Had my SLS in the shop for a tune up...plugs, plug wires, ERG valve...95,000 miles. Got it back and when you barely have pressure on the gas pedal it misses at low speeds and high speeds. You can accelerate and you get no miss, but if you are cruising at 60 mph and trying to maintain even speed, it will start missing. At one point I was cruising down the freeway, let off and started to get back into the throttle and I got a backfire.
I called the mechanic that worked on it and he thought it was caused by the fuel injector cleaner that he added to the fuel, but I have sense filled it up and it is still doing it. He suggested that I take it to a dealership and let them have a look at it and then call him, let him know what they did to fix it....
I am open for any suggestions....this has been a great car and I hate to treat it this way....I am also looking for a good mechanic in the Dallas, Rowlett, Rockwall area...I dont think I am going to take it back to the guy that worked on it ( $900 bucks and it is worse than when I took it in.)
I appreciate your help on this.

 

What was wrong before you took it in?

 

bad plugs and plug wires according to the mechanic.....it was starting to miss at all speeds.

 

My standard two things to make sure of are Air and Fuel Filter, just two simple things that should be taken car when any "Tune Up" is done. Now you say the engine backfired, did you check for any codes?

Other things to clean are the MAF and throttle body, use the spray cleaner. It will be hard to keep the engine running and it may code or stall when you do this.

After that I suggest the coils, make sure the posts are good and clean and that the boots are properly set on them. Also make sure boots are on the plugs as well, as often new wires pop off.

I always use AC Delco plugs and wires, not sure what was used but verify they are up to standard. He also use fuel injection cleaner so we will assume the injectors are ok. I am steering towards the coils if you find the wires and plugs in order.

 

I failed to mention that the week before I had the fuel filter replaced. Can you elaborate on the "coils"? Also, no codes after the backfire. The mechanic said he pulled everything and checked his work but couldn't find anything. Historically he has been reliable and primarily works on GM products....I'm lost, I miss the car's of the sixties, give me my GTO's back....

 

Well about the only thing not touched were the coils. There are 4 coil packs on top on the rear bank of cylinders towards passenger side of car. The sell for about $90 each. On your old GTO their was one coil and coils are often responsible for misses. Take off the wire boots and make sure their is no visible signs of rust or dirt or grim on the posts. Check for cracks in the outer casing. See what your mechanic says about the coils. He needs to attach his engine analyzer and drive the car, when it misses it will tell him which cylinder is missing and you should be able to pinpoint it. The car should be smooth as silk after everything you've done unless you got a bad coil or two.

 

Have you had an engine wash recently, that can screw up the coil packs also, it recently happened to a friend of mine with a 93 allante.

 

I haven't washed the engine lately. A friend of mine recommended a mechanic in Rockwall. I took the car by his shop and after showing him what had been done. He did some of his own testing. He noticed that the miss was between 1100 and 1800 rpm under load. I am going to take the car back to him tomorrow for more testing. He is leaning towards the coils since they are original equipment. He said he would test them tomorrow. I hope we are going in the right direction.
Thank you for your help and suggestions.

 

Good Luck, the coils are about the only thing left to look at. Too bad about that miss, just in the range you need for in city driving.

 

Well, just my luck....Mechanic ran two different diagnostics and got no codes. Pulled, cleaned and checked connections, even wet the engine down to see if problem would jump out at him, no luck. He has another mechanic at a shop nearby and he is going to let him have a shot at it, see if he can isolate problem. If he cant isolate problem I am looking at replacing components to see if it fixes problem. I am not looking forward to writing the check on this issue......I am starting to get the image of a funnel that I am going to be pouring money in....It gets me that it only misses when running down the road in that 1100 to 1800 rpm range and the one time it backfired that was strange.