[goats]

Working on a Fuel Pressure (I think) problem with a friend. Recent rebuild of his motor and complete R/R of subframe as background. 24V LS car with Q runners.

FP measures barely 40psi at idle, and drops heavily under engine speed (30 ish PSI at 4500 /5K rpm). Car dies under real load at higher RPM (5K, nothing near limiter), seems to go lean (like not enough fuel-- throws 1223). No vacuum leaks that we can find, did brake cleaner trick everywhere and brand new intake hose. Tried new FP regulator, no go. Tried spare fuel pump assembly. R/R fuel filter. No Change. Can get to 58 PSI when clamping off return line, gets there quickly. Vacuum is there from air box. We;ve isolated the Fuel Pump by jumpering the relay -- no change. Fuel pump is getting 13 ish volts and the ground is solid to chassis (~ 1 ohm). I think we ruled out motronic by jumpering the relay and we still see the pressure drop/car go leaned out to the point where it dies (5K ish RPM).

I expect a reasonably steady fuel pressure even under higher RPM -- FP should not drop by 1/3 or 1/2 under higher RPM, in fact it should go higher per the manual (vaccum increases, FP regulator increases pressure). Don;t think its coil packs or other causes since the FP drops so much.

Im left with thinking its a clog somewhere in the supply line thus limiting the fuel delivery to the rail. As soon as engine demand (higher RPM) overloads supply (from the rail) the pressure has to drop and things lean out. At least thats the working hypothesis.

Few questions.
1. Would you expect FP to drop so much at higher RPM? To me this seems like the prima facie evidence for lean out and eventual fuel starvation.
2. Anyone seen anything like this at all? Before we tear out the fuel lines and try to find a blockage.....???
3. I am thinking that we have to solve the FP issue first before we do anything else.

TIA

 

[Gubi]

I expect a reasonably steady fuel pressure even under higher RPM -- FP should not drop by 1/3 or 1/2 under higher RPM, in fact it should go higher per the manual (vaccum increases, FP regulator increases pressure).

You've got it backwards. The FPR maintains a constant fuel pressure between the rail and the intake plenum. As the vacuum increases (plenum pressure drops), the fuel pressure in the rail should DROP.

With the vacuum hose to the FPR disconnected you should see constant fuel pressure of whatever the regulator spec is. With the hose connected you'll always see lower than this (e.g., if it's a 40PSI regulator and you're at 5PSI vacuum the fuel pressure should read 35PSI)

 

[goats]

Thanks Tom. Even so, would we expect a 10psi delta in the intake manifold (vacuum) that would result in a 10 PSI (or more) drop in rail fuel pressure? Maybe I;m overthinking this, but upon increased demand (more injector pulses/second) thus the fuel flow RATE should increase as well (to meet the increased demand rate) and since rate is not independently controlled, if pressure drops then so does rate.

 

[Gubi]

Yes, at 10 psi vaccuum fuel pressure will be 10 psi lower than regulator spec.

Fuel pressure is always kept constant relative to plenum pressure. Fuel volume is controlled by injector duration, not pressure.

 

[goats]

Hmm I am asking a slighly different question I think.

What is the normal operational range of plenum vacuum? What vacuum would one expect, relative to idle, at WOT?

In other words, if plenum is at 0 PSIG at idle, which corresponds to lets say 42 PSI fuel pressure, is it reasonable for plenum vacuum to go as high as -10PSI (which would drive FP to 32 PSI).

 

[Alfisto Steve]

What is fuel flow into a beer bottle with engine off and fuel pump relay sockets 87 to 30 jumpered and key on?

Have you all done a fuel pump pull and inspect?

 

[144]

This is from the Motronic 4.1 GM description of the fuel system. 2.5 bar = 36 psi

The pressure regulator is fitted on the outlet side of the fuel rail and maintains an even pressure of 2.5 bar at the injectors during idle conditions. The pressure regulator consists of two chambers separated by a diaphragm. The upper chamber contains a spring which exerts pressure upon the lower chamber and closes off the outlet diaphragm. Pressurized fuel flows into the lower chamber and this exerts pressure upon the diaphragm. Once the pressure exceeds 2.5 bar, the outlet diaphragm is opened and excess fuel flows back to the fuel tank via a return line.

