I have a '99 4runner, 3.4L (5vz)and a manual transmission. Ever since it turned over 150,000 miles, it has sounded like a diesel engine when the air conditioner is on. Thankfully, I don't run the A/C much. It is especially prominent when rolling heavily into the gas (like merging on the interstate or pulling long grades). The only modifications I have done are the Intake Silencer with a K&N filter in the airbox. Other than that it's stock. I am not horribly worried about it, since I usually roll with the windows down, but I am worried about the longevity of the engine. I am not sure if it is a valve issue (need adjustment, etc.) or if I need to be looking for something more serious. I doesn't seem to be as bad on 93 octane. Any ideas? Thanks in advance

 

First thing you need to do is isolate the noise. Find out if its coming from the compressor or some other place. Drop the skid pan and slide under there and have someone turn the A/C on and off.

When an A/C system on a car isn't used much or not used for a long time it causes problems. One of the biggest problems is the rubber seals on the connections and on the main shaft of the compressor dry out due to lack of oil circulation (carried with the refrigerant).

When you say it sounds like a diesel engine, is its clattering or knocking sound? Also if your parked in neutral and step on the gas with the A/C running and bring it up to 2-2500 rpms does it also make the noise??

 

Fogrunner, thanks for the reply. As for what it sounds like, it has the distinct sound of valves rattling, and it tends to get faster with the engine RPMs. I will take your suggestions, as soon as I get the chance and have someone around to help me out. As I said before, the engine performs flawlessly with out the a/c on. When it is on though, there is a noticeable loss of power, the aforementioned rattle, and the confusion (is that detonation? what broke? oh wait... the a/c is on). As soon as the A/C is turned off, the whole situation goes away.

As for video, that would require more technology and technological know-how than I presently have at my disposal, but an otherwise good idea.

Thanks guys for the help! Let me know if there's something I have missed!

 

If your truck performs good with out the A/C on (hard acceleration) then there is nothing wrong with the engine or should I say the load the compressor is putting on the engine isn't causing the engine any issues.

99% of the time when you have a rattling/clattering that only is heard when you turn on the A/C compressor its going to be the A/C compressor is dying.

If this is the case you should replace it soon. Too many people just say they will keep running it till it stops working and then fix it. But what they don't realize is that as a compressor is dying its dumping out metal shavings in the A/C system that can clog up critical compoants.

 

Fog, I regard you as "the man", sir. On my next trip back to Knoxville, I will be having the rig worked over, as you have suggested. It's about timing belt time anyway, and I noticed a little bit of a hesitation today, so a tune up may also be in the works. If only I had the time to get a good port and polish job and maybe some camshafts... and a little cylinder wall refresh...

Like a buddy of mine puts it: "It's just money, right?" (Note that he says that with an overbearing sarcastic tone)

Thanks again for the advice! YT members pull thru again!