Code 56 - ZR-1 Net Registry Forums

mmkkpro · 10-20-2019

Hi guys, hope everyone is well. I'm working on my last code a 56, vacuum sensor circuit. The FSM says if the ECM is reading a low or high condition the ECM and wiring is OK. Have any of you guys had this code and if so what corrected it?
Now, when I switch the ignition on my little vacuum pump runs for about 10 seconds and shuts off but comes back on fairly quickly, which tells me I have a leak. When I had the plenum off I checked all hoses and secondary parts, tested it and it held vacuum. With the engine idling the pump does not run. Yall think the small vacuum leak would cause the code 56? I have 18 inches of vacuum at idle hot. I have been looking for another sensor but no luck yet. Thanks

secondchance · 10-20-2019

It’s been a few years so my memory is a bit foggy. However, I think when when the manifold differential pressure sensor ( looks just like a MAP sensor) went bad I got code 56.
Manifold differential pressure sensor was under the plenum at least for 94 ZR-1.

mmkkpro · 10-20-2019

On the 1990 the sensor is mounted to the front left bottom of the ECM.
I pulled the plenun and nothing is leaking as far as vacuum lines or components. When I put the plenum back on a leak reappears. With it back together and the switch on the pump runs till I pull the line off the drivers side and plug it with my finger. Soon as I reconnect it to the plenum the pump keeps running. It's like either the plenum is pushing on a vacuum line or connection or there is a leak in the plenum itself. I have checked all plenum vacuum connections, I don't know I'm missing something.

Demps · 10-20-2019

Not sure about the code 56–I can look it up & Trace it.

The under plenum vacuum check valve is likely bad with your symptoms. A $5, easy check is to pick up a vacuum check valve & rubber line for female-female connector to the plenum...put it between line from under plenum & plenum.

Ted

mmkkpro · 10-20-2019

Thanks Ted, I just got done reading a post by Dynomite about that check valve and in his post he describes the exact problem I'm having, and I feel confident that will solve the 56 code as it is discribed as low or high vacuum at the sensor. I appreciate your time, thanks.

jss06c6 · 10-24-2019

Agree with Ted.. I moved this check valve from under the plenum to the side where it can be accessed without pulling the plenum.. if you look at the attached photo, you will see the check valve between #3 & #5 intake runners on the plenum.

Sent from my SM-G950U using ZR-1 Net Registry mobile app

mmkkpro · 10-24-2019

Thanks jss06c6, I like that idea putting the check valve where it's accessible. I'm getting good at taking the plenum off not sure if that's good or bad. That's a nice looking LT5 you got there. I'm going to replace the check valve Saturday, I will let you guys know how it goes. Hoping this solves the code 56, tires are next then tags and insurance. Can't wait to take her on her maiden voyage. I have driven her around the block a couple times but I want to get this bad boy up to speed, I'm sure I will have a few more bugs to work out but all in all it's coming along very well. Thank you for your reply, several of you guys have been very helpful. 👍👍🏁

jss06c6 · 10-24-2019

Quote:

Originally Posted by mmkkpro

Thanks jss06c6, I like that idea putting the check valve where it's accessible. I'm getting good at taking the plenum off not sure if that's good or bad. That's a nice looking LT5 you got there. I'm going to replace the check valve Saturday, I will let you guys know how it goes. Hoping this solves the code 56, tires are next then tags and insurance. Can't wait to take her on her maiden voyage. I have driven her around the block a couple times but I want to get this bad boy up to speed, I'm sure I will have a few more bugs to work out but all in all it's coming along very well. Thank you for your reply, several of you guys have been very helpful. [emoji106][emoji106][emoji469]

You'll love the LT-5!! Its worth the effort. Once you get it sorted out, get out and drive that car and enjoy the smiles!

Steve

Sent from my SM-G950U using ZR-1 Net Registry mobile app

mmkkpro · 11-02-2019

Hi guys, I have another question relating to the code 56.
Should I be able to hear air when I blow through the vacuum line that connects to the MDP sensor? I was checking everything over and decided to see what happens when I disconnected the vacuum line that runs from the MDP sensor. When I blow from the sensor end I hear air at the secondary vacuum solenoid. When I was checking everything I asked Marc about the fact that the port to the right is open, and air flows through. Marc told me that's the way it works. To my understanding the port is open until the solenoid is energized. I know I'm not suppose to have any vacuum to the MDP until the secondaries are energized. I put new check valves in and checked for any leaks, my electric pump runs for about 3-5 seconds and shuts off. I bought a new MDP sensor from Jerry and am installing it this morning. I was just wondering about the vacuum line running to the MDP sensor being open in valet mode. Thanks.

mmkkpro · 11-02-2019

Hey guys, apparently it was the MDP sensor, I installed it and just started the car and all is well. Going to let her get good and warm and see what happens. Got ride control light and infl rest lights next. On to the body work now. Thanks for everyone's advise and replies.

