P0300 code - 1998 Lumina - barely runs

Hi I am new to the forum. Just purchased a 98 Lumina with 132K miles, Aware of existing problem prior to purchase. Car barely runs feels like 4 or 5 cylinders, has some knocking and exhaust smells heavily of unburned gas and sulfur. Have to hold the accelerator part way down sometimes in order to get it to start, then it barely idles. (A chuggle is a better way of describing the idle). It will accelerate and shift through the gears. The brakes also have to be held hard to stop the car (could also be from it sitting parked a few weeks) Checked for loose or broken vacumn lines and electrical connectors, could not find any. Started by repalacing front three plugs (AC Platinium) and wires, still have problem. Figured I would start the rear three tomorrow but in the meantime I tried spraying water on all electrical (wires, plugs coil packs) cant see any evidence of arcing. Would like some experienced advice on how to proceed. Is it logical that a fuel filter could still be the culprit with these symptoms? or Fuel injectors, coil pack, ICM, or mechanical problem?

The previous owner claimed he was driving to the store and the bucking problem and check engine light started flashing. BTW I have ordered Proscan to try to help out but it wont arrive in time to really help with any diagnosis.

Thanks for any advice in advance!

The ignition system uses a wasted spark scheme. That requires that both spark lugs on opposite cylinders can fire reliably, since they are both part of the same secondary ignition circuit. In other words, replace all the spark plugs, test all the wires for continuity, and inspect all of the coil pack connections.

Other conditions can cause the PO300, including a jumped timing chain, wiped cam lobes, incorrect fuel pressure, fuel regulator leakage, a substantial vacuum leak, valve train damage, piston damage, a wet air filter element, and numerous other possibilities.

The ignition system uses a wasted spark scheme. That requires that both spark lugs on opposite cylinders can fire reliably, since they are both part of the same secondary ignition circuit. In other words, replace all the spark plugs, test all the wires for continuity, and inspect all of the coil pack connections.

Other conditions can cause the PO300, including a jumped timing chain, wiped cam lobes, incorrect fuel pressure, fuel regulator leakage, a substantial vacuum leak, valve train damage, piston damage, a wet air filter element, and numerous other possibilities.

Thanks for the response. Tested ICM and coils primary and secondary today all checked out OK, replaced back three plugs (Ac platinium) and wires to complete the set. Still rough idle, and acceleration. Engine started and ran slightly better, e.g giving about 1/4 throttle (just before the engine cuts out in park) engine runs fairly smooth. If I drive approximately 50 - 70 the motor feels smooth and somewhat responsive. I will have to try that fuel rail pressure test .. Any specs available for 98 3.1 fuel pressure? Is a trip to the GM dealer for top end cleaning warrented?

one thing I forgot to mention is during reassembly I noticed that the threaded elbow attached to the hose from the PCV valve (secures to the side of engine just under the ICM) seemed loose enough to be able to fall out with little effort. Could not find this rhreaded elbow at autozone. wondering if this connection not being snugly in place would contribute to the symptions.

IT would help to include what engine your car has, as there are different factors for each engine which will cause your symptoms.

The 3.1 is famous for puking intake gaskets, which will pump coolant into the oil. This thins out the oil, allowing it to escape past the rings and into the combustion chamber.
The 3.8 is famous for the EGR passage eroding the plastic upper plenum which will cause the coolant to get sucked into the combustion chamber.

In both cases, the coolant gets burnt and sent out the exhaust. It then gets stuck in the converter substrate, causing the sulfur smell.

IT would help to include what engine your car has, as there are different factors for each engine which will cause your symptoms.

The 3.1 is famous for puking intake gaskets, which will pump coolant into the oil. This thins out the oil, allowing it to escape past the rings and into the combustion chamber.
The 3.8 is famous for the EGR passage eroding the plastic upper plenum which will cause the coolant to get sucked into the combustion chamber.

In both cases, the coolant gets burnt and sent out the exhaust. It then gets stuck in the converter substrate, causing the sulfur smell.

Thanks for the response, Forgot to mention that the motor is a 3.1. Did the compression test today and found #3 cylinder dead, now my focus will be on searching the wrecking yards for a suitable replacement.

You may have just a bad head gasket. I would suggest tearing down your current engine, as you know it's history. The chance of you finding a good 3.1 in a salvage yard is slim, and most rebuilt engines (the ones sold at auto parts chain stores) are shoddy at best. If your engine isn't knocking or extremely noisy, have the head gaskets replaced, and take the heads to a machine shop to be inspected for warpage and cracks. Make sure you (or the shop you take it to if you don't have the ability) use the new redesigned metal framed intake gaskets, not the junk OEM plastic framed ones.

I'd make sure it doesn't just have a rocker that fell off or the stud pulled out...won't show any compression if it can't inhale....

I'd make sure it doesn't just have a rocker that fell off or the stud pulled out...won't show any compression if it can't inhale.... You guys are right, after I slept on it pulling the valve cover to look for the low hanging fruit or pulling the head if necessary makes the most sense. Seeing as though its #3 cylinder I dont want to deal with the frustration of it though, so I turned it over A former Chevy mechanic who is going to tear it apart, from the valve cover down to pulling the head if necessary for $200 max labor. I can live with that, as it looks like worst case for the parts is around $300 (If the problem resides in the head)

I will post pics if anything interesting shows up. Thanks for the responses.

I'd make sure it doesn't just have a rocker that fell off or the stud pulled out...won't show any compression if it can't inhale....

Well it turned out to be a broken valve spring, less than $100 to fix. Whew!!! Will post a pic.

one thing I forgot to mention is during reassembly I noticed that the threaded elbow attached to the hose from the PCV valve (secures to the side of engine just under the ICM) seemed loose enough to be able to fall out with little effort. Could not find this rhreaded elbow at autozone. wondering if this connection not being snugly in place would contribute to the symptions.
glad you got the big stuff fixed and it was a minor repair


BTW the tube you are talking about is GM#2450-4646 and is ~$12. Wont find it at a general parts store, you have to go to GM.

the 3 ribs on the end of the connector is flimsy rubber and it flattens out in time, best to fill the gaps between each rib with some silicone safe RTV to give it some strength... you can do this to your old tube and it will make a good seal, but I don't think it really affects performance with a small leak.

let it tack and dry pretty good, before putting it back in. And before you put the tube back in, wipe clean the hole on your valve cover where it plugs in for the best seal.