Help! Idle problem with hot engine

[pinkflydxx]

1999 Avalon XLS - 6 cyl, 140k miles

I have posted this problem before and tried the recommended solutions with no real resolution to the problem. My car start perfectly and runs fine - until I get it on the interstate and run it for about 20 minutes at a high speed (70 mph or so). When I pull off and come to a stop, the engine tach rev's back and forth from 750 to 1250 or so and an unusual odor from the exhaust. The idle is also extremely rough (almost like it is miss firing). As long as I have the accelerator down, the engine appears to be running fine (as best as I can tell), but when you let off of ithe accelerator pedal, you can notice the tach "bouncing", both at a stop and on the highway. Of course, the vacuum actuator on the throttle body is moving in and out when this problem is occurring.

If I turn off the engine, it will restart, but still idles poorly. However, after I let the engine cool for a few minutes, it starts fine and no idle problem.

In every occurrence, the check engine light has NEVER come on. Therefore, I have no reason at this time to believe that it is related to a sensor (but still have not ruled this out yet). The problem still exists after I have tried the following:

1. Replaced PCV valve
2. Cleaned the throttle body and butterfly valve (without taking it off, using cleaner and getting as much "gunk" out of it as I could reach by inserting a cloth through the opening)
3. Checking the vacuum hoses for cracks as well as possible (have not sprayed with ether to check)
4. Checked if problem was coolant engine temp sending unit (by disconnecting it while problem was occuring - not sure if this was an accurate check for this item and diagnosis)
5. Changed plugs to Denso iridium

I have heard that idle problems can be caused by the IAC without it throwing a check engine code. I am ready to try taking the throttle body off and thoroughly cleaning it. I wonder if the IAC can be cleaned as well, or if I should even try it. Could it even be caused by the IAC if the problem occurred only when the engine gets hot.

How about the Throttle position sensor?

Although it seems unlikely, is there any chance that a sensor somewhere is bad but not sending a proper check engine code?

Any other suggestions/recommendations would be greatly appreciated because I seem to be running out of ideas and options on what would cause this problem and would really like to get this properly diagnosed and repaired as it is the primary vehicle for my wife and children.

Thanks

[tfeld]

I would not rule out a cracked intake hose. They can have cracks on the underside that would be very hard to see--best diagnosed with some TB spray (ether will work, but it's dangerous to be spraying around a hot engine). Since you have sprayed the TB, you obviously removed the hoses, so you or someone else may have inadvertently cracked them when pulling them off. Make sure all your hoses between the MAF and TB are tight too. Any leaks there can cause what you're describing.

Cleaning the TB with it installed is good for a sticking butterfly, but won't get the small ports clean. For a really thorough job, you have to remove it IMO. At the same time, you can remove the IAC and clean it and the port which can get full of funk. IAC is easy to remove and clean.

If it's the TPS that's acting up, try pulling the vac hose to it when this is happening. If it settles down, it may indicate a bad TPS or bad wiring/connection. Setting the TPS is a PIA, so don't remove it or change it's position unless you're sure it's bad and needs replacing.

[pinkflydxx]

Tfeld,
Not sure how a crack would just occur, or how it would occur after getting engine hot, but certainly worth the time to make a check as suggested.

I assume that you are referring to the hoses between the MAF and the TB and not necesarily between the MAF and the air filter housing? I will also check if the hoses are reattached tight with the spray check.

Also when checking with TB spray or ether, I presume that if a crack or loose hose is the problem, then the engine will increase RPM rather abruptly as the spray eventually enters the cylinders?

If the spray checks do not identify a problem, I will then check the TPS. However, I do not see a specific vacuum hose going to it the TPS? I can see the TPS electrical connector but not the vac hose going to it. Can you tell me where the vac hose is located relative to the TB and TPS, or which vac hose to the TB you are referring to? Or are you referring to disconnecting the electrical connection to theTPS while the problem is occuring instead of the vac hose? Please explain.

(FYI the TB is a single inlet port type - 99 V6 Avalon)

Ultimately, I will take the entire TB off and clean it and the IAC thoroughly. The Haynes manual states to not use the solvent to clean the IAC or the TPS? Use Electrical cleaner instead? And do you have to remove both from the TB in order to clean the TB, as well as the TPS and IAC properly?

