Greetings, I thought I would relate my experience after doing what I thought would be a not too difficult valve cover replacement.

Spoiler: The problem was that I had a air leak in the lower intake gasket.


Now for the Tale and the Troubleshooting:

Vehicle:
1997 4Runner Limited, Automatic with the 3.4 L V6 engine, 235K miles.

Issue:
I was getting tired of smelling burnt oil, that seemed to be leaking from the valve cover gaskets. After researching a bunch of articles and videos from this forum I decided to order new valve cover gaskets, Cam Plugs, Spark Plugs and intake manifold gaskets and do the job.

It took me about 8 hours, with a good 4 hours cleaning the valve covers, removing crud, cleaning the 4 1/2 moon cam plugs, installing the new spark plug grommets.

When it was all finished, everything connected, I went to start the car and it idled terribly. At 750 RPM it would die. Above 1500 was OK. I was bummed!

I spent the night looking over this forum trying to think what it could be the problem. I'm a big believer in cause and effect, so what did I change that would cause this problem?

The next morning the first thing I tried was to clean the very dirty engine ground on the driver side of the intake manifold..... No change.

I then tore everything apart with the exception of removing the valve covers, double checked, put it back together... The idle was a little better, but still rough. I went for a 7 mile test ride and then check the engine codes and saw:
P0116 Engine coolant temperature sensor Rage
P0171 system too lean Bank 1
P0306, 0305, 0303, 0302, 0301 (misfires on 6,5,3,2,1)


Cleared codes, ran again, sprayed carb cleaner near center of intake manifold near fuel rails.
RPM increased (maybe leak in the gasket, it is a hard metal gasket)?

Got: P0130 02 Sensor Circuit (Bank 1 Sensor 1)
P0133 02 Sensor Circuit Slow Response (Bank 1 Sensor 1)

Pulled ELM fuse, reset it, no difference.

As the engine is running I started spraying carb cleaner around the vacuum hoses and intake, and near the front of the intake on the bottom, I could hear a RPM increase.

This morning I pulled of the intake, and noticed on the bottom of the lower gasket there was a small 1/4" roundish buildup of smudge, that almost looked like compressed graphite. I cleaned up (again) all 4 mating surfaces of the intake system, put in new gaskets (again, but this time I tried the fel-pro brand), the previous new gasket was LIMICAR brand.

I put everything back together, hit the key and it started right up and idles like it should. Went out for a 10 mile test ride, and it runs great! No idle issue, no engine codes, will idle at 750 RPM just fine.


Thinking about the issue with the air leak in the lower part of the intake it all makes sense.

When at a low RPM, below 1500, Too much air was getting into the engine causing random misfires on each cylinder and also other engine codes including the P0171 system too lean Bank 1. At a higher RPM the small amount of air sneaking in was not enough to affect the larger volume air entering the engine.

Well I spent about an additional 10 hours trying to understand/fix this issue, but I learned a few good lessons.

1: If something is not working, what is the last thing you changed.
2: Engine Idle/Misfire issues will be related to fuel, air, spark. First check what you worked on. I was pretty sure spark was good (new plugs), fuel should be good, I did not touch the fuel system, that leaves air.
3: A small leak on the intake manifold can disguise itself as different issues.
I had all these codes all because of the intake leak:
P0116 Engine coolant temperature sensor Rage
P0171 system too lean Bank 1
P0300 Misfire detected
P0306, 0305, 0303, 0302, 0301 (Misfire on specific cylinder)
P0130 02 Sensor Circuit (Bank 1 Sensor 1)
P0133 02 Sensor Circuit Slow Response (Bank 1 Sensor 1)


Attached are pictures of the offending issue:

-andy

Attached Images

   

Any air getting into the engine that is not going through the MAF sensor will screw things up royally.

When I put the engine in my 97 I somehow put the throttle body gasket on wrong. I got a system lean code as the only indication. Brake cleaner found it. But my idle was normal with a smooth idle at 750. My pounds of air from the MAF sensor was off compared to the other 97 in my fleet.


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You did a good job in troubleshooting on the why that was happening.
Keep it up, and welcome to the forum!

You did a great job figuring out your problem. But, here's some advice. You pretty much can't go wrong with OEM gaskets or OEM anything for that matter. If you would have went OEM to begin with, you probably wouldn't have had this issue. I haven't used OEM for everything, but for most things I have. A lot of times it comes down to how much you value your time. I've heard guys admitting they are on their 5th steering rack replacement because they got a lifetime warranty one from an auto parts store or somewhere else. Well, that's all fine and dandy that you keep on getting free replacements but what is your time worth?

Something to think about as you move forward and do other jobs on your rig. Quite often the money you saved going aftermarket ended up not quite paying off in the long run.

I'm still shocked what how that little air leak going directly into the combustion chamber made such a huge difference at idle.
When I started the 4runner for the first time after the fix, it was idling well, but I heard a whistling sound. I had forgotten to hook in the large PVC hose that attached into the rear side of the air box, even with that out it idled well, and sounded much better once connected, but that little air leak at the bottom of the intake manifold caused all these mysterious problems.

I agree OEM is always the best choice, unfortunately I was in a time crunch (or thought I was). The gasket I put on probably would have been OK if:
1: I had double checked it for flaws.
2: Double checked the mating surfaces to make sure they were clean.

I still don't know if it was the gasket or my poor preparation of the intake, either way I should have double checked first, lesson learned.

Thanks for the post, good info to share on here...