Smog Test and Check Engine Light Due to Low Compression

I have a 2005 Toyota 4Runner. 355,000 miles. No problems with it until now. About 2 weeks ago Check Engine Light came on. I replaced spark plugs (they were due), checked coils and fuel injectors and still have Check Engine Light on with code P0301 (Cylinder #1 missfire).
So compression tests were done and cylinder #1 had 120 psi and others 170. I suppose 120 isn’t terribly bad but is significantly lower than 170.
Question 1: If I reset the check engine light (turn it off), do you think it will pass California smog test?
Question 2: What do I need to cure the problem? Engine rebuild? JDM? or retire the truck?
Thank you in advance for your suggestions.

In California, if the Check Engine light is on, the vehicle fails the emissions inspection.

If you turn off the Check Engine light/disconnect the battery, the readiness monitors need to be reset.

And if the vehicles readiness monitors don’t reset prior to inspection, the vehicle fails the emissions inspection.

Look it up.


A 120 PSI is bad; 170 is very good considering the high miles. The misfire could easily be due o the low compression on No. 1.

I would suspect that the No. 1 cylinder has a cylinder head valve issue. That can be determined easily enough.

Considering the age and miles I would not spend big money to rebuild or replace the engine.
If the problem is related to a valve my suggestion would be to do a complete valve job on the existing heads or replace them with good used ones.

+1 @ok4450 valve job. Anything over 10% is bad.

@ok4450 How can you test if it is the valve or the piston rings? Remove the head for inspection?
Thanks for your insight.

Do a wet compression test.
Squirt a little oil in the spark plug hole and redo the compression test.
If the compression rises a lot then it’s the rings.
If the compression only rises a little, say to 130, then it’s a valve.

Circuitsmith is correct.

Could it just be a tight valve, needing adjustment?

It could well be caused by a tight valve. The problem is that the valves most prone to tightening are exhaust valves. They run almost red hot and the constant pounding of the valve on the seat causes stem stretch. Once the valve starts getting a few ten thousandths of an inch off of the seat the valve head cannot transfer the heat to the valve seat. That leads to the valve running hotter and exhaust gas passing between the seat and valve face.

It only takes a few miles for damage to start in this condition. Adjusting the valve to spec and removing the excess tightness may work for a time but how much time is anyone’s guess. It all depends upon the microscopic damage on the face of the valve.

If it is a lash issue it’s certainly worth a shot adjusting it. Due to the high miles of the car it’s possible that properly adjusted the engine could soldier on decently until the end.

Doing a valve job at mileage this high would be prohibitively expensive as it’s possible that many valves would not clean up on the grinder and some valve seats would need to be replaced along with guides and possibly surfacing the heads for flatness.

Some thoughts . . .

Is the truck running very badly?

Yeah, I know 120psi is low, probably low enough to cause a misfire

But . . . have you verified that it isn’t caused by something else, such as #1 injector, for example

Or even a bad pigtail for the coil and/or injector . . . this truck has high miles, and rotten wiring would not be out of the question. In fact, I’ve seen quite a few misfires caused by wiring

In all likelihood, the misfire is solely due to the low compression. But I would first rule out everything else

By ruling out, I mean inspect and test, not just throw parts at the truck

It’s always a good idea in this situation to check the valve clearances before doing anything drastic. A valve clearance check (vs adjustment) is usually an inexpensive test.

355 K on the same engine is a pretty good accomplishment in any event. Good for you for taking good care of your car. Since the problem is limited to just one cylinder, it’s entirely possible the low compression problem can be corrected without having to rebuild the whole engine. It might just be a sticking valve for example. You could listen to what Tom and Ray said about a common problem with Camry valves sticking a show or two ago.

It’s hard to say what you’ll need to do to pass Calif emissions, but it’s 100% certain you won’t pass if you disconnect the battery or otherwise turn off the check engine light. The smog folks are on to this technique already. You car’s computer will tattle on you, and you’ll just have to come back for a re-test. So better to save on re-test fees and get whatever’s causing the check engine problem fixed first. Best of luck.

I believe the recommended lash inspection interval is about 60k miles and if this car is like most, lash inspection is usually one of those out of sight, out of mind, and dismissed things; until an issue surfaces.

If the lash has gone 355k miles without servicing then it’s certainly exceeded expectations by a country mile and then some.

Thanks for all the helpful comments.
I have checked spark plugs, coils, fuel injector (13ohm and can hear opening and closing with a stethoscope).
The only problem that has come up so far is a low compression of 120 vs 170 on other cylinders. The fuel economy seems OK (around 20-22 mpg on freeway), no significant shake or hesitation during idle or acceleration.
Occasional hesitation when starting motor in the morning. Then Check Engine Light comes back on after driving the car for 5 - 10 miles.
Thanks to @circuitsmith for suggesting a wet compression test to determine if its the valves or piston rings and all others for your input. I will post more as I find out more.
Thanks again.


Glad to hear you’ve checked out the injectors, and pretty much ruled them out