Aecer

My 1990 4Runner has a fast idle when cold at about 2000 RPM. When it warms it drops to 1000 RPM. I finished a head job about 2 months ago and replaced right side head (not left). Idle was OK before the head job. Idle seemed OK after the job, but then temp dropped and with the dropping temp this problem appeared. (Idle may have been somewhat high immediately after the head job when the weather was warm, but I was so relieved it started and ran after all that work, I wasn't focused on details.)

Within one tank of gas after the head job, I got a check engine code 5-2 for disconnected anti-knock sensor. It intermittently showed that code for a month. It was always off when started, then appeared usually during first hard acceleration, but sometimes it would run for a few trips and not appear. It has been off for a month now. I suspected the contacts on the anti-knock connector were corroded and they have now settled down to making contact.

Obviously, to get the head off, everything was disconnected, but I haven't changed or adjusted anything - yet. It's a 3.0 liter 3CV engine, with 285,000 miles. Currently getting 17 MPG and more power than before the job. That seems low on the MPG, but I'm not certain.

I did the right side only because the right cylinders were all below 75 psi compression before job while the left were all above 175 psi.

Cylinder head gasket was replaced twice before - once when it failed (hydraulic lock!) and once shortly thereafter when they realized those head gaskets were all crap. (I suspect the all good compression on one side and the all bad on the other came from something done poorly during the two previous head gasket changes, but can't be sure.)

Anyone have any recommendations on what to check for the fast idle I'm experiencing now? Thanks in advance.

Victor

I think broken knock sensor wiring is very common after work like you described. Not doing the other head doesn't seem like a very good idea. Now you have the other side that may fail soon and you'll be tearing it all apart again. Just 2 cents...

thook

2k rpm? That's really high. Have you checked the base timing on it? What are your compression readings like now?

thook

Unlikely with good compression readings like his.

Unless....you're burning oil?

Wait!!!!! Scratch that. Above 175psi is actually pretty high....overly, in fact. Ummm.....whe's the last time you had the valves adjusted?

Aecer

I thought long and hard about the decision not to do the other side. The body is rusting away and at 286,000 miles, I don't expect it to last that much longer. Another winter or two is all I wanted out of it. Since the compression on the good side was perfect, and it was going to be another several hundred to do the second side, I gambled.

Compression is now above 175 on all 6 cylinders.

I should have mentioned another symptom:

Once or twice the idle has gone into fluctuations when it's warm and I come off the highway. It seems to drop to about 700 RPM quickly, like it wants to die or go to normal idle, then quickly jumps back to 1100-1200.

BTW, on the way home - outside temp mid 40s the idle started at 1750 and dropped to about 850 after fully warm. It's only with temps below freezing that I see the really high idle.

Aecer

Haven't checked base timing yet - (it's cold here for a shadetree mechanic) could mis-timing give me this high idle?

Aecer

Not burning oil. I know compression is high - but I'm guessing it's my crappy gauge. The cylinder walls and piston tops looked great on the right. Not much carbon - no scoring. The valves on the left weren't touched, last adjusted about 60K miles back, the right side was done with the head.

thook

Oh yes. If the timing's too far advanced, it will.

The rec'd service interval on valve adjustment is 60k, if that means anything to you. Tight exhaust valves would cause high compression, and it's even possible they were tightened too much the last time. This kind of condition leads to blown headgaskets. But, make sure the gauge you're using is correct. No point in tearing it all apart because your equipment isn't doing the job.

You can rent compression tools from local parts stores....ie, O'Reilly....to eliminate your gauge as a problem.

Victor

Another suggestion I have is to clean the throttle body very thoroughly. I had a fluctuating idle (not as high as yours) and cleaning the TB fixed it nicely.

Matt16

A gummed up TB plugs up the air passaged which bypass the throttle plate. The idle is turned up to compensate for the low idle. When the TB is cleaned, the idle increases because the little bypass passages are unplugged and more air gets past the closed throttle plate.

Try thoroughly cleaning the TB, then resetting the idle afterwards.

Aecer

To all: Thanks for the suggestions. I'm going to check timing, clean the TB and reset the idle. Today (45 degrees) the idle started at 1700 and dropped to 1100 after a long highway run.

Aecer

OK, I've checked timing (good), cleaned the TB (not perfect, but better than you might expect after 286K miles), and not (yet) touched or reset the idle. No change was seen.

Today (40 degrees) the idle started at 2000, dropped to 1700 within a minute and dropped to 1100 after a long highway run. Immediately after I came off the highway run and stopped at the first light, when very warm, it began to fluctuate between 1100 and 900. It would settle to a smooth idle at 1100, but as soon as it settled to a smooth idle it seemed to want to decrease the idle, and the moment it dropped at all, it would pop back up above 1100 and begin to settle again towards 1100, then repeat the cycle. It never would get below 900. A cycle took perhaps 3-5 seconds.

It continued this for 6-8 cycles. To stop the fluctuations, I tried tapping the throttle and it then settled at 1100. It's then fine until the engine is cold, but with the 1100-1200 RPM idle. I've seen this behavior several times.

In light of the above - does adjusting idle sound like the correct next step or is there something else I should look at first?

Victor

Another thing to consider is the Throttle Position Sensor. If it is bad - or out of adjustment - then you can have a fluctuating idle. Basically it tells the computer how far open the throttle butterfly is. I don't know that the one on the 3.0 is adjustable but your symptoms sound fairly common for a TPS. Try adjusting the idle once the engine is warm and see if that fixes it - if not then do a search and see how to test a TPS.

SEAIRESCUE

The throttle body dashpot may be giving you problems since its old. I would remove it to see if it corrects the problem. Toyota puts an Idle Air Control on most, of not all, of its engines. Not sure if it integrated into the throttle body or a separate item on the fenderwell. I had trouble with one of those on a 22re engine. The brass screw on the throttle body needs to be removed and lots of carb cleaner needs to be sprayed in that hole. I had idle problems with a 3.0 and engine blowby had gummed that passage.

Hope these ideas help.