Sorry for so many other questions, but the manual does not give a lot of details. Thanks again

[tfeld]

Cracks can occur on older vehicles as the rubber tube becomes brittle. Yes, it's the one between the MAF and TB that I'm reffering to. When the hose is flexed or removed to access the TB, etc, it can get small cracks--often on the underside where it's difficult to see. It's not as uncommon as one would hope, especially in hotter climates (don't know where you live). Usually, the problem is noticed when you give it gas and the engine torques just a bit, which opens the crack and lets additional air into the engine without benefit of the MAF sensing it. This messes up the A/F ratio. It may not be your problem, but is pretty easy to check and eliminate. If there's a crack, you will see/hear an increase in rpm when you hit it with the spray. Spray a bit on the MAF housing too, just in case the gasket is bad.

I'm not familiar with your particular year/engine--I have a 2001 with dual TB setup. But all the other Toyotas I've worked on have a vac line to the TPS diaphram (throttle opener--it's on the side of the TB opposite to the actual TPS electrical part). I'm not aware of any entirely electrical versions of this. If yours is totally electrical, then I'm lost on this bit. Sorry if I've confused you on this: there is a TPS sensor itself which is electrical, and there's a throttle opener which is connected to the TPS by a shaft running thru the TB. The opener has the vac hose.

As I said earlier, DON'T remove the TPS from the TB unless you absolutely have to--it's a PIA to readjust. The IAC is easy to remove and clean. I know it says not to use solvent, but I've done so in the past and really don't know why they say this. I think it's safe to use a cloth with mineral spirits to remove the crud that will be on the piston. Otherwise use some electrical cleaner. Once it's off, you can hit the TB with pretty much any solvent you want (just try to keep it off the TPS).

If you have the factory service manual, you should also check the resistance of the TPS. Good that you have one--most folks don't bother. I would also check the resistance of the MAF to make sure it's working right--it too could be a problem spot, especially since your problem tends to be temperature sensitive (the TPS and MAF both operate in a temperature dependent manner). Just in case you only have a Haynes, here are the resistance specs for my 2001 (which may be different for you car):

MAF: Terminal THA-E2
13.6-18.4 ohms @ -4F
2.21-2.69 ohms @ 20F (i.e., cold engine)
0.49-0.67 ohms @ 140F (hot engine)

Also, with the MAF, you should test with air flow. Connect meter to (+)VG and (-) E2G, turn igition to ON, and blow thru the MAF. You should see a voltage fluctuation

TPS: Disconnect the vac hose from the TB to the throttle opener, and apply vac with to the opener as needed (with a Mighty vac or similar). Measure resistance across VTA and E2 terminals:

Throttle Valve Open (i.e., vac applied): 2.0 - 10.2 kOhms
Throttle Valve Closed (no vac): 0.2 - 6.3 kOhms

[LGB0027]

I am also having this exact same problem with my 1998 Toyota Avalon. Almost freaky how exactly similar. Did you ever find the solution? Hopefully you still check in with this forum. It would be great to know if you ever got the problem fixed.

Thanks -

Larry

[pinkflydxx]

Yes, I did resolve the issue. The problem was in the EGR tube and/or valve. Sorry but it has been so long ago that I had this problem I can't remember all the details.

I remember I took the EGR tube off (it's a stainless tube mounted with 2 bolts on a flange on each end). I cleaned it and a spring activated valve very well with carb-cleaner. This spring/valve may be part of one of the EGR sensors. The spring itself comes in contact with the EGR gases. I remember I could not easily reach that spring and had to use a screwdriver into the tube/hole to work in up and down. I can't remember the actual location of the spring valve, but it is in the EGR system and I guess over the years it had clogged up with deposits. It may be built directly on to the EGR tube itself, but once you find it, you will understand what I am trying to describe. I cleaned it well with the carb-cleaner and used a screw driver to work the spring back and forth with the carb cleaner saturating it. I worked it several times to get it to work as freely as possible.

After I reinstalled them, I had not further problems.

Sorry I do not remember all the details, but I am sure that what I did fixed the problem after no other sensor changes, etc worked. Best of luck and I hope this information helps.