Hi, hoping someone has experienced something similar, without having to start replacing parts.

Issue: Car is hard to start when cold, won't catch on the first try. Catches usually on the second attempt. Recently has extended to needing 3-5 attempts to start when cold. After running, re-starting it starts right up.

Background: Did the intake manifold seal replacement recently and this issue only started surfacing after it. And it has gotten progressively worse.

Diagnostics done so far:
1. Cold - With ignition on, engine not running, the fuel reads 18psi. Cycle key on-off a few times and pressure does not go above 18psi.
2. Started - idles at 32-38psi, needle oscillating between them quickly.
3. Started - rev engine from idle, pressure steadies and stays around 38psi.
4. Started - pull vacuum line and pressure increases to 40-42psi. No fuel leaking out of FPR, either when running or off.
5. Started - use Mity Vac to put a 25inHG vacuum. Same oscillating 32-38psi seen in #2 above.
6. Fuel filler cap removed both when running and when off, no trace of pressure relief from tank, shouldn't there be?
7. After running for a few minutes, shut down the motor and bleed off pressure manually w/gauge. Ignition back on to run position, and get 22psi. Cycling ignition from run/off/run/off while waiting a few seconds to build pressure, does not go above 22.
8. After running for 10 minutes, shut down the motor and bleed off pressure manually w/gauge. Ignition back on to run position, and get 28psi. Same cycling ignition run/off and keeps at 28. Even if I relieve the pressure and back to run position it is right back at 28psi.

I'm really hoping it is not the fuel pump. I could do the filter as a preventative/process of elimination.

You guys have the experience and knowledge, really appreciate your help.

Tom

 

When you turn the key to “RUN” the pump should prime for 2 seconds, and shut off. At shutoff, pressure should be at least 40 PSI. Cycling the key should build pressure (slightly of it reaches 40 PSI. After the pump shuts off, the pressure should hold, and not drop quickly. A drop of 10 PSI in 10 minutes is borderline.

Start the engine. At idle, remove the fuel pressure regulator vacuum compensation hose. Pressure should be 43.5 PSI. GM accepts a range of 41-47 PSI.

Reconnect the vacuum line. Pressure should drop proportional to intake manifold vacuum. A stock cam will drop the pressure by 8 to 10 PSI. Example 43 without vacuum drops to 33-35 with vacuum. A more aggressive than stock cam will drop less. Pressure at idle should be relatively steady. Jumping around usually indicates erratic intake manifold vacuum. But the fact pressure jumps around with a constant 25”Hg from the vacuum pump suggests the pump or the regulator is the problem. Possibly air in the fuel line? ? ? That might explain the low (but slightly variable) pressure when you turn the key to “RUN”. Not a problem I have seen before. Fuel vaporizing to form bubbles could explain it, but you indicate the problem is there with the engine cold.

Opening the throttle at idle reduces vacuum briefly, hence increases rail pressure. But if you open throttle and hold it there, the pressure will drop part way again and hold steady.

The fuel tank is vented through a pressure regulator, into the EVAP canister in the driver side rear fender. There should be little if any pressure or vacuum in the tank when you remove the cap. There is also a “safety” valve that relieves excessive tank pressure or vacuum.

 

Thanks Fred. All great info, think I am on the right path, just need some ideas what to do next.

What could cause the fuel pressure to only stick at 18psi on cold starts before it starts? I had the fuel rail off when doing the manifold gasket, so wonder if maybe an injector is not sealing fully. I don't see any fuel leaks anywhere though...

Do you think it's worth it to start replacing fuel filter and pressure regulator to start with?

 

If an injector is sticking open, the pressure would drop almost immediately when the pump shuts off. Ditto if the check valve in the fuel pump was not working, or if the fuel pressure regulator was leaking, either at the diaphragm or not fully shutting off return flow. But as low as your pressure is you indicate it holds when the pump shuts off.

I'm struggling to think of a cause.

Have you taken it out on the road to see how fuel pressure holds when you go WOT at high RPM?

 

If you hold the throttle open just a bit, does it start right away? If so, it could be a problem with the IAC motor.

 

I’m really not into ‘parts changing’ but this can really be a 1 of 2 and 2 is a lot harder and more expensive than 1.
First I’d just change the fuel filter and fuel pressure regulator. Easy, cheap, and pretty much maintenance stuff.
If that doesn’t do the trick drop the tank and do your fuel pump. You’ll be set for another 10-20 years depending.

 

Edit...nevermind.... noticed i replied to the wrong thread...

 

Last night I tried putting 12v on the fuel pump prime jumper under the hood and I was still getting the same 18 PSI at that point.
When I went to start it to take it out for a ride this morning I had 0 PSI. Cycled the key a few times and stayed at 0 PSI. Had my hand on the Fuel pump solenoid and could feel it click on w/key and off after 2 seconds so I know that is working. Kept at it and finally got 10 PSI. Tried starting and it turned over for about 5 seconds before it caught and the pressure jumped up to 30. Was able to take it for a spin and at WOT high RPM it drops to 20-22. Hovers around 32 cruising. Got it back and shut down, pressure holds at 30, and starts right back up.

