Fuel pressures under load???

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EVILSPOTTER

EVILSPOTTER

10 Year 15 Year
Joined
Oct 4, 2007
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8
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1993 22re w56 single cab 4WD HILUX. 4 burned exhaust valves, piston & bore damage.
Replaced with one of my personal spares, long block consists of: seasoned short block .20 over with mahle flat tops & rotating assy balanced by my local machinist, new aftermarket head complete with stock sized valves & springs + EB 268 cam, rock auto timing cover + water & oil pumps, good used oil pan & valve cover etc. No surprises, nothing special.
Install manifolds & accessories from core, drop in spare. No surprises nothing special.
Prime with fuel, start, set timing. Check for leaks. Cool down. Restart, warm up, check for leaks, check timing. Test operate: finding, low power above 2500 RPM, return to shop. Cool down. Install SST in place of cold start injector. Perform pressure test per FSM.
FSMP.webp
 
Finding: up to 2500 rpm will dither between 25-35 psi, under load 15 seconds of WOT will cause pressures below 25 psi. Under heavy load 20 seconds of WOT lowered fuel pressure to 15 psi.
Once engine speed was brought below 2500 RPM dithering would resume between 25-35 psi. At 1500 RPM or less rough throttle movement could produce a snap down in pressure reading, presumably a strong vacuum signal causing a proportional pressure change. Observed that recovery of lost pressure takes much longer than shedding pressure.
 
Finally on to my question:

All pressure testing according to the FSM occurs in the shop, at key on or idle condition. The pressures that we are given as test parameters must only apply in these static conditions. FSM is law. FSM is truth.

Have any of you guys observed fuel pressures under load? I should perform the same testing on one of our 22re's, but none of them are here right now. I have an FZJ80 to try, I believe that it has a similar fuel pressure regulator, but I think comparing those results would amount to apples & oranges.

Am I out of line to think that if I am supposed to have 38 to 44 psi to idle, I should at least get that much to go down the road?
 
I had to back burner it until Thursday, I need to do a couple of paying jobs first. If I have time I'd really like to go get Jim's 4runner & see what kind of pressures it generates under the same conditions. The problem is that up until this point it just hadn't occurred to me to take these measurements on a truck that is running well. As it turns out the only 2xr's here that aren't partially disassembled are all carbureted.
 
For future reference:
Under load the fuel pressure should hit the set point of the regulator & stay within a few psi no matter what the load or engine rpm. At speed the fuel pressure also should react to manifold pressure. @ change from cruising to WOT intake plenum pressure goes up & fuel pressure goes up. Rapid throttle movement produces rapid fuel pressure change (44to48psi), smooth throttle movement produces smooth fuel pressure change (same 44to48psi). Closing the throttle blade causes the inverse pressure change.

Note:
Before this test run I had changed the fuel pump & filter, after this test run I thought that 48 & an occasional reading as high as 50 might be too high so I changed the fuel pressure regulator & repeated the test. This last test run produced the same results almost exactly.
Evidently the fuel pump was too weak to perform properly, but just strong enough to pass when the engine was idling.
 
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