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hard starting LT1 1994

 
Old 05-13-2019, 03:15 AM
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tblu92
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Default hard starting LT1 1994

I have a new toy a 1994 LT1
It has a long cranking start when cold and just somewhat better but not much when warmed
I checked the fuel pressure It spkes to about 40 PSI while cranking then immediately drops to about 10 PSI after a few seconds and will drop to 0 with no start
Could this be a bad fuel pressure regulator or a bad fuel filter ? I thought these fuel filters had a bulit in check valve to prevent the fuel from bleeding back into the tank
The car seems to have full power at WOT--at about 44 PSI
Can it be a faulty fuel pump ? or fuel pump check valve ? Also the fuel gauge does not work sending unit bad as well ??
PLEASE any help wou;d be appreciated as I am a LS guy-------THKS
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Old 05-13-2019, 05:54 AM
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What is the fuel pressure when cranking?
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Old 05-13-2019, 08:42 AM
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KyleF
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Originally Posted by bjankuski View Post
What is the fuel pressure when cranking?
Originally Posted by tblu92 View Post
I checked the fuel pressure It spkes to about 40 PSI while cranking then immediately drops to about 10 PSI after a few seconds and will drop to 0 with no start
The way the system works is the ecm kicks on the prime, then the fuel pump relay kicks in on start. There could be a few issues here. LS guy or not, the LT works like every other engine in the world... air, fuel, spark, then BANG!

1. I would start with the most basic item about fuel flow. When was the last time the fuel filter was changed? It's cheap and can cause problems if clogged. Not suggesting you throw parts at it, but changing the fuel filter regularly is a good maintenance item anyway. I would change it just to eliminate it and mark off a maintenance item.

2. Fuel pressure test: You are half way there, need to also do a leak down test. Turn key on, let it prime, don't crank, and monitor fuel pressure. Should hold that 40 and maybe only bleed off a pound or 2 over 30 minutes. If it falls off quickly, you have an issue with leaking injectors or a faulty fuel pressure regulator. Being that it's an issue starting and you are seeing at least 40 I would think more towards the injectors, but 40 is low psi on an LT1 with no vacuum.

3. Check voltage out of the fuel pump relay.

4. What do the plugs look like? When was the last time it had plugs and wires? Again, may not be your issue but it is a maintenance item that can cause hard starting, but should have no effect on fuel pressure.

Hope this helps getting you on the right path.
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Old 05-13-2019, 09:48 AM
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bjankuski
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My question is what is the fuel pressure during the entire cranking time frame? It is unclear in the original post, it says jumps to 40, then drops to 10 and finally to zero with no start. If the fuel pressure is holding at 40 PSI during the actual engine cranking the fuel pump, filter and regulator are not the issue for the no start and can be ruled out. That is why I want to clarify that point. The fuel pressure holds at 44 PSI at WOT so as far as i am concerned the pump, regulator and fuel filter can already be eliminated as reasons for the no start.

-It appears to me that either the amount of injected fuel is wrong or no spark is present, we need to chase those issues. (I want to verify correct operation of the fuel pump relay and computer while cranking)
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Old 05-13-2019, 12:28 PM
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Originally Posted by bjankuski View Post
My question is what is the fuel pressure during the entire cranking time frame? It is unclear in the original post, it says jumps to 40, then drops to 10 and finally to zero with no start.
Guess I am reading into this that as Key comes on, it primes to 40, cranks it, it bleeds off in a few seconds, goes to zero and doesn't start. That paints the entire cranking time frame to me.


Originally Posted by bjankuski View Post
If the fuel pressure is holding at 40 PSI during the actual engine cranking the fuel pump, filter and regulator are not the issue for the no start and can be ruled out.
Eh, maybe. However, the OP is saying the pressure doesn't hold while he cranks. It is gone in a few seconds. The GM spec is 41-47psi with the key on, engine off. So, he is setting a bit low initially, but not low enough to be too concerned about it. At idle, pressure should drop 3 to 10 psi and slowly increase as vacuum drops (throttle opens).

Checking voltages and replacing a filter of unknown age will never hurt and might fix the problem, but doubt it.

Originally Posted by bjankuski View Post
-It appears to me that either the amount of injected fuel is wrong
That is why I suggested a leak down test. One injector or multiple may be hanging open and flooding some cylinders while others have no pressure remaining when they are called to open. Could have flooded cylinders that won't fire and lean cylinders that won't fire.

Originally Posted by bjankuski View Post
or no spark is present
No spark has no relation to fuel pressure going away. While, it can't be stated for certain there is only one issue here, these should have two different root causes unless it's electrical. In which case I would assume then that the car should also fail to crank. Also, this is why I asked what the plugs look like. Can see a lot of issues about the running of an engine based off what the plugs look like.

Originally Posted by bjankuski View Post
we need to chase those issues. (I want to verify correct operation of the fuel pump relay and computer while cranking)
Voltage test... is voltage leaving the relay and is it reaching the pump.

My initial thought is that the relay is not sending power to the pump after prime. I would just try to verify a few things before just swapping the relay...but they are cheap if you want to just start there.


Big question to OP, if you crank and it fails to start. Do you crank it again and again, and again and it finally starts? If so, watch your oil pressure. Once you have enough, the oil pressure switch becomes a secondary switch to a power source to the fuel pump. As long as the oil pressure stays up, so does your fuel pump. If this is the case, it is most likely the relay.

Last edited by KyleF; 05-14-2019 at 08:45 AM.
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Old 05-13-2019, 02:30 PM
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bjankuski
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OP, one more question, are there any codes stored or a check engine light?
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