Feel like if it was the pressure regulator there is no way I could have had 0 PSI this morning. If the filter was clogged to a point where it blocked to 0 PSI, I feel like it wouldn't free up to allow any flow, where the pump maybe is just on the brink of failing completely.

 

Should hold at least 40 PSI at WOT/high RPM.

32 PSI cruise is OK if the accel pedal is holding steady.

 

How fast does the pressure bleed down, after the pump stop priming?

Air in the system may explain it, question is how could the air be getting in the fuel system.

Possibly - just thinking out loud.....

What if you had a pinhole in the flex pipe that connects the pump to the sender? That could allow fuel to run out of the line to the rails. At very low flow, might actually be educating air into the line through the pinhole. Air prevents or limits pressure building during the 2-second prime. But puts enough fuel in the line after repeated key cycling to at least get the engine started. Pump does not restart until the PCM gets the “fuel enable” signal from the VATS system, and gets the low resolution cam position pulses from the Opti.

Pump starts, air has exited the system via the injectors, flow increases, and the pinhole leak doesn’t bleed enough off to keep the FPR from controlling the pressure. But at high engine load the pinhole bleeds enough fuel to be unable to meet fuel demand.

But, odd this would all coincide with the manifold repair. As much as I hate replacing parts without a logical reason, the filter change may be due. Can’t think of a specific failure in the FPR, but those two parts involve a lot less labor than going into the fuel pump.

Another wild guess.... if you disconnected the fuel lines to pull the manifold, did you reinstall the supply and return on the correct hard lines? They're different sizes, so seems unlikely to happen, if even possible.

How reliable is the fuel pressure gauge?

 

Prior to it starting, if the pressure is at 18, it holds and does not leak down completely for a few hours. After being started, the pressure holds around 30 and loses a few pounds in 30 minutes and then takes hours to lose fully.

I don't see/smell fuel leaking out anywhere, so while a good thought, not sure how a pinhole could be. I can tell the pump does not restart as you stated until the PCM sends signal, because i watch the fuel pressure drop at start while turning over and then it kicks up to around 30.

I will disconnect and re-connect the fuel lines this evening and see if something there could be causing it, making sure they are seated properly.
After that, I agree the filter and FPR are my next steps to keep my time down.

The pressure gauge is brand new, so i feel like that can't be the problem, even if the reading were to be off a few psi because i am getting both high and low readings at various points.

 

The flex line leak would be inside the tank, wouldn’t smell it.

If the feed and return lines were switched, the issue would be the impact on the self-actuated FPR. Connections would probably leak, but that was just an off-the-wall attempt to think outside the box.

I freely admit.... this has me puzzled and I'm grasping at straws.

 

A few more things i have tried...

Pulled the fuel lines and confirmed there is no way to hook up backwards, can not even force them on, different sizes.

While the return line was off, i held my finger over the regulator and had someone turn the ignition to on position, to see if any fuel could be leaking by the regulator, in fact i still got a 18psi reading.

Swapped out the fuel filter for a new one and no improvement, still getting 18 or so.

Being that the pressure is up in the 30's while idling, I'm beginning to wonder if there is a voltage issue when off. The battery is measuring 12v, but maybe it's not all getting to the fuel pump. So the question i have is, does putting 12v on the fuel pump jumper under the hood go directly to the fuel pump, or through the solenoid? I'm going to see if i can measure the voltage at the solenoid when key on for the 2 seconds to see if a full 12v is there.

After that, if nothing found, I'll probably pull the fuel rail and re-seat the injectors in the rail and intake manifold.

Then the fuel pump if need be. I'm trying everything to avoid the fuel pump, refuse to cut a trap door and very much not looking forward to dropping everything out of the way to do the pump.

 

In 96/97 they revised the fuel pump relay, switching from the 4-pin relay used 93-95 to a 5-pin relay. Then, instead of running the fuel pump prime voltage directly to the connector near the rear axle, the fuel pump prime voltage was routed through the extra pin on the 5-pin relay. When the relay is not powered, the internal contact defaults to the position that connects the fuel pump prime voltage to the wire that goes to he connector at the rear axle. When the PCM energizes the fuel pump relay, the internal contact moves from the prime voltage position to the always hot 12 volt supply from the fuse #3.

I've been through this with someone else in the last few months, and provided the fuel pump wiring schematic in the thread. Do you have the free download of the 1996 factory service manual. If so I can give you the page number.

https://www.mediafire.com/?40mfgeoe4ctti

 

See post #6 in this thread, with wiring diagrams attached:

https://www.camaroz28.com/forums/gen...3/#post7009493