Diagnose this car engine

Factual Questions
1

2000 Chevy Prizm, with 258K miles.

Check engine light went on - fortuitously, a few days after passing state inspection - and AutoZone says it’s two codes: P0300 and P0301. What these mean is that there is a misfire in a “random cylinder” and Cylinder 1, respectively. (I’m not sure if a misfire in Cylinder 1 can also trigger a P0300 or if this code means that some of the other cylinders are also misfiring on occasion - my impression is the former.)

I did a dry/wet compression test, and got the following results:

Dry: 90, 180, 180, 150
Wet: 125, 210, 200, 245

So it looks consistent with Cylinder 1 having some sort of issue causing the low compression and possibly the misfire as well.

The fact that the numbers went up for the wet test is apparently an indication of worn piston rings, but I would think the fact that Cylinder 1 was only 125 even when wet would indicate that it has another issue as well. (The 150 on Cylinder 4 is OK on its own, but is apparently an issue in conjunction with the other numbers. I don’t know what to make of the 245 reading on the wet test, but it might have something to do with my having accidently poured in too much oil for the wet test?)

The spark plugs looked fairly normal (bit of whitish coating).

One thing I did notice is that the upper (non-firing) end of Cylinder 3 was coated with oil, and Cylinder 2 had some as well. There were other signs of a leaky cover gasket, and having taken the coils out anyway, I replaced the gasket. But my impression is that this wouldn’t affect engine performance.

In terms of driveability, I’m not seeing any major signs of misfire, not that I’m an expert or anything. The idle is probably a bit rough, but not terrible, and gas mileage has declined over the years (though this is clouded by the fact that it’s been converted to an around-town car from a commuter car).

Anyway, the bottom line is that as above this is a very old car and I’m not going to be putting in major bucks for an engine overall or the like. And I don’t know that it’s worth spending a lot of money on sophisticated diagnostics if the likely result is that it needs major engine work. Having just passed inspection I can drive with the CEL until July 2018. But what I’m wondering is to what extent it’s possible to narrow down the options and/or rate the odds of it being one thing versus another based on the above info.

2

A misfire in any cylinder can set P0300. The code indicates the computer could not definitively identify which cylinder misfired at the time.

That’s the textbook explanation, but in the real world it’s often not the case.

Almost certainly. Excess oil can take up a significant percentage of the combustion chamber volume, which noticeably affects the compression ratio.

You’re right, it wouldn’t.

This indicates it’s not a dead miss (i.e., constant and complete)…yet. I’d say it’s running on “three and a half” cylinders.

Leaking engine valve or worn cam lobe are probably the most likely scenarios. It could cost a hundred or three to get a definitive diagnosis, and perhaps many hundreds to fix.

3

At 250k, major engine work is almost always a third or fourth choice. Tearing down part of the engine is likely to cause other fragile parts to fail, compounding the repair costs. Unless the car is of exceptional value or rarity (uh, not the case here, IMVHO) or one of the few models known for even longer basic engine duration, either a complete rebuilt mill or a new car would seem to be the optimum choice. Several hundred on diagnostics to pinpoint either major block or head work… would be DTOD, I think.

4

I think I know what the issue is:smack::rolleyes:. Is that the original engine? You seem to have above average level of auto mechanic skills, able to perform a compression test. You must have some tools to remove the spark plugs, and carry out the test. The engine is wearing out. You can fix it or drive it till it’s dead dead. I suggest you start saving for a new used car, possibly a Toyota Corolla of the same vintage as the Prizm as your existing tools will be compatible.

Good Luck!

5

could be a few things:

  1. cylinder taper (the cylinder has worn larger at the top of the piston’s travel, and the rings no longer seal well,)

  2. valve seat erosion to the point one or more valves no longer seal

  3. head gasket failure in cylinder #1, but that usually brings other noticeable symptoms too.

the fact that the “wet” compression readings went up that much on all cylinders makes me think all of the cylinders are tapered to some extent. The engine’s just getting worn out.

note that in my experience, when people say the piston rings are “worn out” it’s usually actually cylinder taper. The piston rings are made of hardened steel, and the cylinder/liner is usually some form of (less hard) iron. piston ring breakage is known to occur, but that’s usually spurred on by repeated, heavy detonation.

6

I just remembered one thing that I forgot to mention in the OP.

When I removed the coils, the coils for Cylinders 2-4 were firmly fastened to the spark plugs, and had to be pulled hard to get them off. But the Coil for #1 came off very easily. Similarly, when I reinstalled them, #2-#4 clicked on to the plugs, but #1 just slid on and didn’t click into place despite being firmly pressed down. Thinking something might be wrong with the coil itself, I switched the coils for #1 and #2, but the same thing happened - the #1 slid on and the #2 clicked into place. (Should have switched the plugs too, but I didn’t think of it at the time.) So I started to wonder if that might be connected to the misfires in #1.

7

Much less likely, I would think, than the low compression in #1.

If you want to be sure, swap the current #1 & #2 plugs, and the current #1 & #3 coils. Clear the code memory. If P0301 resets, it’s neither the plug nor the coil causing it. (If P0302 sets, that plug has a problem; if P0303 sets, that coil has a problem.)

8

I would get a rebuilt engine or a new car. A rebuilt engine would be MUCH less expensive!

9

Granted. But the thing is that even the wet test for #1 is a lot lower than the other cylinders. So that suggests to me that all cylinders have some wear, but that cylinder 1 has something else going on as well. Which leads me to wonder if perhaps fixing that something would make #1 more similar to the other cylinders - somewhat worn but still with OK compression, and possibly fix the misfire.

OK. (And thanks a lot for your detailed responses and professional expertise.)

But what would cause something like that? (The plugs look identically shaped to me.) And could that itself be a problem as well even if not the cause of the misfire?

The problem with that approach is that the transmission also has 258K on it and any number of other components are the same. These are running fine now, but if you put in a rebuilt engine now, you could be looking at spending some more serious bucks shortly thereafter if and when these other components fail. Or you might not, but at any rate it’s a pretty serious risk. And if that happens, then your costs might end up being a lot higher than a new car.

10

The compression figures for #'s 2, 3, & 4 are fine (well, 2 & 3 are fine, 4 is okay). The difference between dry and wet is not significant. The fact that the figure for #1 is about half of the others is significant. It’s almost certainly not due to a problem with its piston rings or cylinder, and very unlikely it’s due to a head gasket problem. Review the last line in post #2.

11

A new car is $20,000.00.

How does a rebuild engine plus a rebuilt transmission cost even half of that?

12

I meant a “new to me” used car. I wasn’t thinking of buying a new car. I’ve never bought a new car in my life.

I bought this particular car 10 years ago for $3,600, with 102K miles. I’ve had it for 10 years, put 156K on it, and had no real major repairs other than the standard brakes, tires, muffler and the like. It’s paid its dues. Rather than put $2K into it at this point it would make more sense to try to get another deal of this sort.

13

Yes, if low mileage, then that WOULD be better!

14

Wow, does anyone else find it amazing that a car today is capable of hitting 250k miles? I remember back in the 80’s if you had a car approaching 75k you started to get worried it was going to be a hover goner soon.