Help! 1st Time Removing Plenum....now rough idle

[jon_d4]

94 ZR-1

I just removed the plenum to fix an oil leak from the crankcase vent and tubes. Got everything back together, and I'm getting a really rough idle and the car won't rev above 3k. It runs fairly smooth up to 3k, then it just pulses and shakes violently refusing to go any further.

I've already pulled the plenum back off once to check all of my vacuum lines and that I have a good connection on all of the plug wires. It's still possible that I've missed something, but I've rechecked as much as I can.

It's also throwing a code, but since it's a 94, I can't get the ECM codes by just jumping wires on the connector for the ALDL.

Advance? What is the most likely offender? Thanks for the help.

[mike100]

you can get the codes to display on the orange speedo lcd on the 90-up cars.

but my guess is that maybe a pin on the ignition module got bent trying to get that long connector on the back plugged in. Take off the map sensor bracket and get your 1/4" socket out...

[jon_d4]

Originally Posted by mike100

you can get the codes to display on the orange speedo lcd on the 90-up cars.

but my guess is that maybe a pin on the ignition module got bent trying to get that long connector on the back plugged in. Take off the map sensor bracket and get your 1/4" socket out...

So, after checking the MAP sensor vacuum, I pulled the plenum a third time to check the connector for the ignition module. I was having trouble getting to it without lifting the plenum up. All the pins on the connector look good. On the bright side, I'm getting a lot faster at removing/reinstalling the plenum....

[cvette98pacecar]

Originally Posted by jon_d4

So, after checking the MAP sensor vacuum, I pulled the plenum a third time to check the connector for the ignition module. I was having trouble getting to it without lifting the plenum up. All the pins on the connector look good. On the bright side, I'm getting a lot faster at removing/reinstalling the plenum....

Pull the water seal on the DIS plug back, I guarantee you one of the pins is pushing out when you install the plug, with the water seal pulled back you will be able to tell which pin it is within seconds. I don't remember if these pins are 100 series or not, They are easy to change if needed.
Don't cut or shorten the wire, Take your time and open the crimp connection. Sorry to tell you but the tool is special and a good GM terminal tool is going to cost you about $100.00.

[jon_d4]

Originally Posted by cvette98pacecar

Pull the water seal on the DIS plug back, I guarantee you one of the pins is pushing out when you install the plug, with the water seal pulled back you will be able to tell which pin it is within seconds. I don't remember if these pins are 100 series or not, They are easy to change if needed.
Don't cut or shorten the wire, Take your time and open the crimp connection. Sorry to tell you but the tool is special and a good GM terminal tool is going to cost you about $100.00.

Thanks for the help. My connector was okay. I checked that all the pins that had wires running to them were still present in the connector. I also checked the ignition module to make sure I didn't damage anything on that side. This is good advice, as I'll now pay extra attention to this connector each time I remove/install it.

[Billy Mild]

Did you remove the Throttle body at all?

Could you possibly get a video of the car running? Even a cell phone video would help for this.

[jon_d4]

So, just wanted to wrap up this post and fill you guys in on the results. Sorry for the delay. I was traveling for work this last week and wasn't able to work on the car until yesterday.

Car is now running great! I think my issue was actually the vacuum lines going to the MAP sensor....I pulled the plenum and checked the connector going to the ignition module like was suggested; everything looked good. Well, when I got it all back together the car wouldn't run at all... I was out of time to work on it, so that's where it sat until yesterday.

I pulled the plenum again to recheck everything and discovered that I had two of the ignition coils swapped on the drivers side of the car. When I pulled the plenum the last time to check the connector for the ignition module, I also pulled the ignition coils and the plate above the starter out to retrieve a dropped bolt and must have got them mixed up then.

I got it all back together and double checked my vacuum line going to the map sensor....fired right up and didn't throw a code! Thanks for your help, guys!

