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New Plugs and Wires but Still Misfires?

Hey I just bought a 2001 Buick Lesabre and it was driving fine until this morning when it misfired on the highway. I was going about 40 mph and I felt a little jerk in my engine and my CEL started to flash. I pulled it over and got it towed to my house and my code reader says a p0300…which is a random misfire code. So I figured i just need a tune up so I went and bought new plugs and wires and replaced the old ones. And to my surprise…still a noisy shakey rough erratic start and idle. Before this there were no problems the car shifted properly to all gears and picked up and accelerated like a champ there were no obvious issues. Now I cant get it out of my driveway. I was wondering if anyone knows any other problems could cause this misfire, and how to eliminate problems without buying parts? Thanks!

When you replaced the spark plug wires, are you sure you got then in the right order? It is also possible that one or more of the new wires or spark plugs is defective. When you perform a maintenance function and the car runs worse after, you have to review the maintenance function first.

If you replace the wires, it is often also a good idea to replace the cap* and rotor* because they age similarly.
(*A LeSabre still has those, I think).

Im sure that they are in the right order and the spacing on the plugs are where they are supposed to be. I did make sure of this. Fortunately the car had the original buick wires on them and had a number on the wire. Just replaced them one by one and followed the original routing of the wires and it doesnt seem to be any worse but its not getting any better. And what do you mean by review the maintenance function first? I dont understand what that means can you please elaborate? And I think if one of the new plugs were bad it would show up on my code reader wouldn’t it? As a separate code p0301(cylinder 1) or p0302(cylinder 2) etc? But instead I am getting the same flashing CEL and p0300 code.

I may have mis-interpreted you original post, if so I apologize. I assumed that it only started and idled roughly after you replaced the plugs, but you may not have even tried to start and idle the engine after the tow.

You might have an issue with the crankshaft (or camshaft) position sensor. Even though you still have a distributor, the computer still controls the firing and timing of the spark plugs based on an input from the CPS, among other things. Maybe the connector came loose or got water in it or the sensor itself is bad.

When you get a P0300 out of the blue like that, it is not likely that several spark plugs failed at the same time. It is more likely that it is a common or shared item. That would include the crankshaft (or camshaft) position sensor, coil to distributor wire, coil, rotor, or ignitor.

I am not sure but i dont think I have a distributor car/ rotor assembly…I think I have a ignition coil packs but I am not sure if that even has the same function as what you are talking about. I really dont know but I really dont want to take this in to the mechanic…they make too much and do too little!!!

Thanks to all for the advice I really only know a little about cars but enough to do little things like tune-ups, brakes, suspension, but no engine repair, electrical or diagnostics. Is there a way I can test these parts before removing them or replacing them? Or a way to determine what 1 or multiple parts need replacing?

Thanks Keith I will take your advice about the CPS. But are there any other problems I can take a look at before I go to the mechanic?

Your Buick doesn’t have a cap & rotor the ecm tells the coils when to fire. i would double check the wires to make sure correct wire going to the right plug. it cold be a crank sensor or a coil going bad the car has 3 coils on it. the coils how do they look where the wires conect to them is the post clean or is it pitted or corrioded

I thought so big but I wasnt sure and I didnt want to sound too confident but yes I do plan on replacing the coils before I send it to a mechanic.Thanks!

Several of us folks on here who help others are REAL mechanics. If you would like help I would suggest watching the insults. So what’s your career? Do you invest hundreds of dollars a month in your career? We do!

Did a little research for you but my internet died, just got it back up. Found a link that might help, but its for a 98 model 3800 engine. If yours is the quick start 3800, it won’t apply.

On older GM 3800, the CKP (crankshaft position sensor) was located behind the harmonic balancer. Because of the difficulty in getting that off, you would be better going to a mechanic. The quick start 3800 uses two CKP’s and a camshaft position sensor, so that gets a lot more difficult to troubleshoot.

I googled “CPS GM 3800 motor” and saw that there were a couple of videos on YouTube showing how to troubleshoot a CPS or CKP. You might want to look at those.

“And what do you mean by review the maintenance function first? I dont understand what that means can you please elaborate?”

I mean go over your work, which it sounds like you did. Even the best can make a mistake.

If mechanics do so little for their pay why are you here? They are trained in troubleshooting/diagnosis and is part of what they get paid for beside just making the repair. I’m not a mechanic, but do most of my own repairs. I’m only able to do this after lots of studying and trial and error over the last 36 years. Regardless of what you think a mechanics job IS NOT EASY and tools and equipment are not cheap. Many times even after you diagnose the problem and are making the repair unexpected things will come up like rusted/stuck/broken fasteners which also have to be removed or repaired. If you can’t do it yourself you have to pay the price. I’d rather pay someone to find the problem than to waste time/money throwing parts at a problem which it sounds like you’re getting ready to do, then still have to take it to a mechanic to have it repaired properly. Already replaced plugs/wires and going to replace 3 coils before sending it to a mechanic for diagnosis. Although there are a few mechanics who start throwing parts at a problem a real mechanic does the diagnosis and finds the real problem before replacing parts. In your case you need to invest in a repair manual study, diagnose and repair before wasting a lot of money/time on unneeded parts. A Chilton’s manual will tell you how to test most parts without having to buy a new one to find out if the old one is bad or not.

When I replaced my wires, I didn’t get one on all the way (unknown to me) and also had a cross fire. It was so bad I had to have it towed. So as said, first thing is to go back and check your work to be sure. Then I agre with Keith on the crank sensor, coil, etc.

maf sensor is easy to clean and check, A proper analysis can save more than throwing parts at a problem, good luck!

You would be better off finding out which cylinder(s) are misfiring, and then directing your efforts toward that specific cylinder(s). Any competent mechanic who does too little will have a scan tool (not just a code reader) that will give you real time data and misfire counters for each cylinder. Expect this diagnosis to cost $50-$100.

Alternately, since the wires and coils are so easily accessible, start the engine and pull the plug wires off of the coils one by one until you find the one(s) that don’t make the engine run worse. Then you’ve found your misfire.

I am going to go another way with this, your lesabre has a 3.8 V6… These are known for bad intake gaskets… I have a 98 Lesabre and when it started to go I got a missfire code, and flashing light. I also thought plugs and wires… The missfire would come and go before and after the plugs and wires, and I noticed I was loosing coolent with no external leaks.

This maybe happening to you as well, is your coolent low?