Mechanics

94 4.3L CPI VORTEC RUNNING RICH - SUSPECT INTERNAL FUEL LEAK

1994 GMC Sonoma • 6 cylinder 4WD Automatic • 230,000 miles

Truck: 94 GMC Sonoma SLE, 4X4, Ext cab, 4.3L "W" Engine with CPI, 230,000kms, doesn't burn oil, overall decent shape and never abused.
Symptoms:
1) for last 6 years it gave 20 MPG. Last fill up it dropped to 13MPG. Air filter was changed a couple months ago and is clean.
2) black soot from tail pipe and plugs fouling
3) dtc 45 (O2 sensor detects rich exhaust). Service Engine Soon light only comes on at highway speeds but goes off at slow speeds. I am guessing it only tests for this condition at high speed.
4) stalling and hard starting, requiring accelerator to be floored (to clear a flooded condition) before it will start again
5) once you baby the gas pedel to get it up over 1500rpm, it seems to run great (but with poor economy) but idles really rough and when warm, idle fluctuates (hunts) from 1500 to almost stalling to 1500.

What I did:
A) checked TPS - varialble resistance 7.18Kohms WOT and 1.94Kohms throttle closed - resistance changed smoothly. When I disconnected this sensor the engine would not start. Maybe it is required for starting/running. I think it is OK.
B) disconnected O2 sensor - idle didn't change (still hunted) indicating that removing the O2 sensor had no impact. The soot out the exhaust pipe indicated a rich mixture and so did the O2 sensor so I think this suggests the O2 is good. I think if the O2 sensor is disconnected, the computer should simply use a default map anyway. Since the idle was still hunting this rules out the O2 sensor.
B) Temp sensor: 800ohms when warm, 280ohms when warmer. Seemed to decrease resistance as engine kept running. I think it is OK.
C) PCV valve - when engine running and I put my thumb over it I can hear clicking. I think it is OK.
D) MAP sensor - disconnected, idle still hunted. When disconnected, I think the computer defaults to a default map.
E) I did not check the air temp sensor in the rubber intake tube that feeds the throttle chamber.
F) fuel pressure on rail is 52psi with key on. Slightly under spec (55-60 I think) but remember I am suffering a rich condition so I think it is OK. I have read a lot about leaking pressure regulators, nut kits (high pressure feed and return fuel lines) and spider wires/poppets so I took the penum off to have a look. This is what I found:
-Plenum base and cover on high pressure/return lines (drivers) side was dirty and had some fuel pooling in the bottom of the plenum. No clean surfaces anywhere.
-Plenum base and cover on the other (passenger) side was definately much much cleaner and seemed to have a bit more fuel pooling that then drivers side. This side is a lot cleaner likely due to the excess fuel washing down that side of the plenum.
-plenum gasket (between plenum top and bottom) had a break in the middle of the plenum between 2 of the front venture tubes that lead to the intake. Though we don't want gaskets to be compromised ever, in this case it should not impact things to strongly since the vacuum would not be altered (it was an internal leak, not a plenum exterior leak).
Turning ignition key on did not reveal any leaks in "nuts" or injector or regulator.
Cranking engine did not show any spider or poppet leaks (I had the igntion coil connected with high voltage line grounded since I read this is required to make the injector cycle during cranking).

Question: with no visible leaks in the "nuts", regulator, injector, spider legs or poppets, is there any other possible explaination for the fuel collecting in the base of the plenum?
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Mrman
December 31, 2010.



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