Where to get fault codes scoped?

Hey Guys,
I had my plugs changed and now my car runs terribly, like it’s only running on 5 or 6 cylinders instead of 8, including backfires. My neighborhood mechanic did the work, and ran a basic OBII test that said P0300 Multiple misfire random cylinders, but I obviously need a more specific readout or I’ll be changing coils, plug wires etc not knowing what is needed.
I want to have tests run at a place that will give me the results without selling me on superfluous repairs I don’t need.
PS I know I should have done the plug wires too!
thanks ya’ll

One thing that can happen when you disturb old plug wires is you end up with cylinder misfires.

Replace the plug wires and then see how the engine runs.



These days spark plugs often outlast plug wires. And when the wires begin to fail and internal resistance goes ballistic voltage spikes back through the coil(s) and into modules, etc.

But then maybe COP ignitions were developed to avoid the plug wire problem.

I figured you’d say that, thanks, I will…I can see minor burns on the front wires where they lay on the block. Any idea what I should expect to pay for changing wires (labor, I know what wires cost), I have a V8 Northstar so the cover has to be removed, coils too I think. It took my shadetree mechanic 2 hrs to swap the plugs $80

I would change the wires myself, making sure I do them one at a time to make sure I hook the new ones up correctly.

That raises one possible cause of your problem: Your neighborhood mechanic might have connected the wires to the wrong plugs. It would really take a careless person to mess up this way, but it is possible.

To avoid the cost of towing, see if you can find a knowledgeable friend to make sure the plug wires are connected correctly. If you can confirm they are connected correctly, buy new ones and change them out your self, but make sure you do one at a time.

Found this for a '98 Aurora:

Picture 5

I agree. And when you replace the plug wires, make sure you check the correct order. It’s possible that this guy removed all the plugs, replaced them, and then screwed up the firing order. He would not be the first to do this. Many years ago, in the days when all engines had distributors, it was a common novice mistake. When changing wires, they should always be done one at a time to prevent crosswiring.

I see Whitey and Insightful commented on the same thing. I’ll leave my post as an emphasis to theirs.

with all that effort to do the plugs, did this neighborhood mechanic suggest you replace the wires at the same time you did the plugs?

If not, I’d stop using this mechanic.

See your other post but Tester beat me to it here. The other thing with the Northstar is that the coils can go bad frequently.

Hey Whitey, yeah, very unlikely he crossed wires, in the Northstar V8 it has 4 up front and 4 in the back by the firewall, so he was really only dealing with 4 at a time. I would like to know if you think the plug wires are simply worn and arching, I can see scoring where the wires upfront were laying on the block

I assumed you’do tell me this, thanks, what a stupid move trying to save $60 hoping the wires were ok…the car ran twice as bad once he changed the plugs…the whole it was clattering when I revved the accelerator cable,it even caused the cam to shake mildly at the pulley where the serpentine belt is…it must be running on like 5 or 6 of the 8 cylinders

You don’t think this could have knocked the timing off too, do you. I don’t have a timing light

Please don’t tell me you think the coils are now bad too.

Yes, that is certainly a possibility, but there are others too. Unfortunately, there is no way for me to know for sure without replacing the wires and seeing if the vehicle runs better.

Unfortunately, running the car with the wires hooked up in the wrong order can damage your engine, so I have to strongly recommend you not start it again until you have confirmed they are connected in the right order. The fact that your shade tree mechanic worked on four of them at a time alarms me even more. He should have worked on them one at a time.

I feel as though I cannot over-stress the importance of making sure they’re connected in the right order. It’s not hard to do, so there is no downside to confirming it.

1 Like

If the engine is running worse after replacing the plugs you need to go back to the beginning. Remove and inspect the plugs, replace the wires and inspect the coils.

But I don’t believe that there is any adjustment on the ignition timing to knock off.

Well, the wires sound like they are worn out anyway but a hamfisted repair could damage wires by yanking them off without exercising proper care. They get stuck on the plug tower and can be a bear to remove. They make (and I still have) tools to help pull wires without damaging them. But the black soot marks at the spots where they were touching the block mean they have been questionable for some time.

I doubt he got the wires mixed up. They are in wire looms in order and pretty hard not to get them right. At any rate if they are arching against the engine they need to be replaced and they are grounding out that plug. Good OEM wires have been over $100 for a long time. On that car though I replaced coils as a routine maintenance item from Rockauto. Replace the plug wires though and check for good spark to see if coils are bad. The wobbly wheel though is of concern. Have fun replacing the water pump or radiator or the starter motor or even the serpentine belt without the factory manual. Oh yeah and cleaning out the EGR passages requires pulling the transmission. Another happy former Aurora owner.

1 Like

The OP mentioned backfiring after the plugs were replaced and that does bring up the possibility of crossed wires for me @bing. And also were the plugs used the brand that was on the McParts’ stores monthly sales quota? Bosch likely makes great plugs for Mercedes Benz and VW engines but I have seen their GM and Ford plugs cause some problems. And often shade tree mechanics damage plugs when installing them on the back side.

BTW, I don’t recall ever working on a GM V8 engine with a firing order other than 1-8-4-3-6-5-7-2.

Keep in mind this was done by a shade tree mechanic who either didn’t notice badly damaged wires or noticed them and didn’t bother saying anything.

1 Like

Not to disagree but my factory manual for the Aurora 4.0 says the firing order is 1-2-7-3-4-5-6-8. Now I’ll shut up.

1 Like