"98 Oldsmobile Aurora Codes P0300B Misfire and P0440B Evap Emmission

I have a 1998 Oldsmobile Aurora with 103,000 miles. I’m receiving the codes P0300B Misfire and P0440B Evap Emmission.

I use Premium Unleaded gas (not at a cheap station) so I don’t think it’s the gas causing the misfire code P0300B. Also I’ve put on a new gas cap but I still get the Evap Emmission code P0440B.

Does anyone have experience working on a 1998 Oldsmobile Aurora with these 2 codes?
Thanks for your help.

I’ve got a 95 and my book doesn’t have those codes in it. Do you have the Northstar V8? The Northstar has a history of eating coils so the coils and plug wires would be my first suspicion. I was warned to only use the $100 GM wires also. Don’t know about the evap, only had EGR issues with mine. Still do but it only sits now.

coil packs or wires.

Thanks for commenting on my post. It’s a V8 but I’m not sure about the Northstar; how would I know if it was the Northstar?
While looking at the car this morning while the engine was running I noticed one of the spark plugs/wires shaking. I took the car in for a test and yes the problem is there was no connection being made on the one spark plug/wire. I bought the Duralast wires and ACDelco plugs. I’m going to try to find someone to help me put them on; is this a hard job. The shop was going to charge me over $600 (said it was a 3 hour job).; a car dealer s would probably cost more to install but maybe I will consider buying the GM brand and have someone help me put them on. Why should I use the $100 GM wires? What are coils? I hope this solves the misfire problem.

Regarding the other code for the Evap Emission, I was told I would have to have the car smoked to find the leak. The shop I was at today didn’t have a machine to smoke the car so I will take it somewhere else.
Another question/concern, why do these car sit so low, is there any way to make them higher from the ground; seems I’m always scraping the bottom of the car. I sure hope I can drive this car for awhile; it only has 103,000 miles on it. Thanks again for your help. I appreciate it.

I didn’t mean to click on ‘disagree’ to your comment.
I have the spark plugs and wires on my car.
What are coil packs?

You can undo a disagree simply by reclicking it. A coil pack is the set of individual coils, four in your case, which are all mounted on the electronic ignition module. They can be found at the other end of the spark plug wires.

Your evap problem may be as simple as a gas cap not sealing. In that case you just simply replace it.

Thanks My 2 Cents.

I buy the best wires I can get because the insulation is thick and they last longer.

Pic of module location

Update on my "98 Oldsmobile Aurora, V8, 4.0: I changed the spark plugs and wires (but not the coil pack) about 10 days ago. The only other code I got after replacing plugs and wires was P0440 Evap Emission Control System Leak or Malfunction and the dash board light “Service Engine Soon” was on.

Then I drove the car around town and on the hi-way putting about 75 miles on the car. I drove the car because I was told the code P0440 would not damage the car. The car drove fine to me. Then today I took the car to a shop to get smoked to find the leak for code P0440 and found out now I have 3 codes: P0300 Misfire, P0440 Evap & P0742 torque converter clutch solenoid stuck on. The shop would not smoke the car because it has the P0742 code.

So changing the spark plugs and wires did not fix the misfire. Does anyone have suggestions where else the misfire may be coming from? Can I get the car smoked before fixing the P0742 code problem. Is the code P0742 a major problem or hard to fix; is it inside the transmission? If the transmission was recently rebuilt should this problem be covered by the transmission warranty?

Also the car is only getting 15 mpg; could this have to do with the evap code? I also had the fuel filter replaced 5 months ago and was getting better gas mileage before it was changed.

Thanks for your help.

The reduced mpg is probably due to the misfire, not the evap problem. You have a general misfire, not specific to one cylinder. If specific to one cylinder, you can swap stuff around, like the coil packs, to see if the problem follows the swap. But a general misfire is harder to diagnose.

What’s happening is the computer is firing the spark plug and sometimes noticing that the crank shaft isn’t speeding up a little due to the firing. Which it should, right? An explosion in the cylinder should force the piston down and speed up the crankshaft.

So you occasionally have no explosion. What you need for an explosion is

  • a good spark
  • at the right time
  • with compressed fuel and air in the cylinder
  • and the correct mixture of fuel an air

The problem is almost certainly among the above list. A good mechanic should have the equipment to measure for good spark. A diy’er might use one of those in-line spark testers. Spark problems can come from crank and cam sensor too. And from the igniter module. Timing is something that is usually pretty easy to check. And compression should be next on the list if the spark is good and at the right time. You may have developed leaky valves or rings. Mixture is harder to determine, but shops would start with making sure there’s a new engine air filter, then a fuel pressure test probably, then verify all the injectors are pulsing like they should.