[2ljd]

Originally Posted by jon_d4

So, just wanted to wrap up this post and fill you guys in on the results. Sorry for the delay. I was traveling for work this last week and wasn't able to work on the car until yesterday.

Car is now running great! I think my issue was actually the vacuum lines going to the MAP sensor....I pulled the plenum and checked the connector going to the ignition module like was suggested; everything looked good. Well, when I got it all back together the car wouldn't run at all... I was out of time to work on it, so that's where it sat until yesterday.

I pulled the plenum again to recheck everything and discovered that I had two of the ignition coils swapped on the drivers side of the car. When I pulled the plenum the last time to check the connector for the ignition module, I also pulled the ignition coils and the plate above the starter out to retrieve a dropped bolt and must have got them mixed up then.

I got it all back together and double checked my vacuum line going to the map sensor....fired right up and didn't throw a code! Thanks for your help, guys!

jon_d4,

I just got through doing my first plenum pull and encountered the same problem you described...it starts and runs, but really rough, almost like a few cylinders are missing.

I noticed early on you mentioned checking the MAP vacuum line initially and it still wouldn't start. But, on your last plenum pull you thought it was the MAP vacuum line. Was there something you noticed the last time around that led you to believe it was the MAP vacuum line? Too loose? Did you have to fix the seals as some have done with crazy glue?

I inspected my MAP vacuum line twice and it seemed alright...although I can't say for sure that it somehow isn't leaking despite my visual inspection.

Just trying to get a feel for what your problem was before I pull the plenum again. By the way, I changed my plugs, wires, coils and injectors, so I have a lot of investigating to do...

It ran fine (other than a slight miss I figured had to do with the injectors; original with only 7K miles on the car) before I pulled the plenum.

[Paul Workman]

Originally Posted by jon_d4

So, just wanted to wrap up this post and fill you guys in on the results. Sorry for the delay. I was traveling for work this last week and wasn't able to work on the car until yesterday.

Car is now running great! I think my issue was actually the vacuum lines going to the MAP sensor....I pulled the plenum and checked the connector going to the ignition module like was suggested; everything looked good. Well, when I got it all back together the car wouldn't run at all... I was out of time to work on it, so that's where it sat until yesterday.

I pulled the plenum again to recheck everything and discovered that I had two of the ignition coils swapped on the drivers side of the car. When I pulled the plenum the last time to check the connector for the ignition module, I also pulled the ignition coils and the plate above the starter out to retrieve a dropped bolt and must have got them mixed up then.

I got it all back together and double checked my vacuum line going to the map sensor....fired right up and didn't throw a code! Thanks for your help, guys!

Nothing like a digital camera and some "before" pictures to sort things out "after", I find. Especially when working on something new!

[Hib Halverson]

Originally Posted by jon_d4

94 ZR-1

I just removed the plenum to fix an oil leak from the crankcase vent and tubes. Got everything back together, and I'm getting a really rough idle and the car won't rev above 3k. It runs fairly smooth up to 3k, then it just pulses and shakes violently refusing to go any further.

I've already pulled the plenum back off once to check all of my vacuum lines and that I have a good connection on all of the plug wires. It's still possible that I've missed something, but I've rechecked as much as I can.

It's also throwing a code, but since it's a 94, I can't get the ECM codes by just jumping wires on the connector for the ALDL. (snip)

Oh...but you can indeed!
For your future reference, because partial OBD2 capability was only on 94 base cars, you can get codes from the LT5 ECM two ways.

1) Ground pin 6 of the 16-pin "OBD2" DLC connector, then key-on/engine-off and the MIL will flash codes.
2) Ground pin 12 of the 16-pin "OBD2" DLC connector, then key-on/engine off– and observe the IP Display. When the sequence gets to module 4, any codes there will be ECM codes.

You can find more detailed descriptions of these procedures in the 94 Factory Service manual. Section 6E for flash code diagnostics and Section 8D for CCM diagnostics.

Glad you got the engine running. Mixing up those plug wires does it every time, be it an LT5 or otherwise.