94 LT1 6MT. Accelerated in 3rd with about 50% throttle and the motor started missing and the car was bucking. Went to neutral, gave it a few revs, seemed fine, put it in 4th and it died when I hit the gas again. Was able to restart as it coasted, but it would die as soon as I hit the gas. Pull onto a side road, and was not able to restart it. Left it, figured the fuel pump was dead. Was able to limp it home later between two five minute drives which would end with it dying again. This was over the weekend.
Tested the pressure of the fuel line today, was at about 35-38 on idle. I let it idle for about 10 minutes, revving it to 2500 occasionally, and to my surprise it didn't die. Drove around my neighborhood, and it started missing again around 2500 rpm. It'll rev to 3500-4000 in neutral without missing.

So! Is 35-38 PSI low enough to indicate bad fuel pump? I've read it should be at 40-42.
Also, I was at 3 bars of gas when this started. Is that low enough to start some kind of issue?

Any other ideas/things to check?

 

Dare I say....optispark ...?

 

Any chance you can test it at WOT? I prefer to see what it does under load since it can be supplying all you need at idle but not at WOT under load.

 

When the Fuel level is low (1-2 bar) the car is practically out of gas.

my experience is the car wont rev over a certain rpm and then, as you drain the tank further, it wont rev over a certain rpm (even lower than before). bucking, rough running is common as its running out of fuel.


if you told me you had a high fuel level and had this, i would say for sure its an optispark problem


so repeat this experiment at higher fuel level and report back

 

IDK. I have done WOT runs at less with no sign of pressure loss. If he has that issue, the pulsator may be leaking. I always fill up at empty which is one bar or sometimes less. I maybe his float is not at the same angle as mine.

 

Gosh I hope not, I had that replaced November '18.

So I filled the tank on the way back to the house. I did the pressure testing and driving around my neighborhood with a full tank.
It seems to be running better now with a full tank than when it stranded me when it was nearly empty.
But its for sure not back to normal.

 

I think we're looking at the pulsator. Not sure if you have tried to WOT it and do pressure reading.

 

Fuel filter maybe! Has this been changed? Scan it with the snap on (Brick) . Original Opti? My car ran like that with a Opti that filled a shot glass with rust and you could not even spin the water pump shaft with pliers! Once caught in a rain storm at 15,000 miles. These opti are not sealed on the circumstance of the device, run a bead of marine sealant not window caulk and it will give life to it.

 

Ok, I thought it'd be easiest to make a video for y'all to see whats happening. Here's the link:
I think it's interesting how the tach bounces when it starts to die. Is there something to that?
And could this be a warm up/time based issue? It would rev well in the garage, but as soon as I get it on the road under load, it dies. And once it dies, it doesn't even want to rev in neutral.

and @fake , the fuel filter was replaced when the Opti was done last year.

 

Why would the tach not bounce as it is dying? I am leaning towards a fuel supply issue. I would test it this way. Turn the key on but don't crank. See what pressure we have and how long it holds after you turn the key off. We don't know what the pressure is at this point as far as whether the regulator holds or it can get up to pressure. After which, I would check what it is at WOT. That tells us if the pump can supply the pressure under load where it uses more fuel than you revving the motor.

 

Sounds like a fuel delivery issue to me too. It should be 41-42 psi (very narrow range for the LTx motors). If fuel delivery checks out, then spark plugs are the easy next thing to check. But, again....the symptoms scream FUEL DELIVERY to me!

 

Also you can disconnect the fuel pressure regulator hose on the L98. Not sure about the LT motors to simulate WOT conditions BUT that doesn't give you the picture under full load of the engine running.

 

@aklim Ok, I did what you said (essentially a leak down test) and here are the results.
Turn the key to on, 46 PSI as pump runs, 42 once it stops. Then, after
2 min- 40
5 min- 37
10 min- 34
15 min- 32
20 min- 31

Then I drove it, NOT letting it warm up before. Drove a couple miles in my neighborhood, and started doing WOT pulls in 1, 2, 3 gear. Made a quick video again cause I want y'all to see whats happening. It definitely gets worse with time and under load.
@Paul Workman , the PSI is not staying in the range you described. It seems to be going too high on WOT and too low on idle after.

