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Discussion Starter · #1 · (Edited)
1994 Trans Am GT LT1 6-speed:

It's been doing this for as long as I've had the car, I just now have the money to fix it. It drives fine but is only an issue on starting.

When you prime the fuel pump KOEO it jumps to 20psi, gives up then drops immediately back to 0. Our possible culprits are leaky injector(s), bad fuel pump check valve, bad Fuel Pressure Regulator (FPR).

Based on random people's advice, spec pressure at idle is 40psi, and 43psi Wide Open Throttle (WOT). When you pull the vacuum line off while running, it should simulate WOT close enough for testing. The hardcore guys want you drive around with your gauge hooked up...but screw that too sketchy.

So I have conflicting results on FPR diagnosis:
FPR is OK:
Pressure jumps under simulated WOT
43-45psi under simulated WOT

FPR is bad:
SOME gas in vacuum line (not pouring out as people report)
36psi @ idle vacuum attached
34-36psi at 3500 RPM vaccuum attached.

Now most guys who replaced their FPR and fixed it were still able to get it to hold pressure for an hour before it slowly dropped, and were not able to simulate WOT. However, they also reported it running below spec which I am experiencing.

EDIT: I have an aftermarket Cold Air Intake (which I want to convert back to stock soon) but I don't think it should make a difference.
 

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the first test that should be done to a fuel pressure regulator is with a hand vacuum pump/gauge. engine off, key off. about 20" of vacuum should be applied to the vacuum line to the regulator, and it should hold for at least a minute. if it passes that test, move on

fuel pressure regulators can fail without leaking vacuum or having fuel in the vac line. the entire purpose of the regulator is to hold a certain amount of pressure, and dump any excess fuel down the return line to the tank. if it's failed without leaking, it might just be from some crap stuck holding it open, and a little bit of 'fuel injector cleaner' in the tank might free it up.

as far as leaky injectors, pulling your fuel rails takes 5 minutes. it's 4 bolts. hang them, jumper the prime connector for a few secs. see if any fuel drips out. replace any injectors that are dripping, and see if it holds pressure after that

you could have a leak in the high pressure side of the system too, jumper the prime connector, leave the pump running, and look carefully for dripping near the tank (fittings near the pump would drip down.)

if all your tests pass, it's also possible for the line between the pump itself and the pump head to crack, especially if a walbro pump with a new soft line has been swapped in there. you wouldn't be able to figure that one without pulling the tank.
 

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Discussion Starter · #3 · (Edited)
Thanks for the vacuum specs, I'll go buy a Mityvac kit tomorrow. I was thinking of doing that first but couldn't find the specs anywhere and didn't want to break it. When you say 20" do you mean hg??

I already ordered a new FPR from the dealer, since even if it isn't the cause of the slow start/pressure drop I didn't like that there was some gas in the vacuum line. It runs rich sometimes so I suspect it was on its way out.

As far as dirt goes, I only use BP gas and put in $25 every couple weeks. I drive the car at least twice a week. I've put Seafoam, gas dryer, and injector cleaner at various points the past few months and it has helped the slow start a little.

For testing the injectors, do I absolutely have to jump the prime conn or can I just get my dad to cycle the key without starting? Also, I kind ruled out injectors because I can prime the line dozens of times and have never flooded the cylinders. I would think it would take more than a drip to cause that much pressure drop.
 

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Steveo is right about the fuel rails taking a few minutes to remove. To change your fpr you will most likely pull the rails to do it imo it would be easier so check your injectors before you disconnect your fuel lines from the rails. When I got my car the previous owner had just installed a new airtex pump from autozone and I had the same starting issue and after testing and still not sure much like you I pulled the fuel cap with the tank low and listened down the fill tube after priming for just a second and heard fuel spraying in the tank when i pulled the pump i found that the flexable tube was leaking where it was crimped on at the pump so you might try that as well and see what you come up with.
 

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Discussion Starter · #5 ·
Thanks for the idea, I'm the second owner so it may very well be a junk pump assembly. Sadly it would cost more for me to do the injectors myself than to pay the dealer to drop the rear and do the pump, so if the FPR is ok I hope its the pump....My dash gauge is 100% worthless so that's very likely haha.
 

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Don't forget about the check ball in the pump. It is one of the things that keeps pressure from bleeding off quickly after pump runs then shuts off.

Also you don't have to drop the tank to change the pump. You can cut a trap door in the sheet metal and get the job done in a short time. Unless of course, your a purest who doesn't believe in short cuts.
 

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Thanks for the vacuum specs, I'll go buy a Mityvac kit tomorrow. I was thinking of doing that first but couldn't find the specs anywhere and didn't want to break it. When you say 20" do you mean hg??
sure, hg

20" isn't a "spec", that's just close to what your car will make for vacuum at idle, so that's a good amount of vacuum to apply for testing. anything on your car should be able to hold 30" of vacuum without breaking

that mityvac is the most useful thing you'll ever buy, i swear.. go through every single vacuum operated thing on your intake with it; i bet you'll find all sorts of minor leaks that you can fix.
 

