fuel rail pressure drop - how long for it to drop?

Bacondoggy

Registered User
ok, I want to take a look to see if I have a leaky injector - I know the rail should be at 40PSI, but how long should it stay like that? I know it "should" stay there for hours, but my goal here is to see if I have a leaky injector, so what can I expect to see if I have one vs. if I don't. I know I can jump the pump to run, but this does not solve what I am trying to do here... Also, if your FPR is bad, you would notice a pressure drop also, correct?
 
What you are talking about is individual 'injector leak down'. Since ours share a common rail, attempting to do this in the car won't isolate any faulty injectors.

The proper way is to remove the injectors and have them tested individually by an injector testing service.

Other than that, I'd start pulling and looking at spark plugs...
 
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yes but won't I get a general idea what is wrong if I do a pressure drop test on the fuel rail and say, the pressure goes from 40PSI to 20PSI in a minute, or in 1 hour? This was that I was trying to accomplish. I know it wouldn't tell me which injector would be leaky, but it would tell me that one or more out of the 6 has an issue, and would require me to tear the car apart...
 
If you have a leaky injector or regulator, the pressure drop will happen almost instantly. If it takes 30 minutes for pressure to bleed, down then it's not much of a leak. Since the fuel isn't compressable and the volume of fuel under pressure is rather small, any leakage results in a large drop in pressure (if pump is off).

David
 
By the time you wait 30 minutes, you could have examined the plugs and know exactly which, if any, injectors are leaking. Of course, with the time it takes to pull plugs on these cars in most cases, this could be less than fruitful :)
 
well with unleaded fuel - the plugs don't color much... so its hard to tell. I will do the pressure drop test and see what happens...
 
>the plugs don't color much...

If you have an injector that fails in a big way, leaking down immediately, I'd expect a wet plug...not concerned so much with the color, unless of course it's midnight coal.

What do you expect to find, based on what?
  • If you're expecting one or more injectors that drop instantly, I'd also expect an engine that barely runs if at all.
  • If you're expecting an aged injector that leaks slowly when the engine is off, you might find a wet plug.
  • If you expect an intermittent issue, a cylinder balance test via a code reader might help pinpoint which one.

You seem to already suspect an issue with the fuel injection system in general - a pressure drop test will just say the same thing, without pointing to a specific cause.

I'm just trying to get you to jump ahead and have a chance at isolating/pinpointing the issue you already think you may have.

If you're just checking things out in general, and not even sure if an issue actually exists, a quick pressure test is the ticket, of course.
 
wel my CEL comes on after 10 min of driving or so even after re flashing the eec. its saying the drivers side is running lean. I suspect a bad FPR, but this is my daily driver, so I havent had the chance of tearing it out yet. I noticed a faint smell of gas in the oil before I changed it and figured it might be a leaky injector... on the other hand I may have a clogged injector causing the lean condition. The car drives nicley, but the lean signal concerns me. I have tried cleaning the MAF, new tps/tb new o2 sensors... Would I be able to put #36 injectors in the car without needing a chip? What is a good price for 6?
 
>the lean signal concerns me.

I believe there are recall/TSBs that discuss carbon buildup on the valves that act like sponges to soak up and then later release fuel - bad enough and this causes the system to lean out.

How many miles on the engine?

Has it always had quality fuel?
 
well the car has about 70k original miles. The engine has been completley rebuilt and bored .030 over about 3-5k ago, so I dont think its an issue with the valves...

do I need to change to a different MAF if I got #36 injectors? Is $90 shi9pped a good deal for them?
 
It would be better if you went with a C&L with the correct sample tube for those injectors, later a tune will help optimize what you have but you don't need to be in a hurry for it.

Just realize that mileage will suffer in some areas. At startup there is no adaptive applied so the car will be richer than normal, and could possibly be a problem when your starting up cold. At idle until the car is warm enough it will be rich until adaptive can be applied. At Wide Open Throttle the car will be richer than before which will hurt performance and use more fuel.

Really the only time the larger injectors won't cause a rich condition is when the car is cruising along.

Not a big deal with the 36's, just wanted you to be aware whats could happen.

Frit
 
Fuel Pressure...

I just did this on my '95 SC to isolate the fuel issue -

I cut the intake fuel like just prior to the rail, leaving enough hose on each side of the cut to insert a barbed brass connection. I then used some brass fittings (with plumbers tape) to connect the the barbed connector to my fuel pressure gauge. CAREFULLY I turned the key on so the pump would run - turning the key back off quickly being careful to not let the pressure build over 40 (no FPR to let excess pressure go.)

If the fuel pressure dies (assuming no leaks) then it's the pump. If it doesn't, it's the injectors or FPR. After removing the fuel rail too many times now trying to isolate the bad guy, my advice is to pony up the bucks and replace all injectors and FPR if you isolate the the problem to the rail.

To finish up, use the brass fittings to connect the two ends of the fuel line back together.

Hope this helps (if it's not too late.)

Cheers,
Stigs
 
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