What is the expceted Fuel Rail Pressure when driving

[QB93Z]

I am trouple shooting a performance issue on my 1993 ZR-1. The car seems to idle OK but under load, there is a slight missfire or "pulsation" in the engine feel. During a full throttle run, I have gotten a DTC 55, which is fuel lean condition for more than two seconds.

The trouble shooting guide in the FSM says to investigate fuel pumps, puel pressure regulator fuel injectors and the fuel filter.

I did the fuel pump tests in the FSM. KOEO fuel pressure goes to ~ 50 psi and the drops off a little. This check is OK for both #1 and #2 pump.

I put a fuel pressure gauge where I can see it while driving. I get about 40 psi when driving.

Now my question is, what should the fuel rail pressure be when driving?

If fuel pressure is ~50 psi with the engine off, then when there is a vacuum in the plenum, does the Fuel Pressure Regulator lower the fuel pressure to maintain a constant differental pressure at the injector?

Am I over-thinking this?

Jim

[mike100]

Fuel pressure goes up when the vacuum on the regulator drops. I don't know about the LT5 system, but it is usually about 4 or 5 lbs difference hose on/off. Basically it provides an instantaneous mechanical enrichment when you step on it and manifold pressure approaches atmospheric.

[Aurora40]

Hey Jim, I generally saw >40psi, around 44-45. When hammering it, it goes up to >50.

Pressure with the engine off will be higher, because to the FPR, it is a "WOT" condition (no vacuum in the intake).

Vacuum on the FPR lowers the pressure. You could use a vacuum pump to simulate this if you want. Though the pressure when driving should really tell the story about whether fuel is being supplied properly.

I don't follow the constant pressure differential thing. The pressure goes up when vacuum goes down so that an injector can deliver small/precise fueling at low throttle, yet still pump out large quantities when the engine is sucking it down. The pressure is higher at high load so that a "smaller" injector can still deliver a higher amount of fuel.

Let me know if you want a hand with anything.

[Marc Haibeck]

Hi Jim,

For the LT5 the fuel pressure should be 43 psi +/- 1 psi at idle for and engine with normal manifold vacuum.

I think that the best way to diagnose fuel delivery is to observe the pressure at WOT. Attach a fuel pressure gauge to the windshield and recruit a co-pilot. Run the car at WOT from 3000 to 7000 rpm. The fuel pressure should snap from 43 to 51 psi +/- 1 psi.

If the fuel pressure drops to 47 psi or less you may observe reduced power, knocking and a lean fuel code if it persists for two seconds.

A common situation is a failed fuel pump. At WOT on a single pump the fuel pressure will be 51 psi until about 5500 rpm and then the pressure will sag to about 45 psi at peak power.

Best of luck.

Marc

[Paul Workman]

Hi Jim,

For the LT5 the fuel pressure should be 43 psi +/- 1 psi at idle for and engine with normal manifold vacuum.

I think that the best way to diagnose fuel delivery is to observe the pressure at WOT. Attach a fuel pressure gauge to the windshield and recruit a co-pilot. Run the car at WOT from 3000 to 7000 rpm. The fuel pressure should snap from 43 to 51 psi +/- 1 psi.


It can draw attention tho. I stopped next to a bar to re-tape the gauge to the windshield. Musta been 10 dudes surrounded me to say "WTF", soon as I opened the hood, followed by the (usual) "I never knew...":-D

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/FPtestLarge.jpg

But, while we're on the subject, Marc, where is that handy place to measure total FP current draw?

P.

[mike100]

^^^ btw, a failing dc electric fuel pump draws less current.

[QB93Z]

Thanks for all the responses!

Paul, the setup in your picture looks exactly like mine, I even used the blue tape.

I will try to get some testing time this afternoon and I will post an update.

Jim

[Demps]

Curious as to the 'extension' to the gauge. What/where did you get it?

Thanks, Ted.

[Marc Haibeck]

Paul,

The fuel pump test connector is a single wire connector with a red wire located near the wiper motor. On ?90 cars with a VIN less than about 500 the connector is near the right side of the battery. On some of those cars it is out of sight and very hard to find.

Connecting battery power to the connector drives both pumps. The OE pumps draw 5 amps each. 5 + 5 = 10 amps if both pumps are working normally.

[QB93Z]

Checks I did without an assistant to actually drive the car.

Fuel pressure at idle: 40psi

Fuel pressure at idle with the vacuum line to the FPR disconnected: 50 psi

Fuel pressure at idle while I used a vacuum pump to pull vacuum on the FPR: pressure drops from 50 psi to 40 psi.

So If I assume that the difference between my reading of 40 psi and Marc's comment that fuel pressure should be 43 psi may be gauge tolerance, I conclude that the Fuel Pressure Regulator is responding properly.

Next step, drive the car. So I am waiting for a ZR-1 assistant driver certified for WOT operation.

Jim

[tomtom72]

Jim if you decide that you want to try a set of new pumps I have two in the box. I could send them to you & you could see what if anything happens.

Just saying if it gets to the point that there are no definitive sign posts. The spares I have are Delphi pumps from that batch of fuel tank units they were selling awhile back.

:cheers:
Tom

[Paul Workman]

Paul,

The fuel pump test connector is a single wire connector with a red wire located near the wiper motor. On ?90 cars with a VIN less than about 500 the connector is near the right side of the battery. On some of those cars it is out of sight and very hard to find.

