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Old 12-29-2006, 08:54 PM
Bill Wiser Bill Wiser is offline
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1990 chevy truck hard to start

hey I have a 1990 chevy truck with a 350 auto when it is cold or has been sitiing for a while you have to crank it for about 1 minute before it starts , what i have noticed is as soon as the oil pressure gauge rises it sends fuel to the throttle body and it starts and runs fine even when you shut it off and try to restart unless it sets for about a hour or so then it is the same thing all over again. I have changed the oil pressure sending unit and fuel filter no change, and it runs fine after it starts , thanks for the help Bill
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Old 12-29-2006, 09:30 PM
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justen justen is offline
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Get a fuel pressure gauge on it, pump maybe be weak.
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Old 12-30-2006, 11:39 AM
Bill Wiser Bill Wiser is offline
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pressure

it runs fine it is like when the computer registers oil pressure it allows the injector to open for fuel and not until that happens, at exact time the oil gauge rises fuel will come out of the injector
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Old 12-30-2006, 11:50 AM
dougbfresh dougbfresh is offline
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That's how it suppose to work BUT there is a start mode where the pump runs for several seconds when the key is turned on. When you turn the key ON (not starting), can you hear the pump run?
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Old 12-30-2006, 06:03 PM
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On a cold start a the engine requires in your case at least 45-50 psi. Once the engine runs, it only takes 27 psi. If your trying to start a engine with 27 psi. Your going to have a hard starting situation. We call that a weak fuel pump at work. Just because the pump works some what doesn't mean it's good.

It takes me about 3-4 hours for a truck like yours to change the fuel pump, between lowering the gas tank disconnecting the hoses and remove the fuel pump lock ring. Clean up. Assemble and reconnect everything. It will probably take you longer, I usually do this type of job once every two weeks. And it's easier if you have a lift. But can also be done on the ground.

Your best friend right now is get a fuel gauge on it. And see what your psi is. When cold.


The computer is not connected to the oil pressure gauge, that's a separate unit. There is one wire that runs from the oil pressure sending unit to the gauge. Oil sending unit wasn't integrated into the computer until 1996 when emission were required and stiffened.

Whats happening is, When you sit and wait and watch your oil gauge move, your giving the fuel pump enough time to build up enough pressure, Not a lot. But enough for a hard start. A good fuel pump takes 2 seconds to pressure up. If it takes longer then that. It's a bad fuel pump.

My little blazer samething My oil gauge goes bouncing around from 100 psi to 20 psi. It needs a new gauge. But also it has a hard start problem. I have put a fuel pressure gauge on it and only getting 55 psi. That's way below it's minimum requirements. So I'll have to drop the gas tank one of these days at home and put a new fuel pump, and new fuel filter in and also replace the faulty oil pressure gauge. As of now it will not start unless I leave the key on for at least 10 seconds. Then it starts but a few cranks when it should only crank over 4 times at most. It's a 1996, It's due for a fuel pump. Fuel pumps are usually replaced in 6 years. So it has last well over it's predicted life span.
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Last edited by justen; 12-30-2006 at 07:26 PM.
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Old 01-01-2007, 10:49 PM
m1k3sthemannn m1k3sthemannn is offline
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The oil pressure start indicates your fuel pump relay is dead, although obviously not the only potential culprit.

GM used an oil pressure switch (either integrated with the oil pressure sender, so 3 wires, or separate of the oil pressure gauge sender, two wires) to allow for engine starts with the relay malfunctioning. You can test this with a (typical) empty wire lead hanging off the relay connected to 12 volts, or off of the ALDL connector.

Switch closing is right around 5PSI.

One minute is a bit much for enough pressure to be generated by the starter, so you might be dealing with something else, or something in conjunction to the fuel pump relay. Just test relay operation. Easy and 100% yes or no answer.

Last edited by m1k3sthemannn; 01-01-2007 at 10:50 PM.
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Old 01-01-2007, 10:58 PM
danny danny is offline
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doesn't the fuel pump relay cause the fuel pump to work along with starting?
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Old 01-01-2007, 11:53 PM
dougbfresh dougbfresh is offline
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There is a 2-5 second startup mode where the fuel pump is energized to start, if oil pressure does not come up (i.e. the engine does not start) in that time, the pump is cut off. He needs to determine if the pump energizes KEY ON for several seconds on a cold start.
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Old 01-01-2007, 11:59 PM
danny danny is offline
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the start up mode is controlled through what?
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Old 01-02-2007, 10:47 AM
dougbfresh dougbfresh is offline
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Quote:
Originally Posted by danny
the start up mode is controlled through what?
I seem to recall the ECM does, it looks for KEY ON and a crank signal but they may have used a different scheme 17 years ago. Need to find the shop manual for that truck to be sure but I'm fairly confident that his problem is the pump is not priming correctly. The pressure test would tell is a heartbeat, with the key ON ONLY, the pressure should come up, then the pump should de-energize in several seconds if not started. Many GM's had a diagnostic pigtail in the engine bay to send +12 directly to the pump to test it, it this does, energizing the pump for a second or 2 and then trying to start it would also help tracking this down. If it starts easily after the pump has been primed that would really point to this being the problem.
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