Missing at WOT with full power

[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

[Kevin]

probably the injectors. you can test them by ohming them should be 12.5-12...but i'd jsut replace the at this point

[ZZZZZR1]

Dont just replace secondary injectors, replace the primary as well

All injectors fail because of ethanol, even with low miles and little use

Check out FIC (they support the registry and give excellent customer service)

John Banner. www.fuelinjectorconnection.com

Get stainless ones and he sells Jerry's gaskets as a package (everything you need under the plenum)

:cheers:

David

[A26B]

The slight vacuum bleed down is not a problem.

It could be injectors.

Test #1Weak spark test:
No code will be incurred for weak/no spark. On fairly level road, 6th gear & 50mph. Throttle up smoothly. It should accelerate without missing or bucking. If it does not, then do a spark test w/plug to ground for each cylinder. At idle, it should throw a blue spark appx 3/4". if the spark is orange, it's weak.

As each plug wire is removed, apply a small amount of silicone grease to the around the inside of each boot.

I have, on several occasions, seen a single cylinder fail, even intermittently, in spite of the waste spark system, with new plug wires, probably due to bad terminal contact. A weak spark is most evident as a miss under a load, hence the suggested test.

Note any cylinders with no/poor spark & replace that particular coil. If & when the plug wires are removed, I would suggest ohming each wire out individually to confirm. You may want to considersaving the original wires & replacing them with a new set of similar wires while the plenum is off.

Test #2 Accordian Duct
After a hot run, check the accordian air inlet duct by squeezing it top to bottom. if it feels really soft, it may be partially collapsing at WOT. Normally doesn't cause a miss, just a low power, bog.

Test #3 Secondary Fuel Pump.
Yes, there is a secondary fuel pump. Both the Primary & Secondary fuel pums go through the same in-line fuel filter. I would suggest a search of the forum for Fuel Pump Test, as it has been written up several times. Testing the pumps separately is recommended.

Note: The Fuel Pressure Regulator is very seldom a problem. If you experience low fuel pressure while the engine is running, squeeze close the vacuum line to the regulator. if fuel pressure goes up, then the regulator may be at fault.

If none of the no cost foregoing (no DTC) tests for problems yield positive results, then injectors are most likely to be the problem, especially for 90~92 models.

[Blue Flame Restorations]

I had an intermittent miss. Ended up being the fuel pumps.

[scottfab]

I believe you need to start at the secondary fuel pump (which others have said). I certainly would not jump to the injectors to start and I would think a problem with spark would be at low rpm and dissipate at higher rpm.
Attach fuel pressure gauge and tape to windshield. Go for a drive. At no time should the pressure go down to 45.

However, I am simply amazed it's not setting a code. Try driving to the miss and hold it there. See if you can force a code. The expected code would be a lean condition.

If fuel pressure ok.....plenum time. Left bank or right may be sticking. Ask me how I know.

[Blue Flame Restorations]

My car gave no codes. Missed at high rpm, not low rpm. Pumps. If the original injectors are still in the car, you'll replace them soon anyway.

[A26B]

........ and I would think an problem with spark would be at low rpm and dissipate at higher rpm. ..........

That was not the case with mine. Sometime completely unnoticable in normal driving, 6th gear acceleration acted like a cammed engine at too low rpm, intermittent miss at highway speed, and sometimes would express as a full blown miss at high rpm. Replaced the coil and all was good, in fact performance was better than even when miss was not aparrent.

It's a simple, no cost test for a potential problem that throws no codes. I've seen it on 2 other ZR-1's where injectors were mistakenly diagnosed as being the problem.

I'm not saying its NOT Injectors, but there's no harm & no cost to check it out.

[scottfab]

That was not the case with mine. Sometime completely unnoticable in normal driving, 6th gear acceleration acted like a cammed engine at too low rpm, intermittent miss at highway speed, and sometimes would express as a full blown miss at high rpm. Replaced the coil and all was good, in fact performance was better than even when miss was not aparrent.


And you had no high rpm miss with key off? This was the symptom given. No miss with key off. I would think if it were coils they'd have a miss at low rpm also.

[XfireZ51]

And you had no high rpm miss with key off? This was the symptom given. No miss with key off. I would think if it were coils they'd have a miss at low rpm also.


Agree. Probably fuel.

[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]

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]

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]

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]

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 :handshak:

[SteelBlueZR1]

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