Diagnose - 2,500 rpm Stumble Miss - 1991 8k mile ZR-1

[SteelBlueZR1]

My 1991 ZR-1 with 8,500 miles has developed a stumble, miss, lack of fuel feeling right at 2,500 rpm at all times, except when feathering the throttle to avoid engaging the secondaries. Right after the stumble, the engine starts pulling again, but not as hard or smooth as normal.

Do you think this is fuel pump failure related to sudden demand for fuel when secondaries are engaged? Any help or ideas are appreciated.

Note that the car sat for 20 years and has 16 new SS Delphi fuel injectors, and car driven weekly since I bought it 2 years ago. Symptoms started last week after filling up gas tank, and prior owner apparently left gas tank low or empty while storing (rusted fuel sender?). Car idles fine, but is a bit slow to start.

[FU]

Bert.....shtick a fuel pressure gauge on it and go for a ride. S/S Delphi's new or rebuilt ? Sound's like a bad pump any way.

[SteelBlueZR1]

The Delphi's were New and were installed 1.5 years ago at 3,500 miles. I am going to track down my Wazoo brothers and see if they can help me confirm the diagnosis and replace the pump ASAP. Thanks for your input.

[zr1mom]

Sounds like some possible water in the fuel. You can try some dry gas. Maybe even a bottle of fuel injection cleaner.

[SteelBlueZR1]

Problem solved
The first thing I checked after work was the MAP sensor Vacuum line... It was loose and not an original part, but a hodgepodge of soft and hard vacuum lines held together with ties. I have ordered the correct MAP vacuum hose from Jerry's Gaskets.
I remembered that I had started the car turning the key for too short a time period, and the car backfired slightly... So, my fuel pumps are fine, and the stumble rough run is gone.
Thanks everyone for your input and help.

[zr1mom]

Originally Posted by SteelBlueZR1

Problem solved
The first thing I checked after work was the MAP sensor Vacuum line... It was loose and not an original part, but a hodgepodge of soft and hard vacuum lines held together with ties. I have ordered the correct MAP vacuum hose from Jerry's Gaskets.
I remembered that I had started the car turning the key for too short a time period, and the car backfired slightly... So, my fuel pumps are fine, and the stumble rough run is gone.
Thanks everyone for your input and help.

Ah ha! Had we known it was right after a backfire us old folks would have known instantly. :-)

In most cases the backfire causes the hose to completely come off which in that case your ZR-1 wouldn't even idle.

Congrats!

[Dynomite]

Originally Posted by zr1mom

Ah ha! Had we known it was right after a backfire us old folks would have known instantly. :-)

In most cases the backfire causes the hose to completely come off which in that case your ZR-1 wouldn't even idle.

Congrats!

Hold starter longer to prevent backfire TIPS

[SteelBlueZR1]

Alright, so the saga of intermittent hesitation/stumble at 2,500 rpm and beyond, with stumble and reduced acceleration at higher RPM, yet only with power key on. Car runs normally when power key is off.
Drove car 80 miles today, hoping it was carbon build-up that could be blown out, but car exhibited behaviour 1/2 the time I got on it and always at 2300 to 2500 rpm.
I have replaced the Map Hose, checked and tightened/zip tied the fuel pressure regulator hose, checked and tighened the air snorkel intake (which has stiffeners installed in it), and checked for vacuum leaks (the vacuum pump runs for 2 seconds with car off every 20 seconds).
The car has 8k miles with new stainless steel delphi fuel injectors, but original coils and wires. I have filled spark plug wires with dialectric grease, and car has new AC delco non platinum plugs.
My gut is that I should check fuel pressure and change fuel pumps and gas filter, but I don't have fuel pressure gauge, nor do I want to just throw parts at the problem.
Thoughts? Where is best place to get oem GM fuel pumps assembly and / or pressure gauge?
Thanks for the help,
Bert

[Funracer]

"I don't have fuel pressure gauge, nor do I want to just throw parts at the problem."

I too have been chasing what sounds like the same stumble at
2500 rpm. Here is what happens when you start throwing parts at the
problem. I have tested or replaced at least the following items over
the last 8 yrs trying to get rid of the stumble:

IAC
TPS
Cleaned all the grounds I could get to. There are others.
All 16 injectors with new Accells
Both fuel pumps
Fuel filter
Looked for vacume leaks.
Fuel Injector Relays
MAP sensor hoses
Replaced all the under plenum vacume lines.
All new plugs, wires and coils.
Replaced chip.
Checked compression
Both O2 sensors replaced
No codes

And whatever else I cant remember. One thing I have never done
is have someone check the car with a scan tool (Tech 1 or whatever).
Not totally sure that would find my issue but if I had to do it all
again I would have done that earlier rather than never.

I guess that is my point. For all the $$ I have spent on parts
not to mention the time involved I would give that a shot if the
basics check out for you first.

Why haven't I done it?

Just stubborn I guess. I'm going to find this thing eventually.

Good luck

Neil
'91 with 32K miles

[zr1mom]

Well it could be the secondary fuel pump. Pumps are cheap around $80.

It might be a bad plug wire, even though the car has low mileage the wires are over 20 years old.

For the heck of it if you could borrow someones chip you could try that.