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]

Originally Posted by 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.

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..."



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