Cylinder 6 Misfire - Valve Adjustment?

Hi there I have a 2002 SR5 w/ 252K (5VZFE) that I have only owned for a year and I have no info on previous maintenance. It is in excellent condition for the most part, but I have been having intermittent cylinder misfires (P0300/P0306) which turned out to be low compression on cylinder 6 (110PSI). No fluid leaks or burning oil and it runs perfect most of the time. It idles slightly rough but only occasionally. Before I ran the compression test I replaced plugs and wires and reset the computer and it ran fine for 3k miles before throwing up the check engine light again. I swapped the coils and reset the code again and it ran smoothly for another 2k miles with no engine light. It just recently went back on and cylinder 6 was still the culprit (coils are good) so I checked the compression and it came back low at 110PSI. At this point I did not have the tools to do a leakdown test so I took it to a mechanic to try to pinpoint the issue. He believes that the best case scenario is a tight valve on the exhaust side which will require having the valves checked and adjusted for $700 (includes new gaskets for valve covers, plenum, etc).

After scouring just about every 4runner forum out there I cannot find a single case where a 5VZFE with a misfiring cylinder was solved with a valve adjustment. Seems more likely that it is going to be a burnt valve or an issue where the entire head needs to be pulled and I am worried that paying for labor to take off the valve covers and have a look is going to be like opening pandora's box. Given that this problem seems very intermittent and is not causing any significant issues besides a rough idle every couple months on hot days, I would like to avoid dropping thousands of dollars into it. If anyone has experience with a similar case I would love to hear it. Thanks.

 

honestly it sounds like a blown head gasket based on the symptoms youre describing (low pressure, rough idle, misfire). id take it to a mechanic and see what they they of it, but that sure is what it sounds like

 

Wouldn't I be seeing a loss of oil/coolant, a mixing of the two or issues with overheating if it was a head gasket? I've been doing oil changes every 3k miles and getting out about 1/2 a quart less than I put in and it does not look to be contaminated (seems pretty good for an engine with this many miles). I also flushed the radiator 5k miles ago and it was still very full and showed no signs of fluid loss.

I've also put 5k miles driving 6+ hours at a time across the desert in 110 degree weather since first detecting the misfire. I changed the spark plugs and harness and reset the check engine light and the car ran perfectly for 3k miles. Then a mild rough idle started again and the light came on. Swapped out the coils and reset the codes and ran another 2k miles before it started idling rough again and the light came on.

I feel like I would have seen serious overheating and fluid issues in those 5k miles if it was a head gasket, but I could be wrong...

 

i had a blown HG and didnt have overheating or significant fluid loss, you have to work to find it sometimes, it isn't always as obvious. i suspect mine was blown for 6-7k miles

 

I had a 2001 Tacoma with the same motor and kept getting P301 miscellaneous misfire cylinder #1 ended up being a bad fuel injector, I replaced all 6. Problem solved.... Hope this helps....

 

Have the same problem with my 96. Replaced coil, switched plugs still problem.

 

Have you changed fuel injectors?

 

Ok

Now I'm not one of those ASE Master guesses but this is my .02 on this issue.

First things first. I would not worry about the 110 lbs just yet. First rule out a blowen head gasket. It's a simple test. Have them run a CO2 test at a dead cold start up through the radiator and not the exhaust. What you or they need to be looking for is carbon dioxide in the cooling system and that will turn yellow at the radiator cap if it fails. Sometimes it takes awhile to ture lime an hour for that head gasket to expand and start leaking. If this dose not turn yellow and stays blue your solid and no headgasket leak.

At cylinder number 6 pull the spark plug also. If it has white crusty crap on it that is coolant burning IE blowen head gasket.

Next- if after doing g a really good CO2 test and it passes then I would starting looking g at the fuel injector as said above. That was my first though really before you brought up the head gasket. Here is the issue with testing the injector. There is really no good way to do it, sucks but that reality. Get on Amazon and get a OE Injector and get a O ring kit for it also. Replace that and see if it clears up.

I honestly feel like you might have an injector issue but I'm not saying you don't have a head issue either.

Something that actually works really well is the Chevron Techron Fuel Injector cleaner. It's really the only thing worth a crap. Use it the next few times you fill up and see if that dosent help. It cleaned up on of my lazy injectors pretty quick and that is all we use in our own automotive shop because it really works.

You really need to rule out the head gasket leak first then go from there. If you have more questions hit me up.

Blue

 

try just changing out the plugs I had an issue with this and it fixed it oh and make sure the wires are in right sequence! they can be the right firing order but if the wire aren't where there supposed to be it will run worse lol