Missing at WOT with full power

[MrsMcNastys]

Thanks for the quick replies guys. Just to clarify - power key OFF, runs flawlessly, accelerating, low speed, high speed, stop-and-go etc.

With power key ON, runs great initially, after full temp, key light remains on, mild acceleration or cruise hi/lo no discernable miss, at hot WOT slight consistent vibration as if it has a dead cylinder, more power than key off but not full power, cannot hear the miss only feel it, no codes forced. Turning the key off at WOT with miss stops the vibration and runs smooth.

I'll search for the pump test, I really appreciate the help and feedback. I've learned alot about these LT5's in the past 2 weeks! Love 'em!

Ed

[FU]

Fuel pump's.

[MrsMcNastys]

Fuel pump test:

Key on: Momentarily to 50 then goes to 42 once pressurized and pumps turn off

Key off: Jumping to the red connector 50

This time with key on the vacuum pump was cycling every 3 seconds which it definately wasn't doing before, sooooo maybe I'm back to a plenum removal.

Ed

[scottfab]

Originally Posted by MrsMcNastys

Fuel pump test:

Key on: Momentarily to 50 then goes to 42 once pressurized and pumps turn off

Key off: Jumping to the red connector 50

This time with key on the vacuum pump was cycling every 3 seconds which it definately wasn't doing before, sooooo maybe I'm back to a plenum removal.

Ed

You may want to isolate the pump first. I mean the leak is probably under the plenum but to be sure, find the place in the hard vacuum line with a connector. Pull it apart and cover the pump side end with your thumb. The pump should run a short time and stop. If it does you're pulling the plenum. Sure sounds like a crack a plastic part under the plenum if the duration of pump running changes each time you look at it. (ie was cycling at 15 sec now at 3sec)

As for the pumps. I don't have the manual in front of me but if it's leaking down to 42 fairly rapidly the check valve on the pumps are leaking.

[zr1mom]

Originally Posted by MrsMcNastys

Hi Guys,

Great forum. I have a customers 14,000 mile 1992 ZR1 in my shop. Has not been driven much last few years. He has 2, plus some other toys but decided he wanted to drive this one regularily for a while. He brought it in last week, missing, running rough, no full power. Pulled the codes and located a loose hose on the MAP sensor. Fixed. Car road tested good, full power no problem.

He brought it back this week with a miss. No codes. Tested the vacuum system. Pump good. Very very slow almost imperceptible bleed down on the plenum side. (Is this plumbed to maifold vacuum also? Plenum side of line pulls vacuum at idle) Road tested the car - great with full power initially. After a while at full temp, slight engine vibration at full power WOT. Flawless in full power off. Engine vibration and loss of power with key full power key on only. He reported those symptoms, then would go fine, then a miss again.

I am thinking secondary injectors? Is there a secondary fuel pump and/or fuel filter? One fuel filter was changed at our local useless GM dealer in 2010.

Any other possible culprits before we pull the plenum? Any help would be appreciated.

Ed

Car not driven much, how old is the gas? And did he keep the tank full in storage?

Most likely fuel injectors.

Mom

[scottfab]

Originally Posted by zr1mom

Car not driven much, how old is the gas? And did he keep the tank full in storage?

Most likely fuel injectors.

Mom

Nice to hear from ya mom.
He says runs fine with key-off.
Bad gas tends to affect both no?

[zr1mom]

Originally Posted by scottfab

Nice to hear from ya mom.
He says runs fine with key-off.
Bad gas tends to affect both no?

Thanks, Mom is always keeping an eye out on his children.

True bad gas should affect both but worth taking a look for example octane rating?

But like I mention most likely fuel injectors specially the secondaries.

[MrsMcNastys]

Just a quick thank you to all and to close out this thread.

After the car developed a miss at low speed with the key off, we eliminated the fuel pumps as a possibility as testing (short of an on-road test at WOT) remained the same and pressure within specs.

Ohming out the fuel injectors through the harness showed 3 bad primary injectors. The instructions posted by Marc Haibeck were extremely helpful. I didn't bother to identify which injectors, just that 3 were bad was enough to convince the customer he needed all new ones.

We purchased a matched set from John at Fuel Injector Connection which came with a full set of o-rings for the fuel tubes as well. After pulling the plenum gave us full access to the injectors - a total of 8 tested below specs (3.5-6). Replacement of the injectors and plenum was very straight forward. We also tested the secondary vacuum system for leaks under the plenum, as well as the coils and plug wires for resistance - all good.

Car is done and performs flawlessly. Thanks for all the help guys!

[FU]

We all like a happy ending

[SteelBlueZR1]

Congratulations on resolving this... If it is not the fuel pumps, it is usually the injectors.