Engine will not start at times / low fuel pressure? - LS1LT1 Forum : LT1, LS1, Camaro, Firebird, Trans Am, Engine Tech Forums

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post #1 of 29 (permalink) Old 06-24-2012, 12:19 PM Thread Starter
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Engine will not start at times / low fuel pressure?

New to this board as my usual hangout has gone very quiet.

My 97 Trans Am started to have issues where I had to crank at times up to 30 secs before it would start.

I finally bought a fuel gauge and hooked it up at the Schrader valve after I could not get it going at all and the battery was low from the cranking.

With the battery still weak I put the ignition on and it showed no pressure. Tried one more time and it went up to about 10 - 12 psi. Another try showed 0 again.

Fully charged the battery, ignition on and no pressure. Tried to start and it fired for a brief second then died.

Ignition on and about 10 psi and the engine starts, pressure goes to 33 psi on idle. Take it for a spin. Cruising pressure 33-34 psi and WOT drops to 18 psi.

Back in the garage turned it off and then ignition on with 38 psi. Leaving the gauge on to see if and how quickly the pressure drops. After two hours pressure dropped to 0. Pulled the fuel rail off and with ignition I only get about 12 psi. Pressure dropped back to 0 eventually but no signs of leaking injectors.

Based on what I've been reading, looks like my fuel pump is not performing / on the way out and should be replaced or do I need to look at something else, fuel pressure regulator?

Appreciate you help.

Rene
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post #2 of 29 (permalink) Old 06-24-2012, 09:57 PM
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Yes it could be a bad fuel pump. It could also be the regulator stuck open, clogged fuel filter, or filter sock on pump pickup. Also sometimes the rubber fuel line that connects pump to pickup tube gets a hole or slips off.
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post #3 of 29 (permalink) Old 06-25-2012, 10:56 PM Thread Starter
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Thanks. So I start with the fuel filter, then if still issues to the regulator and finally the fuel pump.

What are the fuel pressures supposed to be? I read in several places 40 - 45 psi at all times. Is that correct?

97 Trans Am A4, Borla cat back
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post #4 of 29 (permalink) Old 06-26-2012, 06:21 AM
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At key on prime 43 psi is what you want. When idling 43 psi, under some running conditions pressure will drop some but 30 psi or below is never good. One way to test regulator is to unplug vacuum hose and pinch off while running. If low pressure goes up to normal reg is problem. Or if you smell gas when you take vacuum line off. Reg can fail one more way where spring inside has failed and allows fuel to go back to tank at all times. When you prime pump with engine off, fuel pressure should stay up for quite a while. If it bleed down quickly reg spring is usually bad. Finally fuel pump has a check ball in it that keeps pressure from bleeding off quickly. If it fails fuel can bleed back to tank but not as fast as bad reg spring.
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post #5 of 29 (permalink) Old 06-26-2012, 08:25 PM Thread Starter
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Thanks again for the detailed description of the checks to perform. I'll report back what the findings are.

97 Trans Am A4, Borla cat back
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post #6 of 29 (permalink) Old 06-29-2012, 10:39 PM
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factory spec is 41 to 47 PSI. fuel filter and regulator should be checked first. Trust me you don't want to drop that tank if you can avoid it. i have a full service shop with a boss that lets me do these things after hours and i hated dropping my tank.
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post #7 of 29 (permalink) Old 06-29-2012, 11:28 PM
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If you get to the point of having to get to fuel pump, here's a page with a pic showing where to cut a trap door for pump removal www.worldisround.com/articles/12533/photo14.html

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post #8 of 29 (permalink) Old 07-02-2012, 07:35 PM Thread Starter
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Thanks guys for all the info. Funny I came across the trap door link back in 2009 and printed it in case I needed it.

Finally got to change the fuel filter today and it did not make any difference but I'm glad it's done. Fuel pressure still behaving strange and being way below 40 psi.

Next step is to replace the regulator which I should receive Thursday.

What is the best way to replace it, pull the fuel rail out?

97 Trans Am A4, Borla cat back
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post #9 of 29 (permalink) Old 07-07-2012, 02:20 PM Thread Starter
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So I changed the fuel pressure regulator today. Now the pressure on idle is lower than before with about 30 psi and below 20 psi at WOT.

What is even worse is the fact that the pressure drops to 0 when the engine is turned off??

Anything else I need to check or is the fuel pump my next step?

97 Trans Am A4, Borla cat back
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post #10 of 29 (permalink) Old 07-07-2012, 02:32 PM
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Might be that the hose that attaches fuel pump to pick up tube has come loose or has hole. You will have to get to top of fuel tank, open it up, to find out. Fuel pump could be failing also.
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post #11 of 29 (permalink) Old 07-07-2012, 02:56 PM Thread Starter
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Thanks, just to make sure I do not miss anything. In the trunk / tank we go.

Any suggestions for a good reasonable priced replacement pump?

97 Trans Am A4, Borla cat back
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post #12 of 29 (permalink) Old 07-30-2012, 06:08 PM Thread Starter
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Finally got to cut the access hole in the trunk as per link above and replace the pump with a kit from Racetronix. Fairly straight forward job.

Upon ignition on, the pumps brings 45 psi and then drops just below 40. With the engine running the pressure is at around 38 psi on idle and when driving it goes up to 45 psi. So all seems right.

The only thing that puzzles me is the fact that the fuel pressure now drops instantly after shutting down the engine??? Any suggestions as to what it could be?

97 Trans Am A4, Borla cat back
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post #13 of 29 (permalink) Old 07-30-2012, 06:40 PM
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There are several reasons your pressure is dropping so quick. 1st suspect is there is a leak where pump hose attaches to metal pick up line. Next is the check ball in pump is bad, followed by an injector or injectors are leaking and allowing fuel pressure to leak down quickly, and finally pressure regulator could be stuck open just enough to bleed off pressure quickly, but not enough to lower pressure when running.
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post #14 of 29 (permalink) Old 07-30-2012, 07:04 PM Thread Starter
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Well as I mentioned, I have a new pump and fuel pressure regulator.

Injectors were checked earlier and did not leak either. Connections in tank are solid.

Come to think of it after replacing the regulator the fuel pressure would drop a lot faster. Anything one can do to get it unstuck?

97 Trans Am A4, Borla cat back
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post #15 of 29 (permalink) Old 07-30-2012, 07:50 PM
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I've seen enough people who knew there fuel pump connection was good, only to go back and check and find it wasn't, to say check it again. A very light tap with a very small hammer might free the regulator up, or it might be defective right out of the box.
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