So, the fuel pressure regulator under the hood seems ok. Does not smell like gas, and does not appear to be leaking. So I'm thinking I should just replace to whole fuel pump/pulsator/sending unit in the tank.

 

According to my Corvette Fuel Injection source by Proobst,, the fuel pressure range for the LTx motors is very narrow - in the 41-42 psi range, whereas the L98s, LSx, and others, the fuel pressure ranges considerably according to throttle position.

That said, I defer this question to someone like Mathew who has a handle on the specific LTx motors behavior.

Far as your leak down test results go, normally the pressure should remain within a couple pounds for 15 minutes or so as long as a component in the delivery system is not leaking, including the injectors, fuel lines and connections from the tank to the fuel rail and regulator, and the check valve in the pump. The readings you got indicate something is seeping a little. However, it doesn't appear to be anything that would cause a catastrophic failure (e.g., affecting all cylinders) like you're experiencing. There may be a hint of "coming attractions" foretold by your leak-down test, but FLOODING is not what you've described. Instead, fuel STARVATION is more in line with what you've described. But, have you examined the plugs for signs of flooding or leaning?

Taping a fuel pressure gauge to the windshield and taking the car for a drive while you (or a passenger) monitors the pressure under various % of throttle and RPM will show if you have a delivery issue or not.(LT5 test shown, refer to FSM for fuel rail Schrader valve location on the LTx motor.)


If FUEL does not appear to be the primary issue, then the SPARK issue comes into view, especially if, as you described, it occurs after the motor has warmed up. "AFTER IT WARMS UP..." is a classic symptom of electrical failures - coils especially. That needs to be verified next.

 

@Paul Workman I dont know if you watched the video I posted before, but watching the pressure gauge and filming it as I drove is what I did. If what you say about narrow pressure range is true, then it would be a fuel issue cause it varies by 10 psi while driving. But if you say you're not an expert and someone else is, I'd like to hear from them.
I don't want to just throw parts at this, but I'm a bit anxious to have it fixed so I'm about at that point.

 

I've never done the fuel pressure tests on my 95 LT1 when I had it. So, I'm referring to what has been an excellent source for all things related to fuel and Corvettes (except C7&C8).. The chart below is from that source "Corvette Fuel Injection" by Charles Probst. And, in the text on the LT motors it was mentioned that the fuel pressure regulation is maintained in a very narrow range on these motors. Your experience is different, so I'd be tempted to "run that one to ground" first.

For what it is worth, I have considerably more experience with the LT5; fuel pressure in particular. And, my measurements range slightly more, primarily pushing the envelope downward on the low side by a few pounds. What the heck does that have to do with YOUR motor? Nothing, except to point out that my own data agrees with that shown in the chart: relivance/verification of the source - is my point.

Also, beware that others might post their anecdotal information based on the L98. Well, as you can see the pressure range is different for the L98. So, I'd be interested in anecdotal information from someone(s) with the same motor as yours. (Mathew Miller is a mechanic and one that I often find as a reliable source.)

Anyway.... Take it (my book info) for what it is worth. Just info, usually reliable, but not verified by actual tests. I'd want to verify the source info first with same motor data from other motor or two. But, that said, if you dynamic pressure drops to under 40psi, e.g., into the low or mid 30s, then book be damned...that is a problem!!

Wish I could be of more specific help.. Good luck. But, before I break off, IF you experience low pressure, you might want to change you fuel filter. I changed mine at around 40k and it was FULL of crap! Changing it resulted in restoring pressure, primarily at WOT.

 

Ok, thank you @Paul Workman !

 

Ordered a new fuel pump/sending unit from Corvette Mods. I'll update when its installed.

 

Word to the wise- Check codes before throwing parts at your car! Car still didn't run after new fuel pump, only then did I realize I should check some codes.

Found that I had the killer codes for Opti. Replaced that and its good as new.

So thanks all for your help, but I led us on a wild goose chase.