Rough Idle/Shaking


Asked by Jun 07, 2010 at 10:24 AM about the 1989 Pontiac Grand Am

Question type: Maintenance & Repair

I have a 1989 Pontiac Grand AM LE 2.5ltr. and It has been a process bringing this car back to drivability since I've purchased it back in 2007 for a mere $800.00. Currently, the car is shaking at the front end and has a hard vibration that can be felt inside the car while running at idle, and driving. Prior to this, the car would stall out with all of my lights coming on in the dash while stopped at a light or intersection. Often it would not start again until I would let it sit for awhile, and eventually stall out while coming to a stop. The matter got progressively worse through the months. I've taken it to several places and was told that it was my ignition control module. I've changed that out three times in the last six months because I was told that they fail often. The alternator is only a year old... so is the water pump... I've also changed the Head Gasket... Intake and Exhaust manifold gaskets... the coils the spark plugs and wire set.... I've just put in a new PVC valve... MAP sensor... O2 sensor and Idle Air Control Valve.... I've changed the fuel filter, the fuel pump, and the Sending Unit in the Fuel tank. Changed my throttle body gasket that had a bubble in it...and even changed the computer to the car... Still when I turn the car on... the motor shakes and the vibration is felt inside the cab of the car and can be heard out the exhaust... The Valves and lifters are fine... and getting a good supply of oil. Lifting the oil filler cap at the top of the valve cover and listening inside... there is no irregular purrs. Has anyone out there experienced anything like this? Or, does anyone have any suggestions of what I've missed.... Oh... I also have a new crankcase sensor installed too... so, that's not the problem either... Someone please help me

11 Answers


I wanted to add also that I've purchased an Actron Code reader, and no codes came up either with the old computer and the new one... If anyone has a suggestion, I would appreciate it... Not sure if I should change the Cat. Converter or if I have stuck exhaust valves, or if My alternator has the wrong amps to operate the car...

1 of 1 people found this helpful.

Sounds like your car is "missing." I would check the plugs are good and properly gapped and inspect the wires for any damage. Inspect ALL vacuum lines, since your fuel pressure regulator is vacuum controlled, any loss in vacuum would cause the car to "miss" badly. If those are good I would remove the coil pack and inspect for any cracks. I had a bad coil pack on my cavalier and if shook and run like crap. If good there, remove the EGR valve and inspect for carbon buildup and to ensure it is closed. If your EGR is sticking open, it would create a "choking" symptom on your car causing the same problem you are experiencing. Know the previous symptoms you had before the car started running so ruff, my assumption is that the EGR is bad. If all is still good, a clogged up cat would also create similar symptoms to what you are experiencing (and honestly, being an 89, prolly should be replaced anyway).

4 of 4 people found this helpful.

Mike, Thank you for your input. I do appreciate it. What started happening with the car at first was that it would just cut off while at a stop light or idle. I replaced all of the hoses because some were so worn out that you could literally see cracking in them. I changed out the fuel system, pressure regulator, pump, strainer, and sending unit and the filter. Changed out the coil pack three times, the coils, and the plugs and wire set. I changed the Ignition control module, and the crankcase sensor as well... and the EGR is only two months old. Changed that too. I was thinking that it was my CAT Converter too... but, don't know how to tell if it is... I will say this much.... When changing out the Ignition control Module, I had to unplug it from the car, and take out the bolts. Turns out that the ground wire is connected to one of those bolts, and when I tried to take it out, the connection broke off because it was fused in between the bolt head and an nut that held it all together. I took the two wires and reconnected them using another connector and mini torched it all together and rebolted it to the Ignition Control Module when I put the new coil pack, and coils on. I don't see how a ground wire could be causing that type of problem...So, My next thought was the Caty... Let me know what you think of that ground wire thing... Should I even be messing with it, or, is it really the Cat Converter?

1 of 1 people found this helpful.

I am sure that ground also served as a "block ground"l. So long that it was hooked up in the same place as before. I would rule out that as an issue. Personally, I would remove the cat and see if the idle smooths. That will let you know if that is a problem. You can not run the engine for any long period of time because lack of a cat is not only illegal in most states, but it also can cause a whole other can of worms in regards to problems. But honestly, you can pick up a high flow cat on summit for less than 50 bucks. And for being a 20 year old car, I would just replace. FYI, most of the items you have changed so far rarely go bad.****************And I just thought of something...... Check the wires coming off your knock sensor. They tend to crack and break, and get burned by the exhaust.. That little bugger will retard your timing if it is not working right.. Causing the same issue you are having now...For a $20 part, may even be ideal to replace as well.. It is overdue to be changed!!!**********************

1 of 1 people found this helpful.

