SL2 intermittent rough idle in gear

Hello! My daughter and I have restored a 1994 SL2 . It has a new short block, head, and exhaust with Cat. We finished the mechanical back in October, and replaced the interior and paint over the winter and spring. We had filled the tank and drove the car approx. 60 miles. The car basically sat for about seven months, inside the garage, and outside in cold weather. We drove the car the other day for the first time. The car runs fine, but intermittently idles rough in gear. I added octane booster and drove the car on the hiway until it was empty and filled up with high octane fuel. It still idles bad in gear. Okay… Tech stuff… Vacuum gauge shows 17-20 hg. When running good , 5 hg. When running rough. When you shift the car into neutral it idles up to about 2,000 rpm and slowly goes down to 750-800 rpm. (acts like a vacuum leak!!!) I have checked for vacuum leaks everywhere, I even resealed the throttle body with a new gasket. It has a new IAC, MAP, EGR(non linear), Ignition coils, IAT, O2 and CTS... scan tool shows engine temp to be on the money. I even flushed the injector rail with a carbon clean machine, and ran 1L. of cleaner through each injector wide open. The cylinder compression readings are 175 psi. each, and hold for awhile. Drive cycles have been learned, and fuel trim is normal. Anyone have any ideas??? I’m pulling out my hair! The only thing I can imagine is possibly random ignition module timing issues or sticking valve!?! Oh…It does it hot or cold, open or closed loop, and NO SES light/ or codes. I’m pulling the head off Friday night. Sorry for being so long winded! I know how easy it is to over look something obvious when you over think things. I hope thats all that I'm doing! Thank You for your time!!!!!:banghead:

I dont think pulling the head is needed yet. You said you rebuilt the engine. How did you break it in?

Who rebuilt the head? Was it pre assembled?

Type of plugs? No platinum. NGK Copper only

Make sure the back of the Ignition module is clean along where it mates to the tranny.

Your car has two ECTS. The one with the two wire connector MUST be brass. THis is what the computer uses, not the reading from the gauge.

Make sure the grounds are secure including the two on the back of the engine under the 4th intake runner

Hi RC1488! Thanks for the advice. I guess I should mention that I'm a factory trained Saturn Tech from Tennessee, and am ASE Master certified. I admit though, I haven't worked for Saturn since 2000, so I'm CERTAIN that I'm out of practice with them. I have dotted my I's and crossed my T's. The coolant temp sensor is operating correctly. The plugs are OEM NGKs. The engine is a "crate" from Seattle via Napa. I got it for the cost of shipping as payment for a favor. I've considered the low compression thing but the engine runs so well when not having this problem, plus the cylinders all have the same reading, and the sleeves are all 0.75 over. I'm leaning toward a possible sticking valve from the possible "crappy gas"... Maybe dump in a bottle of Techron. Or a weak spring maybe? Because of the fluttering needle on the Vacuum gauge. What do you think? Also have you ever come accross intermittent ICM issues... like timing changes??? Thank You Sir !! Regards Jeff in Alaska

Jeff ...In Alaska!?! haha wow lol

Whats the temp up there?

Another saturn tech eh!?! Few of you guys around these days :)

Hmmm...I agree, the low compression is intriguing, but at as you said they are all the same so most likely not the issue.

Its possible its a sticky valve/bad spring, but they are rather rare i feel. Most times its something small instead.

The weird vacuum reading is odd. My first thought is the EGR Valve. Even new, sometimes they dont work. Try this:
http://www.youtube.com/watch?v=8YRq2Xl58Oo

Yes I have had issues with the ICM Module causing rough running. Usually its just in need of a good cleaning on the back and the tranny. Since you have new coils, Id just double check to make sure they are on secure.

The other thing, is how long it sat for. It may be worth your time to do a MMO (marvel mystery oil) Soak, which may loosen the rings and perhaps increase your compression. 7 months is a long time :) I suppose some Techron cant hurt either.

Last is to double check your wiring. Really make sure everything is secure and nothing is loose (grounds). Ive thouhgt everything was good but things do come loose.

Good luck!!!

Yep... Anchorage! We have had a record summer so far, high 70's... The sunshine up here can feel like the 90's though! Without A/C the house can get a bit warm. Makes for bad fishing too.

So I Ran a half of a bottle of Techron through the PCV hose. Took care of it ohhh maybe 90%. Which leads me to believe my first gut instinct which was a sticking valve. I overhauled the transmission and stuck that motor in there back in Oct. and we put maybe 60 hiway miles on it. Then we took off the body panels and left the car under a tarp outside in -30 for about two months. Around christmas I threw a heater under the engine compartment and let it Idle for about a half an hour before pulling it in the garage, and it hadn't been run again untill Memorial Day weekend. And I don't know how long it sat in a crate before I got it. Hind sight I think maybe I should have done the body first and the engine in the spring! I'm sure it was hard on the valve guides being new and tight and then me hardly running it, plus that old gas. STICKY!!
I thought about a top engine soak, but I have never poured Marvel directly into the cylinders. Only in the crank case or for soaking lifters in. What do you think it would do to a brand new cat? I cant imagine much, but... I'll do it like the old GM top engine cleaner solution. Pour a few ounces in to each cylinder and cold soak it for a couple of days and blow it out without the plugs, throw some old used ones in there just to run it out, and put the new plugs back in after a few miles.

Thanks Again!

Sometimes all it takes is another good brain to get you going in the right direction! I've got so much useless information in this old pee brain of mine I've forgotten some of the old time basics! :screwy: I'll let you know how it works out!

Regards Jeff

Yeaa Im thinking the sitting with the weather could be your issue. Its good that the techron helped alot.

We mostly suggest MMO soaks to loosen up the oil rings, but I have used it to loosen up the compression rings also. I had an engine sitting outside for 2 years. Did 2 MMO soaks and compression was back up to an average of 179psi across on a 200K engine! :) Your crate engine should def be above 200 I would think.

How you descibed the process is exactly the procedure. You can Seach MMO soak that I posted here on the forums a little while back.

As for the cat, it will be fine. Just blow out the excess oil out the spark plug holes as you said.

I then would use the rest of the MMO in the oil (as they are hydraulic lifters) and then just drive the car. Put some miles on that engine a little bit with some nice clean gas , then change the oil and filter, and see if that helps.