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)



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]

Originally Posted by scottfab

........ 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]

Originally Posted by A26B

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]

Originally Posted by scottfab

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.