Jen97

Hello, I'm new to this site and hoping (praying, actually) that someone here can offer some insight and possibly better news than I got from my mechanic. I have a 97 4runner with 3.4L V6, plus supercharger. It's been running rough lately, and check engine light is on. OBD codes for lean fuel, misfire in cylinder 6 and knock sensor were showing up. Mechanic found a broken vaccuum hose to fuel pressure regulator, and replaced it, and two of the codes cleared but it's still showing the misfire. Mechanic checked spark plugs, wires, coil test and injector test which all turned out ok. Cranking compression test showed about 150 psi for all cylinders, but running compression test showed cyl. 6 at 25 psi and all others at about 130. He said I will possibly need to replace the camshaft... So my heart sinks. I LOVE this truck, and I need some help/ideas for determining what the problem is, as I understand it could be anything from rings to head gaskets. Should I get a 2nd opinion? Any tests I can do myself? Boyfriend is pretty mechanical and seems to think he can do the teardown, etc. But I'm wondering if there's anything else I can test to see what we're going in for and what to expect for replacement parts. I should add that I am leery of this mechanic since I have never been there before and have had previous problems with misdiagnosis (I once had a mechanic tell me I needed a new transmission and it turned out it needed plugs and wires!!!!)Anyway, ANY help or suggestions will be greatly appreciated. I do have the Haynes book for it, which shows the procedures for inspection, etc. but wondering if anyone knows any alternate tests before we tear the whole thing apart. THANKS!!!!!!!

mastacox

Yikes, sounds like you may have cooked a valve from an extreme lean condition following your FPR mishap. That, added upon the fact that I'm guessing you don't have fuel mods leads me to believe you're going to need a top-end rebuild.

Better talk to Mark (midiwall), he may have some insight for you as he just went through a very similar situation. See for yourself, he roasted cyl. #6 and had piston pitting in #4:

Thread: 3.4l Rebuild

Jen97

Ok, that's not exactly the news I'd hoped for, but I appreciate the guidance. I saw something on another thread about a test to check the cylinder by putting some oil into it. Something along the lines of if oil helps raise compression it could be a ring or if it doesn't maybe a valve? Could anyone elaborate on how exactly to do this test and what to look for? Perhaps that could narrow it down. I'd hate to do more than necessary right now, because I'm pretty low on funds... Also, what's the difference between the two compression tests(cranking and running)? How do you do the running compression test? and which one do I do with the oil test?????

TC22RE

Get your engine ready to do a compression test, just keep it from firing up while you're running the test. Remove your spark plug from the suspect cylinder, install your compression tester, crank engine for a few seconds, check and note compression reading. Remove compression tester, pour a little oil into suspect cylinder, (squirting some through a small tube may help), re-install compression tester, crank engine for a few seconds, check and note compression. If your compression has raised substantially from the first test than your rings may be causing a problem. If not, it could be a valve. It really isnt a surefire way to check for these things and YMMV, but since you asked thats how you can go about doing it. Although, wouldnt a leakdown test be better?

As far as the difference between a cranking and running compression test... I've only ever done/heard of a cranking compression test... which is cranking on the engine without it firing up.

mastacox

Yes a compression test plus a leakdown test should be able to tell you whether its rings or valves, but in either case the top end must come off. I'm not too sure about the "running" compression test either, maybe your mechanic could elaborate to you a little more about the test.

A compression test should be as described above, and the leakdown test is when you pump air into the cylinder and watch to see how long it holds pressure for. Could these be the two compression tests you mentioned?

Jen97

Ok, well just so you know, I am definitely not a mechanic, and I am giving what information was given to me and going on what I have read on such problems, but here goes. I don't know anything about any of these tests, but mechanic said "cranking compression test shows all cylinders with psi of about 150" and "running compression test shows low psi in cyl. 6" (see orig. post) So I'm not sure what the difference is there. And you all would know better than me-would a leakdown test be better????? How is that done? Sorry if I'm not clear, or sound like a amateur, I am!! I'm stuggling to keep up with all this new info, and basically trying to get the best plan of action for the poor guy who gets to do the tinkering (my boyfriend). Thanks for putting up with my ignorance!!

Paul H.

I would definitly ge a second opinion. Ask friends and coworkers for a good mechanic reference.

BruceTS

When running a supercharger without any fuel mods, there's a very good chance of frying a valve or two, due to a lean out condition. I seriously doubt the cam went bad, more likely a valve clearance problem, but this could be caused by a burnt valve.

To me it sounds like the top-end needs to be taken off, very common on SC'ed engines......