10-20-2019	#1
mmkkpro Join Date: Apr 2019 Location: Supply NC Posts: 124	Code 56 Hi guys, hope everyone is well. I'm working on my last code a 56, vacuum sensor circuit. The FSM says if the ECM is reading a low or high condition the ECM and wiring is OK. Have any of you guys had this code and if so what corrected it? Now, when I switch the ignition on my little vacuum pump runs for about 10 seconds and shuts off but comes back on fairly quickly, which tells me I have a leak. When I had the plenum off I checked all hoses and secondary parts, tested it and it held vacuum. With the engine idling the pump does not run. Yall think the small vacuum leak would cause the code 56? I have 18 inches of vacuum at idle hot. I have been looking for another sensor but no luck yet. Thanks

10-20-2019	#2
secondchance Join Date: Aug 2006 Location: McLean, VA Posts: 3,723	Re: Code 56 It’s been a few years so my memory is a bit foggy. However, I think when when the manifold differential pressure sensor ( looks just like a MAP sensor) went bad I got code 56. Manifold differential pressure sensor was under the plenum at least for 94 ZR-1. Last edited by secondchance; 10-20-2019 at 02:30 PM.

10-20-2019	#4
Demps Join Date: Jan 2006 Location: Niceville Posts: 998	Re: Code 56 Not sure about the code 56–I can look it up & Trace it. The under plenum vacuum check valve is likely bad with your symptoms. A $5, easy check is to pick up a vacuum check valve & rubber line for female-female connector to the plenum...put it between line from under plenum & plenum. Ted

10-24-2019	#7
mmkkpro Join Date: Apr 2019 Location: Supply NC Posts: 124	Re: Code 56 Thanks jss06c6, I like that idea putting the check valve where it's accessible. I'm getting good at taking the plenum off not sure if that's good or bad. That's a nice looking LT5 you got there. I'm going to replace the check valve Saturday, I will let you guys know how it goes. Hoping this solves the code 56, tires are next then tags and insurance. Can't wait to take her on her maiden voyage. I have driven her around the block a couple times but I want to get this bad boy up to speed, I'm sure I will have a few more bugs to work out but all in all it's coming along very well. Thank you for your reply, several of you guys have been very helpful. 👍👍🏁

11-02-2019	#9
mmkkpro Join Date: Apr 2019 Location: Supply NC Posts: 124	Re: Code 56 Hi guys, I have another question relating to the code 56. Should I be able to hear air when I blow through the vacuum line that connects to the MDP sensor? I was checking everything over and decided to see what happens when I disconnected the vacuum line that runs from the MDP sensor. When I blow from the sensor end I hear air at the secondary vacuum solenoid. When I was checking everything I asked Marc about the fact that the port to the right is open, and air flows through. Marc told me that's the way it works. To my understanding the port is open until the solenoid is energized. I know I'm not suppose to have any vacuum to the MDP until the secondaries are energized. I put new check valves in and checked for any leaks, my electric pump runs for about 3-5 seconds and shuts off. I bought a new MDP sensor from Jerry and am installing it this morning. I was just wondering about the vacuum line running to the MDP sensor being open in valet mode. Thanks.

10-20-2019	#3
mmkkpro Join Date: Apr 2019 Location: Supply NC Posts: 124	Re: Code 56 On the 1990 the sensor is mounted to the front left bottom of the ECM. I pulled the plenun and nothing is leaking as far as vacuum lines or components. When I put the plenum back on a leak reappears. With it back together and the switch on the pump runs till I pull the line off the drivers side and plug it with my finger. Soon as I reconnect it to the plenum the pump keeps running. It's like either the plenum is pushing on a vacuum line or connection or there is a leak in the plenum itself. I have checked all plenum vacuum connections, I don't know I'm missing something.

10-20-2019	#5
mmkkpro Join Date: Apr 2019 Location: Supply NC Posts: 124	Re: Code 56 Thanks Ted, I just got done reading a post by Dynomite about that check valve and in his post he describes the exact problem I'm having, and I feel confident that will solve the 56 code as it is discribed as low or high vacuum at the sensor. I appreciate your time, thanks.

11-02-2019	#10
mmkkpro Join Date: Apr 2019 Location: Supply NC Posts: 124	Re: Code 56 Hey guys, apparently it was the MDP sensor, I installed it and just started the car and all is well. Going to let her get good and warm and see what happens. Got ride control light and infl rest lights next. On to the body work now. Thanks for everyone's advise and replies.

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