I'm in the process of running some diagnostic checks on my car (See Sig), and when I check the fuel pressure at the Schrader valve on the fuel rail ........ I'm getting like 10-15 PSI during hot idle. I've checked the gauge, and it seems to be correct, sooooooo ..... the question is ....... what is the minimum fuel pressure these LT1 engines will start and idle at?

Also, the fuel pressure measured via the Schrader valve is after the fuel pressure regulator, right? When I disconnect the vacuum line from the regulator I get maybe a 1-2 PSI increase in measured fuel pressure, does this slight increase in fuel pressure still indicate that the regulator is working properly? I suspect a bad fuel pump (it's the OEM original fuel pump with approximately 85,000 miles on it).

 

Pressure at the schrader valve is before the regulator because the regulator restricts return. So you are measuring it in the right place but a lot of fuel gauges don't press the schrader valve stem down far enough to open it when you screw them on and if you take the valve core out (with the engine not running) then try your gauge again, you might get a different reading. As to your original question, I don't know how low you can go and still get fuel through the injectors at idle.

 

Yikes 10-15 PSI means you're going to be HELLA lean and your PCM is going to be commanding tons of fuel to be dumped into that engine. Fuel pump time - get your sawzall and cut the trap door!

 

I would try another fuel pressure gauge.

 

With the vacuum compensation line removed, GM requires 41-47psi. As differential pressure (rail pressure minus manifold pressure) drops, fuel flow through the injectors drops. You do not want to run with less than 40psi differential pressure, which means 40psi at WOT.

 

Kevin ......... great minds think alike! I have removed the Schrader valve stem and retested the fuel pressure and I am still getting 10-15 PSI at idle.

Stl94LT1 ........ I don't have that option, but, I have used the same fuel pressure gauge on my other car (a multi-point fuel injected Lincoln Mark VIII, and the gauge reads a solid 38 PSI at idle on that car. Also, when I purge the gauge of fuel by using tire pressure ..... I get 24 PSI on the fuel pressure gauge ...... the same as the tire pressure I measure with my tire pressure "pencil" gauge. I'm 95% certain at this point that the fuel pressure gauge I'm using is okay.

Meissenation and/or Fred (Injuneer) ............ would a scan tool be able to pick up this "ultra lean" condition by monitoring the O2 sensors for a perpetual lean signal with no flip flop between rich and lean during idle? And, as I originally posted, will these engines actually run at 10-15 PSI fuel pressure?

And finally ......... before I forget ....... Thanks! to all who replied.

 

The PCM can add up to 25% extra fuel before the PCM will set a code for the lean condition. Prior to that, you will just see the long term fuel corrections headed north, from the "normal" 128 (0%) to the maximum allowable value of 160 (+25%).

If you base the calculations on the stock injector rating of 24.9 #/HR, and a pressure of 43.5psi, you will get:

At 28psi, the injector flow would only be 20 #/HR, requiring the PCM to add 25% to the pulse width to get the required fuel into the engine. Any pressure higher than that might not trigger the lean code.

At 15psi, the injector would only flow 14.6 #/HR. The PCM would now have the long term corrections max'd out, you would have a lean code, and the short term corrections would be adding the additional fuel. The fuel atomization and spray pattern would probably be so poor the engine would be struggling to run.

At 10psi, the injector will only flow 11.9 #/HR. Probably wouldn't run - just a guess though.

 

I'm chasing a few codes on my WS6 (Injuneer has been a big help) and in trouble shooting i had to check the fuel pressure at the rail today also. I didn't have a fuel presure guage and tried to borrow my brother's (new years party) so i was able to rent one from autozone. Its $150 upfront, but you get it back when you return it. Its the professional kit, with all the extra fittings (we don't need) and a volt meter too. Just another option if you think your guage is questionable or want a second opinion.

Good luck

 

There's no substitute for full-time monitoring of the fuel pressure.... and other vital signs.



58psi fuel pressure, no vacuum compensation, and 78 #/HR low impedance injectors.

 

Injuneer(Fred) as always you are a wealth of information on all things LT1 related. You may have missed it in my car signiture in my original post but ...... I am running 30#/Hr. Ford SVO (i.e. the "red top") injectors, soooooooo ....... based on an "upwards correction" of your calculations using the stock 24.9#/Hr. injector rating ........ I'll assume the car would indeed be capable of idleing at 15 PSI fuel pressure ..... as it's definately doing, without setting a "lean code". Also, my scan tool reveals that my long term fuel correction is currently 148 (still short of the PCM's max out rate of 160, but definately "north" of 128), and both my primary O2 sensors (right and left side), are outputting less than 100 mv continueously at closed loop (i.e. hot) idle.

Thanks again to all who replied, and finally ...... do any of you know where to get a good deal on a 255/lph replacement fuel pump?

 

Try ebay, I got a Walbro for around 80.

 

Why does it matter? 15psi may run but isnt ok, fix the problem.

 

I just noticed you're in Washington MI. I'm in Macomb Twp, just south of you. If you need any help doing the trap door, shoot me a PM. My dad and I did the trap door on my car, his 1995 Firebird Formula, and a buddy's car from my website. Definitely recommend going that route rather than dropping the tank. Just imagine how frustrating it'd be if you get it all back together just to find out upon firing the car up that something isn't right. A lot easier to just unbolt a door once it's cut to get to the fuel pump again rather than having to drop the tank all over again. Plus once it's done and the carpet is put back no one will be able to tell.

I went with a Racetronix kit from Thunder Racing. Wasn't the cheapest but it comes with replacement wiring for the fuel pump harness. The Racetronix kit is nothing more than a Walbro pump and the replacement wiring.

 

If you choose to go to eBay, be very careful to avoid the Chinese knockoffs, they are of very poor quality. I went with the Racetronix kit, it comes with a hotwire harness that increases the voltage and current to the pump. It is very good quality and worth the extra $, IMHO.

 

Meissenation, thanks for the offer of help, and also for endorsing the "trap door" method as the way to go in replacing the pump, and ....... for giving me the "heads up" that the Racetronix kit does use the Walbro 255 lph pump. I definitely will be using the "trap door" method, and have already dropped several procedures for doing it off the Internet, but right now, working in my unheated garage with this weekends high temps predicted to be around 20F ...... doesn't sound tooooooo appealing to me. I'm now seeing my Z as basically being down for the months of January and/or February, (depending on weather), and doing the pump replacement when my garage gets somewhere warmer that 50F. If I do have a question or two about the replacement procedure, when I finally get around to doing it, ..... I'll PM you, Thanks!

Koolaid, thanks also for the warning about Chinese pumps on eBay. I've already found a California company on line that will sell me a genuine Walbro 255 lph pump for $99 shipped. Considering the difficulty in getting to the pump ..... I'll definitely put a brand new Walbro 255 lph pump back in there!

Thanks again to ALL who replied!