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

[secondchance]

Bert,

It sounds like your secondary fuel pump is on its way out.
You can get Suburban 454 fuel pumps and strainers $80/each. I would just replace both pumps - about $160.
When I get home I'll post part numbers and website. It took me about an hour to replace mine.

[SteelBlueZR1]

Thanks Yun... hope to see you soon! The Secondary Fuel Pump is exactly what it feels like, and my car was stored and delivered with an empty gas tank ( you see the fuel gauge in pictures of many ZR-1s for sale that are at or near empty?!), so after 20 years corrosion could have killed the pump. I still want to borrow a fuel gauge and confirm this while driving. I will ask Jim if I can borrow his fuel gauge. But, I hope that this isn't the start of a long painful diagnosis and the problem goes away with a fuel pump replacement. Jerry sells the whole OEM AC Delco assembly with both pumps for $345 - PN 19180770.

[scottfab]

After reading all posts and seeing that it behaves nicely with key off and that it makes it through the stumble...... I'd say not secondary fuel pump. If it were you'd feel a flattening of torque (pull).
I'd guess at one of the two secondary actuators not able to pull open at the same time as the other side.
Mine did this and I discovered a bit of carbon build up in the way at cyl #7. It was just enough to delay full opening a bit.

In any even a psi test at 2500rmp is next.

[Hib Halverson]

Lotsa guessing on this one, eh?

If I read the OP correctly, this car sat unused for 20 years?

Does the check engine light function properly during bulb check? If so, are any codes set.

Has the light ever been on then gone off?

My guess is this is not a secondary pump problem, but you can be sure if you run the fuel pressure tests in the Service Manual.

I think I'd pull the intake off, then make sure all the secondary port throttles are working and that the actuators are functioning properly.

Once the engine pulls through the 2500 rpm area, how does it run up to 6500 or so?

I noted the car is Steel Blue Metallic. What a great color

[Paul Workman]

Some bullet points come to mind...

fuel pump current draw test





static fuel pressure (also observed when doing the current draw test)
dynamic fuel pressure delivery - testing while driving



secondaries test (you don't have to pull the plenum! See Marc Haibeck's page for proceedure)

accordion "snorkle" intake tube collapsing?? (needs Marc's "hoops" or a sleeve)

the TPS voltage smoothly transistions from idle to WOT?

Sorry to be acutely brief. Gotta run!

P.

[Hog]

What year Suburban fuel pumps are youy guys speccing? The following info may not bve accurate if you guys are using lower pressure TBI truck fuel pumps. I am assuming the higher pressure MPFI Vortec 454 from 96-97.

GM part number 25163473 or A/C Delco number EP-381 are $65 from Rock Auto.
Those pumps are the same for the 1991 4.3 Turbo Syclone and the 92-93 4.3l Turbo Typhoon. Also the same pump as for the 1996-1997 Vortec 305/L30, 350/L31, and 454 L29 for all 1/2,3/4 and 1 ton trucks, SUV's etc. Pretty much all the higher pressure CPI and CSFI 4.3's from 1991 up, NOT the low pressure TBI 4.3's.
This is the same pump that is specced by GM Performance Parts for the 502 hp Ramjet 502, the 493hp Ramjet ZL1 and the 350hp Ramjet 350 crate engines.
That pump is withjout a tank strainer, dont forget to order GM p/n 25055461 or Delco p/n TS7.$6.37.

Aeromotive has released a newer fuel pump that retrofits for these pumps as well. Its Aeromotive p/n SKU: 11169.
It flows 340 liters/hours at 40 psi and 270 liters/hour at 70 psi. And the ZR-1 can use 2!!.'
EDIT The Aeromotive pump is ethanol compatible, they do state that maintenace schedules are shorter with ethanol.


peace
Hog

[SteelBlueZR1]

Thanks everyone for the great substantive ideas and feedback. I am slammed at work, but bummed I can't find time to start doing these diagnostics... I will update when I finally get time in the next week or so. To Answer Hib's questions:

If I read the OP correctly, this car sat unused for 20 years? CORRECT, CAR DELIVERED WITH DEAD BATTERY AND NO GAS.

