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

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.

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

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...

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

Thanks for the responses, guys.

I checked the vacuum line from the MAP sensor to the plenum. I removed it, reinstalled, made sure it was tight, then ran the car again. No change. Thoughts?

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

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....

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

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.

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

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.

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

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!

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

Nailed it. Thanks!

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

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.

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

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.

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

Thanks for the checklist. While some of the items would be nice to do, they'll have to wait a bit.

The purpose of my plenum pull was to change out the injectors and trace a secondary vacuum leak. While I was in there, I changed out the plugs, wires and coils for good measure. I traced the vacuum leak to the solenoid and replaced it. The secondaries worked perfectly after that.

Everything went back together smoothly enough, but upon startup, it felt as though it was running on only half the cylinders. That was last night...tonight, I pulled the plugs to inspect them and only one of them looked slightly discolored (probably didn't run them long enough to affect them).

Then I started the engine again and began to pull each plug wire one by one to see if the idle changed at all. Well, it appears that cylinders 2, 5 and 8 didn't change the idle when I pulled the wires. I know they were all getting spark because it was jumping to the heads.

So I need to pull the plenum and check each injector for resistance as well as for spray, and make sure I pushed the connectors on all the way. If they check out, I'm dreading having to trace back from there...

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

Oh...but you can indeed!:dancing
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.

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

Hi dynomite, what are Air Bag Accelerometer?
Are the two little box on the frame?
I remove them and check the ground, clean the frame and put dielectric grease, reset the error
But I always have the error "inflt rest" and the "man with safety belt"...what I can do???
Thanks in advance!!!

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

Dynomite i deleted the codes, flashing code 12...but if i start engine the error and the light come again...can i check other?

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

Hey Jon

Here are pics of my plenum pull I hope they are helpful and if there is anything I can do I will be happy to help

https://www.facebook.com/media/set/?...1&l=8f46fa4533

Dave

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

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

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

Dynomite, i have error code 26...suggestions???
Thanks guys

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

Yes......your RH INFL REST Sensor is not grounded or otherwise not functioning. There is a yellow connector under the Floor mat on passenger side that needs to be checked.

For 90-93 Cars jumper A to K on the ALDL and the INFL REST light will flash
1990 SIR Codes
14 Front Sensor #1 (LH); Short to Ground
15 Front Sensor #1 (LH); Open Circuit
16 Front Sensor #1 (LH); Sensor Fault
24 Front Sensor #1 (RH); Short to Ground
25 Front Sensor #1 (RH); Open Circuit
26 Front Sensor #1 (RH); Sensor Fault

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

Ok dynomite, in the next weekend ill check the connector and ill let you know.
Thanks in advance.
Max

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

Hi dynomite, i check the connector on the passenger side, it Was loose!!!
I close it and reset the error, the infl rest light and the man with safety belt are disapears!!
Thanks a lot!!!

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

:dancing:dancing:cheers:

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

Yes dynomite...now i have an other light on...abs service!!!
I dont remember the number code but its an issue on rear wheels.
I already clean the abs sensors reset code and after few kilometers the light come on :-(

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

If i well remember the codes are 26 and 36...i hope i m not the only one!!!!!

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

Hi Max,

You may get more response if you bring up an old post with a similar problem to what ever you are facing, or a new post on that issue.

I don't know the code #s but have had ABS faults due to loose wheel hubs, which can happen when you run hard on modern performance tires. Probably worth checking by jacking the car up and shaking each wheel (left/right and up/down), any more than just a little play (both directions) indicates it's time to replace. It usually takes a lot of play before ABS has trouble though.

Hope that helps,
- Jeff