So I have a LT1 from a 93 Firebird swapped into my 92 Third gen. Its been runnning flawlessly for about 4 months now. Then out of the blue I noticed a loss of power when I was on the freeway. It seems whenever I get passed more then an 1/8 throttle the car stumbles then RPM's drop like its going to stall out completly unless you let your foot off the pedal. This is mostly happening under load, then rarely when out of gear.

Im suspecting the fuel pump, so I checked the fuel pressure and heres my readings.

Engine off cycle key 1 time= Fuel pressure jumps to about 38 then drops to 20psi and stays.

Engine Idling=Fuel pressure stays around 35psi

Engine Running and revving up and down=Fuel pressure stays around 38psi but the gauge seems to be very erratic betweein about 35-40psi.

Once again the engine runs very smooth as long as you stay under 1/8 throttle. Anything over that to WOT engine stumbles and wants to stall while under load.

Whats your Ides?

 

Your fuel pressure is right on, Have you checked the small things. Like a fuel filter. Also does the fuel pump prime when you turn the key?

 

From what I have read that Fuel pressure isnt right, Specially it dropping to 20psi worries me. I have not yet replaced the fuel filter, but the pump definatly primes.

 

I would check the fuel filter I had problem with a dirty fuel filter before it would start fine but on Expresway it would bogg down

 

Yeah Im going to switch that out, and also look into an adjustable FPR. Anyone got any advice for which brands to go with?

 

On my LT1 I got Aeromotive EFI FPR _AEI 13101

 

Aeromotive.

Also, just because you have pressure doesn't mean you have flow. Check the filter, (or just replace it) and if you still have problems then you need a fuel pressure gauge with a release port on it to check flow. If you open the valve (dumping fuel into a container) and the engine stays running then you have good flow. If the engine sputters and dies or runs rough then you need a new pump.

 

The drop to 20 concerns me. Could be bad injectors.

Check for a vacuum leak.

 

You can check for leaky injectors by taking the fuel rails off of the intake, so the fuel injectors are off of the intake but still attached to the fuel rails, leave the fuel lines hooked up, then prime the pump by turning the key (but don't try to start the car), then see if any leaky injectors leak out any gas.

If the psi drop wasn't caused by something faulty, it could be the drop to 20psi could be a loose gauge connection or maybe you moved the gauge around. Or maybe it was the first time in a while that fuel was sent through the lines. Anyhow, it does seem weird. All the other fuel pressure readings described are normal.

The problem may not be fuel. It could be air or spark related also. Something as simple as the map sensor connector - the tabs for the connector are known to break off causing the connector to not make a good connection which leads to bad readings being fed to the pcm.

There is also a kind of pressure relief valve for the gas tank. It is attached to a hose that goes to the sending unit on top the tank and located near the rear axle brake line connection. A new valve can be purchased from GM, part 10033125. If the fuel tank builds up too much pressure, the fuel pump has to work harder. I've had to replace them in both my thirdgens. Symptom I noticed is when its 100 degrees outside and you take off the gas cap, you hear a hissing sound for about 20 seconds. Another symptom is that you will hear the fuel pump whine or get a lot louder when that pressure builds up. And yet another symptom is the fuel pump will die or not work as good and the lower the fuel line pressure intermittently. I have capped off the line from the tank that goes to the valve (because I didn't think it was needed at the time) and this lead to burning up a new Walbro fuel pump in under 200 miles.

 

Well Today I was going too diagnose this thing, but has started raining. Its weird because this thing has been running great, daily driven. Then one day the bogging and cutting out started.

 

So I just replaced the fuel filter, it might have helped somewhat. It doesnt completly cutout anymore just sputters.

Fuel pressure
38psi then drops too 20 psi with engine off and key cycled(no matter how many times is is cycled)

36-38psi with engine idleing and it doest change when the vacuum source is pulled off the regulator.

Also, I can open up the valve on my pressure gauge and release fuel, I did this while it was Idleing and it didnt make a difference to the way it was running, So flow is good while its not under load or high rpm.

Now specs for the lt1 are

42-43psi with idleing

47-48psi with vacuum source removed from regulator.

 

Sounds like you need a new pump. It doesn't take much of a drop in pressure to make things run screwy. Chevy trucks from the 90's had a common problem with fuel pumps that would cause all sorts of wierd things that might make you think it was something else... like distributor/ignition problems, when it was the pump all along.

 

The pump was replaced when I did the swap not too long ago, it is possable that it is going bad. Most of the time a pump either works or doesnt. Hopefully it isnt the pump.

Im thinking its the regulator. For one the pressure drops from 38 too 20 when I cycle the key, as if the regulator was open and letting fuel return too the tank. And for two is doesnt bump up the pressure when I pull away the vacuum source.

I just ordered a new one that is adjustable and will let you know when I replace it.

 

sounds good.

 

You may also want to check the coil, my dad was having a similar problem on his '93 that he had, so he checked the coil, and sure enough it was bad...

Just a suggestion.

 

UPDATE!

Thanks for all of the replies, Im definatly not ruling out and of the ignition from being the problem. But there is no doubt that my fuel pressure is about 10lbs to low, So after I fix this problem and there is still issues I will go into ignition.