If the TC lock-up clutch was stuck on all the time, it seems to me the engine would die at stoplights. Since it doesn’t, maybe that’s some kind of sensor problem, and the clutch is actually ok. Do you notice anything weird going on, like a brief or lasting rumbling sensation, at speeds between 35-50 mph?

I don’t see any reason why the evap problem can’t be addressed independent of the TC. Beats me why they said that. Federal regulations require that cars self-test themselves, to insure gas fumes don’t escape from the fuel tank into the air. It’s a complicated system that prevents this from happening, and differs from manufacturer to manufacturer how they do it. But the car either pressurizes or forms a vacuum in the airspace above the tank, then measures to verify that condition holds. Otherwise it will flag a leak. Another common evap problem is caused b/c it can’t vent that space like it want. It’s uses a solenoid valve called a “purge valve”, and purge valve replacements are a common thing here to address evap problems.


There’s absolutely no reason why P0742 SHOULD prevent the shop from performing an evap smoke test

Perhaps they’re fishing for more of your money

In other words, perhaps they also want to get paid for diagnosing P0742

Here’s another scenario . . . they know that if they diagnose and repair the evap problem, the check engine light will stay on, because of P0742. And you might be upset, because you’ve paid for a repair, but the check engine light is still on

If you tell them to extinguish the light, it will come back on, because of P0742

I’ve resolved the codes P0300 and P0472 but can’t get rid of code P0440.
I have had the car smoked and the mechanic could not find a leak and the gas cap is good. I replaced the air filter. If the purge valve is bad wouldn’t the mechanic find this when he smokes the car or is there another way to test if the purge valve is bad? Also I’ve been driving the car and it sometimes stales and dyes at stop lights but starts back up with no problem. The battery checks out good.

Also I don’t know if this is related to the evap problem but the oil pressure psi monitor reading goes from 19 to 60 is this normal? What should the oil psi be for a V8 Olds Aurora?
Thanks again for your help.

Your oil pressure is fine, and it’s got nothing to do with the evap system

GeorgeSanJose or Anyone who cares to answer this.
I just reread your Sept 14 posting above. Although I’m not currently getting the “P0742 torque converter clutch solenoid stuck on” code, the car dies at stop lights…could this be related to the P0440 Evap code? Also you asked if I had noticed any brief or lasting rumbling sensation at speeds between 35-50 mph…what could this mean?
Thanks again for your replies.

I dunno but I replaced all the coils on my 4.0 for a misfire and was told they eat coils. Also when mine was stalling when I let up on the gas at low speed such as coming to a stop light, it was the fuel pressure regulator. It may be time to just pay for the diagnostics but no one really likes to work on these anymore.

These EVAP codes can be almost impossible to solve.

A purge valve will not be detected by a smoke test. A smoke test only pumps smoke into the empty areas of the system checking for leaks. The purge valve, and its associated solenoid, allows fuel vapors collected in the charcoal canister to be drawn into the engine’s intake system to be burned by the engine. The charcoal canister collects fuel vapors whenever the tank becomes pressurized relative to ambient pressure. That happens when refueling as well as when the gas in the tank becomes agitated (starts sloshing around) just from normal driving, and even when the car sits in the warm sun after sitting in the cool air overnight.

There is a vacuum pump that self-tests the EVAP system every time you turn the car on. And some valves to control the flow of fumes when you fill the tank. Usually if there’s a “circuit” (“circuit” including the paths that the fumes follow through) malfunction, a more specific code will trip. There are a few dozen EVAP system codes. A generic code like this one unsupported by a failed smoke test (no detectable leak) suggests the possibility of a saturated or contaminated charcoal canister, causing an inability of the system to breath. Unfortunately, I know of no definitive way to test this.

I wish I could help more. Hopefully one of the other regulars here can.

If you want to guess cheap, then throw an evap purge valve at it and clear the code. Sometimes they stick intermittently, causing extremely hard to diagnose evap codes

The best advice I can give is to replace the part and clear the code yourself. That way, you’ll save on labor, and if the same code eventually returns, at least the purge valve was cheap

The labor to diagnose and repair can easily exceed the value of the purge valve

Just a thought :neutral: