Guys, thanks for the site, fairly new to Chevys, learning a lot.
Getting ready to do a trap door and buy a pump, would love to hear opinions from experienced guys first...

3 months ago I bought a neglected but clean 1995 Z28, 65k miles, been driving it and catching up on maintenance. fluids/filters/belt etc.
all has been as expected until last week when the car started misfiring, chugging, running 20-40 degrees hotter than it always does. (cooling system maintenance done consistent at 180 before this.)
Fuel filter Ac Delco less than 3 months old.
I had the tune up parts on the shelf so I did them.
Plugs, wires, coil, icm, did the spacer mod with some heavy bushings. (old stuff was original 1995 i believe)
Car starts quickly, idles perfect and drives at slow speeds. when I warm up and get out of the 25mph zone it still stumbles and makes low power, runs hotter, seems like the pump sounds different, not sure if that's just in my mind.
When I give it throttle, i can hear the intake opening so to speak, feels like a loss of fuel.
Eventually the power loss gets so bad i have to feather the throttle just to get it home. It never stalls out completely.


--Fuel pressure testing.
key on priming 20psi, multiple key on and off cycles will get it to 30psi, no higher
using the bypass pin to run the pump 30psi, no higher
engine running at idle 34psi, throttle blip with no load 38psi, fpr vacuum line unhooked 41-42psi, throttle blip might get to 44psi
Engine off pressure builds to 38psi after running, it is boiling hot when i bleed it out though, so i assume its just cooking in the rails.
Holds 38psi off for quite a while, longer than the 30 minutes it takes me to get distracted anyway.
Attempting to drive, pressure hangs around 36 to 38 psi, never above 40, Car feels like a 2 liter import with an ebay cone filter (laggy, farty, lots of induction noise, no go)
I will attempt to check voltage at the rear bulkhead plug for the fuel pump, am I missing anything else?
Thanks for reading guys,
Ken in Almont, MI

 

Odd that it only primes to 30 PSI on multiple keys. What is the dash volt meter showing when priming? Sounds like low voltage from a weak battery. The fact that is recovers to normal values with engine running might indicate the alternator is compensating for the weak battery, or a pump drawing excessive amps. If the pump is struggling and drawing excessive amps, it will heat the fuel. Get it hot enough and you can have problems with bubbles forming in the pump inlet.

 

I didn't look at the volt meter while cranking, but I will, battery is fairly new, name brand and I've had no other signs of weakness.(cranks strong and fast, lights bright etc.)
That's why I'm doubting myself though, used to the "mallet on the tank" style fuel pump fail.
The fuel purge on the fuel gauge is brass, when I drain it, it burns my hand, shut car off when it starts to get hot, didn't think about the pump heating as it fails.

 

I'll add that I've changed the filter with no change in readings.
Pressure drops about 10psi per hour roughly.
I also ordered an ALDL cable, hopefully the correct one.

 

I've done this mod, relatively recently, and discussed it in a posting in here. From what you say below, I'm not so sure your fuel pump is bad. (Nice looking car, btw--def worth getting.) The trap-door mod is pretty straightforward, and when you're done, you'll be able to swap pumps quickly. Without yankin' the tank--woohoo! If the car was left sitting for a long period, it's a good idea to siphon out the fuel and clean the tank as much as possible--but that isn't contributing to your immed. issue.

It's tough to be definitive with pressure @ KOEO. Mine is low, even after replacing the pump, but it's running really well. That you get a similar reading using the prime connector is likewise not the last word. The other readings you note are just about what they should be, and you don't get a quick drop in pressure after shutdown, which is good.

Since the car was neglected, I'd follow Injun's suggestion re: the injectors, and also swap the regulator. If the car sat, the diaphram in the FPR is likely toast; mine was, but unlike your issue, it didn't kick over. As Injun suggests, it does sound more like you have an electrical issue. Still, you can swap the FPR and go over the fuel system entirely to at least see where you are. If you don't have a noid kit, you can pick up a cheap scope from Harbor Freight (~$6), and since you idle ok, let it run and listen to each injector individually. You should hear a steady clicking that increases as throttle increases. If you notice anomalies as you increase throttle, test the connectors.

 

Thank you guys for the response, I got a little frustrated today and swapped a new Delphi pump in.
Pressure now jumps to 46psi on first key turn, settles to 42 after the 2 second pump cycle. Seems to actually respond to throttle input too.
Car starts better, still idles and free revs perfect, drive 30mph and drop in 3rd and it bogs completely.
With the temperature increase, the inability to accelerate with any load, and the general crappy performance, I think my exhaust might be clogged.
I will try the vacuum gauge method, and I may "check" the inside of the cat with a hammer and prybar soon.
Any other ideas? I'm listening.
Thank you again.
PS I was daily driving this thing with only a broken trip meter for 3 months with almost no drivability issues (a random stumble cleared up by a spirited throttle pull twice a month or so)\
PPS The FPR has no signs of fuel in it and holds a vacuum indefinitely as far as I can tell with a mighty vac, pretty sure the diaphragm is intact.

 

Sounds like your FPR is ok, but "neglected" is a vague term, and anytime it sits for long periods, swapping the pump, filter & FPR are generally a necessity. Given what you've replaced & the readings you note... I'd still be inclined to listen to the injectors and see if you notice uneven clicking as throttle is increased.

I had a clogged cat some years back, but the symptoms were diff. I heard a whistling noise, and as it got hotter, the engine just flat-out died. Needed to wait for it to cool down before I could get rolling again, but didn't have a throttling problem. Ended up turning the cat into a resonator, and drove it that way until I finally broke down and got a cat. At the least, it won't hurt anything to empty it out.

Getting that cable and running a dataset w/ Scan9495 will tell you a great deal more. Not sure if you knew, but the program is posted in here at a price you can't beat w/ a stick (FREE!). Run it & you'll have a good snapshot of what's going on over time.

 

The FPR can also “leak” by not tightly sealing off the return flow. That's harder to detect because the factory test method requires inserting a valve so flow can positively shut off. Some people try pinching off the OEM nylon return line to stop the flow. That can do serious damage to the nylon line. And I hope you are not using the Aeromotive version of the stock FPR.

 

Thanks for the tips, I'm really looking forward to getting my cable, I have downloaded the program, I have lots of reading to do.

It is the factory FPR and you're right, I thought about pinching lines, but I didn't want to hurt them.

Guys, it was my catalytic converter. My theory is that the neglected fluids and sensors I already replaced were causing this thing to run closed loop for a long time.
It's a model with a single suitcase cat on the bottom, so any fluid sucked in would pool in there. Any crank/no-starts would flood the area with unburnt fuel.
I don't regret changing of 65k mile, 26 year old plugs, wires, coil, and ICM.
I don't even regret changing the fuel pump, 10-20 psi difference in priming pressure difference.
Because the car wouldn't maintain vacuum, because of the exhaust, the FPR wasn't getting vacuum, maybe it worked the pump harder, weakening it?
Regardless, I'm not sorry to have fresh ignition, and fuel supply, also because I spent a long time making a trap door future upgrades won't be difficult.

So a big thank you to everyone who tried to help, I probably could've provided more info, again waiting on my ALDL, anyone know if Red Devil whatever is still shipping?
I'm finalizing my trap door, will drop a pic here to help, cause I've seen some good ones, and some real hacky ones.

The exhaust is really clean, but was plugged inside, integrated into the front Y-pipe so I could not see the front.
My car is faster than it's ever been, for me anyway, I'll probably pick up some MPGs not fighting through a clogged cat.
My mind is wandering, after a week of work I'm having victory drinks, thanks for the support, it did help.

 

PM GaryDoug with any questions on the cable, installation, etc.

Glad you found the problem.

Loss of vacuum doesn’t make the pump work much harder. The purpose of applying vacuum to the FPR is to maintain an exact pressure difference between the injector inlet pressure in the fuel rails and the injector outlet pressure (vacuum) in the intake manifold. The object is to maintain 43.5 PSI DIFFERENTIAL pressure.

