m60 fuel pressure issues

[JGood]

Some of you may know, I've been battling some issues with my m60 swap for a year now. Hesitation/backfiring when getting on the gas after coasting/engine braking, along with stalling when leaving off the gas below 1500 RPM's. Everything has been replaced from top to bottom. Fuel system, ignition system, every sensor, gasket, nut, bolt, DME, harness, etc....

Anyway, I came back to visit fuel pressure testing again tonight, a little more thoroughly this time. I hooked up a pressure gauge in line with the feed line to the fuel rail, immediately AFTER the fuel filter and pump.

Per Bentley, the spec is 51 PSI +/- .9 PSI.

I'm using a Harbor Freight pressure gauge.

With the engine running at idle, I'm sitting at 49psi. While driving, I see anywhere from 45 psi (decelerating, engine braking) to 55psi spikes when stabbing the throttle. But generally returns to about 49 PSI.

If I disconnect the vacuum hose on the FPR, it jumps up to about 57 PSI. And throttle load doesn't seem to effect it very much. I drove around with that vacuum hose disconnected, and my symptoms persisted. They almost don't seem effected by fuel pressure.


Moving on to leakage testing. If I shut the engine off, pressure immediately drops to 45, and falls fast. 1 PSI every 2-3 seconds.

If I clamp the return line after the pressure regulator, it still drops at the same rate.

If I clamp the line right before the fuel rail, it only drops 1 PSI every 30 seconds or so.


This would tell me that the injectors are leaking, correct? However, my symptoms existed with the original injectors that came with the engine, exactly how it is now with my rebuilt Bosch Design III injectors that are installed now.

I'm a bit confused as to what this all means...

FYI: The fuel pump, filter, FPR, injectors, hoses, etc... are all new. The symptoms existed before and after their replacement.

[dude8383]

Could there be a problem with your fuel rails some place? I ask because you did not mention replacing that.

[JGood]

Could there be a problem with your fuel rails some place? I ask because you did not mention replacing that.

I'm not sure, but it's just a metal tube, so I doubt it. Unless maybe it's something with the FPR seat.

http://www.prussianmotors.com/Empower/Pictures/inv_002326_01.jpg

[jlevie]

My first observation would be that the FPR is bad because it allows the pressure to exceed 51.9psi, assuming the gauge is accurate (which isn't a given). Otherwise the FPR is behaving normally. When the vacuum in the intake increases, like on a closed throttle, the FPR should dial back the pressure, which it does.

You left out one test, which is to see what the pressure does if you clamp off both the supply and return lines. That will tell if the pressure is bleeding off through the injectors.

[JGood]

My first observation would be that the FPR is bad because it allows the pressure to exceed 51.9psi, assuming the gauge is accurate (which isn't a given). Otherwise the FPR is behaving normally. When the vacuum in the intake increases, like on a closed throttle, the FPR should dial back the pressure, which it does.

You left out one test, which is to see what the pressure does if you clamp off both the supply and return lines. That will tell if the pressure is bleeding off through the injectors.

So clamp it off between the tank and the pump, and also between the FPR and the tank? In theory, I should lose zero pressure over any time period like this, correct?

I will borrow a gauge to verify actual pressure.

Thanks!

[Jean]

I wonder if jlevie meant clamp both lines going to/from the fuel rail?

[george graves]

I'm interested to see what the problem is....

[JGood]

I wonder if jlevie meant clamp both lines going to/from the fuel rail?

That's what I meant too :)

I just had to clamp the feed line back by the pump, because that's where my pressure gauge was T'd into (no room in the engine bay).

I did that test, and the pressure dropped very slowly. I attribute this to a leaky fitting on the gauge kit. It was dripping maybe once every 10 seconds, probably enough to account for the slow pressure drop. So I think my injectors are good.