tblu92

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

bjankuski

What is the fuel pressure when cranking?

KyleF

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.

bjankuski

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)

KyleF

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.


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.

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.

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.

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.

bjankuski

OP, one more question, are there any codes stored or a check engine light?

tblu92

The ignition system is all new Plugs wires PLUS it has been changed to a MSD distributor
Fuel pressure will NOT hold Turning the key on without start it will spike to 40 PSI and after about 8 seconds will drop to "0"

tblu92

NO DTC's It will crank and crank with no start Then if I turn the key OFF and try a re start it will start easily makes no difference wether cold or warmed

tblu92

Fuel pump relay ? or is there also a relay for low oil pressure as well ?

KyleF

MSD is notoriously a problematic Opti Replacement.

KyleF

The prime should be enough to start the car.

The oil pressure switch to the fuel pump won't come on until your oil pressure goes high from running (normally unless you keep it cranking for an extended period of time)... this is not the primary way to power your pump.

Will the pressure hold if you turn the key on, let it come up to 40 and then turn the key off (no crank?).

bjankuski

Put a fuel pressure gauge on the car and watch it while it cranks and does not start and let us know what it reads. That will help use determine the area to look at.

KyleF

There ya go.

tblu92

Thanks !!!!

bjankuski

Does it stay at 40 PSI for the entire cranking cycle? You say it will not start on the first crank as long as you keep cranking but then starts immediately after you stop cranking and then start cranking for a second time. That description has me asking the question tof does the fuel pressure stay at 40 psi during the entire cranking time on the first no start situation?

What I want to know if the pressure stays up during the entire cranking duration. Engines need fuel and spark to run and this piece of information will help me determine if you have a fuel or spark issue. The pickup in the ignition tells the computer when the engine is rotating and if the fuel pressure stays at 40 PSI during the entire cranking period even with a no start it can eliminate the pickup as an issue. If the pressure falls after a few seconds of cranking that means we have to chase some additional items to determine what item or items have failed.

Brian

C5 Diag

Sounds like a bad crank sensor...that is what you'll see...good fuel pump prime then drops when starting..I'm seeing this quite a few times on the C5’s these last few months...after the 2 second prime without an ignition reference pulse from crank sensor the fuel pump relay will not stay energized (control side feed)...I'm certain the 94 works the same as my 01...I'll check later on my SI program !!..also a quick test is to see if you have Tach movement during cranking...you will not have injector pulse either...disconnect any injector and take 12 volt test light connected to battery POSITIVE...probe the control wire (not power wire which should be pink) and see if test light dimly blinks during cranking...checking when dark is easier to see !!...no crank sensor signal...no injector pulse !!...easier if you have a noid light !!

bjankuski

Bingo, we have a winner! That is why I have been asking for detailed information on exactually how the fuel pressure acts.

C5 Diag

LOL !!...OP said that fuel pressure drops during cranking...that’s all I need to know !!...Tach movement is actually the first thing I’ll check right from the drivers seat...helped a member a few months ago and it was a bad CAM sensor that dragged the 12 volt ref down and affected the crank 12 volt ref (they both share the same circuit) too... he was ready to order a PCM...YIKES !!

tblu92

Thanks so much for all your advice as it's been years sinse I worked on a LT1 I have taken all your advice and will report back once th car gets back from my mechanic---I gave him all the info i was given on the forum from you guys and I'll eventually see what the fix was--The info you guys gave me helped as it short cutted the diagnostic process THKS Tom

C5 Diag

LT-1, LS-1, 4 banger Honda Civic...it the same “diagnostic process” used for all !!