Does the check engine light function properly during bulb check? If so, are any codes set. NO CODES SET

Has the light ever been on then gone off? NOT SINCE REPLACING INJECTORS

My guess is this is not a secondary pump problem, but you can be sure if you run the fuel pressure tests in the Service Manual. GOING TO BORROW JIM'S AND RUN THE TESTS BEFORE REPLACING PUMPS.

I think I'd pull the intake off, then make sure all the secondary port throttles are working and that the actuators are functioning properly. THIS WAS TESTED THOROUGHLY 4,500 MILES AGO (1 YEAR), BUT WILL DO IF PUMPS NOT BAD. PLUS, WILL CHECK FOR CARBON BLOCKING SECONDARIES.

Once the engine pulls through the 2500 rpm area, how does it run up to 6500 or so? STUMBLE AT 2,500 IS INTERMITTENT, BUT CAR PULLS TO HIGHER RPMS WITH LESS VIGOR AND LESS SMOOTHLY, KINDA LIKE STARVED FOR FUEL (BUT SOMETIMES RUNS HARD WITH NO ROUGHNESS NOR STUMBLE).

I noted the car is Steel Blue Metallic. What a great color THANKS FOR THE COMPLIMENT... THAT IS WHY I BOUGHT THIS BLUE/BLUE CAR 1 OF 26 EVER MADE IN THIS COLOR SCHEME. MAKES CAR LOOK REALLY CLASSY AND UNIQUE.

[Hib Halverson]

Originally Posted by SteelBlueZR1

(snip)

Once the engine pulls through the 2500 rpm area, how does it run up to 6500 or so? STUMBLE AT 2,500 IS INTERMITTENT, BUT CAR PULLS TO HIGHER RPMS WITH LESS VIGOR AND LESS SMOOTHLY, KINDA LIKE STARVED FOR FUEL (BUT SOMETIMES RUNS HARD WITH NO ROUGHNESS NOR STUMBLE).

Ok, so the 2500 rpm stumble and the engine's performance above 2500 rpm are both intermittent. Your post implies that the stumble and poor high rpm performance are intermittent in lock step. Is that correct?

I'll be interested to read your account of the fuel pressure check you're going to do.

I noted the car is Steel Blue Metallic. What a great color THANKS FOR THE COMPLIMENT... THAT IS WHY I BOUGHT THIS BLUE/BLUE CAR 1 OF 26 EVER MADE IN THIS COLOR SCHEME. MAKES CAR LOOK REALLY CLASSY AND UNIQUE.

We owners of rare colors must hang together.
I'm Dark Purple Metallic/black 1 of 25.

[SteelBlueZR1]

Sorry for the delay in updating. So, I had previously installed the new Jerry's Gaskets sourced oem map vacuum line as the old one was a hodgepodge of cobbled together rubber and plastic vacuum lines and completely soaked with oil.
I thought I still had a fuel / secondary problem, as the stumble problem continued; but on a whim, I removed and cleaned again the new vacuum line, which still had some remnant oil in it. Maybe the MAP sensor connector being unplugged and plugged in 2x's also cleaned the contacts?
Now the car idles perfectly (Nickel stood on edge for :30) and there is no hesitation when the secondaries open, and the car pulls amazingly strongly and smoothly to red line.
I guess it just took some time for the oil to work its way out and allow correct MAP readings. There was tons of oil in the original MAP line and only a little in the new OEM map line.
I drove the car 30 miles tonight, and the LT5 has never run this well before!
A Big Thanks to Jerry of Jerry's Gaskets for talking me down from buying the fuel pumps pre-emptively and focusing on the MAP sensor vacuum hoses.

[A26B]

Glad it worked out well for you Bert.