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1992 ZR-1 Hesitation

Illiad

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May 22, 2002
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Syracuse, New York
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1973 Coupe & 1992 ZR-1
My ZR-1 has developed a hesitation problem over the past month.
I first noticed a problem about 3 weeks ago when I took the beast to a local car show. I started the car up and I could smell raw gas coming out of the tail pipes. The next week I noticed the car has a long cranking before the car starts when the engine is up to operating temperature. The following week I noticed a noticable hesitation under full throttle acceleration even with the full power switch on. I got the car home and pulled all the plugs and noticed the left bank cylinders (1,3,5,7) are all black, while the right bank (2,4,6,8) are fine. Without taking it to the shop what is the best way to diagnose the problem? I have new plugs on order but I suspect this will not eliminate the problem.
 

Illiad

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May 22, 2002
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Location
Syracuse, New York
Corvette
1973 Coupe & 1992 ZR-1
1992 ZR-1 Hessitation

Yes, I do have the service manuals, Book 1, Book 2, and the 1990-1993 LT5 Service Manual Supplement. How do you retrieve the codes from the computer?
 

Rob

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Actually, all of that information is covered in the service manual. I only have a 1990 Service Manual and some codes may have changed as well as the diagnostic procedures. You should check your service manual.
 

Illiad

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May 22, 2002
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Location
Syracuse, New York
Corvette
1973 Coupe & 1992 ZR-1
1992 ZR-1 Hessitation

Rob, I ran the codes (a simple procedure) the first time and codes H74, H31, H44, H55 came up. I ran the procedure a second time and no codes came up. I'm really surprised no codes came up under ECM module because the check engine light came on when the car started hesitating then would cycle on and off every 5 min while driving home. I'm going to pick up the plugs today and take her for a test drive then I will run the codes again and let you know what happens. Thanks for pointing me in the right direction. I need to get the beast ready for Carlisle.
 

Rob

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Out of curiosity, what rpm do you feel the hesitation?
 

Illiad

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May 22, 2002
Messages
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Location
Syracuse, New York
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1973 Coupe & 1992 ZR-1
Rob, It starts around 3000 RPM and how do you get the computer to display the history fault codes. It displayed them the first time through the sequence and I can't get it to do it again?
 
S

secondchance

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Not as severe as yours but I'm having a simmilar problem. Could you post as you go thru diagnosis?
By the way, it was nice to meet you in person, Rob.
 

Rob

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Illiad said:
Rob, It starts around 3000 RPM and how do you get the computer to display the history fault codes. It displayed them the first time through the sequence and I can't get it to do it again?

Have you checked your secondary port pump operation? Turn the key, but do not start the car. You should hear the pump turn on for about 5-10 seconds and then shut off. If you don't hear it at all....it could be burned out. If it doesn't shut off, you could have a secondary port throttle vacuum leak under the plenum.

To test for normal pump operation, you can disconnect the vacuum line that runs along side the oil filter going under the plenum. When you do that, with the key turned on, the pump should turn on. Put your finger over the open end of the hose. It should shut off.

The only reason why I mention this is that the hesitation is occuring when the secondaries are starting to engage. Since vacuum problems are common on the LT5 engine, they are fairly easy to diagnose and at least rule out of the diagnostic equation.
 

Illiad

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Joined
May 22, 2002
Messages
20
Location
Syracuse, New York
Corvette
1973 Coupe & 1992 ZR-1
ZR-1 Hesitation

Rob, I installed the plugs tonight and no difference. I still smell raw gas coming from the tail pipes. I took the car out for a short spin and the problem is still their and the check engine light came on. I pulled the codes as soon as I came home and got a H44 code under ECM Module. This says the left oxygen sensor is lean. This makes sense since all the left plugs are black. I think the next step should be changing the left oxygen sensor. I'm not sure who would carry it here in Syracuse? Rob, are you the gentleman I met, who trailered a 1994 Black ZR-1 to Breese Chevy for the Destiny Club show?
 

Rob

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No, I'm sorry. I own a 1990 ZR-1 and live in northern Virginia. Have you tested the ignition wires and run any diagnostic checks on the coil packs?
 

Illiad

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Joined
May 22, 2002
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Location
Syracuse, New York
Corvette
1973 Coupe & 1992 ZR-1
ZR-1 Hesitation

Rob, No I have not tested the wires or the coil packs. That would involve removing the plenum. I think it is the left oxygen sensor since I just pulled the new plugs on the left side out and they are black after only a short drive. What else would cause only the left side plugs to go bad. I also rechecked the fault codes and H44 is still present, which is the left O2 sensor running lean. Where have we met in person?
 

Rob

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Actually, you should be able to check to see if there is spark getting to the plugs without removing the plenum. Again, check the service manual for proper procedure.

I'm not sure. I've been to Carlisle a number of times. I was at the Legend Lives event in April 1995, to see the last ZR-1 come off the assembly line and I was there again in January of 1997 to see the unveiling of the C5 on January 7.
 

Illiad

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Location
Syracuse, New York
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1973 Coupe & 1992 ZR-1
1992 ZR-1 Hesitation

Rob,

I replaced the O2 sensor and this corrected the problem. The car runs strong and the spark plugs are now clean. Thanks for pointing me in the right direction. Hope to see you in Carlisle!
 

Rob

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Awesome!!! Glad to hear it. How was the replacement of the O2 sensor? Was it difficult? Any recommendations?
 

Illiad

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Joined
May 22, 2002
Messages
20
Location
Syracuse, New York
Corvette
1973 Coupe & 1992 ZR-1
Rob, Changing the O2 sensor was easy. All you need is a 22mm open end wrench, just make sure the car is hot when you loosen it. The next thing I have to change is the back-up light switch. Any suggestions on that?
 

Rob

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Is or isn't hot?

Backup light switch.....errrrr....I'm pretty sure I replaced that too....I think it's under the dash near the steering column and next to the center high mounted stop lamp switch. Again, check your service manual. I don't remember the exact procedure. It's not easy as it's extremely difficult working in that area. You have to become a human pretzel in order to fit under there.
 

Illiad

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May 22, 2002
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Location
Syracuse, New York
Corvette
1973 Coupe & 1992 ZR-1
Rob, the back-up switch is located at the top of the transmission. I have a offset 22mm wrench to loosen it. The problem is getting your hand up there to reattach it. I could remove the slave cylinder, but not sure whats involved with doing that. I read on the ZR-1.net site this is a very common problem.
 

Rob

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That's right, I'm thinking of a different switch.

In your email above, you said to make sure the car is hot. Are you sure about that?
 

Illiad

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May 22, 2002
Messages
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Location
Syracuse, New York
Corvette
1973 Coupe & 1992 ZR-1
Rob, What I mean is to have the exhaust pipe hot just to loosen the O2 sensor. The new sensor has a special anti-seize coating on it. It's very easy to get to and the whole operation should take 5 mins. Are you going to Carlisle, if so, where do they have ZR-1 parking? On the Corvette forum someone said we could park at the ZR-1. net tent or the LT-5 registry.
 

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