Connecting battery power to the connector drives both pumps. The OE pumps draw 5 amps each. 5 + 5 = 10 amps if both pumps are working normally.

Thanks very much, Marc!:notworthy

[Paul Workman]

Curious as to the 'extension' to the gauge. What/where did you get it?

Thanks, Ted.

Ted,

I'm pretty sure I got it at AutoZone. I'll be by there tomorrow anyway, and I'll see if I can find one there. I'll let you know.

P.

[QB93Z]

Curious as to the 'extension' to the gauge. What/where did you get it?

Thanks, Ted.

Hi Ted,

My fuel pressure rig looks similar to Paul's. I bought the basic fuel pressure pressure tester at AdvanceAuto and then cut the hose and added a couple of feet of fuel line. It took a couple of barb fittings and hose clamps.

The AdvanceAuto fuel pressure tester is nice because it has a pressure release valve built in at the gauge end so that you can relieve the fuel pressure before you unscrew the shrader fitting from the fuel rail. This greatly reduces the fuel sprayed on the back of the alternator. :-D

Jim

[QB93Z]

Jim if you decide that you want to try a set of new pumps I have two in the box. I could send them to you & you could see what if anything happens.

Just saying if it gets to the point that there are no definitive sign posts. The spares I have are Delphi pumps from that batch of fuel tank units they were selling awhile back.

:cheers:
Tom

:saluting:

Thanks for the generous offer Tom. I have a couple of spares on hand. The only reason I haven't just put them in is that I am curious to see if I can diagnose the problem instead of just doing wholesale part swapping. But if it comes down to that, I am ready to go. ;)

I am very sorry to hear that your Z got rear-ended. I know it must have been a very sickening feeling. I am sure you will get it fixed up good as new.

Jim

[QB93Z]

Trouble shooting update. Engine is running rough at all speeds and indicates lean on the left bank.

I replaced the left bank O2 sensor. The problem did not change.

Did some WOT runs with a fuel gauge in stalled. Fuel pressure tracked properly with manifold vacuum and at WOT fuel pressure was 51 psi.

I think that I have ruled out a fuel filter or fuel pump problem.

I still get DTC 55, Fuel Lean, on every WOT run.

At WOT, Left Bank O2 read 4 mvolt (and averaged < 200 mvolt) on a Tech 1A.

My next step is to check the plugs and plug wires and coil secondary circuit. Then I plan to pull the plenum and check the coils and replace the injectors with a low miles OE 1993 set that I have.

Any other thoughts or suggestions?

Thanks,

Jim

[sammy]

dont know if this is it but i had one side of a coil go out. and it was rough at idle and kinda smoothed out when i hot on it . gerry was the one that pointed me in that direction (thanx jerry(

[scottfab]

Was this problem ever ironed out? I was experiencing something similar. It was a fuel pump that would fail at WOT. How I found it was that it got progressively worse then finally failed once shortly after start-up.

I just received two new fuel pumps from Rockauto.com.
They were $36 ea.
Brands were Performance and part # P-240 KC.
They work fine. Pressure is back up to 55psi at start then
at 42psi at idle and
the pressure holds at 35psi when ignition turned off so
the check valve is holding.
The worn ones only managed 38psi at start up and dropped to 0
as soon as the ignition was turned off.
I think this problem is solved.
Time for a test drive. 70degs, sunny and no smog.
yippie


PS I always call in my CC when ordering from Rockauto.
Two bad experiences when entering CC # online.

[tomtom72]

Scott, I do believe that this turned out the be a pinched wire on one of the primary injectors. Jim figured that it got pinched upon a plenum re-install.

:cheers:
Tom

[QB93Z]

My problem turned out to be injector related, not the fuel system. One problem that complicated the trouble shooting was that the fuel pressure tester I was using has a problem with the fitted that attaches to the schrader valve on the fuel rail. When the fitting was screwed on tight, the pressure gauge would not display fuel pressure. Very annoying.

Jim

[tomtom72]

My problem turned out to be injector related, not the fuel system. One problem that complicated the trouble shooting was that the fuel pressure tester I was using has a problem with the fitted that attaches to the schrader valve on the fuel rail. When the fitting was screwed on tight, the pressure gauge would not display fuel pressure. Very annoying.

Jim

I have one of those models also....very annoying!

[scottfab]

My problem turned out to be injector related, not the fuel system. One problem that complicated the trouble shooting was that the fuel pressure tester I was using has a problem with the fitted that attaches to the schrader valve on the fuel rail. When the fitting was screwed on tight, the pressure gauge would not display fuel pressure. Very annoying.

Jim

Wow, makes sense for the code 55 with an injector problem. I suppose my attaching a fuel pump to this thread is dubious. Oh well, someone searching for "fuel pump" will hit on it. :dancing

[QB93Z]

Wow, makes sense for the code 55 with an injector problem. I suppose my attaching a fuel pump to this thread is dubious. Oh well, someone searching for "fuel pump" will hit on it. :dancing

The tech manual trouble shooting chart for a lean condition starts out with fuel system diagnosis, so I eliminated that first before I pulled the plenum to look at the injectors and wiring.

Jim

[Dream car 90]

I'm also showing the infamous code 55 , I replaced the fuel filter and checked fuel pressure for primary and secondary fuel pumps both read @ 50 psi , so next step is to remove plenum and tackle the injectors .