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02 Sonoma 4.3Won't Start ...Intake Inj?

08-30-2007, 07:23 PM
The engine cranks with no problem. Fuel pump is pumping, and there's pressure at the schrader valve behind the intake after each attempt. Engine will run for a second when I spray starting fluid into intake, so there's no spark problems. I've checked all the fuses and relays... Any ideas?

Thanks, John

08-30-2007, 08:31 PM
Fuel pressure is not monitored by the On Board Diagnostic (OBD) system and therefore cannot set a Diagnostic Trouble Code (DTC), or illuminate the Service Engine Soon (SES) light.

Fuel pump operation:
The fuel pump circuits are designed to energize the fuel pump through a relay continuously any time the key is in the START position. When the key is in the RUN position the fuel pump will operate provided the engine is running. If the engine stalls, or is not running for any reason, the PCM/VCM will shut off the fuel pump after approximately 2 to 3 seconds.

Fuel pressure test:
Install the fuel pressure gauge to the test port near the distributor. Turn the key to the RUN position but do not start the engine. While the fuel pump is running, fuel pressure must be 60-66psi. After approximately 2 to 3 seconds, the fuel pump will shut off. Fuel pressure must remain between 55-60psi for several minutes. A drop in pressure below 55psi indicates excessive leak down. Possible places to leak are the fuel pressure regulator, the fuel lines under the upper intake manifold, a fuel injector that is stuck open, a leaking fuel line or connector, or the check valve in the fuel pump assembly.

Here is a link to an inexpensive yet ample fuel pressure tester: Accurate diagnosis of an internal fuel leak requires additional test equipment. In some cases it requires the removal of the upper intake manifold.

08-30-2007, 09:21 PM
Thank you for the test procedure, but I am skeptical of it being the fuel pressure. The fuel pump assembly is new from 6 months ago when the truck kept getting harder to start (had to repressurize the line each cold start), and I get a good strong spurt from the schrader valve after every starting attempt when I've checked. (To be honest, I first removed the truck bed today thinking it was a bad fuel pump again, but I tested it w/ lines removed, and it SHOT gas out the outlet port. THEN, I thought fuel filter might be blocked, but I keep getting good pressure from the test valve under the hood. I admit to not having tested the pressure, but my instinct tells me that fuel delivery to the intake isn't it.)

The problem happened instantly, so I thought electrics might be the problem. I did disconnect the battery earlier in the day, but reconnected it only a few minutes later. Could the fuel computer have been zapped? I drove it once after reconnecting the battery, then I couldn't start it again. I wouldn't think the computer was zapped, because I wouldn't have been able to drive it even that ONE time.

Can the fuel injector (?throttle body injector?) be bad? is there another test for it?

08-31-2007, 09:03 AM
As Old Master said you need to check that fuel pressure first.
If you do not have full fuel pressure the injector will not squirt fuel.

08-31-2007, 10:06 AM
Ok, Master and MT, you were right! I've got myself a new fuel pressure gauge and it only reads 18psi at the test valve. So, this could be as simple as fuel filter replacement? Or fuel pump? Or pressure regulator?

Thanks again!


08-31-2007, 10:38 AM
Doubtful but possible. Your not gonna like this but your going to have to pull the inlet fuel line off and check the pressure there. If it's low at the inlet line before the intake then you have a prob between the fuel pump and the manifold. But if it's good there you have a leaking line inside the manifold, possibly a regulator or injecter line. Let us know what you find.

09-03-2007, 09:49 PM
Ok, It was the fuel pump. Thanks to everyone for their advice.

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