94 Ultra - Idle is misbehaving

[Cntrysthbst]

Sometimes when I stop at a light, the idle increases to 1200 rpm and back to 800 rpm. Keeps going back and forth for a few seconds, then stops at normal idle. Sometimes it does it sometimes it doesn't. Just replaced the TPS a couple weeks ago. IAC is clean and working. K&N filter is clean to. No codes. ???

It just started doing this a day ago or so.

[wrightz28]

I'd start with making sure the TPS is fully secured in place and look fo rvacuum leaks.

[Cntrysthbst]

TPS is secure, and I see or hear no vacuum leaks. Nothing disconnected.

[Bassasasin]

Things that make you go vroom.

Without codes.
Fuel, AC, TPS, Camchain, MAF, plugs, wires, dirty intake idle area.


Anyone else have a comment, correction?


Else...on topic...

Reset the PCM. Disconnect the battery 10 seconds.

[Cntrysthbst]

Seemed like today the idle went up to about 1100 and stayed for a few seconds and went back down, instead of it going back and forth. I thought I felt it do this in driving too. Does this point to something more specific?

[Cntrysthbst]

Looks like I'm on my own on this one. Thanks for the help.

[HotZ28]

Originally Posted by Cntrysthbst

TPS is secure, and I see or hear no vacuum leaks. Nothing disconnected.

Looks like I'm on my own on this one. Thanks for the help.

HANG ON THERE YOUR NOT ON YOUR OWN YET

You may not be able to “here” small vacuum leaks. With the engine running, try using a propane torch (unlit - of course) and place the nozzle around different parts of the TB, manifold & vacuum lines. If you find an area that is leaking, the engine speed (rpm) will increase. This could be a vacuum leak (mentioned in two post above) or, un-metered air leak, past the MAF. (TB gasket)?

Other items that should be inspected include, the oxygen sensor and feedback control system. A sluggish oxygen sensor, for example, may not allow the fuel mixture to change back and forth quickly enough to keep the converter working at peak efficiency. While observing the 02 with a scanner, you should see a “rapid flash” of a rich/lean. If the oxygen sensor has died altogether, the fuel mixture will remain fixed and the engine will probably run too rich causing an increase in fuel consumption as well as emissions.

Also, in your other post, you said your exhaust system is getting too hot. Converters can fail if they get too hot. One reason that they get hot, is due to unburned fuel in the exhaust. Contributing factors include a rich fuel mixture, ignition misfire (a fouled spark plug or bad plug wire) or a burned exhaust valve that leaks compression.

Fuel in the exhaust has the same effect as dumping gasoline on a bed of glowing embers. “Things get hot fast”. If the converter's temperature climbs high enough, it can melt the ceramic substrate that supports the catalyst, causing a partial or complete blockage inside.

This condition increases backpressure, preventing the engine from exhaling and robbing it of power. Fuel consumption may increase and the engine may feel sluggish at higher speeds or, if the converter becomes completely clogged up, the engine may even stall after starting, then not restart.

Now with all that said, what should you do?

1) Do the propane test (cheap).

2) Monitor 02 sensor performance and other engine sensor performance, on a "scanner". Remember, a failing 02 sensor usually will not set a code until it is nearly non-functional!

3) Use an infrared temperature instrument, to monitor exhaust temperature at various locations. (Any good muffler shop should have one and know how to use it)