So I purchased an adjustable FPR from Aeromotive. Very nice looking part, Install wasnt perfect but it worked out. After Installing it and cycling the key checking for leaks everything was going well. But my fuel pressure has not changed! I can max out the adjustment on the FPR too increase or decrease pressure and the fuel pressure stays the same right between 35psi-38psi. There is no difference in pressure when vacuum is applied or removed from the FPR.

Once again, fuel pressure goes too about 38psi then drops too 20psi when key is cylced

its about 36-37psi when idleing and stays about that no matter what rpm or with the vacuum hose disconnected.

Im gonna say its the pump, knowing I got it off ebay and it wasnt a specific brand. What are your guys' thoughts?

 

Probably the pump but test for leaky injectors first since thats a lot easier.

 

Are your injectors "high pressure?" If so, you may want a fuel pump that moves more fuel.

 

When I did my regulator I had to pulled the rails out, and I didnt notice any leaking. I wish my FPR was in the position of the 94-97 LT1's. The 93 is a real pain!

 

WHAT? no seriously....>WHAT!?!?

 

Idk what he was trying to get across there.

 

Sorry. What I'm trying to say is, maybe, if you have more powerful injectors, you may need a more powerful fuel pump.

 

Bigger flow injectors require bigger pumps, yes. But not an issue in this case. He's not close to getting to the limits of the stock pump.

 

This problem doesnt sound like it involves the injectors, especially "more powerful" ones. Please review the below so I dont have to retype it.

 

it's probably a good thing you used the "cutting an access panel" method so the replacement can be done relatively quickly. You did cut an access hatch, didn't you?

 

I didnt cut a whole in my car, no way. So I solved my fuel problem, I have good pressure now but it is still bogging. Im going to replace the coil and if that doesnt do it Im going to buy a new OPTI

 

What did you figure out was the cause of your fuel problems?

 

The little rubber hose connection between the pump itself and the hardline inside the tank had come lose, it was not completly off but I could tell that fuel was leaking right there and just going back into the tank. I replaced the pump anyway with a walbro 255.

But Im still getting this misfire, it is kind of intermitant. I noticed it will sometimes go away when it is really cold outside. Ill put my food to the floor, then at about 3500 rpms and anything above that it starts misfiring and cutting out. I have disconnected the TPS sensor and then drove around, still same problem. Then disconnect the MAP sensor and still get the same problem. Most likely the opti.

 

agreed.... sounds like opti to me

 

So I have replaced the opti-spark with the summit racing brand. Im still getting some misfiring, also the idle is high and really random. Next im going with injectors, and if that doesnt do anything Im starting from scratch and buying a new LT1.

 

first mistake is buying the summit opti....GARBAGE...i personally have had two LT1 4th gens....both needed opti's....best bet is the GMPP one. i've heard of people having luck with the Chandler Motor Sports one which is a remanned GM opti. a faulty opti will throw a code....it did on my 94 Z28 and my 95 Formula.....right before it stopped working all together

 

Thanks for your opinoin on part brands? Anyway you got any insight on my problem?

 

ask Chandler to put that in writing. I do not believe you will have luck getting them to say that.

 

yup

first, do not even think of tossing what you have. The LT1 is a stout engine.

It's problems like this that drive all of us crazy.

First, you need to address the idle. That could be an IAC, TPS or simple adjustment. Has anyone ever messed with the idle screw? See if turning it down brings down the idle speed. You also need to check the voltages. It's simple with an ALDL cable and logging software. You then see what the computer sees.

I only know about clearances of an LT1 in a fourth gen (and I have long tubes) but inspect/clean/replace your plugs.

Do any look like you've been running rich?

If the idle has been addressed, and the plugs addressed, and it still backfires etc then I have a question. Does it do it all of the time? If you are cruising along is it hitting on all 8? If you punch it, does it randomly stumble and at times backfire? If so, the logging software is probably showing that you are running rich and your O2's are probably bad.

Yes, the opti can be a culprit as well but you need to be methodical and cover the basics. Oh, I've used a Summit opti with no problems until my NAPA waterpump decided to have one of the press fit pipes come off and spray coolant everywhere.......buh bye opti.

I then went through 3, yes 3 brand spanking new GM optis until I received a good one.

 

It runs awesome, untill you hit WOT. The idle issue I can take care of, it just started happening. I have disconnected one sensor at a time, and driven just to see if it made any difference. Did this with the IAC, TPS, MAP. Nothing changed. I have not wired up my ALDL because its a swap and the wiring isnt perfect. This all started happening after getting gas a shell station... Im suspicious

 

So, I replaced the injectors, and not suprised no difference. It also runs alot better when the air outside is really cold. But if its warm out its a dead miss. You put the throttle to the floor, there is a hesitation then rpm's climb with a dead miss.

I got around to doing a compression check, I only got 6 down but it was enough to make me believe it needs a rebuild. There is a difference of about 32psi between some cly. The highest being 180 and the lowest in high 140psi range.

I raced a swapped civic with a dead miss and still pulled about 4 car lengths, this was starting in 3rd from a roll and I backed off by the end of 3rd. Closed course of coarse haha.

Anyway I will be doing a rebuild, with the gm hot cam kit, larger throttly body, bolt ons and tune. People are putting down 350whp and 360 ft. lbs down to the wheels as well. Id would be completly happy with these numbers.

So it looks like the misfire is due too compression issues.