Thank You, Again Mike!!! You and I are on the same page with this one... The ground wire was actually placed right where the elect connections to the ICM are. Right through one of the bolts that attached the component to the block...Not too sure if that is considered a block ground, or, if I should ground it directly to the block and not the component. Would I have to solder if I wanted to add more wires, or would it be a crimp that would do it? I was just researching about the Knock Sensor in my Haynes book when I checked to see your response... I will check that today... I was thinking the same as you that it probably is bad and needs to be changed. My last resort will be the Cat. if all else is good. I will check the wires too... Tell me what you think about that ground though


To be honest.. I am not 100% sure on an answer.. I guess I would have to see it to help better... But if it is a block ground (also know as engine ground), it should not be going through anything. IF it is a block ground and it is grounded through your ICM you would be getting alternator feedback through the ICM... Although, I don't think this would cause your symptom (since the system is grounded), it will greatly reduce the life of both your ICM and Coil Pack.

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Best Answer Mark helpful

You are exactly what I needed to bring a method to this madness... Thank you very much... I'm glad that I'm on the right page with this, and getting your feedback is helping me in that aspect. I have checked the wires...and none of them are burnt, freyed, or broken... all is in tact... Knock Sensor is good as and it's connection... What I will try next is disconnecting the Ground from that ICM bolt and attaching it somewhere directly to the block... I think that will fix it...but... will keep you posted... You are right, though, it feels and sounds electrical... almost as if I have a faulty Voltage Regulator in the Alternator...but, the Alternator is practically brand new... I think that because the ground is not directly connected to the block, its causing these symptoms. If this does not to work I go on the Cat. Take Care


Mike... I replaced the Cat. and rewired the ground to the ICM, the car runs smooth now... But, I used a Universal Cat. and I'm not too sure but, even though the rough Idle is over, and the shaking is gone... When I take off from a complete stop at idle like at a Stop light, the Check Engine light will come on and while driving it would stay on for a few minutes and then go off... But, the car is running okay... The engine is the Tech 4... The Iron Duke... so it is noisey... but, the car is sounding the way she did when I first got her... I used the Actron Code finder and I brought up the Code #45... Which is O-2 Sensor, or circut, Rich Exhaust detected. Could this mean that Using a Universal Cat. be the wrong choice? I heard that the catalyst in a universal was not good or consistant with alot of engine functions. What do you think?


Sounds like you are having an improper back-flow issue from the universal cat. Personally, If the car is running good, I wouldn't worry about it.. But you will prolly have to change the spark plugs more often then normal.


I was thinking the same thing... I was told that the catalyst is not very good in the universal, and I'm kicking myself for cutting that corner. The car has its challenges in running... I cannot take off very fast from an Idle like at a Stop Light. If I do, there is a popping sound out of the rear exhaust, and the car hesitates in stalled out on me once...but, has not since then... Further, Initial starts once the engine is turned off can sometimes be a challenge as well... Have to use starter spray in the carb sometimes... After that she is running. So, my question to you Mike is, with the 45 Lean exhaust code, and the Back Flow concerns, what is the best fix for that, or is there one? Do I just change the plugs, or get another cat all together? Let me know what you think

1 of 1 people found this helpful.

iac or coil planium top or a bad cat or a timing chain has went bad cousing it to stall out shack and rattle at stops but runs perfect while driving it finding out the cat will plug up and glow hot red at the back side of the cat but the back pressure from that being plugged up will cause back pressure to the engine causing the engine to spin backwards that then it streched the timing chain and causing the timing chain to skip a tooth or 2 ahead of time throwing the timing off on your engine i just went through all this this past week i kicked the cat out the exhaust system what that means is i cut the stupid cat out put a straight pipe in place of it cost me about 50.00 dollars with the clamps and hot heat weld putty and new timing chain set with guides and as good as new again those cats can cause a bad fire can blow up your car i changed it to a straight pipe and clamps and heat weld putty around the joints and good as new and o2 sensors work with this god bless my master degree in mechanics pays off . they do this on race cars too and on certain other vehicles as well . cats are no good but if you want to put a replacement one in go ahead you can get a replacement cat for 100.00 dollars ok have a nice day.

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