reason is the regulator, then spend the extra bucks and get an adjustable one. Set it for GM's max pressure, and be impressed with the way your car now runs.

 

Thanks all. Just drove it to confirm, 38 psi key on engine off. 30 psi driving. 40 psi wot. The adjustable fpr makes sense, think I might exchange the one I just bought first before I spend more $.

 

If your going to drop the extra bills then I highly recommend the Aeromotive unit. Its made from solid aluminum billet and is very well constructed. I was able to make one pound adjustments and it seems to repeat very accurately. It only takes about fifteen minutes to replace it and a couple more to adjust. New bosh III injectors made a big difference but the adjustable regulator brought it right where it needs to be. I think the regulator made one of the biggest increases of all the things I've done to the engine. Here is a pic.