A vacuum hose connects the upper chamber to the inlet manifold so that variations in the inlet manifold pressure will not affect the amount of fuel injected. This means that the pressure in the rail is always at a constant pressure above the pressure in the inlet manifold. The quantity of injected fuel thus depends solely on injector opening time, as determined by the ECU, and not on a variable fuel pressure.

At idle speed with the vacuum pipe disconnected, or with the engine stopped and the pump running, or at WOT the system fuel pressure will be approximately 2.5 bar. At idle speed (vacuum pipe connected), the fuel pressure will be approximately 0.5 bar under the system pressure.

 

[goats]

Ok 1 bar = 14.5 psi. The excerpt above says WOT = 2.5 bar or roughly 36 psi. I guess I need to get a vacuum gauge on the plenum along with the FP gauge to see exactly what is going on

 

[144]

The Alfa values are a little different than the GM ones, I think. This from the 12v Alfa Motronic manual.

Does the MIN/MAX switch test good? At full throttle operation, the MAX side of the switch is closed, the ECU commands a slightly richer mixture to prevent detonation at full power demand.

 

[goats]

Yep this is close to what is in the 24V book but 24V book doesnt show the 35-50 PSI during driving. As I was saying I think the extremes we see in this fuel system are extreme -- Spikes to <30 easily. But have to verify with FP jumpered -- just to try to rule out electrical stuff--
thanks will report back with more info as available

 

[Alfisto Steve]

The Alfa values are a little different than the GM ones, I think. This from the 12v Alfa Motronic manual.

Does the MIN/MAX switch test good? At full throttle operation, the MAX side of the switch is closed, the ECU commands a slightly richer mixture to prevent detonation at full power demand.

No min/max switch on 24v engines. It has a throttle position sensor (TPS).

It is a variable potentiometer. Now what effect it can have on fuel pressure or injector pulse width I don't know?

 

[Spitfire]

Ok 1 bar = 14.5 psi. The excerpt above says WOT = 2.5 bar or roughly 36 psi. I guess I need to get a vacuum gauge on the plenum along with the FP gauge to see exactly what is going on

In my experience with the ford, the vac gauge on the plenum may not tell you all that much with respect to fuel.

I always understood that you pull the vac line off the regulator to simulate WOT. Basically neutral manifold pressure. You can only recreate this for a fraction of a second in the garage before you take the engine to rev limit.

You'll have the gauge showing vacuum, and when you open the throttle blade full, the gauge will go to zero or close to it, but then you'll be revving the crap out of an unloaded engine. You'll have to close/modulate the throttle blade to control rpm and the gauge will be back into vac.

I've seen Steve's test for fuel flow utilized in other applications.

 

[Alfissimo Int.]

Is this Chases car? The one I had the pleasure of trying to figure out as well. I am wondering about that fuel rail valve that is located on the inlet of the fuel rail. Could that be clogged or causing an issue?

 

[goats]

Yep Chase's car. That's the pulse dampener and easy to remove and check I would think. Jason do you have a link to any discussion on the BB about this that we can go back and reference?

 

[Alfissimo Int.]

Yes, go up to my name and look through the posts I made about this car. I will check when I can. Problem is the car went to a shady mechanic after me and who knows what this moron did.

 

[Alfissimo Int.]

http://www.alfabb.com/bb/forums/164-168-1991-1995/162106-95-ls-problem-175f.html

 

[ChiapC]

Thanks for the link Jason, yup, it's my car/problem now! Thousands into a rebuild on the engine, only to have the exact same problem.

I'm fortunate enough to have Goats in my backyard for some expert help!

This is going to be interesting, for sure. I hope we can get this situated!

I already MAJORLY owe Goats.

 

[Alfissimo Int.]

Call me if you have questions, I had that car for awhile. Again I have no idea what happened after it left me and it went to the gorilla who may have screwed it up more.

 

[goats]

Uh - Oh -- now I remember this ---- what have I gotten myself into????

WE are going to FIX THIS CAR. You can bet on it.

 

[Alfissimo Int.]

Haha. You will figure it out. I was on the verge but the owner took it to some grease monkey who knew nothing about these cars. Good luck I think once you find the issue that it will be super helpful for all as I have seen this issue with others.