The injector is an orifice. Flow through the injector is proportional to the square root of the pressure difference between the inlet and the outlook. If the differential pressure is held constant, the flow rate through the injector is a constant value, and the PCM knows the flow rate through the injector at all times. It just needs to vary the injector pulse width based on engine load.

At WOT the FPR maintains 43.5 PSI in the rails. At WOT there is no (or very little) vacuum in the intake manifold. Differential pressure is 43.5 PSI minus 0 PSI vacuum = 43.5 PSI. At idle the vacuum is typically 20”Hg, which is about minus 10 PSI. So the vacuum compensation line pulls up on the spring activated FPR diaphragm, reducing the rail pressure to 33.5 PSI. The differential pressure is 33.5 PSI - minus 10 PSI = 43.5 PSI. Remember minus a minus = +.

The spring is pressing the diaphragm down on the return flow, controlling the rail pressure. The vacuum is pulling up on the diaphragm, pulling the flow path open, allowing more bypass flow, producing a lower rail pressure. So, yes, the vacuum moves the pump flow a bit lower on the flow/pressure curve, and the pump works just a little bit less.

I run a Weldon outboard fuel pressure regulator, without vacuum compensation. The rail pressure is always 58 PSI. The lack of differential pressure control is compensated in the fuel tables in the aftermarket MoTeC M48 Pro engine management system. The LS1 engine fuel management works that way too. It doesn’t have a recirculated fuel system, it is “dead end”. The FPR is back at the tank, and is a “bypass” regulator, simply dumping a bypass stream of fuel back into the tank. There is no pvacuum compensation. The LS1 system operates at 58 PSI rail pressure (minus any pressure losses due to flow through the line.

 

Very cool. Dunno if you're planning to swap the cat, but I went w/ the SS Magnaflow. It includes the Y pipe, and you'd be able to swap the cat alone in the future (it's not welded to the pipe). The install was a bit hinky--had to use 2 jacks to position it right--but it fits w/ the stock manifolds & exhaust (3" flange). While I was at it, went ahead and upgraded the cat-back to 3" w/ the Magnaflow Street Series.

For the trap door, I used hi-density foam and made 2" zinc rails to close it down. Riveted the zinc to the door itself, sandwiching the foam in between, and then pressed it in the orifice, carefully drilled some holes and screwed it down w/ SS screws. It took a bit longer, but no possibility of exh fumes, while still being easy to remove (which I actually hadda do--the 1st pump I got was bad). I've seen write-ups where other guys did it diff, but as long as your cut lines are straight & you get a good seal, you're in good shape. While I was at it, took the suggestions I got in here & swapped the rear fuel lines to -6AN and rerouted 'em. It's a lot easier to work back there now.

 

I did a similar thing using aluminum and riveted in (I did a strip across the top too, just forgot another pic) I put down a layer of aluminum tape between the top and bottom, and then 2 layers across the top. seems all is well.
It's way less sketchy than my usual cars anyway.
My ALDL cable is at the post office, looking forward to what else I can learn.
You're saying Magnaflow has a y pipe and cat that bolts to the stock manifolds? I would consider it for a future upgrade for sound and exhaust smell.

 

Yep: https://www.autohance.com/magnaflow-...converter.html. Can't remember if I got it from these folks, but it was ~$450. I thought about doing long tubes, but it was a whole lot easier bolting onto the stock manifolds. The Street Series cat-backs have a nice growl, but not too loud to **** off the neighbors--or draw the attn. of law enforcement. They do have a Comp setup that's somewhat louder.

It's funny you mention the smell. After I did the deed, I noticed my exhaust smelled kinda sickly sweet, but I gather it was from the chems they use during manufacture. After ~100 miles it dissipated. Never noticed that on any other exhaust I've done, but this was the 1st time I went completely SS.

Depending on what state you're in, you can elim. the cat altogether--just like back in the true muscle-car era.