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 :dancing :-D :)
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]

Problem solved :dancing :-D :)
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]

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 (http://www.zr1.net/forum/showthread.php?p=139294)

[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.

[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

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/Fuelpumpcurrentcheck004.jpg
http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/Fuelpumpcurrentcheck001.jpg
http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/Fuelpumpcurrentcheck005.jpg

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

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/FPtestLarge.jpg

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]

(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.

[SteelBlueZR1]

Thanks for all the feedback... Since the stumble and hesitation, followed by rough running and loss of power from 2,500 rpm up was intermittent, everything I did seemed to help, but then the stumble returned.
I just replaced the fuel pumps with OEM AC Delco's, and the stumble hesitation continues, but the pumps did test weak, and now the car starts in 1 second or less vs. 4-5 seconds with old original pumps, plus the power key off drivability is improved with new pumps.
I am just driving car in power key off mode, but really want to solve the problem. I will have WAZOO help me diagnose with Tech 1, before throwing more parts at it.
I did pull codes and had a 61 -low secondary port vacuum system pressure. I checked the secondaries operation, using Haibeck's probe the pink/black wire at the ECM with plenum on instructions, but my car battery died during the test, so it was unclear for now if the secondaries are opening properly.
I am thinking the Secondary Port Vacuum Actuators are bad, and/or the Secondary Port Control Solenoid, and/or the Secondary Port Vacuum Reservoir, but only a plenum pull and vacuum test process will answer this ongoing question.

[SteelBlueZR1]

Update: we ran the Tech 1 at a Wazoo Carlisle get together (thanks Kevin!)... and it diagnosed a bad secondary solenoid switch. switch likely damaged by backfire from bad Map line.
So after doing my first plenum pull and replacing the the Secondary Port Vacuum Solenoid, and my vacuum resorvoir, tested secondaries and my car runs like a champ; the intermittent loss of power and rough running over 2,500 rpm is now a distant memory. I think the bad MAP sensor hose caused a backfire that damaged the selonoid.
Thanks to everyone for their help.

[VetteVet]

Glad to see that you got it resolved!

Jep