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oh wait wait wait

there WAS gas in the vac line?

yeah it's your regulator, then. that's a garuntee that the FPR diaphram is torn. but play with the mighty vac anyways, so you can see what a leaky accessory behaves like
 

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Discussion Starter · #9 ·
Ya I'm pretty sure it was gas and not just some other gunk from a 20 year old car. Not much like other people claim, though.

I've used a mityvac at my old shop on several occasions and it is damn useful.
Fixed a stalling S2000 by changing the MAP sensor o ring after the vacuum tested good. Nice and easy 20min diagnose and fix.

If only we had the luxury of OBII.....
 

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you have pretty much all the diagnostic capabilities of a modern OBD-II on your car, with the exception of misfire detection. pretty much all the same stuff can be logged..
 

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Discussion Starter · #11 · (Edited)
Alright steveo, you are clearly the f-body king.

So I vacuum tested the FPR and it held for over 5mins before I got bored. BUT, this time, I had my mom cycle the key with the vacuum on and off and rather than just watching the gauge I listened for leakage back into the return line. Without the vacuum line attached, I'm pretty sure that I heard it splash back into itself. This would explain why even though my car starts slow every time, it acts better when I start it back up within 5 minutes of turning it off. Since WITH good vacuum it holds up but after sitting it loses it's ability altogether.

At the same time, my fuel pump hose in the tank is 100% trashed lol. So this is why it cannot be primed. I opened up the fuel filler valve while my mom cycled the key once. After the pump gives up it sounds like an olympic dive team all diving into the gas tank. I don't mean that I heard some drips after the pump stopped, I heard splashing for a solid 5 seconds after the pump gave up.

I should have the new FPR installed over the weekend, but I'm going to let the dealer put the new pump in since I can't drop the rear end...and I don't have a shop to use at the moment. I'm hoping to convince them the cut the access panel in the trunk while they have the tank out, since I've seen even the factory pumps go bad before they turn 10 years old.
 

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Not sure who owns this pic or took it but it can be cut with the tank in the car using caution on cutting blade depth don't think the dealer will do something like that might be against there policy's

 

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that noise you're hearing could just be pressure bleeding from the FPR down the return line to the tank too

but at least if you're hearing splashing and shit, you know it's not the check valve on the pump, you wouldn't hear that, since it's sunk in the bucket..

weird thing is, if it held vacuum for 1 minute plus, doesn't explain why there was fuel in the vac line. it shouldn't hold vacuum in the diaphram is wrecked
 

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While you're doing good by listening for flowing fuel and all, proper fuel system diagnosis is done with fuel shut-off valves which will let you pin point the problem. Just sayin'...
 

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Yup, I prefer using shut off valves with gauges on either side. You can see where you hold pressure and where you lose it. I had both a bad pump and regulator and only knew doing it with shut offs and gauges on either side.
 

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Discussion Starter · #17 ·
Alright everyone....this FPR removal is incredibly more difficult than I anticipated. ..I'm jealous of Corvette owners right now.

So it looks like someone has been here before and installed the torx screw that clamps both lines UPSIDE DOWN. I couldn't get my socket in it because of an electrical connector under it, then naturally dropped the T25 socket somewhere behind motor.

So I'm not sure what to do here. I had everything taken apart in just 15 minutes, but this reversed torx screw had me stumped for an hour. Once it's out and the lines are free I'll be able to pop it out of the rail.

I've considered removing the rail but I'm not sure how to separate the line fittings. I can jiggle them in and out but I don't see any obvious way to remove the lines from the rail. Any advice?

I'm probably just gonna go buy a "low profile" torx ratchet but I'm not even sure that there's enough clearance for that. Clearly the last guy did it on the bench.
 

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there's a fuel line disconnect tool, it's best to have that when screwing around with any fuel line fittings on your car that dont have that plastic retainer thing on them.
 

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Discussion Starter · #20 ·
Alright Update:

I caved and bought the special torx ratchet and socket set (only $7) which is slim enough that I was able to spread the bracket fairly easily...yet very carefully. Doing this did not require disconnecting the fuel lines or rail...I'm very skinny.

After piecing it all back together, putting on the new FPR and double checking that annoying metal clip on the line 100 times, I'm glad to say that it had improved my readings!

Now the fuel pressure does not drop IMMEDIATELY back to zero, after two or three primes it levels off at 40psi for several seconds before slowly dropping both with and without artificial vacuum. This is a large improvement over the old FPR.

So I'm bringing it to the dealer next week to have them dig up some new vacuum lines for me since mine are dried and torn, and change the PCV and CCV valves and grommets. After that I'll see how it behaves, but its looking like the fuel pump will have to go as well.

They want $1,000 to do it the book's way and I'm considering it. I'm not sure if I feel comfortable doing a fuel pump completely by myself and I'm nervous that cutting the access hole will increase road noise....
 
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